vf: sensory, module a

Threshold Determination

-Ascending limits -Descending limits -Staircase -Constant stimuli -Adjustment -Forced choice

Hypercolumn

A complete set of ocular dominance columns (both eyes) and orientation columns (all eyes) forms a hyper column Each hyper column has dimensions of ab 1 x 1 mm

Classes of horizontal cells

H1: receive input primarily from M and L cones (little input from the S cones) H2: Strong connectivity with S cones, and also receive input from M and L cones Horizontal cells show graded responses and do not generate action potentials Because photoreceptors and horizontal cells both hyperpolarize in response to light, the synapses connecting them are referred to as sign-conserving synapses

Bilateral visual deprivation

If the occlusion is removed later in life, vision still remains profoundly disturbed

Stiles Crawford Cont.

If there is a decentered pupil for some reason, the cones orient themselves such that they now point toward the center of the pupil, maximizing their effectiveness in capturing photons of light

Demonstration of our reduced sensitivity to low temporal frequencies

In the figure, if you stare at the top X, the blurred border will disappear; but fixation of the bottom x does not cause the sharply focused border to disappear This is bc even when fixating a target, small involuntary eye movements occur continuously When these eye movements occur across the blurred border (a gradual change in illumination), the changes in retinal illumination are of a very low temporal frequency, approaching 0 Hz. Because we are NOT SENSITIVE TO LOW TEMPORAL FREQUENCIES, the border disappears In comparison, eye movements across the sharply focused border result in the introduction of moderate temporal frequencies (its a sharper change in illumination). We are more sensitive to these moderate temporal frequencies, so the border doesn't disappear

Refractive errors early in life

Infants often have astigmatism One study showed half of neonates manifest astigmatism between 0.75 to 2 D Axis may depend on infants race: -Caucasian: ATR -Chinese: WTR Amount of asigmatism generally decreases over the first 5-6 years of life Unusually high amounts of astigmatism at a young age may way warrant correction (we just don't want to interfere with emmetropization) Significant amounts of astigmatism, esp ATR, may produce asthenia, which could discourage a child from reading So clinicians may consider prescribing lenses for young school age children w/ at least 2 D of astigmatism that shows no signs of abating

Preferential looking

Infants prefer visually interesting targets (like a grating or flashing light) over ones that are less interesting Used to assess infant grating acuity as well as other functions, like contrast sensitivity and vernier acuity For grating acuity, the patterned stimulus (a spatial grating) and a non patterned stimulus (uniform gray circle) have the same avg luminance, eliminating it as a cue. By observing the infant's direction of gaze, the examiner guesses which side the pattern is on. From trial to trial, the positions of the stimuli are randomly interchanged. The examiner must rely on observation of the child's gaze to determine it's location. Since the examiner is required to guess which side the pattern is on, the procedure is referred to as forced-choice preferential looking Several trials are conducted for different spatial frequencies and a psychometric function is plotted. As the SF decreases, the percent correct increases up to 100% For the highest SF, the percent correct is 50%, which represents a chance performance for a 2AFC procedure (i.e. the infant cannot resolve the SF grating). Threshold is represented by the midway pt between chance performance and 100% performance (at 75%). The SF that elicits this percentage correct represents the infants grating acuity

Deprivation Studies

Info from the two eyes is first combined in striate cortex, with most visual cortex neurons receiving input from both eyes BUT most neurons do not receive equal input form the two eyes: one eye tends to dominate a given cortical cell The ocular dominance histogram is constructed based on recordings from individual neurons in one hemisphere of the adult striate cortex Cells in categories 1 and 7 are monocular with category 1 cells receiving input from only the contralateral eye and category 7 cells receiving input form only the ipsilateral eye Neurons in category 4 are binocular and have equal input from both eyes Cells in the remaining categories are also binocular, but dominated by one of the eyes

Spatial Summation (scotopic vision)

Lots of photoreceptors (rods) for each ganglion cell Patient will see one spot of light in the example bc 10 quanta are activating the ganglion cell Excellent spatial summation, poor spatial resolution (you cant see that there are two patches of separate light)

Handheld color filter

May allow certain pts to distinguish otherwise indistinguishable objects; useful because if you have a red contact only in one eye all the time, you might suppress one of your eyes It is important for the pt to understand that the filter will be useful only if objects are alternatively viewed with and without it

Demonstration of the contrast sensitivity function

Moderate SFs are seen at LOWER CONTRASTS (higher in the CSF and in img) than low or high SFs

Parvo and mango neurons

Parvo and magno cells account for 80% of the retinogeniculate pathway (70% parvo, 10% magno) Rods primarily feed into the magno pathway Theres less info on konio neurons

Grating Acuity

Pt is required to distinguish a grating form a uniform patch of light of equal luminance A form of RESOLTUION acuity An example would be 40 cycles/degree

How does an early cataract affect the increment thresholds measured during visual fields?

Reduce the amount of light reaching the eye but! this means that it not only blocks the increment, but also the background (both intensities decrease) Therefore, the Weber's law fraction remains the same, just decreases both the increment and background intensity. This means that the contrast remains the same (the ratio between the brightness of the increment and background), and therefore the visual field increment threshold remains the same. Threshold would've changed if the contrast/ratio between the brightnesses changed.

Dorsal processing stream

Referred to as the parietal pathway or "where" system Predominant info from the magno pathway

30 Hz fast flicker ERG

Reflects the electrical activity of ON AND OFF cone bipolar cells

Where does the N95 potential for the pattern electroretinogram (PERG) originate from?

Retinal ganglion cells If your pt doesn't have a N95 but a and b waves are still present, its probably a ganglion cell problem.

Where does the scotopic threshold response (STR) originate from?

Retinal ganglion cells and amacrine cells

Scotopic Vision

Rod mediated Operates under nighttime conditions High absolute sensitivity (only 10 quantal absorptions required; sensitive to very dim lights) Poor contrast sensitivity Poor VA (20/200) Color blind

Presbyopia and the elderly

The ability to accommodate decreases with age When it becomes symptomatic (resulting in blur or asthenopia) the patient is said to have the condition of presbyopia Usually happens in the mid 40s. One of the primary reasons patients seek eye care Could be due to decreased elasticity of the lens capsule

Color Vision in elderly patients

The ability to discriminate between colors is reduced in elderly patients Reduction in retinal illumination and yellowing of the lens are contributory factors The decreased color vision manifests as a shift toward a tritan defect, with BY discriminations being harder as people age

Development of Vernier Acuity

Vernier acuity, a form of hyperacuity, matures rapidly during the first year or so of life (similar to grating acuity) Then develops more slowly than grating acuity, reaching adult levels at slightly older ages (ab 6-8 years of age); this is not surprising bc vernier acuity is thought to depend on cortical processing, and there is slow maturation of the cortex

Multifocal VEP

Visual field mapped with mini VEPs

Mango Cells

Weak/ no color opponency Regardless of the stimulus wavelength, these cells generally give a response of the same sign (not capable of contributing significantly to wavelength based discrimination)

Determining infant VA

-Optokinetic nystagmus (OKN): not established that OKN relies on the same pathways as conscious visual perception, so pt could have impaired vision, but a normal OKN response -Visually evoked potentials (VEPs): give results that are at variance with those obtained with FPL. VEP shows adult levels at 6-8 months (as compared to FPL's 3-5 yrs). This may be due to the fact that there are greater cognitive demands associated with FPL -Tracking eye movements elicited by visual stimuli: Infants will follow (using smooth pursuit eye movements) resolvable moving targets, and these eye movements may be measured objectively. Advantage is that it may allow screening to be performed by less skilled examiners

CPD to Snellen

1 cycle = 2 MAR in mins 1 min = 60 degrees

Assume that the absorption of 10 quanta of 510 nm by an eye that contains only rods (the eye of a rod monochromat) results in vision. If the wavelength were 450 nm, how many quanta would need to be absorbed to produce vision?

10 quanta (no matter the wavelength!)

Cones

3 types: L cones, M cones, S cones L (long wavelength) cones contain erythrolabe (max 552 or 557); red M cones contain chlorolabe (max 530); green S (short wavelength) cones contain cyanolabe (max 426); blue Half life of cone photopigments is 1.5 mins (regenerate faster) S is a little strange Only the L and M cones contribute to the behaviorally determined photopic spectral sensitivity function If S cones were completely lost, VA would still be 20/20 because they are less important to photopic spectral sensitivity

Anomalies of Color Vision

4.5% of the population manifests anomalous color vision Anomalies of color vision are classified according to the pigment affected and the nature of the defect Most prevalent are INHERITED anomalies (non progressive and pose no threat to vision); close to 8% of men have inherited/hereditary red-green color vision deficiencies The prevalence of red-green color anomalies in preschool girls is closest to 0-0.5% Acquired color vision anomalies are less prevalent and are secondary to disease or drug toxicity

Saturation

A desaturated color, such as a pastel, appears as though it has been mixed with white. It is not bold, but rather washed out A saturated color appears full of color Saturation is a function of wavelength (and also how much white is in the mixture) Yellow is the least saturated color (578 nm)

Metacontrast

A form of BACKWARDS masking (target precedes the mask) In metacontrast, the mask and target are spatially adjacent The visibility of a briefly presented target is reduced by the subsequent presentation of a spatially adjacent mask Lateral inhibition w/ in the retina is thought to contribute to metacontrast. Stimulation of the faster magno pathway appear to be critical for producing this effect

Masking

A mask reduces the visibility of a stimulus referred to as the target In simultaneous masking, both the mask and the target are present at the same time. For example, a spatial grating (the mask) may interfere with the detection of a stimulus composed of a similar frequency (the target). Since both the mask and target may share the same spatial frequency channels, there is a reduction in the visibility of the target gratings Simultaneous masking is more pronounced in AMBLYOPIC patients. An amblyopic eye's VA may be considerably worse when measured with a standard eye chart than with isolated optoytpes. The reduction in acuity caused by surrounding spatial patterns is referred to as the CROWDING PHENOMENON For backward masking, the target precedes the mask. The mask still reduces the visibility of the previously presented target. This may occur when the mask is substantially brighter than the target (enables the mask's neural response to reach central visual areas first) Forward masking: the mask precedes and reduces the visibility of the subsequently presented target

Dichromat

A person who has two photopigments with overlapping absorption spectra Have some degree of color discrimination but it is limited; they have color discrimination if the absorption spectra of the photopigments overlap If you mix wavelengths, two patches can be made to appear identical when the intensities are adjusted so that each patch has the same effect on the visual system (one patch has two wavelengths, and the other patch has one wavelength; three wavelengths, two patches); but two diff wavelengths cannot have their intensities adjusted so that they look the same Better color discrimination than a monochromat

Monochromacy

A person with only one photopigment has no color discrimination When a photopigment molecule absorbs a light quantum, it does not encode for the wavelength of that quantum. All info regarding its wavelength is lost, which is referred to as the PRINCIPLE OF UNIVARIANCE A monochromat's hypothetical absorption spectrum shows that wavelengths of light have different probabilities of absorption, so if two different wavelengths emit the same number of quanta of light, each wavelength may result in a different number of absorptions, and therefore one may appear brighter to the monochromat than the other, and they may interpret this difference in brightness as a difference in color If two patches of light that contain different wavelengths produce the same number of quantal absorptions, the monochromat will not be able to tell the difference between them; this shows that a person with one photopigment cannot make wavelength based discriminations because different wavelengths can always be made to appear identical by adjusting their intensities -- he or she is unable to distinguish objects on the basis of wavelength alone Still, monochromats may very well be able to correctly label colors under certain conditions. If three wavelengths emit the same number of quanta, and are absorbed differently, and therefore appear to have different brightness levels, they may be able to assign color labels to these patches (long story short-- they use brightness to label colors; they've heard other people label dim objects as red, etc)

Blindsight

A pt with blindsight has no conscious vision This is secondary to a lesion that has destroyed all of the striate cortex Sometimes a pt will respond to a visual stimulus while claiming that no stimulus is seen If you have them do a forced choice test, they usually perform above chance. This can be explained because it is possible to activate the extrastriate cortex while bypassing the striate cortex (possibilities include projections through the superior colliculus and pulvinar); although it will not result in conscious vision, it may facilitate non conscious vision, also called blindsight

Test Retest Variability

A pt's measured VA may vary upon reputation even when there is no change in the pt's visual status (aka, is it variability or did the pt's vision actually worsen) It has been suggested that the low SF content is an important contributor to TRV

Treatment with colored filters

A red contact lens worn on one eye is sometimes used with the goal of improving color discrimination The lens is a long pass filter, blocking short wavelengths, and transmitting longer wavelengths The original absorption curve will be displaced towards longer wavelengths; when two patches of light emit the same number of quanta, more of one wavelength will be absorbed for the eye wearing the contact, and the patches will be distinguishable. If we were to increase the intensity of one of those wavelengths for the eye with the contact so that they appear the same, the unaided eye would now be able to distinguish between the patches. Takeaway is that the individual (in this case were using a monochromat) is able to distinguish the two patches if they compare the two eyes; in this way the person with monochromacy is behaving as if they are a dichromat (each eye has a diff absorption spectrum, resulting effectively in two photopigments-- one in each eye) The comparison with the eye not wearing the contact lens is how we obtain the improvement in color vision Glasses with colored filters used as a "cure" for red green color anomalies may work for a standard color vision test, but would fail a test that takes into account the chromatic properties of the colored glasses; color discrimination may be improved in certain regions of the spectrum, while colors that were previously discriminable may no longer be discriminable

Simultaneous Contrast

A stimulus of constant luminance when viewed against backgrounds of various luminances demonstrates that brightness depends on the background Consistent with Weber's Law, the contrast of a stimulus, not its luminance, is the key factor is predicting its appearance

Ganglion cells that contain photopigment

A subset of ganglion cells contain the photopigment MELANOPSIN In rats, these cells project to the suprachiasmic nucleus (SCN) of the hypothalamus (responsible for circadian rhythm); Melanopsin containing ganglion cells (also called intrinsically photosensitive retinal ganglion cells or ipRGCs) found in the macaque retina show peak sensitivity at ab 483 nm and project to the LGN It has been suggested that stimulation of these cells suppresses the release of melatonin, a sleep promoting hormone produced by the pineal gland

Contrast sensitivity as we age

After 65 years old, contrast sensitivity declines more rapidly Senile miosis and nuclear sclerosis (which are normal aging processes that reduce the amount of light reaching the retina) probably contribute to reduced contrast sensitivity. Also likely contributing is the aging of natural elements both in the retina and centrally.

