3.3 Vison
Object Recognition -Humans Recognizing Faces
Activation of fusiform face area (IT) in humans viewing faces.
Color-sensitive cells are spatially segregated in V1
"Blobs" containing color-sensitive cells receive inputs from the parvocellular (P) layers of the LGN (3-6). The blobs are identified with a stain for cytochrome oxidase. These cortical cells are part of a color- analysis subsystem. Columnar Organization of V1: Blob Columns- "Blobs" extend vertically throughout the cortex except for layer IV. The cortical cells in the blobs are part of a color- analysis subsystem.
Tritanopia
"blue" pigment (430 nm) is missing. Male - 0.0001%; Female - 0.001% Chromosome 7 The most common color confusions for tritanopes are light blues with grays, dark purples with black, mid- greens with blues and oranges with reds.
Herring Grid Illusion
*The brain only knows about intensity of light at a point in space from the frequency of action potentials in the ganglion cells whose receptive fields coincide with that point. *Receptive field sizes are smallest near the fovea. When you 'look' at something, you direct your fovea to that spot. Because receptive field size is small, the entire receptive field is in the light at the center, so the retinal ganglion cell doesn't respond. *Because Retinal Ganglion Cells are Only Interested in Differences in Light Intensity, Our Perception of Brightness is Distorted eg. Receptive field B has its ON-center completely activated by light, but only a little bit of its antagonistic OFF surround activated. Receptive field A has its ON-center completely activated by light, but lots of its antagonistic OFF surround illuminated. Thus, retinal ganglion cell A is less active than nearby receptive field B. The brain interprets this difference as less light falling on receptive field A than on receptive field B. In other words, there must be a dark spot at the intersection of the white lines.
Rod Photoreceptors - Phototransduction (2)
-A single activated rhodopsin (Rh*) activates approximately 800 Transducins (Tαβγ ● GDP). Each GTP●Tα activates only one phosphodiesterase enzyme, but each activated phosphodiesterase enzyme converts cGMP to GMP at the rate of 2000 molecules/second. Shining a light on a photoreceptor hyperpolarizes the receptor because the decrease in [cGMP] causes cGMP gated channels to close -In the dark, photoreceptors have a depolarized membrane potential of ~ - 40 mV because Na+ flows into the neuron through the open cGMP gated channels, the dark current -Photoreceptors do not generate action potentials. the strength of light determines strength of hyper-polarization
Turning Off Phototransduction
-The all-trans-retinal cytoplasmic tail of activated rhodopsin (Rh*) has numerous phosphorylation sites. -Rhodopsin kinase phosphorylates these sites. -The more phosphorylated the tail, the more likely arrestin will bind and inactivate the tail preventing Rh* binding to inactive transducin (Tαβγ ● GDP). -The all-trans-retinal dissociates from the opsin allowing the opsin to bind an 11-cis-retinal.
Vision: Cones and Color Vision
-We Only See A Small Portion of the Electromagnetic Spectrum Each color we see has a specific wavelength. We perceive longer wavelengths (wavelengths that cover larger distances in space) as red and shorter wavelengths (wavelengths that cover smaller distances in space) as violet. Remember ROY G BIV - red, orange, yellow, green, blue, indigo, & violet. -How Many Types Of Photoreceptors Do You Need To See All Of The Colors that We Perceive? Each photoreceptor type is characterized by its spectral sensitivity, the relative efficiency of detection of light as a function of wavelength. Rod photoreceptors respond best to a greenish-yellow light, but also respond weakly to lights that are red and violet.
Photoreceptor Adaptation In Darkness
1. In darkness, Ca2+ enters through open cGMP-gated channels and binds to two target proteins: recoverin and GCAP (guanylate cyclase activating protein). 2. With Ca2+ bound, recoverin inhibits rhodopsin kinase prolonging and strengthening the light response. Activated rhodopsin Rh* can continue to activate transducin (Tαβγ ● GDP). Ca++ binding reduces GCAP binding to guanylate cyclase thereby decreasing the production of cGMP. This change increases the sensitivity of the photoreceptor to light in darkness.
