Sensation and perception 5
Striate cortex (V1, Primary visual cortex, Area 17)
6 layers of cells (all neocortex has 6 layers) Topographically/retinotopically organized(what reflects to the retina maintains that map in V1) Cortical magnification(More neurons in cortex to process info from fovea compared w/ info from peripheral retina) Visual acuity decreases w/ eccentricity (the further away it is from the center of vision) visual crowding in periphery is another reason we don't see well in pheriphery.
Pupil
Hole in the center of the iris where light passes through. Size of pupil (dilated or constricted) determines how much light can pass.
Crystallins
densely and regularly packed proteins that make up the lens, and whose organization (dense and regular) gives the lens its translucence
Red eye
dim light, pupil dilated, too much light in...reflected back from fundus(back surface of eye, including retina, optic disk, macula, fovea), thru choroid (latter gives reflection red appearance)
Cone pathway
parvocellular LGN
Strabismus
(1 eye is turned out) leads to 2 different images on the 2 foveas. also known as lazy eye, one eye is looking in another direction
So we have many types of cells in striate cortex (which, remember, is primary visual cortex)
200 million cells in striate cortex Respond to set of stimulus properties: stripes, edges, gratings; orientation; width or spatial frequency; movement Some are simple Some are complex Some are end-stopped Some are not end-stopped Respond preferentially to one eye or the other Respond preferentially based on where in visual field stimulus is located
The structure of rods and cones & the cool stuff inside of them
3 regions in each photoreceptor Outer segment :retina and choroid. Contains lamella: A layer of membrane in the outer segment containing photopigments (the actual receptors for light). Several hundred lamellae per photoreceptor cell. As many as 10 million photopigments per cell Inner segment:contains nucleus Synaptic terminal
How does accommodation work for close up and far away?
Accommodation is only for close up. when light is far away from our eyes, it stays in a more parallel path. The ciliary muscle is relaxed and further away from the lens. so the zonules are tugged a little more tightly and pull on the lens in all directions which then flattens the lens. when we are looking at something close up, light spreads apart as it comes into the eye and we need to make it come together to focus it on the retina. we need the lens to be more heavily curved and bulge so it will bend the light more. ciliary muscle contracts in towards the lens and there is less distance between the ciliary muscles and the lens so the zonules are not pulled as tightly and are more loose, lens isn't being pulled tightly and can bulge into more curved organization or position, allowing us to bend the light even more as it passes through So as opposed to other parts of the eye, the refractive power of the lens is dynamic: can refract light more when its bulged and less when its flattened
Lateral Inhibition
Antagonistic neural interaction b/t adjacent regions of the retina. In other words, input from neighbors is inhibitory. Achieved primarily by Horizontal Cells. so ganglion cell activity is influenced by more than one receptor cell
Vitreus Humor
Between lens and retina Clear, jelly-like fluid. 80% of internal volume of eye Refracts light. Maintains shape of the eye. "Floaters": bits of biodebris that drift around in vitreus (you see them maybe in bright light)
Presbyopia
By 40-50 yr old, can't accommodate enough to focus on things within arm's reach Why? Lens becomes harder and capsule surrounding it loses elasticity Some vertebrates, like fish, accommodate by moving lens in relation to retina Like camera lens (move forward to focus on nearby object; back to focus on distant object)
Receptive fields in striate cortex: Those picky cells!
Cells in V1 are They are linear; they respond to lines (bars, edges, gratings) they are still breaking down parts of the visual world. For each cell in striate cortex, the line has to be of a particular angle. Cell responds best to that angle; Called orientation tuning. More cells are tuned to horizontal and vertical lines than to obliques. Each line also must be a particular width; cells are tuned to a line of a particular angle & width. Some cells respond best when that line is moving in a particular direction. Based on what they respond to, we call these cells simple cells or complex cells. Both get a direct line from LGN.
iris
Circular band of muscles (dilator, sphincter) that controls the size of the pupil, therefore controls light entry. Pigmentation of the iris gives "color" to the eye. Very complex... better for i.d. than finger prints
How are these picky cells arranged in cortex?