Perceptual Learning and the elderly

Age related impairments in form perception, object and face recognition, and visual processing speed may have a cortical component Amblyopia, which is due to cortical abnormalities, may be treated with perceptual learning. Perceptual learning may improve visual performance in the elderly

CIE Color System

Allows us to precisely specify any possible color A color is specified by the relative amounts of three primaries which when mixed tg, produced the color The CIE primaries are imaginary mathematical transformation of real primaries (this conversion is performed to avoid specifying negative quantities of primaries)

Just Noticeable Difference

Also known as difference limen (DL) It is the threshold increment As the background intensity increases, the JND (threshold increment) also increases, such that the ratio of the JND to the background intensity (contrast) remains constant

Fourier Analysis

Also known as linear system analysis; a mathematical process Sine waves of the proper frequency, contrast, phase, and orientation can be used to construct more complex spatial stimuli Any achromatic scene (black and white) can be though of as consisting of sine waves of the appropriate frequency, contrast, phase, and orientation

Ventral processing stream

Also known as the temporal pathway or "what" system Receives input predominantly from the parvo retinogeniculate pathway

Binocular neurons

Although a majority of striate cortical neurons are binocular, most are dominated by one eye Stmulation of a neuron through the dominant eye causes a stronger response than stimulation through the other eye Ocular dominance is laid out in more or less a regular pattern of alternating right and left ocular dominance bands, sometimes called ocular dominance slabs or columns (R and L in the figure) An electrode that penetrates an ocular dominance slab perpendicular to the cortical surface (i.e. electrode 1) tends to encounter neurons dominated by the same eye, that are tuned to the SAME orientation as you go from neuron to neuron; Similarly an electrode traveling parallel to the cortical surface (like electrode 2) will encounter cells driven by only the left eye and VARY systematically in orientation sensitivity, as you move from one neuron to the next Orientation columns do not extend through layer IV (cels in this layer have a concentric organization) Orientation selectivity is organized in a pinwheel configuration (each sector of the pinwheel designated by a different color represents a diff orientation)

Rods: under scotopic conditions

Although rods contribute to both midget and parasol cells, their contribution to parasol ganglion cells is significantly stronger

Where do oscillatory potentials (OPs) originate?

Amacrine cells

Useful field of view (UFV)

An attention based measure of visual function that may be able to tell us which elderly drivers are at risk for involvement in an automobile accident Traditional visual fields do not fully characterize the usefulness of the pt's field of vision; a pt may have a normal visual field, yet not be able to effectively process information presented within this field of vision The UFV test requires the pt to divide his or her attention between two stimuli; the task is to identify a centrally/foveally fixated target, while also identifying or detecting a target presented in the peripheral visual field The UFV decreases beyond 50 years -- a majority of studies suggest that there is a relationship between impaired UFV and car accidents in the elderly Interestingly, impaired performance on a paper test that assesses divided attention, visual processing speed, and other higher level visual functions by asking the pt to connect dots with letters and numbers (i.e. 1, A, 2, B, etc) is associated with a history of motor vehicle collision in the elderly

Modulation Depth

Analogous to spatial contrast (resolving the grating and not resolving the grating because of contrast) A temporally modulated stimulus of low modulation depth may not be resolved, appearing as steady. As modulation depth increases (like in the bottom figure), it may be resolved and seen as flickering

Retina

Analyzes the optical image that falls upon it, encoding it into a neural signal that is transmitted to higher visual centers

Meridional Amblyopia

Animals raised in an environment consisting of only one orientation have disproportionately large number of cortical cells sensitive to this orientation Meridional amblyopia apparently occurs when a pt progresses through the critical period w/ one visual meridian in sharper focus than the other The more focused meridian presumably wins in the competition for cortical cells, dominates more cells, and manifests better visual resolution

Midget or diffuse

Another category for bipolar cells Midget bipolar cells have a smaller soma, less dendritic trees, and smaller receptive cells than diffuse bipolar cells The centers of the midget bipolar cells in the central and mid peripheral retina manifest input from a single M or L cone; some believe that this arrangement contributes to the high level of VA seen in primates. In the periphery, the receptive field centers of midget bipolar cells receive input from more than one photoreceptor, consistent w/ the reduced VA associated w the peripheral retina Both on and off center midget bipolar cells manifest color opponency This is bc a single M or L cone forms the receptive field center (in the central and mid peripheral retina) and H1 horizontal cells apparently contribute to the surround. Therefore, midget bipolar cells manifest different center and surround spectral sensitivities, creating color opponency S cones also contribute to color opponency, but apparently communicate with a distinct class of bipolar cells, referred to as S cone bipolar cells Diffuse bipolar cells are non color opponent A diffuse bipolar cell's receptive field center is formed by 5-10 M and L cones. This causes the spectral sensitivity of the center to be very similar to that of the surround. Manifest both high spatial resolution and color opponency Many scientists believe that a receptive field center formed by only a single cone evolved first to provide for high resolution acuity So VA is maximized by this arrangement, which also creates the basis for color opponency

Appearance of the spectrum

Around the neutral point, wavelength discrimination is best The reduced brightness at long wavelengths experienced in protanopia is due to the absence of erythrolabe; they can tell the dimness is another color Reminder: Deuteranopic Function: intersects the abscissa at ab 498 nm Protanopia function intersects the abscissa at ab 492 nm Where it intersects the abscissa, the wavelengths appear white-- totally desaturated-- and are called NEUTRAL POINTS (the neutral pt in tritanopia is approx. 569 nm)

Dynamic VA

As stimulus velocity increases, resolution acuity remains relatively constant until the stimulus velocity reaches approx. 60-80 degrees/second Beyond this velocity, the ability to resolve a moving stimulus, commonly referred to as dynamic VA, deteriorates; this is likely due to the inability to accurately follow the stimulus with tracking (or following) eye movements, referred to as SMOOTH PURSUIT EYE MOVEMENTS Dynamic VA (resolving a moving stimulus) is therefore limited by the speed of our smooth pursuit eye movements Higher dynamic VA is associated with improved ability to detect hazardous situations while driving

Astigmatism and age

As you age you get increasing dioptric power in the horizontal meridian, so you shift towards AGAINST THE RULE

Development of Face Processing

At 6 months of age, human infants are capable of discriminating between two human faces as well as two monkey faces Older infants and adults are less capable of discriminating between the monkey faces, revealing that the ability to distinguish among faces becomes more specialized as the infant matures. This suggests there may be a critical period for the development of face recognition

How does the dark adaption curve look if the stimulus is 650 nm?

At 650 nm the photochromatic interval is smaller --> so you won't see rod-cone break You would also see this if the stimulus is very small (0.5 deg) and central fixed because centrally fixed means at the fovea (there are no rods, so it would be just cones in the graph)

CSF High Frequency cutoff

At this spatial frequency of a 100% contrast grating is increased, a pt is reached where the grating can lo longer be resolved This pt is represented on the CSF as the high frequency cut off. For a young healthy adult, its ab 60 cycles/degree As the pt reads down an eye chart, the optotypes become smaller and their details become finer (high SF content increases). The point at which the details of the optotypes can no longer be resolved is the pt's VA (basically the high frequency CSF cutoff determined w/ optoypes rather than gratings)

Saccadic Suppression

Ballistic eye movement between two fixation points is a saccadic eye movement (during this time, the visual world remains still and clear) Vision is suppressed shortly before, during, and shortly after saccadic eye movements. This phenomenon is variously referred to as saccadic suppression/ saccadic omission; it enables us to look from one object to another without a smearing of our vision that would otherwise be caused by the rapid movement of images across the retina The magno pathway is sensitive to this high velocity movement, so the magno pathway is suppressed to avoid a smearing of vision There is evidence that a primary contributor to saccadic suppression is an EXTRARETINAL signal

Isoluminant gratings

Bars made up of different hues, creating chromatic contrast Thought to be poor stimuli for the magno pathway Under certain isoiluminant conditions, the perception of motion is impaired (it may be weak or slower than the actual movement of the bars) Adding luminance contrast (changing the stimulus so that the red, green, and yellow bars have diff luminance) improves the perception of motion -- this luminance contrast presumably stimulates the magno pathway

Spatial modification transfer function

Bc of ABERRATIONS inherent in any optical system, the img becomes more degraded at higher spatial frequencies, even when it is in focus. These same aberrations have little effect on optical img quality at low and moderate spatial frequencies Defocus (like moving the screen away from the plane of focus) causes a further reduction in img quality for high spatial frequencies (low/moderate frequencies are less affected) Translucent lens degrades the img: roughing up the Len's surface w/ sandpaper causes light scatter and a resultant reduction in contrast at ALL frequencies (not only high-- as is seen in optical defocus)

Why is determination of a threshold complicated?

Because humans and other animals are not perfect observers and therefore threshold may vary on repetition of its measurement

Distinction between inherited and acquired anomalies (QUIZ 4 STARTS HERE)

Blue-Yellow anomalies are so rarely inherited that it must be assumed that such an anomaly is acquired until proven otherwise

Chromatic properties

Both parvo and konio neurons are characterized by COLOR OPPONENCY: excited by certain wavelengths, and inhibited by others The sign of the response (excitatory or inhibitory) encodes info about the stimulus wavelength, pointing to the critical role these cells play in wavelength based discrimination Parvocellular layers (midget ganglion cells send input here): red green opponency Konio layers (input from small bistratified ganglion cells): blue yellow opponency Certain cells in the parvocellular region of LGN manifest BY properties (not sure if these are truly parvo cells or if konio cells are not confined to the interlaminar regions of the LGN) RG and BY neurons project to separate layers of the striate cortex

Rod and Cone Anatomy

Both rods and cones have an outer segment containing disc-like structures with photopigments that absorb light quanta (that initiate processes that lead to vision) Outer pigment of the rod has free-floating discs that contain photopigments (rhodopsin) Light travels through the inner retina and then hits the rods; the whole idea is that photopigments absorb the light, so having the photopigments stacked up this way is to optimize light absorption/ light capturing ability (if the light incident on a disc is not absorbed by the first or second disc, it can be absorbed by the third disc); for a single rod to be activated only one molecule of rhodopsin needs to be bleached (only takes one photon) Rods outer segments are continuously produced and shed during the daytime, and cone discs are shed at night The discs are shed and phagocytized by the RPE Inner segment of the rods contains nuclei and synaptic ending referred to as "spherule" Cones' disks are not free floating, and the synaptic ending is called a "pedicle"

Rabin Cone Contrast Test

CCT Relatively new Allows assessment of functionality of each of the three types of cones Administered using a computer Each column contains optotypes that are detectable based on contrast as defined for only one of the three cones As the pt reads down the column, the cone contrast decreases until a threshold is reached This test allows a cone threshold to be obtained for each of the three different cones The computer based version of the CCT uses an interactive staircase procedure with randomized letter presentation to determine each cone threshold The CCT allows a refined QUANTITATIVE measurement, so you can track the progression of an acquired color vision deficiency to determine if it remains stable, deteriorates, or improves; you can quantify the degree of the deficiency and monitor it overtime) The CCT is affected in glaucoma and has the potential to play an important role in assessing color vision in clinical practice

What is the effect of retinal illumination on the high frequency CFF?

CFF increases approximately linearly with the log of the retinal illumination, with the CFF considerably higher under photopic conditions (compared to scotopic); probably due to the speeding up of retinal processes that occurs at increasing levels of light adaptation; therefore, temporal resolution improves as the background illumination increases For scotopic the CFF is ab 20 Hz For photopic the CFF is ab 70 Hz The CFF also increases with the log of the stimulus area; flicker is more likely to be perceived if the stimulus is large Also, we have higher temporal resolution of the peripheral retina; For example, you may notice that a computer screen appears steady in central vision but flickers in the periphery It has been determined that flickering light causes DILATION of retinal blood vessels; this is localized to the stimulated area, suggesting it represents a response to local metabolic demands

Development of Critical Flicker Fusion Frequency

CFF is ab 40 Hz at 1 month and reaches adult levels of approximately 55 Hz by 3 months The retinal and cortical immaturities that slow the development of grating and vernier acuity have little effect on the maturation of temporal resolution

Dark Adaptation in Congenital Stationary Night Blindness

CSNB wipes out the rods, so the dark adaptation curve will just have the cone part

Inherited Anomalies

Can interfere with the performance of certain visually related activities Color coded visual aids at school can place young patients at a disadvantage; Pseudoisochromatic plate tests should be administered at a child's preschool eye examination, but it is not necessary to repeat the test on subsequent visits because inherited color vision losses remain stable Acquired color vision loss in a child could indicate a CNS lesion and must be thoroughly investigated Parents should be reassured that inherited color vision anomalies are common, stable, and pose no threat to vision

Contact lenses

Can produce corneal edema and the swelling of the corneal stroma layers which may result in increased light scatter similar to that produced by a cataract, with resultant reduction in contrast sensitivity across all frequencies; pts may have normal or near normal acuity, but complain of poor vision (aka 20/20 but they say things are really blurry) Scarring of the stromal layers bc of trauma or refractive laser procedures can also cause scatter -- with a resultant reduction in contrast sensitivity at low and moderate frequencies; Swelling of the stroma in Fuch's dystrophy can also lead to a loss in sensitivity at these frequencies; corneal epithelial disruptions that occur in dry eye can also have similar effects

What are the clinical implications of rod saturation for a patient who has rod monochromacy (no cones)?

Cannot see in daylight bc no cones Vision is the same at night bc he still has rods Best possible vision is prob about 20/200 vision (bc they only have rods and scotopic VA is usually 20/200) We can try to make sure the patient is under scotopic conditions all the time by prescribing lenses that are very dark. Red tints may be most effective due to the decreased effectiveness of long wavelength light (650ish) at bleaching rhodopsin!

Absorption by photopigments

Causes photoreceptors to become hyperpolarized which lead to a sequence of events that allow for vision

Occupational issues

Certain occupations related to public safety may have color vision standards that exclude individuals with anomalous color vision For some occupations, normal color discrimination can be an advantage (like optometry, ophthalmology, dentistry, surgery, etc) Individuals w/ red green anomalies may have difficulty distinguishing amount colors that are dark or desaturated if these colors fall along the red green confusion lines The ability to discriminate and name colors is improved by increasing the level of illumination Certain non neutral sunglass tints (other than gray) may make it more difficult for individuals with inherited anomalies to detect and recognize traffic lights (colored sunglasses should not be recommended for pts who have anomalous color vision) Protan anomalies have been associated w/ more rear end car accidents bc they cant really see the red tail and brake lights as easily. They also have issues with red traffic lights

Motion Perception

Changes in the spatial distribution of light, over time, can lead to the perception of motion Research suggests that motion is processed along a specialized visual pathway, the dorsal processing stream

Köllner's rule

Changes in the transmission properties of the crystalline lens, which commonly occur as people age, and outer retinal disease (photoreceptor layer) result in blue-yellow color vision anomalies; ARMD primarily affects the photoreceptor layer (outer retina) Disease of the inner retina, optic nerve, visual pathways, and visual cortex results in red green anomalies; inner retina is affected by diabetic retinopathy, leber's, and toxic amblyopia Not correct in every instance and important exceptions have been reported The nature of an acquired defect can change overtime, i.e. a blue yellow anomaly may change to red green and vice versa. Moreover, a pt may manifest blue yellow and red green anomalies at the same time: a condition known as NONSELECTIVE LOSS Köllner's rule predicts RG anomalies in optic neuritis, but this disease may be associated with nonselective loss

Desaturated Lanthony D15 Test

Chips are less saturated than Farnsworth D15 test Therefore, the radius of the hue circle formed by the desaturated chips is less than that formed by the chips of the D15 test; the inner circle from the previous CIE diagram Tast is more difficult, so it may detect anomalies that would be missed by the Farnsworth D15 test May be especially useful in the detection of subtle acquired losses that occur in certain eye diseases, such as glaucoma

Color Vision

Chromatic contrast allows us to see objects that would otherwise be indistinguishable from the background in which they exist, thereby adding greatly to our visual capabilities Early investigators thought we had a unique cone for each color we perceive-- not realistic, you'd need thousands of diff cones Thomas Young posited that color info is encoded by a limited number of cones, perhaps three, and the relative activities of these cones encode color -- known as the TRICHROMATIC THEORY