Ventral Pathway
1. Loss of Color Vision From V4 Damage- Area V4 receives inputs from primary visual cortex via V2. The receptive fields are large and sensitive to color and shape. Loss of V4 produces achromatopsia - a loss of color vision although cone function is normal. 2. Monkeys Recognizing Faces- Area IT is the inferotemporal cortex in the temporal lobe. It has receptive fields that require complicated spatial stimuli for activation. Some receptive fields respond best to faces. The same area where you find face cells in monkeys is activated in humans when they see faces. Damage to this region can cause prosopagnosia - an inability to recognize faces even though vision is normal.
Primary Visual Cortex - Orientation Columns
1. Neurons in a vertical column of visual cortex (pia to white matter) respond best to the same line orientation at the same point in visual space. 2. When moving tangentially (horizontally) through visual cortex, the preferred orientation and receptive field location change. An entire set of orientations (organized like the arms of a clock) repeat approximately every 1 mm. Regions of primary visual cortex are activated by specific orientations of visual stimuli. These data indicate that V1 has columns of neurons going from the pial surface to the white matter that are responsive to a specific visual stimulus orientation.
Primary Visual Cortex - A New Type of Receptive Field
1. Simple cells respond best to bars and edges with a specific or preferred orientation. The receptive fields are made by combining multiple center-surround LGN receptive fields with the same polarity, e.g., all ON-center OFF-surround. This pattern creates a cortical receptive field with inhibitory and excitatory regions. Simple cells are sensitive to bars or lines of a particular orientation. (Shining a bar of light of the proper orientation on the on-center increases neural activity. Turning off a bar of light of the proper orientation in the inhibitory off region also increases activity.) 2. Other cells (called complex cells) are sensitive to stimulus orientation independent of stimulus location, within broad limits. 3. Still other cells (called hypercomplex cells) are stimulated best by lines of a particular orientation and length. These cells are part of cortical systems that analyze the form or shape of an object, without strong constraint on its location.
Looking in the Eye is Looking at the Brain
1. The retina develops as an out pouching from the diencephalon during embryonic development. 2. The outer covering of the eye (white of the eye - sclera) is the dura mater that protects the brain. 3.The retina inside the eyeball is brain tissue.
Disorders of the Eye - Age-Related Macular Degeneration (AMD)
AMD develops slowly and does not produce complete blindness. The loss of central vision where we have our highest acuity prevents much of day- to-day activities such as reading and driving. develop yellow deposits behind the retina between the back of the eye ball and the retina called drewzin. macula fovea damage.
Higher Order Visual Cortical Regions
Although it is tempting to think that the visual cortical areas form a linear hierarchy, their interconnections are more complex. Parallel Processing-For example, blob neurons in V1 project to neurons in the thin stripes of V2. Those neurons project to area V4 to create a visual stream of color and form information. Motion & orientation sensitive neurons in V1 project to thick stripes in V2. Those thick stripe neurons in V2 project to V5 to create a visual stream of motion
OFF Center ON Surround Ganglion Cell Receptive Fields
An OFF center ON surround retinal ganglion cell receptive field responds like an OFF center ON surround bipolar cell receptive field to light shown in the center and surround except that retinal ganglion cells generate action potentials. When the light goes off ( ) in the center, the ganglion cell discharges because the center darkened. Remember it's an OFF center.