Columns: orientation, ocular dominance (column specific to an eye), hypercolumns All cells in a column respond to the same type of stimulus. ex. maybe they all respond to a straight vertical line moving in a particular direction coming from the right eye. this would be an orientation column Neighboring columns respond to similar (but not identical) stimuli.
How do photons in environment become what we see?
Control light entry Focus light on receptors Transduction (photons to electrical)
The development of columns in striate cortex requires experience
Critical period for development: 1st 3-4 months in cats & monkeys: if do not have normal vision in this period, they may not ever be able to see normally and could result in a permanant change. 1st 3-8 years in humans Cortical neurons are still wiring with rest of visual system If one eye doesn't receive appropriate stimulation, the neurons destined to respond to that eye don't become properly connected. Cataracts can reduce/blur image into 1 eye
Transduction in the visual system in dark
Dark: Lamella in outer segment of photoreceptor (in this case, a rod) This is in the dark, without photons to activate the photopigments Sodium channel is open, so photoreceptor is depolarized. Sodium entering cell in dark is called dark current In the DARK, with NO LIGHT, the photoreceptor cell is DEPOLARIZED If it's depolarized, then it is releasing a neurotransmitter, glutamate, which is INHIBITING the bipolar cell Glutamate acts at mGluR->2nd messenger closes a nonspecific cation channel So message stops there.... No info to ganglion cell...no info to brain... which is fine, b/c no light (no info to send)
Acuity-Sensitivity Tradeoff in fovea and in peripheral retina
Different types; Different wiring Peripheral vision: Diffuse bipolar cell->M ganglion cells: One bipolar cell gets input from many (~50!) photoreceptors->pools info...sends along to ganglion cell. All rods and peripheral cones Great for increasing sensitivity; terrible for acuity. Fovea: Midget bipolar cell->P ganglion cells: One bipolar cell gets info from ~1 photoreceptor (cone) Great for acuity; bad for sensitivity
Functional consequences of ganglion cell receptive fields
Each ganglion cell responds best to spots of light of particular size. Therefore act as filter, by editing info they send to brain Ganglion cells are most sensitive to differences in the intensity of light in the center & in the surround; unaffected by average intensity So pick up contrast. Average light differs b/t environments; contrast is less variable and way more important.
2 simple cells: edge detector and stripe detector
Edge detector likes light on one side of its receptive field and darkness on the other side. Stripe detector likes a line of light of a particular width surrounded on both sides by darkness.
Different types of opsins
Exact structure of opsin molecule determines maximal sensitivity to wavelengths of light Long wavelength: red light Medium wavelength: green light Short wavelength: blue light Rods: rhodopsin It would be much more accurate to call these L-, M- and S- cones instead of red, green and blue cones, b/c at this point, there is no color, only wavelengths of light...
Hyperopia
Farsighted Eyeball is too short for refractive power of eyes, so focus behind retina...again star looks like a blur. If young, can correct by accommodating Correct with plus (convex) lenses to converge rays before they enter the eye Most newborns are hyperopic b/c optical components of eyes are well-developed at birth compared w/ length of eyeballs
Aqueous humor
Fluid derived from blood Just behind cornea Supplies oxygen & nutrients to cornea & lens Removes waste from cornea & lens Helps maintain shape of eye & intraocular pressure (too much production or too little drainage in glaucoma)
Superior colliculus of midbrain
From optic nerve Orienting to visual stimuli (tectospinal tract) Sensory map
optic nerve
From retinal ganglion cells Mainly to thalamus... Also to hypothalamus (just dorsal to chiasm) & midbrain
Suprachiasmatic nucleus (SCN) of hypothalamus:
From specialized retinal ganglion cells in retina For light to entrain circadian & circannual rhythms Eventual projection to pineal gland (melatonin)
Review of ganglion cells
Ganglion cells have concentric on-off receptive fields Ganglion cells differ in the size of their receptive fields Small in fovea (info from cones->midget bipolar cells) Large in peripheral retina (info from rods->diffuse bipolar cells) Above feed in to different parts of thalamus and are useful in different aspects of the stimulus
hypercolumn
Hypercolumn: 2 sets of columns, each covering every possible orientation (0-180 degrees), w/ 1 set preferring input from the left eye and 1 set preferring input from the right eye. So, hypercolumn includes: 18-20 adjacent orientation columns (enough for all orientations of a stimulus). orientation columns for left and right eye
When the light hits the surface, it can be reflected, absorbed or transmitted.
