PY793 - Exam 2

Parallel Processing

Each area analyzes specific location and attribute(shape,color,movement)&distri info

Projections of the Optic Nerve and Tract

(retino-geniculate pathway-retina to lateral geniculate nuclei(LGN) of thalamus(90% of optic nerve axons)input via geniculo-cortical projections);10% innervate other subcortical-likepulvinar nucleus of thala&superiorcolliclus of midbrain-both play role in visual attention,retino-collicular more primitive visual system) in the brain,each optic nerve divides into pathways that differ with respect to where they terminate within the subcortex. The retino-geniculate pathway, the projection from the retina to the lateral geniculate nuclei (LGN) of !he thalamus contains more than 90% of the axons in the optic nerve and provides input to the cortex via the geniculo-cortical projections. The remaining 10% of the fibers innervate other subcortical structures including the pulvinar nucleus of the thalamus and the superior colliculus of the midbrain. The superior colliculus and pulvinar nucleus play a big role in visual attention, and the retino·collicular pathway is sometimes viewed as a more primitive visual system.

Rod Properties

- High sensitivity to light, specialized for night vision - More photopigment, capture more light - High amplification, single photon detection - Low temporal resolution: slow response, long integration time - More sensitive to scattered light - Low acuity: not present in central fovea, highly convergent retinal pathways - Achromatic: one type of rod pigment

Cone Properties

- Lower sensitivity, specialized for day vision - Less photopigment - Lower amplification - High temporal resolution: fast response, short integration time - Most sensitive to direct axial rays - High acuity: concentrated in fovea. - Chromatic: three types of cones, each with a distinct pigment that is most sensitive to a different part of the visible light spectrum

Two Ganglion Cell Surrounds

1) The ganglion cell inherits whatever surround comes with the bipolar cell input (mediated by horizontal cells "suppressive sound" reduce bipolar input) 2) Ganglion cells can also receive amacrine cell input (mediate ganglion cell surround responses at the opposite on or off sign from that of the center, antagonistic surround)

Activation Of Rhodopsin Starts With The Absorption Of Light

Activation of rhodopsin starts with the absorption of light, which causes retinal to change from the 11-cis to the all-trans configuration. This reaction is the only light-dependent step in vision. As a result of this conformational change, retinal no longer fits into the binding site in opsin. The opsin, therefore, undergoes a conformational change to a semistable conformation called metarhodopsin II, which triggers the second step of phototransduction.

Cat Retinal Ganglion Cells

12 diff types(9antagonistic center-surround organization,3w/other org,9center surround grouped(4oncenter/offcenter PAIRS&1 color coding))/

The Pretectum (NOT,PO,PP)

3 bilateral retinal inputs(nucleus of optic tract(NOT),olivary pretectal nucleus(PO),posterior pretectal nucleus(PP)

Lateral Inhibition in Limulus

A demonstration of lateral inhibition in the limulus. The records show the response recorded by the electrode recording from the nerve fiber of receptor A when receptor A is stimulated and (a) no other receptors are stimulated, (b) the receptors at B are stimulated simultaneously, & (c) the receptors at B are stimulated at an increased intensity.

Hyper&Depolarizing BC

All synapses contain glu All are excitatory except the synapse from the photorecep to the depolarizing BC MGluR6(inhibitory)activates the closing of a non-selective cation channel, resulting in depolarization of the bipolar cell, when less glu

Cone System Has High Spatial Resolution

Although rods outnumber cones by roughly 20 to 1, the cone system has better spatial resolution for two reasons. First, because many neighboring rods converge onto a single bipolar cell, differences in the responses of the rods are pooled in the interneuron. Second, cones are concentrated in the fovea, where the visual image is least distorted.