ON Center OFF Surround Ganglion Cell Receptive Fields
An ON center OFF surround retinal ganglion cell receptive field responds like an ON center OFF surround bipolar cell receptive field to light shown in the center and surround except that retinal ganglion cells generate action potentials. When the light goes off ( ) in the surround, the ganglion cell discharges because the surround darkened. Remember it's an OFF surround
The Lateral Geniculate Nucleus is NOT just a Relay Nucleus
Approximately 75% of LGN neurons are projection or relay neurons that terminate on layer IV primary visual cortex (V1) stellate neurons. Remaining 25% are local GABAergic interneurons in the LGN, which inhibit LGN projection neurons. Projection neurons also innervate neurons in the reticular nucleus of the thalamus. These neurons inhibit LGN projection neurons. ----- Neurons in layer VI of primary visual cortex excite projection neurons and local inhibitory interneurons in the LGN. The LGN receives ~80% of its inputs from primary visual cortex. Layer VI neurons also excite neurons in the reticular nucleus of the thalamus. Primary visual cortex controls the information it receives from LGN by how strongly cortex activates projection neurons and inhibitory reticular nucleus of the thalamus neurons.
Bipolar Cell Receptive Field- OFF
Because of the interconnections between photoreceptors, horizontal cells, and a bipolar cell, bipolar cells have a 'center- surround' organization. •Horizontal cells produce negative feedback at the synapse between photoreceptors and bipolar cells (i.e., the action of the horizontal cell opposes the direct action of the photoreceptor on the bipolar cell). •In the case of OFF-type bipolar cell in this illustration, horizontal cells cause the bipolar cell to depolarize. •Horizontal cells receive inputs from many more photoreceptors than the bipolar cell, so the spatial extent of the response mediated by horizontal cell feedback is larger than the extent of the photoreceptors connected directly to the bipolar cell.
Vision: Bipolar, Horizontal, and Retinal Ganglion Cells
Bipolar cells and horizontal cells receive synaptic inputs from photoreceptors. Retinal ganglion cells receive inputs from bipolar cells. Retinal visual receptive fields
Retinal Ganglion Cells - Brightness and Contrast
Brightness is our perception of light intensity. Contrast is our perception of the difference in light intensity between two areas. Because our visual system responds to differences in light intensity, our perception of brightness is easily distorted by retinal ganglion cells. -------------------- Retinal Ganglion Cells Respond To The Difference In Light Falling Across The Receptive Field Not The Absolute Amount Of Light This on-center off-surround neuron produces the same number of action potentials regardless of whether its receptive field is completely in the dark (1), completely in the light (3), or half in the light and half in the dark (2).
On and Off Bipolar Cells
Each bipolar neuron is connected to many photoreceptors. There are two types of bipolar cells. OFF bipolar cells that hyperpolarize when light is shown on the photoreceptors to which they are synaptically connected. ON bipolar cells depolarize when light is shown on the photoreceptors to which they are synaptically connected. *Bipolar cells do not generate action potentials.*
Layers Of The Cerebral Cortex
Cortex has 6 layers, I (pial surface) to 6 (white matter). Pyramidal cells are the primary excitatory neuron in all layers but layer 4. Layer 4 contains stellate cells that receive synaptic input from thalamus.
Sensing Color Using Our Three Types of Cone Photoreceptors
Each of the three cone types responds to a different range of light wavelengths. People with normal color vision are Trichromats - they have three cone types ------- The Organization of Color Opponent Retinal Ganglion Cells Explains Much of Our Color Perception -A red/green opponent ganglion cell can have a red excitatory center and green inhibitory surround (arising from L cones and M cones, respectively). Other red/green ganglion cells have green excitatory centers and red inhibitory surround. There are also blue/yellow opponent ganglion cells. The yellow results from combining inputs from L and M cones. The blue comes from S cones. If you adapt all of one cone type in a color- opponent system, then you should see the opposing color when you look at a white screen. This result occurs because continuous stimulation by one color causes the photoreceptors to lose responsiveness. Because the unstimulated opponent color now acts without opposition, you see the colors reverse.
`Ocular Dominance Columns of Primary Visual Cortex (V1)
Inputs from the two eyes are separate in the LGN and remain segregated in layer IV of primary visual cortex. The picture shows the results of injecting a tracer into the right eye that moves trans-synaptically to layer IV of V1. White regions are labelled terminals
Binding Problem.