Color Constancy

Color constancy refers to the approximately constant color appearance of objects as lighting conditions change Assists us in identifying objects as lighting conditions vary (i.e. indoors and outdoors and at various times of the day) Colors are perceived relative to adjacent and surrounding objects (the background) The physiological basis for color constancy has not been fully elucidated Color constancy is not absolute: as lighting conditions change, there are subtle, but important, changes in color appearance. For example, when looking at a natural landscape, lighting conditions provide a cue that allows us to gauge the time of day

Electrophysiological evidence for color opponency

Color opponent cells were found in the primate retina and lateral geniculate nucleus In single unit electrophysiological experiments, a visual neuron is isolated and its electrical activity is recorded. The fovea of the animal is aligned with the fixation point "F" on a screen and a micro electrode capable of recording action potentials is inserted into the primate's LGN (or retina) and a single cell is isolated The neuron's receptive field may be exposed systematically to equal energy spectral stimuli. The neuron's response, as measured by its frequency of action potentials, is plotted as a function of wavelength: -For the neuron in graph A, short-wavelength stimuli cause inhibition, while long wavelength stimuli cause excitation; since this cell responds to one portion of the spectrum with excitation and another with inhibition, it is referred to as a color-opponent neuron (or spectrally opponent neuron); this cell has color coding capabilities -- if we are told this neuron is excited by a stimulus it must be a long wavelength -Graph B shows a non-color opponent neuron: all wavelengths result in an increased frequency of APs; does not have color coding capabilities and is therefore monochromatic (same principle of adjusting intensity applies-- increasing intensity can cause two stimuli to produce the same neural response, so that the neuron cannot distinguish between the two stimuli); The spectral sensitivity function for a typical non color opponent neuron has a broad peak in the mid-spectral region at ab 555 nm. This has lead many scientists to conclude that it represents the addition of M and L cone inputs

Lighting conditions for color vision testing

Commonly used color vision tests are designed for administration under standard illuminant C lighting conditions If an unfiltered incandescent source is used, the pt may perform better than if the proper illumination were used Standard fluorescent lighting is not generally advisable because of its uneven wavelength profile The MacBeth lamp provides illumination very similar to that provided by illuminant C, but it is not readily available; practical lighting options are Varilux F15T8/VLX fluorescent tube for routine clinical practice If an incandescent source is used, the pt should view the test through an appropriately colored filter that renders the lighting conditions suitable Alternatively, color vision testing can be performed using indirect sunlight

Isoluminant gratings

Composed of bars of all the same luminance but varying chromaticities May isolate the parvo system in humans (bars are visible only bc of chromatic contrast) Magno pathway is silenced by isoluminant stimuli, leaving the isolated parvo system to detect these stimuli The perceptual distortions noted when viewing isoluminant stimuli (for example: abnormal motion perception) are consistent with reduced magno contribution Bc spectral sensitivity of magno cells varies slightly from cell to cell, it is unlikely that isoluminant stimuli silence ALL magno neurons. So its probably not the parvo system in actual isolation, but rather it'll be predominantly the parvo system

Photopic Vision

Cone mediated Operates under daytime conditions Poor absolute sensitivity (to dim lights) High contrast sensitivity Good VA (20/20) Color vision

Where does the photopic a wave originate from?

Cones and OFF cone bipolar cells

Color Confusion lines for Tritanopia

Confuse blues and yellows Copunctal point goes through blues and yellows

Color Confusion lines for Protanopia

Confuse reds and greens Copunctal point goes through red and greens

Color Confusion lines for Deuteranopia

Confuse reds and greens Copunctal point goes through purple and green Note: Red and blue are least likely to be confused (do not have a confusion line running through them)

Strabismic Amblyopia

Constant unilateral strabismus present during the critical period, as may occur in congenital (infantile) esotropia, leads to amblyopia in the deviated eye Deviate eye might be suppressed, which presumably leads to amblyopia Amblyopia is NOT expected to develop in intermittent or alternating deviations, which is almost always the case in exotropia, and sometimes the case in esotropia Pts with alternating strabismus, however, may have little or no stereopsis (bc cortical neurons do not receive similar images simultaneously from each eye during the critical period)

Parasol cells

Constitute approx 10% of ganglion cells in the central 20 degrees, and up to 15% at greater eccentricities Have large dendritic trees, synapsing with more than one diffuse bipolar cell Their receptive fields are typified by large surrounds Prevailing view is that the high frequency resolution of parasol cells is less than that of midget cells

Contrast continued

Contrast ranges between 0% and 100%. It is IMPOSSIBLE to make delta l greater than lave, so you cant ever have more than 100% contrast. It is NOT POSSIBLE to have less than 0 luminance. Note: the trough of the luminance profile is at 0 luminance when delta l = lave. Delta l = the diff between the peak and the avg luminance lave = the avg luminance of the grating (the average of the light peaks and dark troughs)

The effect of the subject's criteria

Correct rejects: there is nothing there and you said there is nothing there Hits: when there is something there you say there is something there Lax criteria: you rarely miss a stimulus, lots of hits, but lots of false positives too Strict criteria: Very little false positives, but a lot of misses If the stimulus is the same, it is possible that threshold criteria is different, so you get different responses

High Pass VA chart

Corrects the source of variability by removing low spatial frequencies from the optotypes, creating what are called HIGH PASS optotypes and placing them on a background of the same avg luminance They're also referred to as vanishing optoptypes bc they become invisible at ab the same time they cannot be resolved. COMPARED TO STANDARD CHARTS, THERE IS A REDUCTION IN TRV

Edges or gratings

Cortical neurons respond well to sine wave gratings and are selective for a particular spatial frequency (one cell may be finely tuned to 3 cpm, and another to 6 cpd) Does not prove however that the visual system's primary operation is like Fourier analyzer

Visual fields cont.

Critical to managing glaucoma Darkest spot is the blindspot (dark area. located temporal and inferior to the fixation point) Loss of vision: darker shading indicates higher threshold (lower sensitivity)

Farnsworth Dichotomous Test

D15 15 chips that form a hue circle w/in the cie diagram Pt arranges the colored caps such that they follow an orderly progression of color Choose cap that looks most like the reference cap, and then the cap that looks most like the cap you just chose and so on and so forth This is an arrangement test Diagnosis of an anomaly is made by noting the axis of the crossovers (corresponds with protan, deutan, or tritan color confusion lines; but for nonselective loss there may be no discernible axis); The crossovers correspond to color confusion lines Differentiates between protan, deutan, and tritan anomalies but not dichromacy from anomalous trichromacy This test has low sensitivity so certain individuals with anomalous trichromacy may pass the test (this may not be a serious disadvantage when screening for inherited anomalies bc minor inherited losses are unlikely to have practical consequences for the pt) An advantage is that it can detect tritan losses!!

Contrast

Dashed line in the figure of the luminance profile graph represents the luminance of the grating (the avg of the peaks and troughs) Two gratings of different contrast can have the same average luminance or lave. The difference is that the one with the higher contrast will have a larger difference between its peak and the avg luminance value (bottom figure is higher in contrast, and you can see the peak of the second grating is way higher than the lave)

Under daylight conditions, a red and green surface look equally bright. Compared to green, the red surface most likely emits:

Daylight = photopic Photopic system is most sensitive around 555 nm (green), so green is absorbed most easily. Therefore, the red surface most likely emits more energy to be equally as bright as the green surface. The red surface is not as efficient at being absorbed

Hue

Depends on the wavelength, but also stimulus brightness Most monochromatic stimuli change hue slightly as their intensity is adjusted (Bezold Brücke phenomenon) except for three INVARIANT WAVELENGTHS and INVARIANT HUES: -Unique Blue (478 nm) -Unique Green (503 nm) -Unique Yellow (578 nm) For stimuli shorter than unique green, as intensity increases, the stimulus appears more blueish For stimuli longer than unique green, as intensity increases, the stimuli appears more yellowish Example: if a stimulus has a wavelength of 600 nm, it appears orange. If you increase the intensity, it appears more yellowish (as defined by the rules above); If you decrease the intensity, it will appear more reddish

POAG

Diagnosis is made using visual field loss/ appearance of optic nerve head Condition cannot be detected with these standard diagnostic procedures until a substantial proportion of ganglion cells have died Since early treatment delays the progression of visual loss, it is important to make the diagnosis as early as possible POAG autopsies reveal axons of large neurons may be damaged prior to those of small neurons, leading to the proposition the magno pathway is more vulnerable to glaucomatous damage than the parvo pathway Tests of frequent doubling: as the temporal rate of a flickering sine wave creating is increased, the pt will note the apparent spatial frequency of the grating doubles. The perception of frequency doubling is IMPAIRED in POAG Physiolgoical underpinnings of this technology are not fully understood

Classification of inherited color vision anomalies

Dichromacy: missing one of three photopigments -Deuteranopia: missing chlorolabe -Protanopia: missing erythrolabe (have trouble seeing red objects) -Tritanopia: missing cyanolabe Anomalous Trichromacy: Three photopigments are present but the absorption spectrum of one of these photopigments is displaced to an abnormal position -Deuteranomalous trichromacy/ Deuteranomaly: the chlorolabe spectrum is displaced toward longer wavelengths -Protanomalous trichromacy/protanomaly: the erythrolabe spectrum is displaced towards shorter wavelengths

Anomalous Trichromacy

Displacement of cone photopigments from their optimal positions results in deficient color discrimination The greater the displacement, the more severe the color vision anomaly The solid curves show the normal positions of the cone absorption spectra, and the dashed curves show the locations of the displaced spectra Remember: L (long wavelength) cones contain erythrolabe (max 552 or 557); red M cones contain chlorolabe (max 530); green S (short wavelength) cones contain cyanolabe (max 426); blue

ARMD

Disruption of the macula may lead to distorted vision; lines get curvy A new hyperacuity test in which the pt is asked to identify which circular target on the iPhone is distorted may allow at risk pts to monitor their vision at home and seek care at an earlier stage than otherwise possible There is evidence that performance on vernier acuity tasks (and therefore processing of position info) is adversely affected in glaucoma

Monocular Deprivation

During the critical period, monocular deprivation results in an abnormal striate cortical architecture When an eye is deprived, the columns receiving input from the non deprived eye are widened at the expense of those associated with the deprived eye The underlying structure for ocular dominance columns develops independent of visual experience. Once the basic cortical architecture has been laid down, environmental manipulations can be disruptive

Reading disability

Dyslexia is a selective impairment of reading skills in spite of normal intelligence, vision, hearing, instruction, and motivation Prevailing view is that its a cognitive disorder Data suggests it might be an issue with processing high frequency temporal information (they also have abnormalities in flicker fusion rates and ability to temporally resolve two consecutive stimuli), so it might be the magnocellular pathway. But this might be overly simplistic

Summary of development of visual functions

Each has a different critical period Need to know this

Snellen Chart

Each line is designated by its foot size which is the distance at which each of its optotypes subtends an angle of five mins of arc

Molecular genetics of cone photopigments

Each molecule of cone photopigment contains a chromophore and an opsin The chromophore is identical for all cone photopigments and is called retinal. Absorption of a quantum of light by the chromophore initiates a series of events leading to vision Opsin (a visually inert chain of amino acids interlaced into the disc membranes of the outer segment) determines the absorption characteristics of the photopigment molecule Each class of cones has a diff opsin Cone photopigment genes are homologous to the rhodopsin gene, suggesting that all four share the same ancestor M and L cone opsin genes are exceedingly similar to each other, showing 98% homology -- pointing to recent duplication (in img, red is same aa, green means a diff amino acid) M and L cone genes are found on the X chromosome (explains sex linked inheritance of color vision deficiencies in which M or L cones are missing or altered) Gene for S cones is on chromosome 7 Gene for rhodopsin is on chromosome 3

EOG

Electrooculogram Electrodes are on the skin, pt moves eyes back and forth Standing potential drops when lights are turned off. Standing potential increases when you turn the lights back on; you take the ratio of the standing potentials to see THE RPE FUNCTION Very specific to the function of RPE cells

ERG

Electroretinogram Used to assess the function of neurons in different layers of the retina In response to a flash of light We place an electrode on the cornea of the eye whose function we're interested in; then you have a reference electrode placed somewhere else on your face and then you flash light in front of the eye of interest As the retinal neurons respond to the light, they alter their electrical activity This electrode picks up the current change as a voltage change You ultimately see the sum of all the responses of each subset of retinal cells You add them together and get a complex waveform Rod mediated (scotopic) ERG: -Dark adapted conditions (dim flashes of light) -Least amount of light for which you can generate the response -All major classes of retinal cells in the rod pathway contribute responses to the ERG Cone mediated (photopic) ERG: -Light adapted conditions -All major classes of retinal cells in the cone pathway contribute responses to the ERG

Opponent Color Theory History

Ewald Hering: -Found that colors are either red or green, but not both (same holds true for blue and yellow) -Also found that a chromatic stimulus elicits an afterimage of a complementary color -Proposed the opponent color vision theory which posits that color is processed by bipolar hue channels referred to as the red-green and blue-yellow channels (the channel can signal only one of the two attributes at any given instant); also hypothesized that brightness was coded by a separate black-white channel

Cone Monochromacy

Exceedingly rare disorder, VA is normal but pt manifests monochromatic color matching Seems to involve a defect in postreceptoral processing of color info

Spatial Summation

Explained by post receptor connections One quanta of light will activate a photoreceptor. You need 10 QUANTA of light to activate a ganglion cell-- that will result in vision**

Retinocortical pathways

Extracellular recordings reveal that parvo, magno, and kongo cells manifest DIFFERENT visual sensitivities Parvo cells: sensitive to red green color contrast, but not fast movement Magno cells: largely monochromatic and sensitive to rapid movement Konio cells: respond well to blue yellow chromatic contrast Because of their anatomical segregation and different visual sensitivities, parvo, magno, and konio cells can be considered components of parallel retinocortical visual pathways This division is found in the retina and LGN. Also to lesser extent, in the striate cortex and higher cortical areas

Spatial summation (photopic system)

Fewer photoreceptors (cones) per ganglion cell Two patches of light but subject sees nothing because you need ten quanta absorbed to activate a ganglion cell Excellent spatial resolution (two stimuli would be seen), but poor spatial summation

Organization of the parvo and mango pathways from the retina to the cortex

Figure

Example of using the CIE diagram

Figure shows a mixture (M) of two wavelengths 510 and 560 The dominant wavelength (what color must be mixed with white to make the mixture) is found by drawing a line from the W to the M all the way to the spectral locus Excitation purity is a measure of saturation (in the figure M can be made with 75% of the dominant wavelength and 25% white) Higher excitation purity = more saturated Mixture location example: If a mixture is 3 parts wavelength Q and 1 part wavelength W, it will be 3 times closer to wavelength Q

FDF Perimetry

Flicker defined form perimetry Dots on either side of a border are modulated in counterpoise (180 degrees out of phase) The pt's task is to detect the border, which becomes more visible as the dots' contrast increases. FDF perimetry appears to detect glaucoma in its early stages Glaucomatous eyes manifest a disproportionate loss of larger axons, presumably parasol axons; data suggest the magno pathway codes high temporal frequencies (so its not surprising that pts with glaucoma suffer losses in sensitivity to high temporal frequencies) TMTFs may also provide useful info regarding retinitis pigmentosa and ARMD. These findings highlight the criticality of temporal processing in the design of clinical tests

Isopter

For all points that fall on an isopter, the visual system has equal sensitivity

Wavelength Discrimination

For both protanopia and deuteranopia, there is relatively well developed wavelength discrimination in the region of 490 nm but longer than ab 545 nm there is no ability to discriminate between stimuli on the basis of wavelength alone; they are able to discriminate between stimuli above 545 nm if the stimuli differ in luminance

Paracontrast

Forward masking where the target and mask are spatially adjacent The mask reduces the visibility of a subsequently presented, spatially adjacent target

Where does the scotopic/ dark adaptive b wave originate?