If a surface looks lighter, it is reflecting most of the light If a surface looks dark, it is absorbing most of the light If transmitted, some is refracted (bent). Such as when light goes into/thru the water or the eyeball
Pupillary light reflex Aka, Whytt's reflex
Immediate constriction of pupil in response to bright light Cranial nerves 2 and three. the optic nerve and the ocular motor nerve
Thalamus (LGN)
Info came from retina (optic nerve/tract) Parvocellular LGN gets info from cone pathway. Magnocellular LGN gets info from rod pathway Projects to Primary visual cortex (aka striate cortex)
Lateral geniculate nucleus (Thalamus)
Information from retinal ganglion cells 6 layers of cells Magnocellular layers (first two layers) Info from M ganglion cells (rods; peripheral retina) Responds to large, fast-moving objects (so projects to/thru "where pathway" in parietal lobe) Parvocellular layers- last four layers Info from P ganglion cells (cones; fovea) Processes details of stationary objects (so projects to/thru "what pathway" in temporal lobe)
Choroid
Layer of blood vessels providing nutrition for the eye Heavily pigmented (high melanocyte content) so absorbs extraneous light entering eye...this reduces reflection within eye (would blur image) Attached to sclera.
retina
Layer of tissue on back portion of eye Like an outgrowth of the brain (same embryonic tissue) Nerve cells and photoreceptors (rods & cones) to absorb light (photons) and transduce it to neural activity Fovea: point of central focus Optic Disk: hole in back of eye where optic nerve fibers exit and blood vessels enter
Visual fields
Left visual field to left nasal retina and right temporal retina. Right visual field to right nasal retina and left temporal retina. Fibers from nasal retina cross at optic chiasm. Left visual field to right hemisphere. Right visual field to left hemisphere.
Transduction in the visual system in light
Light: Molecule of rhodopsin exposed to light and absorbs a photon->Causes retinal to change shape, so no longer fits in binding site w/ opsin->Retinal is released and binds w/ & activates G-protein (transducin)->G-protein activates a 2nd messenger (phosphodiesterase)->2nd messenger leads to closing of a sodium channel->Photoreceptor is hyperpolarized. so the photoreceptor releases less neurotransmitter. Because the neurotransmitter normally hyperpolarizes the bipolar cell, the reduction causes depolarization of the bipolar cell. The depolarization causes the bipolar cell to release more neurotransmitter, which excites the ganglion cell. Axon of ganglion cell is part of optic nerve, so signal goes back to brain.
Visual pigments (aka photopigment)
Made in inner segment, stored in outer segment in the lamellae Metabotropic receptor 2 parts: retinal and opsin: Retinal: A chemical (chromophore) that absorbs light (photon) & releases energy (activates G-protein) Opsin: a membrane protein whose structure determines the wavelengths it responds to
rods and cone facts
More rods than cones (90 mil vs. 4-5 million) Cones packed in fovea...few cones as move away from fovea Cones for detailed vision (acuity), bright light Rods for dim light Agh! So fovea is blind in dim light
Receptive field for ganglion cells
ON-center ganglion cell Light in the center of the receptive field increases firing rate of ganglion cell; light in surround decreases firing rate. no light in center- decrease in firing rate. no light in surround, increase firing rate. OFF-center ganglion cell Light in the center of the receptive field decreases firing rate of ganglion cell; light in surround increases firing rate. If equal amount of light in center & surround, no response. this (and on-center) due to lateral inhibition More light striking ON areas, more vigorous a response ON and OFF regions have opposite responses to light
Cataracts
Opacities in the lens caused by irregularity in the crystallins. Different types, different locations (which of the many layers of the lens) Interfere w/ vision b/c absorb and scatter more light than the normal lens. things look blurry. Causes/risk factors: congenital (rare), age-related (usually after 50, in most people after 70), diabetes, being struck by lightening (!), penetrating or non-penetrating eye trauma, sun exposure, else?