If different components of the visual world are analyzed in different cortical regions, then how do we perceive a unified visual image? This question is the Binding Problem.
Anomalous Trichromacy
In anomalous trichromacy, the retina has three cone types but the spectral sensitivity of one pigment is shifted. Protanomalous- Sensitivity of long (red) cone shifted toward medium (green) cone Deutranomalous- Sensitivity of medium (green) cone shifted toward long (red) cone Tritanomalous- Sensitivity of short (blue) cone shifted toward medium (green) and long (red) cone. This is an extremely rare condition.
Rods and Cones Work at Different Light Levels
In dim light, scotopic, we only use rods. No color vision at this light level. In bright light, photopic, we only use cones. Rods don't work at this light level. In low light, mesopic, we use both cones & rods. Rods are more sensitive to light than cones, but do not allow detailed vision.
Dorsal Pathway - Motion Detection MT (V5)
It is possible to quantify the ability to detect motion using moving dot patterns for which a certain percentage of dots move in the same direction while the others move randomly The subject's task is to determine whether the dot pattern is moving leftward or rightward. The experimenter alters the correlation of the dots. At 0% correlation, the dot pattern is random and the subject cannot choose the correct direction because there is no direction. The 65% rightward choice by the subject shows that this subject has a bias for choosing rightward directions. Cells in V5 (also called MT, middle temporal area) detect stimulus movement, independent of stimulus shape. The receptive fields of MT neurons exhibit direction tuning, they discharge most strongly (number of action potentials) for one direction. All of the neurons in a vertical column in MT exhibit the same direction preference for movement, similar to the orientation columns in V1. ----------------- Put a microelectrode in a MT column with receptive fields that respond most strongly to a leftward motion. Ask the monkey to determine the direction of dot motion at different correlations between dots. On some trials (●), the experimenter microstimulates neurons with leftward preferred directions. On the other trials (○), no stimulation occurs. The experiment shows that it is possible to bias a monkey's decision as to movement direction by stimulating neurons in MT.
Horizontal Cells
Like bipolar cells, horizontal cells receive glutamatergic inputs from photoreceptors. Like OFF bipolar cells, horizontal cells have AMPA receptors and hyperpolarize in response to light shown on photoreceptors connected to the horizontal cell. Horizontal cells release the neurotransmitter GABA. Horizontal cells are electrically coupled through gap junctions.horizontal cells spread over wide areas of the retina, hitting many photoreceptors, and also contact bipolar cells. •Horizontal cells provide lateral inhibitory feedback at the synapse between photoreceptors and bipolar cells. •Neurotransmitter release from horizontal cells is highest in darkness, when they are depolarized, and decreases during illumination. •Horizontal cells release the neurotransmitter GABA. •Horizontal cells produce a synaptic effect in bipolar cells that opposes the effect of illuminating the photoreceptors that are directly connected to the bipolar cell. •So, an Off-type bipolar cell depolarizes when the horizontal cells hyperpolarize, and an On-type bipolar cell hyperpolarizes when the horizontal cells hyperpolarize.
Horizontal Cells impact on bipolar cells
Mammals express different chloride transporters in dendrites of On and Off bipolar cells. GABA opens ionotropic GABAA Cl- channels OFF bipolar cells express KCC2, which generates chloride efflux in the dendrites that results in an Ecl- = -90 mV. In the dark, GABA opens Cl- channels and Cl- influx hyperpolarizes the OFF bipolar cell. When there is a light, GABA release stops and the OFF bipolar depolarizes. ------- Mammals express different chloride transporters in dendrites of On and Off bipolar cells. GABA opens ionotropic GABAA Cl- channels ON bipolar cells express NKCC, which generates chloride influx in the dendrites that results in an Ecl- = -33 mV in that compartment. In the dark, GABA opens Cl- channels and Cl- efflux depolarizes the ON bipolar cell. When there is a light, GABA release stops and the ON bipolar hyperpolarizes. ------ Bipolar cells make glutamate synapses with retinal ganglion cells. Retinal ganglion cells possess ionotropic glutamate receptors. Thus, a retinal ganglion cell with its input from an ON bipolar cell will exhibit an ON center OFF surround receptive field. A retinal ganglion cell with its input from an OFF bipolar cell will exhibit an OFF center ON surround receptive field. Unlike photoreceptors and bipolar cells, retinal ganglion cells generate action potentials.