Antagonistic Surrounds

An area in which stimulation of the opposite response sign (on vs. off) to the center, generates an excitatory response (That is, an On-center ganglion cell may have an Off-surround, or an Off-center ganglion cell may have an On-surround. Often, different cone mechanisms mediate these two excitatory responses, making the cell color opponent.) Some ganglion cells have only a single surround mechanism, the "suppressive" inhibitory surround, also sometimes called a "silent inhibitory surround Presumably, this surround comes from 1) horizontal inhibition of bipolar center responses, 2) amacrine input in the IPL probably mediates the inhibitory surround over a larger area

Visual Cortex

Area 17 (V1); most posterior pole of the occipital lobe

Melanin

Black pigment that fills the cells in the pigment epithelium, absorbing any light not capture but the retina; prevents light from being reflected off the back of the eye to the retina again (some animals have the tapetum)

Light Is Absorbed by Visual Pigments in the Photoreceptors

Both rods and cones have three major functional regions: 1) The outer segment, located at the outer or distal surface of the retina, is specialized for phototransduction. 2) The inner segment, located more proximally within the retina, contains the cell's nucleus and most of its biosynthetic machinery. 3) A synaptic terminal makes contact with the photo-receptor's target cells.

Topographic Discontinuities

Boundaries bw adjacent visual areas to the topographic rep of external space in the contralateral hemifield

Fovea

Characterized by a depression; about 1.5mm across; only cones, no rods; largely avascular for better image quality; center of fovea (faveola) about 0.2mm across, no rods and virtually no blue cones; about half of all ganglion cells are foveal

Center-Surround Receptive Field of Bipolar Cells

Cone cells in the center of the receptive field of a bipolar cell synapse directly on the bipolar cell. Each cone cell synapses on both on-center and off-center bipolar cells. Cone cells release a single neurotransmitter, glutamate, which inhibits (hyperpolarizes) on-center bipolar cells and excites (depolarizes) off-center cells. In the dark the cones are depolarized (around -40 mV), so that voltage-gated Ca2+ channels in their synaptic terminals are open, allowing Ca2+ to enter the terminals and trigger the release of glutamate. This constant release of glutamate in the dark maintains the on-center bipolar cells in a hyperpolarized state. When illuminated, however, the cones become hyperpolarized, and the voltage-gated Ca2+ channels close, reducing the Ca2+ influx and therefore the amount of glutamate the cells release; as a result, the on-center bipolar cells depolarize. Conversely, cone cells maintain off-center bipolar cells in a depolarized state in the dark. When glutamate release is reduced by light the off-center bipolar cells hyperpolarize. Glutamate produces different responses in the two classes of bipolar cells by gating different cation channels. In off- center bipolar cells glutamate opens a type of cation channel that carries an inward (depolarizing) Na+ current into the cells. In on-center bipolar cells the mechanism by which glutamate hyperpolarizes the cell is unusual and may be different for rods and cones. At some synapses the glutamate transmitter appears to act by opening K+-selective ion channels. At others it closes a cGMP-gated channel that carries an inward Na+ current. In the absence of transmitter this type of channel is kept open by a high intracellular concentration of cGMP. Glutamate appears to cause the closure of these channels in precisely the same way that light causes the closure of cGMP-gated channels in photoreceptors—by activating a specific glutamate receptor that activates a G protein, which in turn activates cGMP phosphodiesterase and lowers the cytoplasmic concentration of cGMP. Cones in the surround of a bipolar cell's receptive field synapse on horizontal cells. Horizontal cells do not make direct synaptic contact with the bipolar cells, however. Instead, they have synapses on cones in the center of the bipolar cell's receptive field. When the surround is illuminated, the horizontal cells depolarize the cones in the center, the opposite effect of light absorption by these cones. Whether this mechanism alone accounts for the antagonism between center and surround in bipolar cells is not yet known.

Cones are responsible for day vision

Cones perform better than rods in all visual tasks except the detection of dim stimuli. 1) Cone-mediated vision is of higher acuity than rod-mediated vision and 2) provides better resolution of rapid changes in the visual image (ie, better temporal resolution). 3) Cones mediate color vision. People who lose cones function are legally blind.

Cone Vision

Cones perform better than rods in all visual tasks except the detection of dim stimuli. Cone-mediated vision is of higher acuity than rod-mediated vision and provides better resolution of rapid changes in the visual image (ie, better temporal resolution). Cones also mediate color vision. Differences in performance are due partly to properties of the rods and cones themselves and partly to the connections they make with other neurons in the retina (the rod and cone systems).