From rod bipolar cells and Müller cells

Where does the scotopic/dark adaptive a wave originate?

From rod photoreceptors

Dorsal Processing Stream

From the retina to the striate cortex, high velocity motion info is processed primarily along the MAGNO pathway The PARVO pathway processes principally low velocity stimuli Amblyopes show reduced sensitivity to low velocities, apparently bc of impairment in the parvo pathway

Cortical posterior superior temporal sulcus (STC)

Functional magnetic resonance imaging (fMRI) reveals the cortical posterior superior temporal sulcus is activated when viewing biological motion, but not artificial motion

AdaptDx Dark Adaptometer

Gives you dark adaptation curves which may be useful in the diagnosis of retinitis pigmentosa, CSNB, and certain other rod/cone degenerations and diseases (these adaptation curves would look abnormal!) Turns out, in AMD there is an early loss of rods (you'd see this in a dark adaptation curve)

Development of motion perception

Global motion processing matures to adult levels at ab 14 yrs old The ability to discriminate between speeds of motion (which of two stimuli is moving faster) develops at a different rate for fast and slow speeds. Speed discrimination for fast stimuli reaches adult levels at ab 11 years of age, while it has not fully matured at this age for slower speeds.

Outer plexiform layer

Glutamate (NT) released by the photoreceptors has diff effects on the two classes of bipolar cells; Don't forget: on bipolar cells have excitatory center and inhibitory surround. Off center bipolar cells have inhibitory center and excitatory surround In the outer plexiform layer, glutamate causes bipolar cells to have either an on center or off center configuration Under dark conditions, photoreceptors continuously release neurotransmitter Light stimulation causes hyperpolarization of photoreceptors, and a consequent reduction in the release of neurotransmitter For on center bipolar cells, glutamate is inhibitory-- you'll get hyperpolarization (reduction of glutamate will cause relative excitation-- depolarization) For an off center bipolar cell, glutamate is excitatory, it'll excite the SURROUND, causing depolarization (reduction of glutamate will cause relative inhibition-- hyperpolarization)

Staircase Method

Good for visual field testing, usually used for visual field testing! Combines ascending and descending limits and is commonly used for research and clinical applications Initially a stimulus may be below threshold and presented in discrete steps of increasing visibility until it is seen. At this point, the staircase is reversed, and the visibility of the stimulus is reduced in decreased steps until the observer cannot detect it. You again reverse the staircase and repeat the process. The threshold is taken to be the stimulus intensity at one of the reversals (for example, the fourth reversal, or the avg of the last few reversals)

Basic organization of the cortex

Gray matter (cell bodies) with underlying white matter (axons) Gray matter is pretty thin (4 mm thick) but the cortical surface area is substantial (ab 2200 cm^2) Human cortex is 3 pounds, with ab 10^10 cells, and 10^15 synapses and 2000 miles of axonal connections 4 Lobes: frontal, parietal, occipital, and temporal Cortical tissue is pretty uniform, but the lobes are organized into diff functional areas or "modules." there are about 20 dedicated to analyzing visual info

Bipolar cells

H1: receive input primarily from M and L cones (little input from the S cones) H2: Strong connectivity with S cones, and also receive input from M and L cones Horizontal cells show graded responses and do not generate action potentials Because photoreceptors and horizontal cells both hyperpolarize in response to light, the synapses connecting them are referred to as sign-conserving synapses

Wiring diagrams

H= horizontal cell B= bipolar cell Inhibitory synapses are indicated by minus signs and excitatory synapses by plus signs Note: the center of a cell is always stronger than its surround (more robust response to light) L and M cones oppose each other to produce L-M opponent cells (R-G opponent) S cones are opposed by an addition of (L and M cones); B- (R+G) = B-Y opponent For the R-G opponent cell, only one L cone contributes to the excitatory receptive field center while the inhibitory surround is formed by the input of many M cones For the B-Y cell, S cones form the excitatory center while both L and M cones contribute to the inhibitory surround The achromatic cell has L and M cone input to both excitatory and inhibitory surround, giving the center and surround the SAME SPECTRAL SENSITIVITY and making the cell monochromatic (non color opponent) The right hand column are bipolar receptive fields

Horizontal cells

Has widely dispersed dendritic tree Manifests substantial spatial summation A horizontal cell sums up input from photoreceptors distributed over a large area of the retina Because of this high degree of spatial summation, an annulus elicits a strong response from a horizontal cell, causing it to hyperpolarize

Signal Detection Theory

Helps us predict how a subject will respond in an experiment Neural activity present in the absence of a stimulus (neural noise) is randomly distributed; sometimes there is a lot of noise, and other times there is very little. A stimulus causes a constant level of neural activation, regardless of the noise present Neural activity bc of a stimulus is added to the noise to make the noise + signal curve High detectability: the noise + signal curve is completely separated from the noise curve

Rods and Cones distribution

High CONCENTRATION of cones (specifically M and L cones) at the fovea, but 90% or more of the cones are not at the fovea. The reason the fovea is where we have the best VA is bc the cones are so densely packed there. If the fovea was destroyed, we would still have the cones in the periphery but they're not as densely packed, so we'd expect the VA to be 20/200. M and L are not present in equal quantities in the eye (ratio differs from person to person) S cones are concentrated on the slopes of the pit of the fovea No rods at the fovea! High concentration of rods outside the fovea

Fovea

High density of cones in the human fovea provides the basis for excellent VA and much of the richness of our visual experiences Occupies 0.01% of the retinal area Fovea encodes a disproportionately large amount of info Foveal damage can occur in ARMD (damages the choriocapillaris and Bruch's membrane in the foveal region, leading to loss in the overlying sensory tissue) Dry ARMD: sensory tissue is damaged in the absence of vascular hemorrhaging Wet ARMD: neovascularization of the choriocapillaris results in leaky blood vessels and sub macular hemorrhaging ARMD is one of the leading causes of blindness, particularly among retirement aged individuals; the use of drugs that block angiogenesis has been proven to be effective in the treatment of wet ARMD

Resolution Acuity and aging

High levels of VA are usually maintained until ab 65-70 years of age. Some studies show a gradual decline past that age, while others indicate a more precipitous decline High contrast acuity is relatively unaffected through the seventh or eighth decade, but this is not true for less than optimal lighting conditions VA measured under conditions of reduced contrast, reduced illumination, or added glare shows marked reductions in the elderly, especially in the very old The time it takes for VA to recover following exposure to a bright light (disability glare recovery) also increases with age

CSF in Uncorrected RE

If the eye is out of focus like in uncorrected myopia, the high spatial frequency cutoff is reduced, which manifests itself as decreased VA Similar to an ophthalmic lens that is out of focus, as discussed earlier

Development of color vision

If the infant can discriminate between the circle and the background, they'll look at the left side bc its more interesting If the luminance of the circle can be adjusted so that the infant does not have a preference, he or she does not have wavelength based discrimination for the stimulus tested RG discrimination arises during the second month of life; discrimination is worse for smaller stimuli, probably bc of poor spatial summation secondary to retinal immaturities. Some people think there is adult like RG discrimination at the end of the first year of life, but others thing RG and BY discrimination improves until adolescence VEPs suggest that the photopic spectral sensitivity function is adult-like in young infants BY color discrimination is not well understood. Some have found BY discrimination in 2 month old infants, but other studies suggest a slower time course for its emergence

In the lab we can control a subject's criterion using a payoff matrix

If you're going to lose a lot of money due to a false positive, you'll utilize strict criterion If you'll gain a lot of money due to a hit and won't lose much for a false positive, you'll utilize lax criterion

Weber's Law

In many psychophysical procedures, the task is to discriminate between the combo of the stimulus and background and background alone

Rod hyperpolarization

In the dark, Na+ flows through ion channels to the rod outer segment -- frequently called the dark current-- producing slight depolarization to -50 mV Absorption of light by rhodopsin initiates the blockage of Na+ channels, and therefore hyper polarization of the outer segment (less sodium in means more minus inside) A molecule of rhodopsin contains opsin and a chromophore -Opsin is a visually inert chain of amino acids interlaced into the disc membranes of the rod outer segment (opsin determines the absorption profile of the photopigment) -The chromophore is responsive to light and consists of retinal (a type of retinol/vitamin A) When unbleached, retinal is in the 11 cis retinal state. Absorption of light causes it to change to all trans retinal. The protein transducin activates the protein phosphodiesterase (PDE) which breaks down cyclic GMP into ordinary GMP. A decrease in cGMP levels leads to the closing of rod outer segment Na+ channels--> rod hyperpolarization The # of sodium channels in the ROS is limited, so there's a constraint on the potential magnitude of rod hyperpolarizaiton When a small amount of a rod's rhodopsin is bleached, all the sodium channels are closed and further bleaching of rhodopsin doesn't result in further hyperpolarization

Striate cortex

In the occipital lobe Named because of the line of Gennari (a band formed by the myelinated geniculate axons that synapse in layer 4B) Also referred to as the primary visual cortex, visual area 1, and V1. Has 6 layers, with layer one being most superficial and layer six being the deepest Dominated by representation from the fovea

Saturation for anomalous trichromacy

Individuals w/ normal color vision and anomalous trichromacy do not manifest neutral points Still, they display abnormal saturation perception (you can see the least saturated wavelength in deuteranomaly is 498 nm, while in protanomaly it is 492 nm)

Stereopsis Development

Infant responses to stimuli that produce retinal disparity can be assessed using preferential looking. Few infants manifest stereopsis prior to 3 months of age, but it has rapid onset between 3 to 6 months. Stereoacuity matures rapidly thereafter, often reaching 1 min of arc by 6 months.

Striate cortex architecture

Info from the two eyes is first combined in the striate cortex, with most cortical neurons being binocular Binocular cortical cells may mediate stereopsis: at a given distance from the eye, a cortical cell's receptive field as determined by stimulating the right eye may be in a slightly different position than that for the left eye (but the receptive fields overlap at a critical distance from the eye); bc inputs form the two eyes are summed, a stimulus at this critical distance maximally activates the cortical neuron. The stimulus distance can thereby be encoded, presumably contributing to the physiological basis for stereopsis In figure, the receptive fields overlap at D

Sine wave gratings and local motion processing

It has become commonplace to use sine wave gratings that undergo a phase shift to produce a grating that appears to drift (move) to the right

Acquired anomalies

It is best to screen adults with a test that can detect acquired color vision losses which are often blue yellow in nature To screen for both red green and blue yellow anomalies: New HRR plate test Standard Pseudoisochromatic Plate test (SPP) D15 Note: testing should be performed monocularly, first testing the eye suspected of manifesting disease, typically that with the worst corrected VA

Temporal Frequency

It is possible to vary the frequency of a temporal sinusoid A LOW temporal frequency may be seen as flickering at a low rate, whereas a HIGH temporal frequency may appear to flicker at a higher rate Temporal frequency typically given in hertz (Hz) 1 Hz = 1 cycle/second As the temporal frequency is increased, a frequency is reached at which flicker can no longer be resolved -- this is the critical flicker fusion frequency --CFF; represents the high temporal resolution limit of the visual system for a given modulation depth; can be thought of as temporal acuity We usually don't see light bulbs flicker (even though the current is in Hz) bc the temporal rate is generally beyond the CFF

Standard cortical architecture

Its development requires normal visual input early in life (can be altered by environmental deprivation, such as may occur in certain forms of anisometropia and strabismus)

CSF Low Frequency Drop Off

Lateral inhibition/antagonsim may be the basis for the CSFs low frequency drop off The SF grating on the top is a more optimal stimulus for this ganglion cell than the lower SF grating on the bottom. This is bc the large bright bar of the low frequency grating falls on both the receptive field's center and surround, resulting in LATERAL INHIBITION, which reduces the neuron's response

Behavioral studies in monkeys

Lesions in parvocellular region of monkey LGN causes: -reduced wavelength discrimination and high spatial frequency contrast sensitivity (remember that parvo pathway is key to color discrimination, and VA) Lesions in the magno pathway: -Reduction in high temporal frequency flicker resolution and low spatial frequency contrast sensitivity (remember, the magno pathway encodes fast movement and low SFs)

Light Adaptation

Light adaptation is a faster process than dark adaptation** Flash a light onto the background and when the patient sees it, this is the threshold Increment is a flash of light on the background Intensity of the background vs intensity of the increment; constant ratio between increment and background Weber's Law: As the background brightness is increased, the increment intensity must also be increased such that the ratio (for scotopic vision, K = 0.14) remains constant; Saying a visual system follows Weber's Law means that threshold contrast remains constant as the illumination changes. In the graph, the third section has a slope of 1 which means that it is the portion that follows Weber's Law. For Scotopic vision Weber's Fraction, K = 0.14 (ratio remains constant!) The ratio of the increment intensity to the background intensity is referred to as contrast**; For the portion of the curve that follows weber's law, as the background intensity changes, the contrast sensitivity (ratio) stays the same. The last section has a slope of infinity because the rods are saturated; at this background illumination, the rods cannot detect the increment stimulus no matter how bright it is because they are overwhelmed by the brightness of the background; only ab 10% of rhodopsin is bleached at the point of rod saturation As you increase the background intensity, the ABSOLUTE SENSITIVITY decreases (your threshold increases, harder to see the increment/flash of light). The RELATIVE SENSITIVITY (Weber's Fraction), however, does NOT change. You still have the same ratio, its just a larger absolute threshold to see the increment.