problem with The Organization of Color Opponent Retinal Ganglion Cells
Note that a color opponent retinal ganglion cell is also strongly excited by a white light that covers only the center of the receptive field. So, a small white light stimulated this cell the same way as a diffuse red light. This potential confusion is sorted out in the synaptic circuitry of color-sensitive cells in the visual cortex
V1 Neurons are Frequently Binocular
Objects at the same depth from the eye activate corresponding points on the retina. α1 & α2 are equal for the two objects. Because the red dot is at a different distance from the eyes than the gray dots, the red dot activates non-corresponding points on the two retinas, retinal disparity. The brain uses this information to perceive the red dot as at a different distance from the eye than the gray dots. Binocular V1 Neuron Tuned for Depth-Binocular neurons can encode depth information by being tuned to specific retinal disparities between the two eyes.
OCT technique
Optical coherence tomography reveals the layers of the retina in the living human eye. really works, and is very affective.
Organization Of The Lateral Geniculate Nucleus
Parvocellular - small (p) cells Layer 3-6 neurons are center- surround receptive fields are color opponent. They receive inputs from p type retinal ganglion cells. Magnocellular - large (m) cells Layer 1-2 neurons are center- surround receptive fields that are NOT color opponent. They receive inputs from m type retinal ganglion cells. Each neuron in the LGN responds only to visual stimuli from one eye. Each layer of the LGN has a map of the retina (visual world) from only one eye. Retinotopic Map
Off Bipolar Cell
Photoreceptors release glutamate. Off bipolar cells have ionotropic AMPA & Kainate receptors, these receptors are opened by glutamate. In response to light, photoreceptor hyperpolarization reduces glutamate release by the photoreceptor, which causes OFF bipolar cells to hyperpolarize.
On bipolar cell (mGluR6)
Photoreceptors release glutamate. On bipolar cells have a metabotropic glutamate receptor, mGluR6 ( glutamate closes channels). In response to light, photoreceptor hyperpolarization reduces glutamate release by photoreceptors, which causes ON bipolar cells to depolarize. This is because the lack glutamate, allows the mGluR6 receptors to stay open. when you look at the retina with the right tools, you can see the different bipolar cells. Gαo labelling in primate rod and cone On bipolar cells, mGluR6 is a metabotropic glutamate receptor that activates the second messenger Gαo.
Recognizing Color With The Retina
Photoreceptors respond to many different colors (wavelengths of light), but are most sensitive to a specific color, e.g., yellow green. An equally bright orange or purple light will produce a weaker response because the photoreceptor is less sensitive to that A single type of photoreceptor will see different colors of light as different intensities of light.
photoreceptors (cones/rods)
Phototransduction occurs in the rods and cones of the retina. The absorption of a photon of light takes place in the discs in the outer segment of rods and cones. these cones and rods hows outer segments where phototransduction(detection of light) takes place (disk filled area.) each one of these disks have a structure in it called rhodopsin. think of it like pancakes stacked on each other. giving light a continuous opportunity to make contact with rhodopsin molecule.. Visual pigment molecules in the photoreceptor outer segment absorb light. Pigment molecules are in multiple layers stacked perpendicular to the long axis of the outer segment. In rods, the pigment molecules are in intracellular disks whose membranes are NOT continuous with the plasma membrane. In cones, the pigment molecules are in membranes that are infoldings of the plasma membrane.
Hypercolumns
Primary visual cortex is organized in hypercolumns that contain neurons providing information on orientation, color, and ocular dominance for one point in the visual world.