Deuteranope

Dichromate without green cone pigment Deuteranopia affects about 1 percent of males and 0.01 percent of females and results in the perception of colors such that a deuteranope perceives blue at short wavelengths, sees yellow at long wavelengths (like a protanope), but has a neutral point at about 498 nm.

X vs Y Cell Functional Differences

Duration-XsustainedYtransient Summation-XLinearYnon-linear RF center-XsmallYlarge Nerve conduction-XmediumYfast Movement resp-XpoorYgood Func-Xdetail,Ychange/motion Xpattern&color

Dichromats

Experience some colors, though with less color "acuity" than trichromats

Projections to the SC

Fibers from the contralateral eye terminate in a dense continuous band in the upper part of the superficial gray layer;from the ipsilateral eye terminate in register in the lower part of this layer,in bands or slabs similar in width to those of the visual cortex.projections of the substantia nigra pars reticulata to the deeper layers of the SC also shows similar bands or slabs, suggesting that periodic vertical segmentation may be a general feature of collicular organization.axons of all types of ganglion cell, except X cells, terminate in the SC. All the cells in the superficial layers of the colliculus respond only to visual stimuli, acquiring their receptive-field properties partly from their retinal input and partly from the visual cortex.These cells are typical of those found along sensory pathways, having small dendirtic fields with little overlap. the cells of the superficial layer(stratum griseum superficiale)project to the nucleus of the optic tract&olivary pertectal nucleus of the pretectum,pulvinar-lateral posterior complex,dorsal&ventral lateral genicular nuclei.Of particular importance is the topographically ordered collicular projection to the pulvinar/lateral-posterior complex to a region termed the interadjacent zone by Updyke. Since the pulvinar/lateral posterior complex projects to the visual cortex, this patway provides a route to the visual cortex that is in parallel with the geniculate pathway.

Retina as a Sensory Transduction Example

First, photoreceptors in the retina are perhaps the best understood of all sensory cells. Second, unlike other sensory structures, such as the cochlea or somatic receptors in the skin, the retina is not a peripheral organ but part of the central nervous system, and its synaptic organization is similar to that of other central neural structures.

Retina Contains...

Five Major Classes of Neurons

Mouse Retina

Harbors more than 30 functional output channels

Question: How do we send the message from 100 million photoreceptors via 1 million ganglion cell axons?

Hint: jpeg and mpeg encoding. Answer: compress the message in space and time.

Stage 1: Light Activates Pigment Molecules in the Photoreceptors

In rod cells the visual pigment, rhodopsin, has two parts. 1) The protein portion, opsin, is embedded in the disc membrane and does not by itself absorb light. 2) The light-absorbing portion, retinal, is a derivative of vitamin A. Retinal can assume several different isomeric conformations, two of which are important in different phases of the visual cycle. In its non-light-activated form of rhodopsin, the 11-cis isomer of retinal fits snugly into a binding site in the opsin molecule

In Rods The (cGMP) Molecule Acts As A Second Messenger

In rods the (cGMP) molecule acts as a second messenger, carrying information through the cytoplasm connecting the freely floating discs, where light is absorbed, to the cell's plasma membrane, where ionic fluxes are altered. In cones, since the discs are continuous with the plasma membrane, a cytoplasmic messenger is not necessary; nonetheless, cGMP is used in these cells in the same way as in rods. Cyclic GMP controls ionic fluxes by opening a specialized species of ion channels, the cGMP-gated ion channels, which allow an inward current carried largely by Na+ ions to flow into the cell.

cGMP 2...

In the dark the concentration of cGMP is relatively high, thus maintaining the cGMP-gated channels in an open state and allowing the inward current they carry to maintain the cell in a relatively depolarized state. Phototransduction then occurs in three stages: 1) Light activates visual pigments; 2) these activated opsin molecules stimulate cGMP phosphodiesterase, an enzyme that reduces the concentration of cGMP in the cytoplasm; and 3) the reduction in cGMP concentration closes the cGMP-gated channels, thus hyperpolarizing the photo- receptor

The Fovea

In the fovea the cell bodies of the proximal retinal neurons are shifted to the side, enabling the photoreceptors there to receive the visual image in its least distorted form. most pronounced at the center of the fovea, the foveola, where there is a "pit".