Flow of info within the retina

Light goes through all the layers and is absorbed by the photoreceptors, and is turned into an electrical signal that is sent to bipolar cells (forward in the eye) and finally to ganglion cells, that transmit the signal to the optic nerve and LGN (lateral geniculate nucleus, a nucleus in the thalamus) It is a feed-forward system Also in the retina we have horizontal integration of info (how cells communicate with cells next door to them) Horizontal cells: mostly in outer, photoreceptor layer Amacrine cells: mostly in inner bipolar cell layer

LGN divisions

Magno cells: large neurons, two most ventral layers of the LGN Parvo cells: smaller neurons, four dorsal layers of the LGN Konio cells: in between the principle layers in the interlaminar regions (intercalated layers); even smaller cells!!!!! The three major classes of retinal ganglion cells selectively synapse in these different layers: Midget goes to parvocellular layer Parasol goes to the magnocellular layer Bistratified cells project to the interlaminar regions The LGN sends its primary projection to the visual cortex

Extracellular single unit recording techniques

May be used to record the activity of ganglion cells

Options of different visual electrodiagnostic tests

Measures of RETINAL function: -Electroretinogram (ERG) -Electrooculogram (EOG) Measures of CORTICAL function: -Visually Evoked Potential (VEP)

Cortical Visual Area 5

Middle Temporal Area, MT, V5, MT/V5 Cells in MT/V5 respond to global stimuli including random dot kinematograms. Damage to MT/V5 can impair the perception of motion, resulting in the condition referred to as AKINETOPSIA Plays a large role in processing motion info The cortical processing stream that originates at the striate cortex, continuing through visual area 5 to the prefrontal cortex is variously referred to as the: -PARIETAL PATHWAY -DORSAL PROCESSING STREAM -THE WHERE SYSTEM This processing stream receives most (but not all) of its input from the magno pathway

Forced Choice

Minimized the effect of the subjects decision criteria because the response "I cannot see the stimulus" is not acceptable. Not all observers use the same criteria when deciding whether or not they see a stimulus If the experiment forces the observer to choose between two alternatives, it is referred to as a two alternative forced choice (2 AFC experiment). Lowest percentage correct is 50%, not 0. Even if the stimulus cannot be seen, the observer is expected to guess correctly 50% of the time. Threshold is typically assumed to be the point midway between chance performance (50%) and perfect performance (100%), so as indicated in the graph, the threshold for a 2AFC experiment is 75% 4AFC is also shown in the graph. Chance performance is 25%, so the threshold is 62.5%. The more alternative forced choice options there are, the steeper the psychometric function will be (more choices there are, the lower the lowest percentage correct can be )

Occlusion Amblyopia

Monocular congenital cataracts are the most common cause of occlusion amblyopia which may also result from monocular lid ptosis It may also be iatrogenic, secondary to monocular patching used in the treatment of a disease or injury

Frequency (sin wave gratings)

More alterations or cycles = high spatial frequency Spatial frequency of a grating is the number of cycles/degree of visual angle (i.e. 30 cycles/degree) or number of cycles per unit space (i.e. 4 cycles/cm)

Random Dot Kinematograms

More complex first order stimuli require the integration of MOTION CUES across a wide expanse of visual space A. Dots are moving in random direction w/ respect to each other: no coherence B. More dots moving in a common direction: 50% coherence C. All dots moving in a common direction: 100% coherence Coherence motion threshold: the smallest percent coherence that results in the perception of motion in a defined direction (up, down, left, right, etc); determined by random dot kinematograms; humans do rlly well on this task, manifesting coherence thresholds close to 1%

Second order motion stimuli

More complex, such as texture defined contours

Michaelson Eq

More practical to measure the max luminance and min luminance, and to use these values to calculate the contrast

Pseudoisochromatic Plate Tests

Most commonly used color vision test Number of plates arranged in a booklet Vanishing plates, the typical design, consist of a figure embedded in a spectrally different backgorund Pts w/ anomalous color vision have difficultly seeing the figure bc the colors that constitute it and the background all fall on a common color confusion line making the embedded figure indistinguishable form the background Pseudochromatic plate tests do not distinguish between dichromatic and anomalous trichromacy but are useful in distinguishing between PROTAN AND DEUTAN anomalies For deutan anomalies (deuteranopia/ deuteranomaly) in the figure they wouldn't be able to see the circle bc purple is on their color confusion line that also goes through white (keep in mind that gray is a variant of white); go back to the color confusion lines if you cant remember For protan anomalies they wouldn't see the triangle bc their color confusion line that passes through white has pink on it (again go back to color confusion lines if need be) Although certain pseudoisochromatic tests (the new HRR and SPP2) have plates designed to detect blue yellow, others like the Ishihara do not Because of the importance of detecting acquired losses the lack of tritan plates can be a serious limitation

Trichromatic Vision

Most humans have this Three diff photopigments with overlapping absorption spectra Shows wavelength discrimination that is superior to those with dichromacy but still limited! Given at least 4 wavelengths divided into two patches, a person with trichromatic vision is able to adjust the relative intensities of these wavelengths such that the two patches appear identical (its a limitation, dichromats and monochromats can do this too); but given 3 wavelengths divided into two patches, a trichromat cannot make the patches look equal even if they adjust the intensities of the wavelengths

Biological Motion

Natural movements of humans and other animals Might be processed differently than other forms of motion

Development of myopia

Nature theory: myopia is an inherited condition Nurture theory: environmental factors cause myopia Its probs genetic + environment; new research aims at determining which inherited and environmental factors place the eye at risk of developing myopia Twins tend to manifest highly correlated refractive errors Most of the growth of the eyeball occurs during the first 6 years of life Myopia is relatively uncommon when children enter school (2% of 6 year old children are myopic) This observation has led to the suspicion that environmental factors, such as prolonged near work or limited time spent outdoors, may trigger myopia in genetically susceptible school aged children Near work has not been proven to cause myopia When an animal's eyelid is sutured closed at birth or the retinal image is optically blurred early in life, the eye develops axial myopia Emmetropization is an active process not only driven by retinal defocus but also by the sign of the defocus -plus lens in front of the eye makes the eye hyperopic -minus lens in front of the eye makes the eye myopic Local factors (not feedback from higher visual centers) may play a role in the regulation of eye growth -- so there can be localized blur Inducing lenses can influence shape: -minus lens makes eye more prolate (longer in axial dimension) -plus lens makes eye more oblate (longer in the circumferential dimension) The shape of the eye determines the location and nature of the retinal blur Peripheral myopic defocus leads to axial hyperopia Peripheral hyperopic defocus leads to axial myopia Reducing peripheral hyperopic defocus might be the key to slowing the progression of myopia Exposure to outdoor lighting during childhood may slow the development of myopia

Perceptual learning

New approach to amblyopia treatment; has been demonstrated to occur in the elderly Intensive engagement of the amblyopic eye with customized video games results in improved VA, stereopsis, and other visual functions Experiments suggest binocular training designed to improve interocular interaction between the amblyopic and non amblyopic eye leads to improved visual function in the amblyopic eyes of adults and children Requiring the nonamblyopic eye to view through a neutral density filter may rebalance input from the two eyes and could potentially be useful in treatment Total light deprivation may restore some degree of plasticity to the visual system (in adult human patients)

Treatment of amblyopia

Nonamblyopic eye is patched or blurred with a cycloplegia, thereby requiring the treated child to use the amblyopic eye Relatively short periods of patching are apparently as effective as longer periods for the treatment of moderate childhood amblyopia

Striate cortex

Normally devoted exclusively to visual processing but may process tactile info in people who become blind early in life; also, when they redirected retinal ganglion cells to the auditory thalamus in newly born ferrets, it resulted in vision AND visually responsive cells in the auditory cortex Rather than remaining fallow due to the lack of visual input, the striate cortex is utilized for non visual functions

Saturation

Not all spectral stimuli are equally saturated (in trichromacy) Deuteranopic Function: intersects the abscissa at ab 498 nm ** Protanopia function intersects the abscissa at ab 492 nm ** Where it intersects the abscissa, the wavelengths appear white-- totally desaturated-- and are called NEUTRAL POINTS (the neutral pt in tritanopia is approx. 569 nm) ** Neutral points can also be seen where the confusion lines that pass through white intersect the spectral locus (on the CIE diagram w/ confusion lines) Wavelength discrimination is best in the region of the neutral point Individuals w/ normal color vision and anomalous trichromacy do not manifest neutral points

Alternating exotropia

Not expected to develop amblyopia May have little or no stereopsis

Ventral and dorsal processing stream

Not independent of one another, there is significant communication between the two cortical processing streams

Chromatopsias

Not true color vision anomalies bc they do not typically produce a decreased ability to discriminate colors Rather, they are a distortion of color vision, similar to looking through a colored filter (pts may report that objects have a colored tinge or halo) May follow a cataract extraction; a nuclear cataract acts as a yellow filter, absorbing blue light-- the pt has probably been used to this for several years prior to extraction so now they're getting a lot more blue light and they perceive blueness (cyanopsia) The effect will wane as the visual system adapts They can also occur secondary to various medications; digitalis and the fluorescein used in fluorescein angiography may produce yellow vision (xanthopsia)

Retinal Prostheses

Now available to bypass damaged retinal tissue Add a microarray to the retina to directly stimulate the ganglion cells The glasses have a camera that send a signal to the retinal implant

Where does the photopic b wave originate from?

ON AND OFF cone bipolar cells; the polarity of the ON AND OFF cone bipolar cell responses are always opposite and are reversed at light onset and offset

Where does the photopic d wave originate from?

ON AND OFF cone bipolar cells; the polarity of the ON AND OFF cone bipolar cell responses are always opposite and are reversed at light onset and offset

Nagel Anomaloscope

ONLY CLINICAL INSTRUMENT THAT CAN PROVIDE A COMPLETE DIAGNOSIS OF RED GREEN COLOR VISION ANOMALIES, INCLUDING THE DIFFERENTIAL DIAGNOSIS OF DICHROMACY FROM ANOMALOUS TRICHROMACY! Through the eyepiece the pt views a bipartite field that is composed of an upper mixture field and a lower test field. The mixture field is made of a mixture of red and green and the test field is made of yellow. Turning the mixture knob controls the relative amounts of red and green (knob at 0 is exclusively green, knob at 73 is exclusively red; between is various combos of red and green) The pt's task is to match the two fields so that they appear identical Important feature of the mixture field is that its LUMINANCE, as measured for normal trichromacy, does not change (no matter the combination of red and green) The lower test field consists of 590 (yellow) of variable radiance. Turning a knob controls only its radiance. 0 is for very dim yellow and higher scale settings are brighter yellow None of the nagel Anomaloscope's three primaries are absorbed by the S cones, so S cones are not involved -- therefore, a person with normal trichromacy behaves as if he has dichromatic vision People with normal trichromatic vision are very specific when making matches. They do not accept mixture field settings that differ much from their preferred setting. They can match the two patches because they are technically a dichromat when S cones aren't involved, and there are three wavelengths divided into two patches. Dichromats can adjust the test field radiance such that it matches the mixture field. Individuals with red green dichromacy only have one photopigment that can absorb light with the nagal anomaloscope, so they have MONOCHROMATIC vision in this test!!! -Deuteranopia: luminance function is ab the same as in normal color vision so the brightness of the mixture field is the same regardless of the setting. So once they match the test field to be the same luminance as the mixture field, they don't need to change the test field setting to match any other mixture field setting. They use the same test field to match any mixture field! -Protanopia has their luminance function displaced towards shorter wavelengths so the mixture field stimuli do not have the same brightness. Green appears bright and red appears dim. So when matching green, they will set the test field to a high radiance. The pt finds it necessary to reduce the radiance of the test field when matching red. Anomalous Trichromacy: If asked to adjust the mixture field so that it has the same color as the test field, they sometimes, but not always, select a mixture scale setting different than that selected in normal trichromacy. They are a bit less specific in their matches than individuals w/ normal trichromacy, but more specific than those with dichromacy. A pt with anomalous trichromacy will be able to match a comparatively broad range of mixture settings -Deuteranomalous trichromacy: adds more green to the mixture field; they will not adjust the radiance because deuteranomalous trichromacy manifests essentially the same luminosity function as an individual w/ normal color vision -Protanomalous trichromacy: adds more red to the mixture field; They will also adjust the radiance of the test field, because red will appear more dim to them.

Motion perception under scotopic conditions

Objects appear to move 25% slower under rod mediated vision than under cone mediated vision Motion perception under mesopic conditions (rods and cones are active) is impaired. May be caused by incomplete integration of rod and cone signals

Frequency of seeing curve (FOS) or psychometric function

Observer is required to detect a light (stimulus) flashed onto a surrounding background; it is repeated for a range of stimulus intensities and the percentage of stimuli detected is plotted as a function of stimulus intensity Ideal observer (hypothetical) manifests an unambiguous threshold: below the threshold intensity, the stimulus is never seen, and above the threshold, the stimulus is always seen (not how biological systems work)-- shown in first graph Real observers: No clearly defined intensity below which the stimulus is never seen and above which the stimulus is always seen (bottom graph-- it is a gradual slope); instead we say the threshold value is when 50% of the time, the stimulus is detected -The diseased eye is also shown: the curve becomes flatter-- harder to determine what the threshold is because there is a broader range of stimulus intensities that could correspond to 50% detection The more alternative forced choice options there are, the steeper the psychometric function will be

Computer based color vision tests that use pseudoisochromatic plates

Offer the potential to screen a large number of ppl for color vision deficiencies But, the chromaticities of the plates may vary from computer to computer due to variations in hardware and software settings Waggoner Computerized Color Vision Test may serve as an adequate screening tool

Receptive fields of ganlion cells

Often have a center -surround organization: light falling on the receptive field's center has the opposite effect of light falling on the surrounding area of the receptive field; this is also called spatial antagonism or lateral inhibition If it increases APs in the center and decreases APs in the surround, the ganglion cell is said to have on center and off surround The response to a stimulus that fills the entire receptive field is about the same as if there were no stimulus--you'd get the smallest response with this type of stimulus; this illustrates that spatially antagonistic ganglion cells do not respond well to diffuse illumination (no change in frequency of APs from the baseline maintained discharge) Ganglion cells are selective for spatial contrast

Why does the visual system show a reduction in sensitivity for high frequencies?

One reason is optical limitations: any optical system, including the eye, manifests a high frequency limitation because of OPTICAL ABERRATIONS (this is the case even when the eye is in focus) Another factor is the packing density of retinal photoreceptors (Nyquist theorem)

Intracellular recordings from the various neural elements in the mud puppy retina

Only amacrine cells and ganglion cells generate APs All other retinal neurons generate graded potentials

Limitations to the snellen chart

Optotypes are not evenly spaced (bottom letters are more crowded tg) so difficult to resolve at the bottom because there is this masking effect produced by the closely proximate surrounding contours; this crowding phenomenon is esp strong is disorders of central vision, including amblyopia Bailey Lovie eye chart was designed to circumvent many snellen issues: -Each line has the same number of optotypes -Spacing between the optotypes is proportional to the size of the optotypes, making the contour interactions approximately the same for each line -Optotypes are selected to be equally legible -Difficulty of the task is determined primarily by the spatial frequency of the optotypes and not other factors -Has footsize and logMAR written on the chart; diff between each line of the chart is 0.1, making it straightforward for the clinician to measure VA at nonstandard testing distances (which you might need w/ low vision pts) -Each letter is given a value of 0.02 log units, and there are five letters for each row adding up to 0.1 -

Retinocortical Visual Projection

Organized along three major pathways from retina to LGN; the LGN is divided into three regions each constituted of a different class of neurons. Two most ventral layers consist of large mango cells, while the four dorsal layers consist of smaller parvo cells. In between these principal layers, in the interlaminar regions (intercalated layers), are collections of yet smaller konio cells. The LGN sends its primary projection to the visual cortex Parvo (small): red-green opponency; midget cells project to the parvocellular layers Magno (large cell): non color opponent, parasol cells project to the magnocellular layers Konio: yellow-blue opponency; bistratified cells project to the interlaminar regions Therefore, primate color vision is trichromatic but encoded in an opponent fashion (at the level of the bipolar cells) It is not clear whether brightness info is encoded by color opponent neurons or by a separate class of non color opponent neurons (magno cells)

Wavelength Discrimination

Our ability to distinguish one wavelength from another varies across the spectrum A test wavelength is on the left side of the split (sometimes called a bipartite field) The right side is initially identical to the left, but then is changed to a different wavelength until the two sides of the field no longer match in hue The difference in wavelength required so that the left and right sides of the field no longer match is a measure of wavelength discrimination (Δλ)-- the smaller (Δλ) is, the better wavelength discrimination you have The regions of best discrimination are at about 495 and 590 nm (two stimuli are different hues even if their wavelengths are just a few nanometers different) A stimulus of 410 nm may be the same hue has 415 nm, however, bc hue discrimination is relatively poor in the short wavelength region of the spectrum (as you can see in the figure-- sometimes called the W curve of color discrimination) Although it is not settled, the reason wavelength discrimination varies across the spectrum may be that color discrimination is best where the slopes of the cone absorption spectra change most rapidly with respect to each other