Protanopes
Protanopia When the long, " red", pigment is missing. Male - 1.0%; Female - 0.02% X chromosome, are more likely to confuse; 1. Black with many shades of red 2. Dark brown with dark green, dark orange and dark red 2. Some blues with some reds, purples and dark pinks 3. Mid-greens with some oranges
Problems With Light Focusing
Refractive Power (diopters) = 1 / focal distance (m) In emmetropia, the cornea's refractive power is approximately 42 diopters (.024 m), bringing the object into focus on the retina. i.e., the diameter of the eye matches the refraction produced by the cornea. With myopia, eye diameter is too large for corneal refraction, i.e. cornea bends light rays too much.(very popular) developed world problem, from doing close work and so on. With hyperopia, eye diameter is too small for the corneal refraction, i.e. cornea bends light rays too little.
Looking Into The Eye
Retina divided into 2 sections, front being the temporal retina, and the back being the nasal retina. The point of highest acuity (our best vision) in the retina is the macula, whose center is the fovea. In-between the two retina sections. Blood vessels avoid the macula to allow light a clearer path to the retina. The optic disc, where the optic nerve leaves the eye and blood vessels enter and leave, is the 'blind spot' of the retina. No vision occurs at this point on the retina. The optic disc is in the nasal retina, the region of the retina closest to the nose.
Rod Photoreceptors - Phototransduction
Rhodopsin activates the G protein transducin. Transducin is a heterotrimeric G protein, like those involved in many neurotransmitter effects that act via G-protein-coupled receptors. Activated transducin reduces cGMP concentration causing cGMP gated channels close. Hyperpolarization of the photoreceptor occurs. More detail: Transducin (Tαβγ ● GDP) binds to activated rhodopsin (Rh*). GTP binds to the α subunit to activate the G protein causing GDP release and dissociation of the Tβγ. Rh* releases the GTP ● Tα and then Rh* can reenter the cycle to bind to another Tαβγ ● GDP. The GTP●Tα activates phosphodiesterase (PDE*Tα●GTP), which hydrolyzes cGMP into GMP. As [cGMP] decreases, cGMP gated cation channels close. w\Because transducin is a GTPase, Tα●GDP rejoins Tβγ and can be reactivated by Rh*
The Photosensor Molecule of Rods: Rhodopsin
Rhodopsin is the combination of a protein molecule, called opsin, and a light-absorbing chromophore molecule, retinal (short for retinaldehyde). Retinal is covalently attached to opsin at a particular lysine residue (residue 296). In the dark, retinal is in the "bent" form (11-cis retinal). When retinal absorbs a photon, it photoisomerizes to the "straight" form (all-trans retinal) *This simple change is the only light-dependent event in all of vision.(11 cis to all trans*
Types Of Retinal Ganglion Cells and Where They Project
The P (X) and M (Y) retinal ganglion cells project to the lateral geniculate nucleus, the primary visual system for form vision.(thalamocortical pathway (also called geniculostriate pathway)) The W retinal ganglion cells project to the suprachiasmatic nucleus for circadian rhythms and the accessory optic system for pupillary light reflexes. (phylogenetically older targets (non cortical) The W and M (Y) retinal ganglion cells innervate the superior colliculus, which plays a role in orienting and eye movements.
Optic Tract & Optic Chiasm - A Bit of Anatomy
The Temporal retina of one eye and the Nasal retina of the other eye both look at the same half of the visual field. Retinal ganglion cells in the temporal retina send their axons to the Ipsilateral lateral geniculate nucleus (LGN). The axons of retinal ganglion cells in the nasal retina cross in the optic chiasm and go to the Contralateral lateral geniculate nucleus (LGN). Thus, the left side of the brain looks at the right side of the visual world and the right side of the brain looks at the left side of the visual world.