Three Types of Primate Cone Cells Contains A Different Pigment

In the retina of primates each of the three types of cone cells contains a different pigment optimized for absorption of light in a different part of the visible light spectrum. As in rods, the visual pigments in cones are composed of two parts: 1) a protein called cone opsin and 2) a light-absorbing portion, 11-cis retinal. Each type of cone pigment contains a different isoform of cone opsin that interacts with 11-cis retinal to shift its sensitivity to different parts of the visible spectrum. The existence of three types of cones with different absorption characteristics underlies tri-variant color vision in humans.

Retina Vertical Section

Inner closer to ganglion; outer closer to photoreceptors; KNOW THE DIFFERENT LAYERS

Suprachiasmatic Nucleus (SN)

Just above the optic chiasm,just lateral to 3rd ventricle Each nucleus receives a bilateral projection of retinal fibers(forms both gray type I and II synapses onto dendrites&dendritic spines Collateral branches of fibers going to the optic tract receives a visual input from the LGNv,overlaps the retinal projection,as well as receiving a number of nonvisual inputs. project to a number of areas, which are otherwise nonvisual. role in neural control of a variety of circadian rhythms

Mach Bands

Just to the left of the contour, near B, a faint light band can be perceived, and just to the right, at C, a faint dark band can be perceived. (b) The physical intensity distribution of the light, as measured with a light meter. (c) A plot showing the perceptual effect described in (a). The bump in the curve at B indicates the light Mach band, and the dip in the curve at C indicates the dark Mach band. Note that the bumps are not present in the physical intensity distribution (b). --- 80, 88, 8, 16, 16 It only cares about the edges and the distinctions

Retina

Lies in front of the pigment epithelium that lines the back of the eye

Photoreceptors Respond with Graded Potentials

Like some other sensory receptors, rods and cones do not fire action potentials. Instead, they respond to light with graded changes in membrane potential. Rods respond slowly, so that the effects of all the photons absorbed during a 100 ms interval are summed together. This helps rods detect small amounts of light, but prevents them from resolving light that is flickering faster than about 12 Hz. The response of cones is much faster; they can detect flicker up to at least 55 Hz.

Cat Layer Destruction

Magno-movement perception,parvo-color,pattern&depth perception

The Rod is Highly Convergent

Many rods have synapses on the same bipolar cell. Thus, signals from the rods are pooled in the bipolar cell and reinforce one another, strengthening the signals evoked by light in individual receptors and increasing the ability of the brain to detect dim lights. In contrast, fewer cones converge on each bipolar cell. In fact, cones in the foveola have small diameters, are closely spaced, and do not converge at all; each bipolar cell receives input from a single cone. (midget system)

Two Stages of Visual Perception

Mechanical: Light entering the cornea is projected onto the back of the eye, where it is converted into an electrical signal by a specialized sensory organ, the retina. Neural: These signals are then sent through the optic nerve to higher centers in the brain for further processing necessary for the perception of form, motion, and color in higher centers.

Rods

Mediate night vision; loss only results in night blindness; Sensitive to light, function well in the dim light, when most stimuli are too weak to excite the cones; more light-sensitive than the cones system, it is achromatic because there is only one type of rod, and a single photoreceptor type cannot code wavelength and intensity in its response; Rods contain more photosensitive visual pigment than cones, enabling them to capture more light. Even more important, rods amplify light signals more than cones do. A single photon can evoke a detectable electrical response in a rod; in contrast, tens or hundreds of photons must be absorbed by a cone to evoke a similar response.