complex cells
Orientation specific Larger receptive fields than simple cells: Stimulus can occur in wider range of visual field. Best response to movement if stimulus is properly oriented & movement is in particular direction. which is movement specificity.
Organization of visual processing in the brain
P cells are the ganglion cells in the fovea. cone pathway; M cells are the ganglion cells in the peripheral retina (rod pathway). Notice that the cone pathway projects more thru the "what" pathway and the rod pathway projects through the "where" pathway
Hermann Grid
Phantom gray dots at intersections but not in lanes. Dots go away when stare directly at them. At fovea: Smaller receptive fields so no difference between intersection and band. more light on center compared to surround so ganglion cell have lots of activation. but at the intersection, there is equal activity so the lane activity is ramped up. we cannot make the color white, whiter. so we make the intersection darker. when you put the intersection on your fovea, it disappears because we have smaller receptive fields here. there is now no difference between lames and intersections, so gray dots disappear.
Pathway through the retina
Photoreceptor cells->bipolar cells->ganglion cells Integration between cells in each layer achieved by horizontal cells and amacrine cells
How does contrast work
Photoreceptors connected directly to bipolar cell. also Photoreceptors connected to each other via horizontal cell. Center photoreceptors stimulated->inhibition of peripheral photoreceptors via horizontal cell So if shine a light on center of receptive field, center (which gets bright light) is activated, and periphery (which gets dimmer part of periphery of light) is inhibited Contrast!!
Receptive fields for ganglion, LGN, and Striate cortex
Receptive fields of ganglion cells are defined by photoreceptors that influence their activity. Receptive Fields of LGN cells are based on ganglion cells that synapse with them. Receptive Fields of striate cortex cells based on LGN cells that synapse with them.
Cells in the Retina
Retina is built backwards receptor cells-rods and cones horizontal cells (integrate signals between photo receptors) bipolar cells (receptor to ganglion cells) Amacrine cells (Integrate signals b/t bipolars, amacrines, ganglion cells) Ganglion cells- Send action potentials to brain
optic tract
Same optic nerve fibers except that once past optic chiasm it is called optic tract
optic chiasm
Some axons cross to other side; some axons stay on same side Axons from nasal retina cross; axons from temporal retina remain ipsilateral No synapse here... just crossing of some axons
how do cortical simple cells get their orientation tuning (Hubel and Wiesel) building a linear receptive field
The receptive field of cortical neurons is based on the LGN cells that feed them. The arrangement of those LGN cells (ie, in a row) helps establish orientation specificity of cortex cells. That's not the entire story (lateral inhibition b/t cortical cells) a bunch of LGN cells each with a circular receptive field but they all feed into the same cell in V1, put circular receptive fields together to get something that is linear.
Crystalline Lens (Lens)
Transparent tissue Bends light that is passing through the eye (to focus image on retina) Accommodation (near-far focus) How does it bend light? By bending itself: Ciliary muscles control its curvature
cornea
Transparent tissue covering the front of the eye. Continuous with sclera (protective coating around back 5/6 of eye). Light is transmitted & refracted thru this No blood vessels Nerve endings to detect touch and to force eye to close & produce tears if the cornea is scratched (to preserve transparency). What nerves, you ask?? Afferent branch is trigeminal (V); efferent is facial (VII)
Cortical Receptive Fields more about simple and complex cells
Used to think simple cells fed into complex cells; now think 2 parallel pathways Simple and complex cell might respond to same stimulus, but complex cell will respond to anywhere (well, not anywhere) in visual field.
retinal exam
Your doc sees retina with an ophthalmoscope Only part of the body where we can see arteries and veins directly, so good diagnostic for entire vascular system
Amacrine cells
also part of "lateral pathway". Connect bipolar cells, other amacrine cells, and ganglion cells. Exact function unclear.