Lens Accommodation for Near Vision
The ciliary muscle is a sphincter muscle, like the iris that changes pupil diameter. When the ciliary muscle is relaxed, the zonule fibers pull on the lens and flatten it out. When the ciliary muscle contracts, the zonule fibers become slack and the lens bulges out. 1. Ciliary muscle RELAXED for distance viewing 2. Ciliary muscle CONTRACTED for near viewing --------------------- Presbyopia - a loss of near vision with aging caused by a loss of lens elasticity. the lens does no bulge out. correct hyperopia with convex lens. Before presbyopia, a person can correct hyperopia with accommodation.
The Eye Is A Light Focusing Device
The cornea and lens bend light rays (refraction) so that they become focused on the retina. **The cornea and (to a lesser extent) the lens refract light rays coming into the eye and bring the light rays into focus at the fovea of the retina.**
Mach Bands
The dark stripe at the dim side of the transition and the bright stripe on the bright side are called Mach bands. These dark and light stripes are not really there in the stimulus! Mach Bands and Lateral Inhibition -Lateral inhibition enhances the perceived contrast in brightness at the border between light and dark regions of the visual field
Vision: The Primate Retina
The foveal pit only has photoreceptor cells. Other cell layers are displaced laterally to allow light to strike the photoreceptors directly, bypasses all the other layers at the fovea. -foveal slope. photoreceptors in primates, only cones.
Primary Visual Cortex - Distorted Retinotopic Map
The map of the visual world is distorted. More of primary visual cortex is devoted to the fovea and macula than to any other region of the retina. Primary visual cortex over represents the fovea because the fovea has more photoreceptors than any other part of the retina.
V1 - Double Opponent Color Receptive Fields
Unlike single- opponent color cells in the retina, however, double-opponent cells are not sensitive to white light or to full- field, uniform color. Double opponent neurons respond best to specific color pairings.
Leaving Primary Visual Cortex
The primary generalization about higher order visual cortical regions is that the dorsal pathway plays a role in spatial location and movement and the ventral pathway plays a role in object characterization. V1 - primary visual cortex V2 - secondary visual cortex MT - medial temporal cortex (motion) V4 - color vision Dorsal and Ventral Pathways dorsal pathway- V1-V2-MT ((parietal lobe) spatial location and action) -Lesioning the dorsal pathway prevents monkeys from making a two choice discrimination based on spatial location, but not object characteristics ventral pathway-V1-V2-V4 (temporal lobe) characteristics of object. -Lesioning the ventral pathway prevents monkeys from making a two choice discrimination based on object characteristics, but not spatial location.
Color Blindness
There are three types of color blindness in which you are missing one cone type. These individuals are. protanopia deuteranopia Tritanopia
Bipolar Cell Receptive Field- ON
There are two types of center-surround receptive fields, ON center off surround and OFF center on surround. The receptive fields are best activated by differences in spatial contrast across the receptive field.
Organization Of Primary Visual Cortex, V1
V1=Striate Cortex LGN inputs to cortical layer IV have a dorso- ventral organization based on whether they are from P (X) or M (Y) LGN neurons. 1. P cells project to layers IVa, IVcβ. The P cells also excite layer VI neurons that project back to the LGN. 2. M neurons project to layer IVcα. Within layer IV, LGN inputs from the two eyes are in different columns, ocular dominance columns.
W cells
W cells have small cell bodies with widely spreading dendritic fields. They are functionally diverse with many different kinds of light responses, and do not participate in the thalamocortical system. Ganglion cells that contain melanopsin respond directly to light with a sustained response proportional to the level of illumination. These W ganglion cells are <2% of the total retinal ganglion cells. They participate in the pupillary light reflex and in circadian rhythms.
Deutranopia
When the medium, "green" pigment is missing. Male - 1.1%l; Female - 0.01% X chromosome Deuteranopes are more likely to confuse:- 1. Mid-reds with mid-greens 2. Blue-greens with grey and mid- pinks 3. Bright greens with yellows 4. Pale pinks with light grey 5. Mid-reds with mid-brown 6. Light blues with lilac
Disorders of the Eye - Diabetic Retinopathy
With diabetic retinopathy, retinal blood vessels leak and/or proliferate, angiogenesis. Effective blood perfusion is lost causing regions of the retina to die or to be covered by blood. Retinopathy is the leading cause of blindness in diabetics. treated by lasering off blood vessels, not good but only treatment.