Dorsal Stream

Mediating guided behavior-dominated by magnoganglion cell input What pathway

Ventral Stream

Mediating perception-dominated by parvo-ganglion cell input Where pathway

Tritanope

Missing blue cones; 0.002 men, 0.001 women; short blue, long red, neutral point 570nm

Object Discrimination

Monkey pick up correct object, temporal lobes removal caused difficulty

Landmark Discrimination

Monkey pick up food closest to cylinder, parietal lobes removal caused difficulty

Anomalous Trichromat

Need 3 wavelength to match any other wavelength just as a normal trichromat. However, they mix these in different proportions. Depending on wavelength shift may be better or worse at wavelength discrimination tasks, some women can be tetrachromats

Complex Cells

Orientation+movement direction(only movement)

Parallel magno&parvo pathways to LGN

P-cells(parvocellular)-synapse in layers3,4,5,6 M-cells(magnocellular)-synapse in layers1&2

Three types of dichromats

Protanopia, deuteranopia, tritanopia

Rabbit Retinal Ganglion

Phenotyped using intrinsic small molecule signals 14 natural ganglion cell classes and 3 amacrine cell classes

LGN

Projects to visual cortex via geniculo-cortical pathway called the optic radiation Fanout of axons exits from the LGN&ascends to the cortex,fibers terminate in primary visual area-occipital lobe Visual info processed by 5 distinct neurons by V1:photorecep,BC,ganglion,LGN,&cortical cells

Protanopia

Protanopia affects 1 percent of males and 0.02 percent of females and results in the perception of colors by which short-wavelength light is blue, and as wavelength is increased, the blue becomes less and less saturated until, at 492 nm, the protanope perceives gray. The wavelength at which the protanope perceives gray is called the neutral point. At wavelengths above the neutral point, the protanope perceives yellow, which becomes increasingly saturated as wavelength is increased, until at the long-wavelength end of the spectrum the protanope perceives a saturated yellow. Question: Why does the protanope see long wavelengths as yellow and not red or green? Answer: This is explained by color opponency in the opponent process theory of vision.

5 Cell Classes in the Retina

R:Photoreceptors, H:Horizontal cells(receive input from photorecep-make conventional synapses onto bipolar cell dendrites), B:Bipolar cells(receive input from photorecep-can synapse directly onto ganglion cell dendrites or only onto amercing cell processes, which then synapse onto ganglion cells), A:amacrine cells(make feedback junctions onto bipolar terminals, feed forward synapses, and lateral synapses on other amacrine cell processes), G:ganglion cells

(Rod)Monochromatism

Rare colorblindness, usually hereditary 10:1mil, no functioning cones, so only rods in both dim and bright, see in shades of lightness (white, grey, black), so actually colorblind, so sensitive to bright light that they need sunglasses, rod system overloaded in strong illumination, creating perception of glare, poor visual acuity

Hubel and Wiesel

Recorded cortical neurons,stimulate different areas in cat,moving lines and spots

Two Types of Photoreceptors

Rods and Cones

Basic Neural Pathway in the Retina

Signal sequence (photoreceptors, bipolar cells, ganglion cells)

Cytochrome oxidase method

Stains enzyme of same name; concentrated in areas of high metabolic activity(darker regions-blobs,lighter-interblobs))

V1 Complex Cells

The RF of a complex cell in the primary visual cortex has no clearly excitatory or inhibitory zones. Orientation of light stimulus is important, but position within the RF is not

Phototransduction Results From a Three-Stage Cascade of Biochemical Events in the Photoreceptors

The absorption of light by visual pigments in rods and cones triggers a cascade of events that leads to a change in ionic fluxes across the plasma membrane of these cells, and consequently a change in membrane potential. A key molecule in the cascade is the nucleotide cyclic guanosine 3°-5° monophosphate (cGMP).

To allow light to reach the photoreceptors without being absorbed or greatly scattered...

The axons of the neurons in the proximal layers of the retina are unmyelinated (relatively transparent)

Fundamental principle of visual perception

The nervous system is most interested in change (spatial: borders, temporal: motion)

Light-absorbing Visual Pigments

The outer segments of rods and cones are filled with light-absorbing visual pigments. Each pigment molecule consists of: 1) a small light-absorbing molecule attached to 2) a large membrane-spanning protein. Rods and cones can contain a remarkably large number of these membrane proteins (as many as 10^8 in each cell), because they have evolved an elaborate system of stacked membranous discs in their outer segments that dramatically increase the surface area of the membrane in these cells.