Temporal and spatial properties

Parvo cells manifest SUSTAINED responses when presented with a long duration stimulus (respond for a relatively long period of time); good for encoding low temporal frequencies; smaller receptive field centers provide parvo cells with higher spatial frequency resolution than magno cells Magno cells respond to the same stimulus in a TRANSIENT manner, with only a brief burst of activity at stimulus onset and offset (might be due to input from transient amacrine cells); transient response properties give magno cells the ability to resolve high temporal frequency stimuli; magno cells transmit APs faster than parvo or konio cells, so stimuli that isolate the magno pathway are expected to result in shorter visual latency The clear division into distinct parvo and magno pathways seen in the retinogeniculate pathway is not so apparent in the cortex (neither appears to communicate exclusively with a particular cortical processing stream)

Adjustment Method

Patient themselves makes the adjustment: pt simply adjusts the stimulus until they see it Not particularly reliable Not really used

PERG

Pattern electroretinogram Transient Contains the P50 and N95 components; ERG recorded in response to each contrast reversal of checkerboard or grating patterns

Nyquist Theorem

Photoreceptors packed at low and high densities Foveal cones subtend a visual angle of approx. 0.5-minute arc. A grating constituted of bars of this width has a spatial frequency of 60 cycles/degree (20/10), suggesting that even if we could correct all the eye's optical aberrations, acuity could not be improved beyond 20/10

Replacement model of deuteranopia and protanopia

Posits the missing photopigment is replaced by a remaining photopigment In deuteranopia, chlorolabe is replaced by erythrolabe In protanopia, erythrolabe is replaced by chlorolabe

Prior Adaptation

Prior adaptation to a spatial frequency grating supports the model of having independent SF channels: if a subject's CSF is determined and then they view an adapting sinusoidal grating for ab 1 min prior to redetermination of the CSF, there will be a DISCRETE REDUCTION IN SENSITIVITY at the specific frequency to which the observer was adapted This is the result predicted by the multiple channel hypothesis If there was only a single channel, you'd expect an overall reduction in the CSF across all frequencies, and adaptation to ANY spatial frequency would have the same effect... it would produce a reduction in sensitivities for all frequencies Single unit electrophysiological data from the primate visual cortex are consistent w/ the notion that the visual system could act as a Fourier analyzer: neurons in striate cortex appear to be finely tuned to specific SFs; A collection of such neurons could form the physiological basis for spatial frequency channels

Contrast of projected/printed charts

Projected snellen chart's contrast is greatly affected by room illumination (turning on the lights reduces the chart contrast with subsequent reduction in VA) Printed charts: increasing room illumination does not substantially change the contrast of the chart optotypes (VA would be about the same)

Further classification

Protan, Deutan, and Tritan refer to the affected photopigment (erythrolabe, chlorolabe, and cyanolabe, respectively) A person w/ a protan defect may either have dichromacy or anomalous trichromacy People with protan and deutan vision tend to confuse reds and greens and are said to have RED-GREEN COLOR VISION ANOMALY (these anomalies are usually, but not always, inherited) Individuals with tritan vision, who confuse blues and yellows, are classified as having BLUE-YELLOW ANOMALY

The Smith Kettlewell Institute Low Luminance (SKILL) Card

Provides an inexpensive and quick clinical method to assess low luminance, low contrast VA Card is designed for near testing One has a high luminance high contrast (90%) acuity card and the other has a low luminance low contrast chart SKILL score is derived by subtracting the VA measured on the low contrast chart from that measured on the high contrast chart The test results are relatively unaffected by optical blur, but room illumination should be ab the same between measurements (bc low contrast acuity is strongly affected by luminance) SKILL performance remains relatively stable up to ab 50 years of age, then begins to decrease Usually this decline is due to preretinal factors (pupillary and lenticular aging changes), and the rest is due to retinal and post retinal factors SKILL card may demonstrate losses that are not apparent using only a high contrast chart SKILL scores have also found to be abnormal in diabetic pts even when these pts do not manifest retinal changes observable during ophthalmoscopy

Recognition acuity

Pt to not only resolve an optotype but also you must label it/ name it aka you need to know the alphabet to do snellen

Cataracts/ light scatter

Pt's with early cataract formation who may manifest only a minimal reduction in high contrast VA oftentimes complain of disabling visual loss. The discrepancy between the severe complaint and the slight reduction in acuity occurs because the cataract scatters light, reducing image CONTRAST across ALL FREQUENCIES The pt is therefore impaired not only at high frequencies, but at moderate and low frequencies as well Therefore the pt's complaints are disproportionate to the reduction in acuity Cataract is an opacity of the crystalline lens This opacity scatters light and REDUCES CONTRAST FOR ALL SPATIAL FREQUENCIES, resulting in a CSF that is depressed at all frequencies Pts with early cataracts often report that the headlights of oncoming traffic greatly reduce their vision. The light entering the eye is scattered by the cataract causing VEILING GLARE that can reduce the contrast of the retinal img BAT: brightness acuity tester: a clinical instrument that shines diffuse light into the eye as the pt view an eye chart; can cause marked reduction in VA if there is a cataract (even an early one) bc of light scatter

Achromatopsias

Rare conditions where the pt manifests monochromatic or nearly monochromatic vision The most common is autosomal recessive (AR) achromatopsia Complete AR achromatopsia: only rods present Incomplete AR achromatopsia: residual L and M cone function Signs/Symptoms: -No or very poor color discrimination -Nystagmus -Photophobia -VA of 20/200 (remember no cones!) DARK RED lenses, which minimize the bleaching of rhodopsin, thereby permitting rod function under brighter lighting conditions, can be recommended for these AR achromatopsia pts X-linked (XL) achromatopsia (sometimes called blue or S cone monochromacy) contain only rods and S cone (but other cone types may be present in certain affected individuals) -- inherited in a recessive manner but EXCEEDINGLY RARE; symptoms are similar to those of AR achromatopsia; MAGENTA colored lenses which minimize rod saturation while providing light to enable S cone function, may provide relief for these pts (think blue and magenta from blue's clues)

Seasonal affective disorder

Reduction in daylight hours during the winter months can negatively impact mood In certain ppl, mood may be depressed to the point that it interferes w/ normal daily activities (SAD) Might be able to be treated using bright light therapy (typically in the morning); melanopsin containing RGCs/ipRGCs may play a role in meditating this therapeutic effect Possible reason as to why smart phones (emit bluish light) at night might suppress melatonin release, interfering with sleep in certain individuals

Amblyopia

Reduction in vision secondary to monocular deprivation during the critical period Amblyopia results from abnormal cortical development, not an abnormality of the eye itself Can be secondary to amblyogenic conditions of monocular occlusion, anisometropia, or strabismus (diagnosis of amblyopia should only be made if there is a history of one of these factors)

CIE Chromaticity Diagram

Relative amounts of the imaginary primaries required to match any real color It is constructed by converting the imaginary primaries' tristimulus values to relative units, referred to as chromaticity coordinates The chromaticity diagram shows only the x and y chromaticity coordinates Coordinate z is calculated by subtracting the sum of x and y from 1 (example, .4x and .2y make a color, what is left is .4 z) Inside the CIE chromaticity diagram are physically realizable colors. Spectral hues (monochromatic hues-- not mixed with any other color) are arranged along the arc of the perimeter of the diagram, referred to as the SPECTRAL LOCUS Non spectral purples fall along the straight line connecting 380 and 700-- purples are not produced by a single wavelength, but by mixing blue and red White is in the center

Sensitivity Regulation

Relative sensitivity of the visual system (webers fraction) does not change as the illumination increases, but the absolute sensitivity does decrease (threshold increases bc background intensity is increasing)

Complex cells

Respond best to an elongated stimulus of a specific orientation Stimulus can be positioned anywhere within a complex cell's receptive field, but the stimulus must move in a specific direction! The receptive fields of complex cells cannot be divided into separate excitatory and inhibitory regions Spatially nonlinear magno cells are thought to provide input to complex cortical cells

What is the origin of the photopic negative response? PhNR

Retinal ganglion cells Photopic negative response could disappear if your cones are degenerated because the bipolar cells would have no input and therefore your ganglion cells wouldn't either. So not necessarily that the ganglion cells aren't working, but that they're not getting input

A new type of retinal prosthesis

Retinal prosthesis can potentially provide vision in pts who suffer from a disease that destroys the photoreceptors Severely limited bc they're designed to stimulate ganglion cells w/ a signal that roughly replicates that of the photoreceptors. In doing so, they ignore the processing that occurs w/ in the retina A different way to restore vision in these pts would be to determine and replicate the encoded message produced by post receptor retinal processing and to provide this input directly to the ganglion cells This may result in an image remarkably similar to that produced by a healthy retina Restoration of near normal vision using a retinal prosthesis would benefit by taking into account post-receptoral processes

Descending Limits Method

Reverse of Ascending Limits A trial commences with a clearly visible stimulus and then is decreased until they cannot see it Used to determine VA (along with forced choice): pt is asked to read down the Snellen chart with optotypes that become smaller from top to bottom (and more difficult to resolve) Very effective in determining VA, but just alone it is NOT good enough (you need forced choice as well)

Determination of absorption profile of rhodopsin

Rhodopsin has an absorption spectrum (some wavelengths of light are absorbed more by rhodopsin than other wavelengths of light) Light that is incident is a single wavelength, you'd do this with a bunch of different wavelengths Transmission is least at 507 nms, so it is absorbed MOST at 507 nms (transmission and absorption spectrums are therefore reciprocals of one another); note: there is absorption of other wavelengths, but with less probability Rhodopsin molecules undergo a transformation once they absorb light (it is bleached); it goes back to its original state in about 5 mins (half life is 5 mins-- 50 percent of the rhodopsin will recover in 5 mins)

Teller Acuity cards

Rigid rectangular hand held cards have a grating on one side, while the other side contains no pattern, but has the same avg luminance as the grating. If the infant looks at the grating, it is assumed he or she can resolve it Presenting a grating several times is often sufficient to determine if it can be resolved. Examiner flips to card to switch switch the location of the grating Clinician presents gratings of increasing frequency until the infant does not consistently tend to the location of the grating. It may give similar results to the FPL method, but there is low test-retest reliability so it may limit effectiveness in diagnosing subtle reductions in acuity that may be associated with clinically significant amblyopia. Teller acuity cards reveal that healthy 1 month old infants have acuities of approximately 20/600 (1 cycle/degree). Resolution acuity improves rapidly during the first year of life, with 1 year old children manifesting acuities of approximately 20/100 (6 cycles per degree) This is followed by a slower improvement in acuity. Adult levels of 20/20 acuity are not reached until 3-5 years of age, a finding consistent with common clinical experience. Immaturities in the retina (particularly the foveal cones) account for the poor acuity of infants during the first year of life

Small distratified cells

S cone bipolar cells synapse onto a distinct class of ganglion cells called small bistratified cells which account for ab 8% of ganglion cells These have an on center receptive field formed exclusively by S cones

The behaviorally determined scotopic spectral sensitivity function has the same shape as the absorption function

Scotopic spectral sensitivity is found by asking the patient to sit in a dark room for 45 mins to regenerate rhodopsin, and then see what the min amount of energy needed is for the patient to see a certain stimulus The minimum amount of energy required for detection of a stimulus is referred to as the THRESHOLD for that stimulus The threshold is lowest at 507 (we need the least light to see 507 nm); so at 507 nm, sensitivity is the highest Sensitivity is the reciprocal of threshold, as threshold increases, sensitivity decreases For example, If you have a low threshold to feeling something on your hand, you are very sensitive there on your hand

Stimulus length

Sensitivity to stimulus length is a feature common to many cortical cells

Human Contrast Sensitivity Function

Sensitivity, not contrast, is being measured Subject asked to view a monitor that initially displays a grating below threshold-- really low contrast so much that it is not visible. Examiner slowly increases contrast until the grating is seen (the reciprocal of this threshold contrast is the contrast sensitive for the grating) Sensitivity is determined for a large number of spatial frequencies, resulting in a CSF: contrast sensitivity vs spatial frequency. NOTE: SF INCREASES LEFT TO RIGHT, BUT CONTRAST INCREASES FROM TOP TO BOTTOM Typical adult human CSF is shown; max sensitivity is approx 4 cycles/degree, with sensitivity decreasing on either side of this peak. Spatial frequency is specified in ANGULAR UNITS, so no matter the viewing distance, the 4 cycles per degree will have the max sensitivity. Note: if you double the viewing distance, the bars of the grating will need to be twice as wide than those at the original distance to subtend the same angle; If you do not recalibrate the bar width, and you were to use a shorter viewing distance (as an example), you would have the same CSF but the peak of the curve would be shifted. So if you had 4 cpd as your peak at a viewing distance of 1 m, when you move to .5 m, the peak of the curve would be shifted to 8 cpd. (Think: when you bring a snellen chart closer, you can read smaller letters/ higher SFs) Arrow pts to high frequency cutoff. Note the coordinates are in log units, the abscissa represents 100 percent contrast

Method of Ascending Limits

Sequence of stimulus presentations: the stimulus intensity is increased systematically until it is visible Results are averaged to obtain a threshold Particularly advantageous in DARK ADAPTOMETRY because the state of retinal adaptation is affected minimally by the stimulus (since you start low intensity) Issue is that bias is introduced because the patient expects the stimulus to come at a certain time-- if the stimulus starts at the same intensity on each trial, they might try to be consistent and anticipate when the stimulus should be seen based on when its was seen on previous trials (method may be contaminated by observer anticipation-- this can be addressed by starting each trial at a different intensity)

Simone cells in the striate cortex

Simple cells: most sensitive to an edge or bar of specific orientation -If the stimulus is a bar, it must be a specific width, and it must be properly positioned within the cell's receptive field -Simple cells CAN be mapped out with small spots of light: it reveals that their receptive fields are divided into antagonistic excitatory and inhibitory regions -Simple cell receptive fields may result from input of LGN neurons whose receptive fields lie along a straight line -Has orientation and position selectivity and distinct on and off regions

Sine Wave Gratings

Simplest spatial stimulus: serve as building blocks to construct more complex stimuli Sine wave gratings consist of alternating bright and dark bars; peak of luminance profile shown in graph corresponds to a bright bar of the grating, while trough corresponds to dark bar Transition from bright to dark bars is a gradual (sinusoidal) transition, NOT abrupt Two most important characteristics are FREQUENCY and CONTRAST

Minimum Detectable Acuity

Smallest object that can be seen How thin can the wire be for you to be able to see it against the blue sky You don't need to resolve or recognize, just be able to DETECT it (similar to an increment threshold) Bc of optical imperfections of the eye, the wire is not in perfect focus on the retina, so the img takes the form of a line spread function In order for the line to be detected, the height of the line spread function must be of critical value; the height of the line spread function diminishes as the line becomes thinner The critical value is the same as in the increment threshold experiment In the example, the line would not be detectable Minimum detectable acuity is limited by how thin a line or small dot can be made and still emit sufficient light so that the increment threshold is reached (determined to be 1 second of arc -- way smaller than that for resolution acuity) Optical defocus will result in diminished height of the line spread function with a resultant reduction in minimum detectable acuity -- but its not as marked as it would be with resolution acuity