Tilt aftereffect
cells that were responding to diagonal are tired, cells that fired the most when you switched, respond to the other orientation bc the others were tired. has to do with what cells are active, not what you actually see.
retina->LGN->V1
circular-> circular->linear
more notes on columns
columns are not only important for orientation and ocular dominance in the retina and v1, we are breaking down the image once cell at a time and at hypercolumns we out some of the information together but as we move further into v2 and association, we put the pieces together to see the while image To summarize: The striate cortex is concerned with analyzing the orientation, size, shape, speed and direction of motion of objects in the world, and it does so using modular groups of neurons -hypercolumns- each of which receives input from and processes a small piece of the visual world.
Radiation
energy emitted in form of waves (light) or particles (photons)
Illuminance
energy from source to object; the amount of light falling onto patch of unit surface area (the sun illuminates the Earth)
Radiance
energy from source; intensity of a light beam (the sun radiates light)
Electromagnetic radiation
energy produced by vibrations of electrically charged material. from the electromagnetic spectrum, we can only see 380-760nms as the human visual spectrum.
Simple cells
have simple receptive fields: responds best to object of given shape and orientation Best stimuli are bars, lines, rectangles with definite edges Also respond to gratings, tuned to spatial frequency, which corresponds to line width
fovea
location of sharpest focus; only cones; clearer light path
Mach Bands
look at picture on slide 21 Ganglion cell A receives no illumination to center or surround, so activity. Ganglion cells C and E each receive equal illumination of both center & surround, so activity b/c of light, but just based on that. (activity level: E > C > A) Ganglion cell B has less illumination in center compared to surround, so it decreases its firing rate, so that region appears darker than area to the left or right of it (importantly, appears darker than A even though no real difference). center is in the dark so less action potentials. some of the surround is in the light, so even fewer action potentials making it seem darker than A Ganglion cell D has more illumination in center compared to surround, so it increases its firing rate, so that region appears lighter than area to left & right of it (importantly, appears lighter than E even though there is no real difference). center is in light, so more action potentials. also some of surround is in a darker area, so even more action potentials.
Rod pathway
magnocellular LGN
light
nature's way of transferring energy through space. We can complicate it by talking about interacting electric and magnetic fields, quantum mechanics, and all of that, but just remember--light is energy.
Myopia
nearsighted eyeball is too long for refractive power of eye, so focus image in front of retina instead of on it... looks like a blur. Correct with negative (minus; concave) lenses which diverge the rays before they enter the eye
Emmetriopia
no refractive error, b/c the refractive power of the eye is perfectly matched to the length of the eyeball
Amblyopia
reduced visual acuity in 1 eye due to abnormal visual experience Can lead to decreased binocular vision & depth perception Can't be corrected later in life
Meningioma
tumor of meningie cell growing inward from back of head can cause a blind spot in the center of the field of view for both eyes. cortex that processes center vision is being damaged Also caused by macular degeneration: the fovea is the central part of the macula and is where central vision is occurring. loss of retinal cells in macula is macular degeneration
Hypercomplex cells
used to be called hc cells, now considered subtype of simple and complex cells): Like simple: Orientation specific (top) or Like complex: Orientation specific, movement specific (right) Plus, length specific: End stopping Good for detecting edges, corners and borders
Components of light
waves are light when it moves around the world (hue). different wavelengths interact differently with the receptors in our eyes. short wavelengths come off as blue, medium come off as green, and long look red. the number of photons present is associated with the intensity of light # of photons = perception of intensity; wavelength = perception of color