Light-absorbing visual pigments

The outer segments of rods and cones are filled with light-absorbing visual pigments. Each pigment molecule comprises a small light-absorbing molecule attached to a large membrane-spanning protein. Rods and cones can contain a remarkably large number of these membrane proteins (as many as 108 in each cell), because they have evolved an elaborate system of stacked membranous discs in their outer segments that dramatically increase the surface area of the membrane in these cells.

Threshold Converted to Sensitivity

The threshold for seeing a light versus wavelength If we take the reciprocal of the curve in, the curve turns over and becomes a plot of sensitivity versus wavelength, commonly known as a spectral sensitivity curve

Sex linked

The two most common forms of dichromatism, protanopia and deuteranopia, are inherited through a gene located on the X chromosome. Since males (XY) have only one X chromosome, a defect in the visual pigment gene on this single X chromosome causes color deficiency. Females (XX) with their two X chromosomes, are less likely to become color deficient, since only one normal gene is required for virtually normal color vision. Protanopia and deuteranopia are called sex-linked because women can carry the gene for color deficiency without being color-deficient themselves, and they can pass the condition to their male offspring, who inherit their X chromosome from their mothers. Many more men than women are dichromats (~1:20 vs 1:400). Most mammals are dichromats rather than trichromats.

Cat Retinal Ganglion Cell Classes From Morphology

The α type 4-7% of the population,largest somata,distributed more or less uniformly across the retina,except the fovea. the β type,half the population,smaller somata than a cells.ϒ cells,half the population,smaller somas,comprises many classes, actually.

Three Types of Cones

There are three types of cones, each containing a visual pigment that is sensitive to a different part of the light spectrum. The brain obtains information about color by comparing the responses of the three types of cones. In contrast, rods contain only one type of pigment and therefore respond in the same way to different wavelengths (except for frogs).

Photopigment in Photoreceptor Disks

These discs develop as a series of invaginations of the cell's plasma membrane, ultimately arranging themselves like a roll of pennies in a bank wrapper. In cones the discs are continuous with the plasma membrane, while in rods they pinch off from the plasma membrane and become intracellular organelles. Like other neurons, photoreceptors do not divide, but their outer segments are constantly renewed. New discs are formed at a rapid rate; in rods about three discs are synthesized every hour. Old discs are discarded at the tips of photoreceptors and removed by the phagocytotic activity of the pigment epithelial cells.

Cone Cells

Those in the center of the receptive field of a bipolar cell synapse directly on the bipolar cell Synapses on both on-center and off-center bipolar cells Release NT glutamate, which inhibits (hyperpolarizes) on-center bipolar cells and excites (depolarizes) off-center cells Those in the surround synpase on horizontal cells-which do not make direct contact with the bipolar cells, instead synapse on cones in the center of the receptive field When surround is illuminated, horizontal cells depolarize the cones in center, the opposite effect of light absorption by these cones Conversely, cone cells maintain off-center bipolar cells in a depolarized state in the dark-glutamate release reduced by light-the off-center BC hyper polarize Glutamate produces different responses in the two classes of bipolar cells by gating different cation channels In off-center BC glutamate open inward(depolarizing)Na+ cation channel In on-center BC MGluR hyperpolarizes In the dark the cones are depolarized (around -40 mV), so that voltage-gated Ca2+ channels in their synaptic terminals are open, allowing Ca2+ to enter the terminals and trigger the release of glutamate. This constant release of glutamate in the dark maintains the on-center bipolar cells in a hyperpolarized state. When illuminated, however, the cones become hyperpolarized, and the voltage-gated Ca2+ channels close, reducing the Ca2+ influx and therefore the amount of glutamate the cells release; as a result, the on-center bipolar cells depolarize.