Snellen Summary

Snellen acuity samples only a small portion of the pt's spatial vision Cataracts corneal edema and other conditions may reduce contrast sensitivity at spatial frequencies other than those measured in an acuity task Complaints of reduced vision that are not proportional to the reduction in snellen acuity warrant careful investigation

Amacrine cells

Some show a center surround organization Unlike bipolar cells, they tend to respond briefly/transiently at the stimulus onset and offset Bc of this characteristic, amacrine cells are thought to play a critical role in coding movement First neurons to display APs (photoreceptors, horizontal cells and bipolar cells show graded potentials)

Photoreceptor

Specialized sensory receptors containing a photosensitive pigment that absorbs light quanta (converts radiant energy to electrical activity); first step in vision Rods/ cones are slightly DEPOLARIZED relative to a typical neuron (resting membrane potential is about -50 mV compared to a normal neuron w/ a potential of -70 mV); when exposed to light, the photoreceptors hyperpolarize to ab -70 mV (so in this case, stimulation causes hyperpolarization!) Degree of hyper polarization is related to the intensity of the stimulus (more intense = more hyper polarization); bc of this, a photoreceptor's electrical response is sometimes called a graded potential

Blobs

Staining striate cortex for cytochrome oxidase reveals an irregular pattern of blobs within its superficial layers and a regular pattern of stripes in adjoining visual area 2 Striate blobs are rich with concentrically organized, double color opponent neurons that result from parvo input These cells are optimally responsive to stimuli that manifest color contrast The superficial region of striate cortex between blobs (interblob region) also appears to receive substantial parvo input Magno pathway apparently bypasses blobs and the interblob regions

Spatial Modulation Transfer Function of a Lens

Start with a grating of a specialized spatial frequency and contrast that serves as the object for the lens. Measure the image's contrast and divide by the object contrast. This ratio will tell us how well the lens transfers info (contrast) contained in the object This is repeated for a spectrum of SFs, ranging form low to high. The result is a spatial modification transfer function (SMTF) that shows the quality of image reproduction as a function of spatial frequency

Method of Constant Stimuli

Stimulus visibility is varied randomly from presentation to presentation Subject has no way of knowing when they'll see it (their expectation is the same throughout the whole test-- they do not know when they will see the stimulus) It is like a yes or no experiment We plot the number of correct responses against intensity of stimulus and read threshold off of that We can determine if the subject has a lax criterion (sees a stimulus when it is not there) by giving them a blank trial. If they push the button, thats a false positive, or false alarm We can determine if the subject has a strict criterion by conducting trials using supra threshold stimuli. When the subject reports not seeing such a stimulus we call the response a false negative.

More on striate cortex

Striate cortex also send major reciprocal projection to the LGN (as well as to the pulvinar, a thalamic nucleus thought to be associated with visual attention, motion processing, and visually guided movement) As a general rule: projections to lower visual centers (like LGN and the pulvinar) originate from the deeper layers of the striate cortex (layer 6); projections to the extrastriate cortex originate from more superficial layers (layers 2/3) Info also flows back towards the striate cortex from the extrastriate cortex via reciprocal pathways (feedback loops); these feedback loops play a gating function, contributing to the receptive field properties of V1 cells

Extrastriate cortex

Striate cortex projects here. Region of visual cortex not distinguished by the line of Gennari Includes visual area 2 (V2), visual area 4 (V4), inferotemporal cortex (IT) and MT/V5, also referred to as middle temporal cortex (MT) or visual area 5 (V5)

Temporal Vision cont.

Studied w/ stimuli whose luminance varies sinusoidally over time These stimuli are the temporal equivalent of spatial sine wave gratings An example would be a computer screen that turns on and off with a sinusoidal time course "A" refers to the amplitude of modulation -- changes to luminance over time

ROCs: receiver operator curves

Summarize the patient's behavior Shows the probability of a hit as a function of the probability of a false positive for the diff threshold criteria Strict Moderate Lax

POAG

TMTFs and related temporal psychophysical tasks offer promise for assisting in the early diagnosis of glaucoma Data obtained on pts with OHT (elevated IOPs w/ normal visual fields) reveal abnormalities at moderate and high frequencies, suggesting that temporal sensitivity may be an early indicator of ganglion cell loss

Temporal and motion aspects of vision and age

Temporal modulation decreases in the aging eye, at all temporal frequencies There are also reductions in temporal resolution as measured by the CFF The ability to detect motion has also been demonstrated to diminish with age (minimum displacement thresholds and global motion) Might be due to central neural changes

Temporal Sensitivity (Photopic)

Temporal summation period is 10 ms Poor temporal summation, but excellent temporal resolution We can see that there are multiple events but it is difficult to summate so it is hard to see things to begin with bc the impulses are sub threshold We like photopic vision better because we have better temporal and spatial RESOLUTION

Two paths of light are adjacent to each other. The conditions are scotopic. One patch emits light of 507 nm and the other emits light of 620 nm. Both patches produce 40 quanta of light. Which patch is brighter?

The 507 nm patch is brighter because it is absorbed more (rods absorb light of wavelength 507 nm the most)

CIE summary

The CIE diagram specifies the relative amounts of primaries necessary to match a color sample The CIE primaries do not physically exist, they are imaginary The CIE system uses imaginary primaries to avoid the use of negative quanitites Any set of color matching functions, real or imaginary, can be converted to another set of real or imaginary functions

Development of contrast sensitivity

The CSF for 1 month old infants does not manifest reduced sensitivity at lower SFs. This reduction that normally occurs bc of lateral retinal inhibitory processes is absent because lateral interconnections have not yet fully developed in the neonatal retina. Limitations in spatial vision present during the first year of life are due primarily to immaturities of the retina rather than the central vision pathways CSF shifts upward and to the right as an infant matures, reaching adult form at ab 9 yrs old. This reflects increased contrast sensitivity and improved VA The CSF peaks at the adult location (ab 4 cycles per degree) at approximately 4 years, and the overall function is adult-like by 9 years)

Where does the C wave of the dark adapted ERG originate from?

The RPE

Short Wavelength Automated Perimetry

The S cone system is apparently more vulnerable to certain pathological processes than the M and L cone systems This may be due to vulnerabilities in the S cones themselves or the blue yellow opponent pathway. The paucity (scarcity) of S cones may also be factor SWAP has been developed to assess S system function in certain eye diseases, particularly glaucoma Humphrey visual field apparatus can be adapted for this function by replacing the standard white stimulus with a short wavelength stimulus to maximize S system sensitivity To suppress the M and L cones, the background is yellow rather than the standard white

Temporal Modulation Transfer Functions

The advantage of sinusoidal stimuli is that they allow the application of linear system (Fourier) analysis A complex temporal stimulus can be constructed by the appropriate combination of temporal sinusoids Temporal vision is characterized by the temporal modulation transfer function (TMTF); to determine a person's TMTF, an individual views a light source like a computer screen that is modulated sinusoidally at a given temporal rate Initially, the modulation depth is small, so the screen appears steady. The modulation depth is increased until the subject reports the screen is flickering. The modulation depth at which the subject first sees the flicker is the threshold. Its reciprocal is relative sensitivity for flicker resolution; the procedure is repeated for a large number of temporal frequencies, from low to high, to obtain a TMTF Typical human TMTF is given in figure; graph shows relative sensitivity (reciprocal of threshold percentage modulation) as a fraction of temporal frequency. Stimuli that fall outside of the TMTF are seen as fused or steady, they aren to resolved temporally; the stimuli that fall under the graph are resolved temporally and perceived as flickering As you can see, the TMTF shows maximal sensitivity to moderate frequencies (like the CSF) A stimulus modulated temporally at ab 10 Hz is seen flickering at lower percentage modulation than are stimuli of lower or higher temporal frequencies The high frequency cutoff for the TMTF represents the highest temporal frequency that can be resolved at 100 percent modulation

Stiles Crawford Effect

The angle at which a light ray strikes a cone is critical. It is not critical for rods. When the light hits the cone straight on it is more effective at bleaching photopigment (appears brighter) Graph shows that a light ray appears brightest when the light rays strike cones perpendicular to their surface

Receptive fields

The cell's receptive field: an area that influences the cell's neural activity Even when light is not shining on the receptive field, the cell spontaneously generates APs -- these APs that occur in the absence of a stimulus are referred to collectively as the spontaneous activity or maintained discharge of the neuron A stimulus positioned w/in a neuron's receptive field can either increase or decrease the neuron's discharge rate, depending on the stimulus' location. Plus signs in the receptive field mean that light positioned there will cause an increase in the frequency of APs. Minus signs will cause a decrease in the frequency of APs

Ganglion cells

The center surround organization found in ganglion cells has its origin in bipolar cells On center midget bipolar cells synapse with incenter midget ganglion cells Off center midget bipolar cells synapse with off center midget ganglion cells Midget cangino cells are sometimes referred to as retinal parvo cells (they make up approx. 70% of foveal and nonfoveal ganglion cells) Foveal midget ganglion cells receive input from only one midget bipolar cell. Consequently, their center is formed by only one cone. This limited spatial summation accounts for their high spatial resolution In the peripheral retina, more than one bipolar cell feeds into a midget ganglion cell center, so theres a larger receptive field center, increased spatial summation, and decreased spatial resolution (compared to foveal cells) On center diffuse bipolar cells synapse with on center parasol ganglion cells, and off center diffuse bipolar cells synapse with off center parasol ganglion cells Parasol ganglion cells are sometimes called retinal magno cells

Color Confusion Lines

The confusion line origin is called the COPUNCTAL POINT Deuteranopia, protanopia, and tritanopia each have a different copunctal point All colors falling along a confusion line are indistinguishable; screening plates are designed to test color confusion lines The color confusion lines for anomalous trichromacy follow the same pattern as dichromacy but may be shorter, not crossing the full length of the diagram

Stereopsis changes with age

The elderly manifest decreased stereopsis Stereoacuity THRESHOLDS become increasingly elevated beyond 50 yrs old

Development of Scotopic Sensitivity

The form of the scotopic sensitivity function is adult-like at 1 month of age Not surprising bc the shape of the function is determined by the absorption characteristics of rhodopsin, and does not depend on postreceptoral processing The absolute sensitivity of the scotopic system (the sensitivity for a stimulus of 507 nm presented under conditions that maximize scotopic sensitivity) reaches adult levels by ab 6 months of age

Serial or hierarchical processing

The formation of increasingly complicated receptive field arrangements from less complicated arrangements (like center surround LGN cells)

Cortical magnification of foveaL vision

The fovea is only 0.01% of the retinal area, yet is represented in at least 8% of the striate cortex May be due to the increased area of cortex devoted to individual foveal ganglion cells, rather than solely to the high density of foveal ganglion cells

Origins of the reduction in sensitivity to low and high temporal frequencies

The high frequency TMTF cutoff is due to neural constraints in coding high temporal frequency information The faster a neural system responds, and the more transient its response, the greater its temporal resolution As a stimulus' temporal rate increases, a frequency is eventually reached where the neural response is neither fast nor transient enough to allow resolution For low temporal frequencies, we can explain this by thinking about lateral inhibition of ganglion cells. The center and surround respond with different latencies (there is a phase lag between the center and surround). For low temporal frequencies, this phase lag can cause the center and surround signals to cancel each other (i.e. inhibitory interactions are maximized), thereby reducing sensitivity

Color Vision Anomalies Molecular genetics

The highly homologous genes for the M and L cone photopigment opsin are positioned on the X chromosome in a head to tail tandem array Erroneous crossover of genetic info could occur when the pair of X chromosomes aligns and exchanges genetic info during meiosis (i.e. unequal homologous recombination) For instance, the gene coding for the M cone opsin could erroneously align with the gene coding for the L cone opsin, leading to unequal exchange of genetic info In B you can see there is INTERGENETIC cross over, and one chromosome has no M cone photopigment gene (offspring who inherit this chromosome may manifest deuteranopia); the offspring that inherit the other chromosome will have normal color vision In C you can see INTRAGENETIC cross over resulting in hybrid genes; depending on the specific nature of the crossover a hybrid gene may result in a normal photopigment, the loss of a photopigment, or a photopigment with an absorption spectra that is displaced from its normal location (could be the basis for anomalous trichromacy); A hybrid gene that differs substantially from the normal gene may result in severe anomalous trichromacy, where the deviant photopigment absorption spectrum almost completely overlaps that of the other long wavelength photopigment

Development and Maturation of vision

The human visual system matures over the first decade or so of life

Inheritance of anomalous color vision

The majority of red green anomalies are inherited and transmitted in an X linked recessive fashion More common in men than women Caucasians have the highest prevalence of color anomalies and blacks have the lowest The most commonly inherited anomaly is anomalous trichromacy Inherited tritan anomalies which are autosomal dominant are extremely rare Note, in the examples of the transmission of X linked, red green anomalies from parents to offspring that a boy ALWAYS gets the defective gene from the mother (must get the Y from his dad)

Maximum displacement threshold (Dmax)

The maximum distance the dots can move and still elicit motion perception

Minimum displacement threshold (Dmin)

The minimum distance dots must move in a given direction to elicit the perception of motion

Clinical determination of the CSF

The most accurate clinical methods display gratings on a video monitor that is driven by a microprocessor More accessible methods employ gratings displayed on a printed chart (VISTECH) or a series of cards (Arden plates) Vistech's reliability has been called into question Determination of CSF is not common clinical procedure bc contrast sensitivity info can be obtained more efficiently with other procedures A Bailey lovie Acuity chart w low contrast is good for this (low contrast, decreasing letter size) Pelli Robson measures threshold contrast using optotypes (size is the same, but contrast decreases) Both can alert the clinician to reductions in contrast sensitivity

Critical/ Sensitive period

The period during which the visual system can be influenced by environmental manipulation Figure shows that the sixth - tenth week of life is still early on for the cat, so the cells would become responsive to the right eye The human visual system matures at a much slower pace than the cat -- most sensitive to environmental manipulation during the first 7-9 yrs of age During the critical period, the two eyes compete w/ each other to dominate cortical neurons; if both eyes have equivalent retinal images, most of the cortical neurons become binocular, albeit many are dominated by one eye When one eye is deprived, the other eye has an overwhelming advantage in the competition for cortical neurons, often preventing input form the fellow eye. This results in an excess of neurons that are monocular

Modern Models of Color Vision

The presence of color opponent neurons in the retina and LGN reveals that receptoral info (trichromacy) is encoded in an opponent fashion at postreceptoral levels. The three classes of cones are "wired" tg to produce spectrally opponent neurons (this postreceptoral antagonism first occurs very early in the retina at the level of bipolar cells in primates)

Dark Adaptation

The process by which our vision slowly recovers (lights are on, we turn them off, and our vision must recover) After the lights are turned off, the visual threshold decreases for about 40 mins As time goes on, the threshold decreases, and sensitivity therefore increases (we begin to be able to see better) The beginning bump in the figure, the cone plateau, represents the photopic threshold, then the tip is the rod cone break, where the rods become more sensitive than the cones, then the rod plateau the rods take over and represents the scotopic threshold, after about 85% of the rhodopsin has recovered At 30 mins the threshold is determined by the rods, so we cant tell color, but if we increased the INTENSITY it would hit the cone threshold, so we would be able to discern color The rod cone break is most visible when there is a large photochromatic interval, and when the target is large in diameter (so it hits both rods and cones); therefore, it'll be smallest the target is centrally fixated and small (only hitting cones in fovea). The primary reason rapid assessment of dark adaptation may be clinically useful is its ability to detect deficits in rod dark adaption

Photopic luminance functions, V(λ)

The protanopic luminance curve is substantially displaced towards shorter wavelengths and the deuteranopia curve shows a very slight displacement towards longer wavelengths (the displacement of the deuteranopia luminosity function is minimal, and for clinical purposes it may be considered normal) The normal V(λ) function (luminance function) probably results from the addition of the M and L cone inputs, so if one of the cone types is absent, as occurs in dicrhomacy, it produces a displacement; the greater dislocation of the protanopia luminance function suggests that L cones play a greater role in generating the normal V(λ) function The luminosity functions in anomalous trichromacy manifest the same general dislocation but its less pronounced; therefore for individuals deuteranomalous trichromacy manifest very minimal displacement of the luminosity function (clinically shows up normal)

Reduction in sensitivity for low temporal frequencies

The reduction in low frequency temporal sensitivity as given by the TMTF curve tells us that very gradual changes in illumination are not seen Think about a minute hand on a watch. You know it moves, but you do not actually see it move An important manifestation of the visual system's reduced sensitivity to low temporal frequencies is the inability to perceive stationary or stabilized retinal images (this is called the troxler phenomenon); Relative to the retina, the retinal blood vessels aren't moving. Only when a moving light is shined into the eye that casts a moving shadow of the blood vessels onto our photoreceptors is when it is possible to visualize our own blood vessels (when it is visualized in this manner, the retinal vasculature is referred to as the purkinje tree)

Gene Therapy for color vision deficiency

The replacement of defective genes in ppl w color deficiencies offers the promise to cure color blindness Scientists have demonstrated that adult dichromatic monkeys infected w/ a virus carrying the missing opsin gene manifested improved color vision Further work is needed to determine efficacy and safety in humans

CSF and VA

The smallest high contrast optotype that falls under the CSF curve represents the THRESHOLD ACUITY The typical clinical acuity measurement only tests a very limited aspect of the pt's spatial vision: the high spatial frequency cutoff A low contrast Bailey-Lovie Chart consists of low contrast optotypes of diminishing size The Pelli Robson chart is constituted of optoptypes of the same size, but of diminishing contrast

What is the effect of visual deprivation on the development of ocular dominance?