Photoreceptor adaption in light
Without Ca2+ bound, GCAP binds to and activates guanylate cyclase, which then synthesizes cGMP at a higher rate. The increased [cGMP] promotes fast return to depolarized state and makes hyperpolarizatiolns to light smaller. Less active recoverin leads to more rapid rhodopsin inactivatio. These changes support light responses in the presence of background illumination.
Disorders of the Eye - Glaucoma
aqueous humor is behind corne, clear liquid vitreous humor is behind the retina, jelly like. The ciliary body produces approximately 2.5 μl / min of aqueous humor. The anterior chamber holds approximately 250 μl of fluid. Aqueous humor must constantly flow out of the anterior chamber or pressure will build up in the eye. Abnormal intraocular pressure causes glaucoma. Glaucoma is treated with medicines, laser trabeculoplasty, and/or conventional surgery. The medicines reduce the production of aqueous humor. Laser trabeculoplasty works by 'blasting' a hole in the trabecular meshwork to ease aqueous humor egress. Conventional surgery makes another hole in the eye to allow the aqueous fluid to escape the eye.
Three Classes Of Retinal Ganglion Cells
non-primate mammals- W cells, X cells, Y cells. In primates- X cells = P cells (parvocellular(small)), Y cells = magnocellular (large)) Y cells have large cell bodies, large dendritic fields, and large center- surround receptive fields. They tend to respond transiently to illumination, prefer moving stimuli, and are not sensitive to color. (5% of ganglion cells) X cells have medium-size cell bodies, small dense dendritic fields, and smaller center-surround receptive fields. They produce sustained responses to illumination and are sensitive to color. (90% of ganglion cells) 1 ) P-cells are color opponent, M-cells are not. 2) P-cells have smaller receptive fields than M-cells. 3)P-cell axons conduct more slowly than M-cell axons. 4) P-cells have a sustained response to light, where as M- cells exhibit a transient response. 5) P-cells are less sensitive to low contrast stimuli than M- cells. Ingeneral: M-cells are important in detecting stimulus movement, P-cells are sensitive to stimulus form and detail.
Vision: Central Visual Circuits
• Different types of retinal ganglion cells • Retinal ganglion cell projections • Lateral geniculate nucleus (LGN) • Primary visual cortex (V1) • Higher order visual cortices
Phototransduction: converting light into an electrical signal
• First, something in the photoreceptor must detect the presence of light. That is, there must be a molecule that can absorb photons in the visible part of the electromagnetic spectrum. • Second, absorption of light by the photosensor molecule must initiate some sort of cellular signal. • Ultimately, the ionic permeability of the plasma membrane must be altered by opening or closing ion channels.
Photoreceptor Adaptation
• Just like some of the skin sensory receptors, photoreceptors also show adaptation during continued presence of light. • In darkness, photoreceptors are maximally sensitive to light, and their responses are large and slow. • In the presence of background light, however, photoreceptors become less sensitive to light and their responses become faster. • This adaptation process is controlled by the calcium concentration inside the photoreceptor outer segment.
Vision: The Retina
• Structure of the retina • The photoreceptor cells & phototransduction • Information processing in the retina • Retinal ganglion cell light travels into the eye, hits the retinal ganglion cell, these cells travel from the ganglion cell layer that go out through the optic nerve (travel through axons), the ganglion cell layer forms synapses on the inner plexiform layer between the retinal ganglion cells and bipolar cells. this takes you into the inner nuclear layer (bipolar cells) from there you go to the outer plexiform layer, where you have synaptic contact between bipolar cells, photoreceptors and horizontal cell. the outer nuclear layer, the cell body for the photoreceptors, this leads to the pigmented epithelium.