Contextual Modulation

V1,modulated by surrounding stimulus

cGMP

WILL BE ON TEST

End-Stopped (Hypercomplex) Cells

Will not fire if the stimulus is too long,feature detectors-detect movement,orientation

Physiologists

area 19 many visual areas

Hierarchy Hypothsis of why so many areas

areas form a hierarchy in which each area elaborates on the the earlier ones(find edges,detect corner,which shape)

Geniculo-Cortical Pathway

axon tract exits the LGN on each side&ascends to area V1 at calcarine fissure at pole of occipital;visual info through-photorecep,BC,ganglion,LGNcells

Bipolar Surround Rules

bipolar cells report local contrast as the difference between their center and surround illumination, the bipolar surround can only reduce the bipolar cell input (less depolarization) the bipolar surround is equivalent to a gain control, normalizing response to overall light level.

Layers in the dLGN constitute a particular representation...

characterized by 3 organizational properties: 1)space-3 layers receive input from one retina&3 layers from the other retina-GC from right temporal hemiretina layers2,3,5 of right LGN,from left nasal hemiretina layers1,4,6-visual info from specific regions in space projected to same LGN bc temporal and contralateral nasal semi-retian responsive to stimuli in the same visual field...2)layer alignment-each layer=topographic map of the retina(& thus external) that is a tight register-object will activate cells in each layer that falls along a line perpendicular to the LGN surface...3)stimulus analysis-cell types in each layer have clear distinction in response&cytoarchitecture

Salience

degree to which the line stands out

Central Projections of Ganglion Cells

dorsal thalamus (dorsal lateral geniculate nucleus-LGNd),optic tectum (superior colliculus),accessory optic nuclei (except some elasmobranchs and teleosts),pretectum,hypothalamus (sμprachiasmatic nuclei-SN),ventral thalamus (ventral lateral geniculate nucleus-LGNv)

Simple Cells

exc&inhib RF but sidebyside so respond to bar in a particular orientation can be stationary-shown in orientation tuning curve

Binocular Projections to the Lateral Geniculate Nucleus

from right hemiretina to right LGN convey representation of left visual hemifield;fibers from contralateral nasal hemiretina terminate in layers1&4,from ipsilateral temporal hemiretina in layers2,3,5;layers1&2magno-3thru6parvo

Glutamate closure

in the same way cGMP gated channels in photorecep-activating a specific gluR-activates G protein-activates cGMP phosphodiesterase-lowers cytoplasmic cGMP concentration

Analytic Parallel Processing in the Visual System

info distributed across distinct subsystems(ex.shape,color,dynamic&movement in space)

Light...

is focused by the cornea (70%) and the lens (adjustable), then transverses the vitreous humor that fills the eye cavity before reaching the retina the pupil controls the amount of light entering the eye

Properties and Circuits of LGN Relay Cells

most LGN cells driven by 1 or 2 retinal GC relay cell responses mo dulated by 1)reticular input from areas outside the LGN that mediate emphasis b/w vision&other senses 2)feedback from the cortex modulate relay cell firing (relay(1):GC(1) equal each LGN relay cell at V1 drives 100-200 cortical cells-divergence,sparse rep)

The Retina is in front of the...

photoceptors

V2

prestriate cortex(in front of striate)(3 regions(thick stripes(M),thin(P-blob),interstripes(P-interblob))

In the fovea, the cell bodies of the proximal retinal neurons are...

shifted to the side, enabling the photoreceptors there to receive the visual image in its least distorted form (most pronounced in the fovea)

Inhibitory Ganglion Cell Surround

stimuli confined to center (more firing), stimuli invading surround (less firing)

70 msec

the amount of time it takes for visual response

Eyes move...

the scenes of interest are projected onto the fovea; The retina also contains a region called the optic disc, where the optic nerve fibers leave the retina. This region has no photoreceptors and therefore is a blind spot in the visual field.

Superior Colliculus(SC)

two subdivisions-superficial(receive afferents from retina,visual cortex,lateral tegmenjtum,termend the parabigeminal nucleus)&deep(below stratum opticum-dont receive retinal projection,do receive afferents from many areas visual or otherwise)

P Pathway

within striate cortex, the P pathways involves a second synapse from which an inter laminar projection carries information in the P pathway from layer 4 to the more superficial layers 2 and 3