The striate cortex of a monocularly deprived animal is markedly different from that of a normal animal Most cells are monocular and only responsive to the non deprived eye For the striate cortex to develop a normal complement of binocular cells, it is necessary for both eyes to provide input during development When you close an immature animals eye, there is an absence or substantial reduction in the number of cortical cells that are responsive to the deprived eye. Behaviorally, the animal has greatly reduced or no vision in the deprived eye and since there is a paucity of binocular neurons, there is little or no stereopsis When you do this on an adult cat, the monocular deprivation has no effect (there is a normal complement of cortical neurons); Although the visual system is plastic/ malleable early in life, it becomes hardwired/inflexible later in life.

What are gross electrical potentials and how are they clinically relevant?

The summed electrical activity of a large number of neurons Can be measured non invasively Can be used to determine the locus of the lesion in the visual pathway Can be used to assess a pt's vision objectively -- useful esp in non verbal, unresponsive, unreliable pts as well as for detecting malingering (faking) Done in conjunction with other tests Referral centers usually do the electrodiagnostic testing You need to be able to interpret the test results and incorporate it in the diagnosis and management of your pts

Adaptational Range of Vision

The visual system can function over a light level range of about 10 log units Under twilight (mesopic) conditions, both rods and cones contribute to vision This capability is facilitated by the existence of two different classes of photoreceptors that are operational under different light levels (this is known as the duplex retina): Rods (scotopic vision) Cones (photopic vision)

The visual system as a Fourier analyzer

The visual system is thought to deconstruct the retinal image into its spatial frequency components This view assumes the existence of independent spatial frequency channels w/ in the visual system; rather than consisting of a single channel that is maximally sensitive to 4 cycles/degree, we can think of the CSF as forming an envelope that encompasses several narrower channels-- each of these SF channels is presumably independent Although there is experimental evidence (prior adaptation experiments) for the visual system acting as a Fourier analyzer, it does not prove that it is correct. Nonetheless, the CSF is a useful and important tool for understanding how the visual system processes spatial info

Global motion perception

The visual system must integrate info from many dots in random dot kinetograms over a broad expanse of the retina This involves higher level cortical motion centers

RGB system of color specification

The wavelength to be matched is placed on one side of a bipartite field On the other side are the three primaries red: 645 green: 526 blue: 444 The amounts are adjusted until a perfect match between the two sides are obtained When this procedure is repeated for each wavelength across the spectrum, we obtain COLOR MATCHING FUNCTIONS as seen in the graph (shows the amount of each primary required to match a given wavelength) -- the quantity of each primary required for a match is referred to as the tristimulus value For the wavelengths that require negative amounts of red, you need to add the red to the wavelength on the left hand side of the bipartite (the wavelength we're trying to match) The CIE system overcomes this problem

Strengthening of synapses

There is a strengthening of synapses for the non deprived eye relative to the deprived eye (from geniculate cells to cortical cells) The synaptic arrangements become permanent as the animal matures, terminating the critical period LGN neurons are geniculate cells

Glaucoma and the magno pathway

There is evidence that the magno pathway is damaged in gluacoma Tests of motion perception have been evaluated to determine if they are useful for the early diagnosis of glaucoma Minimum displacement thresholds are elevated in pts with suspected glaucoma and motion coherence thresholds are elevated in regions of the visual field showing glaucotamous damage A visual field test based on detection of motion -- the Moorsfield Motion Displacement Test-- shows promise as a diagnostic tool Motion deficits may occur early in the glaucomatous disease process and measures of motion perception might prove to be valuable for early diagnosis

Hyperacuity

There is exquisite sensitivity manifested in this form of acuity Bc of humans' excellent abilities at detecting tilt, a type of hyperacuity, it can be very difficult to align a pic on the wall so that it appears perfectly level Depends on the visual system's ability to sense direction A. is vernier acuity; when the two bars represent sufficiently different directions, a threshold for this yperacuity task is reached Thresholds for hyperacuity tasks are quite low on the order of 3 seconds of arc! This is related to the visual system's ability to avg luminance info across space to arrive at a sense of direction; these processes occur at post receptor levels, including the retina and, perhaps, the visual cortex Hyperacuity is rather resilient to optical defocus/blur. Defocus leads to blur circles, thereby reducing resolution acuity. Luminance profiles of a vernier target, however, provide sufficient info to distinguish their positions

Scotopic and photopic spectral sensitivity can be compared by plotting both on the same graph

These functions are determined after the subject was in the dark for about 45 mins and all rhodopsin and cone photopigment has regenerated There are two thresholds: one for the scotopic system, and one for the photopic system Diff between the thresholds is known as the photochromatic interval (difference between the scotopic and photopic sensitivities for a given wavelength); Photochromatic interval is smallest around 650 (red) Scotopic system is most sensitive around 507 nm (think rods absorb best at 507) (blue) Photopic system is most sensitive around 555 nm (green)

First order stimuli for motion

These stimuli consist of linear exchange of light for dark (or vice versa) Includes: -Stroboscopic stimuli -Sinewave gratings Neurons capable of encoding certain aspects of motion (like speed and direction) have been found in the retina

Two patches of light one is 507 nm and one is 620 nm. Each bleach 30 rhodopsin molecules. Which is brighter?

They are equal in brightness They bleach the same amount of rods. The 620, however emits more light because its less efficient at bleaching rhodopsin

Absorption Spectra for Deuteranopia and Protanopia

They manifest monochromatic color matching beyond about 545 nm -- so if the stimuli differ in luminance they'd be able to tell the difference in the stimuli

Temporal Sensitivity (Scotopic)

Time between flashes (impulse interval) Under scotopic conditions the temporal summation period is 100 ms No flash is seen by itself because they are sub threshold, and the impulse interval is greater than the temporal summation period Scotopic system manifests excellent temporal summation, poor temporal resolution

Temporal Vision Quiz 5 starts here!!!

Time related vision Concerned with changes in luminance over time Closely related to motion perception The MAGNO retinogeniculate visual pathway, which has substantial input to the dorsal (parietal) cortical processing stream and its motion analysis areas, plays a key role in encoding temporal and motion information

Relationship of high frequency CSF cutoff to snellen acuity

To legs of the optotype E can be thought of as bars of a spatial grating. To read the E, the pt must resove its detail (bars and gaps) The angle that just resolvable bars or gaps make with the eye is called the minimum angle of resolution (MAR) A typical optotype is designed such that its overall size is five times that of its detail (aka 1 bar is 1/5 of the letter size) Denominator of the snellen fraction is the foot size (distance at which the letter subtends 5 mins of arc) Each line on the snellen chart is designated by the foot size of its optotypes

Alternatives to snellen acuity

Tumbling E: this is just resolution acuity not recognition, bc you don't need to name the letter, just which way is it pointing, etc; same thing would apply to identifying which optotype in a group of optotypes is different than the others Landolt C Cardiff Broken Wheel Lea: the one they use in peds/ w the apple/ matching; thats just resolution (no naming so its not recognition) Patti Pics HOTV tests

Anisometropic Amblyopia

Two eyes have unequal refractive errors Amblyopia can develop when, during the critical period, one eye has a blurred retinal image at all distances, and the fellow eye has a clear retinal image at some distances This most commonly occurs when one eye manifests a hyperopic refraction, and the fellow eye is less hyperopic, plano, or myopic (compound hyperopic anisometropia, simple hyperopic anisometropia or antimetropia) Can be prevented if an appropriate prescription for spectacles is worn during the critical period Just because there are different RE errors though doesn't mean that the pt will be amblyopic, possible that each eye is focused at certain distance (i.e OD -1, OS -4), but because the distances are different, a normal complement of binocular cortical neurons may not have developed-- can result in reduced stereopsis

Metamers

Two stimuli that appear identical but are physically different Like two patches of light that appear the same color -One contains one wavelength -One contains two wavelengths

Routine measurement of VA

Typical VA charts measure only the high spatial frequency cutoff but, there are some clinical advantages to this. High spatial frequency cutoff is reduced by refractive errors, but ALSO, VA is very very sensitive to disruption of the photoreceptor matrix that may occur in foveal disease

Time course of devleopment

Typical healthy humans are born with or develop a slight amount of hyperopia (less than 2.5D) during the first year of life This mild hyperopia tends to decrease throughout childhood

Forced Choice Preferential Looking

Used effectively to determine the VA and other visual capabilities of infants Experimenter/examiner is forced to choose the location of the stimulus based on observation of the infant's eyes. Experimenter observes the infants gaze as the infant views a 2AFC display. By employing a forced choice, the criterion (strict or lax) used by the experimenter is minimized

Imaginary primaries

Using matrix algebra we can transform one set of color matching functions into another For the CIE system, RGB matching functions are transformed into imaginary primaries called XYZ All wavelengths can be matched w/ positive quantities of these primaries

Spatial Resolution

VA Highly sensitive measure of visual function Our visual system's ability to detect and resolve luminance defined stimuli

Visually Evoked Potentials (VEP)

Visual evoked electrophysiological signals extracted from the activity in the visual cortex recorded from the overlying scalp; can be used to estimate VA and contrast sensitivity Minimum of 3 electrodes; recorded from the overlying scalp (inion) Not sure if you need to know these but they were highlighted blue: N1 (N70) reflects the activity of the striate cortex P1 (P100) likely reflects the activity of the extra striate areas; the 100 means 100 milliseconds N2 (N140) likely reflects activity in the parietal lobe

Multifocal ERG

Visual field mapped with mini ERGs Records ERGs from multiple retinal locations simultaneously You'll see a honeycomb pattern

Visual fields and increment sensitivity changes with age

Visual fields are generally unaffected by age related reductions in retinal illumination Senile miosis and nuclear sclerosis cause the retinal illumination produced by both the increment and background to be reduced by the same amount, so the rental image contrast remains unchanged A stimulus that was at threshold before this age related reduction in illumination will remain at threshold afterwards as long as the test is performed in the weber's region

Photopic Vision and Contrast

Weber's fraction for photopic vision is K= 0.015 (weber's is smaller) The photopic system is 10 times more sensitive to contrast than the scotopic system (this is important)

Visual Field

When increment sensitivity is determined for many different points, we obtain a visual field The visual field machine is PHOTOPIC even if its done in a dark room Determines increment threshold along visual field The threshold is lowest CENTRALLY and increases peripherally

Complementary colors

When mixed tg, they produce white To use the CIE diagram to determine the complement for a given wavelength, simply connect the wavelength through white to the opposite side

Apparent or illusory motion

When spatially separated lights are sequentially flashed with an appropriate interval between the flashes, a sense of motion is elicited Referred to as STROBOSCOPIC MOTION or the PHI PHENOMENON: this effect is common in lighted signs that create the illusion of motion (think of a bowling alley sign with a lighted bowling ball moving down an alley-- the sequential flashing, at the proper rate, of the various lights in the sign elicits the illusion of movement) Movies, tvs, and computers use stroboscopic motion

Light Constancy

When the illumination increases, the contrast remains constant because the amount of light reflected from both the optotype and background increases at the same rate, resulting in a constant ratio of optotype to background luminance In the figure, the ratio of optotype to background is the same in both cases, so the optotype is not any more visible

Cerebral cortex

Where visual info is organized and integrated with memory and other senses to produce the rich visual world we experience First stage of cortical processing occurs within the STRIATE CORTEX (primary target for projections from the LGN) Visual info is then disseminated widely throughout the cortex along two major processing streams (the ventral and dorsal streams)

Laterality

Whereas inherited anomalies are bilateral and symmetric, acquired anomalies may be unilateral or asymmetric Any difference in the color vision of the two eyes, as demonstrated on a color vision test, is due to an acquired anomaly This is why it is important to perform color vision tests monocularly when screening for these conditions (first to the eye that is likely to harbor the disease -- typically the eye with the worst corrected VA); if you do it binocularly, the results may be normal, reflecting the performance of the unaffected eye

New HRR tritan test plates

Wouldn't be able to see the X (check color confusion lines)

Inner plexiform layer

You can see the outer and inner sublayers and where the off and on center bipolar cells synapse

Why do you use a bright background for the photopic ERG

You use a BRIGHT background for the photopic ERG so that the rods are shut off

Ganglion cells and amacrine cells

generate APs APs do not decay over distance (graded potentials do) Ganglion cell axons must traverse a substantial distance before they reach the LGN, so it makes sense that they need to produce APs that can last the trek to the LGN. Conductance speed is increased by the myelin sheath along the ganglion cell axons, which give rise to saltatory transmission Ganglion cell axons become myelinated as they leave the eye at the optic disc to form the optic nerve Parasol ganglion cells respond transiently to a flash of light, while midget ganglion cells manifest a sustained response After leaving the eye, midget and parasol ganglion cell axons synapse in the LGN forming the parvo and magno retinogeniculate pathways The kongo pathway is formed by the axons of the bistratified ganglion cells, which also synapse in the LGN After the LGN, the parvo and magno pathways maintain various degrees of independence through the striate cortex, visual area 2, and higher cortical centers Most but nota ll ganglion cells project to the LGN. Certain ganglion cells project to the superior colliculus (eye movements), while others synapse in the pulvinar, a thalamic nucleus that may play a role in visual attention, motion processing, and visually guided movement

Cerebral achromatopsia

secondary to a lesion in the extra striate cortex prior to developing the lesion, the pt has normal color perception afterwards, they perceive a black and white (achromatic) world