The Visual System

Question 1

Discuss the major steps in phototransduction in the outer segments of rods

Answer 1

Phototransduction: the process by which light is converted to a change in membrane
potential by the photoreceptors.
Light absorption causes photoreceptors to hyperpolarize.

To catch light, membrane protein is packed at high density in surface membrane infoldings (cones) or intracellular membranous sacks (rods), photon has 50% chance of capture (absorbed by Vit A bound to protein).

Light stimulates rhodopsin, leads to G prot. transducin activation, which activates cGMP phosphodiesterase–>hydrolizes cGMP. Ultimately light reduces concentration of cGMP, leading to closure of Na channels (hyperpolarization)

Question 2

Describe the receptive field properties of retinal ganglion cells.

Also give examples of the chain for OFF-center ganglion cell and ON-center ganglion cell

Answer 2

Photoreceptors talk to bipolar and horizontal cells.
Bipolar cells talk to ganglion cells. Only ganglion cells make APs, other cells talk by changes in membrane potential and altered NT release.
Retina wired to detect contrast (first level of edge/corner/shape detection).
Receptive field: the best stimulus to get sensory neur to change AP firing rate.

Ganglion cells: donut-shaped receptive fields.
On center ganglion cells: excited by light shining in their centers and inhibited by light in periphery.
Off center ganglion cells (opposite of on).
In fovea, receptive field as wide as single cone. Larger fields in periphery of retina.
Key determ in receptive field type of ganglion cells is the type of receptor on bipolar cells.
Remember: {photorecep are hyperpol by light (less NT), photorecep release glutamate; bipolar cells can be either excited (OFF center) or inhibited (ON center) by glutamate; Bipolar cells always make excitatory synapses on ganglion cells}

OFF center: if glutamate excites a bipolar cell, then shining light on a photoreceptor will lead to INHIBITION of the ganglion cell.

ON center: if glutamate inhibits a bipolar cell, light relieves inhibition, EXCITING the ganglion cells.

Question 3

Describe color-opponent ganglion cells

Answer 3

In fovea, most bipolar cells are connected directly to one kind of cone in the field center (ie red) and indirectly via horizontal cells to cones with a different color preference (ie green) in the field surround–>creates RED ON center and GREEN OFF surround receptive field. (All combos of red-green on-off exist)
=Color opponent cells. (red green and blue yellow opponent cells exist (yellow by converging red and green cones)

Question 4

Identify where color is processed in the cortex

Answer 4

Color info is separated out from spatial info in the retina, and is handled in central regions of hypercolumns called blobs.
Visual cortical area V4 is considered “color” area while V5 is considered motion area.

Question 5

Discuss the receptive field characteristics of cortical simple and complex cells and describe how these receptive field properties are achieved by synaptic inputs from lower order cells

Answer 5

A simple cell might
have an ON area
that is a narrow line
at some preferred
orientation that is
flanked on each side
by OFF areas. Diffuse light is ineffective.
While some simple cells have ON-centers with OFF flanking lines, others are the
reverse.
Simple cells have receptive fields with antagonistic flanking regions; the
shape of the field is a straight line and the orientation of the line is crucial.

Hierarchical processing: Several cells with similar but spatially offset receptive fields (concentric cells) converge on a higher order cell to create an altogether new type of receptive field. (The cortical cell will then have a receptive field that is the sum of the LGN cells’ receptive fields)

Question 6

Describe the meaning of a sensitive period in the development of the cortex and discuss the importance of this concept in diagnosing and treating abnormal development

Answer 6

f

Question 7

Indicate the conditions under which the effects of abnormal developmental experiences can be reversed.

Answer 7

f

Question 8

Intensity corresponds to___, wavelength corresponds to_______

Answer 8

intensity->brightness

wavelength->color

Question 9

Focusing power of eye

Answer 9

Cornea 2/3
Lens 1/3 (under neural control, allows for focusing of nearby objects)

Lens suspended by zonule fibers attached to ciliary muscle

Lens gets fatter shape to focus on nearer objects.

Question 10

What gives rise to the blind spot?

Answer 10

optic disc (where retinal ganglion cells group together in optic disc to form optic nerve)

Question 11

5 types of neurons in retina

Answer 11

photoreceptors face the back of the eye (rods–color insensitive and work best in dim light, cones–color vision, work only in bright light, concentrated in fovea (not rods))
Bipolar cells
Horizontal cells
Ganglion cells (output cells of retina)

Question 12

3 properties of light

Answer 12

reflection
absorption
refraction

Question 13

What mediates the receptive field surround?

Answer 13

Horizontal cells
Behave as though they have excitatory receptors for glutamate released from photoreceptors and make inhibitory synapses on the photorecep in field center.
So: if a spot of light is shined on periphery of an ON-center ganglion recep field, photorecep in surround will hyperpolarize, reducing secretion of NT, reducing activation of excitatory recep on horizontal cells, which will hyperpolarize horizontal cells. This will DECREASE GABA secretion onto field center photoreceptors and will decrease inhibition of photoreceptors, causing them to release more NT. Since this is an ONcenter
cell, the receptors on the bipolar cells in the field center are inhibitory. So the inhibition of
the bipolar cells will increase when light shines on the periphery, which will reduce the bipolar
cell excitatory input to the ganglion cell, which will reduce the firing rate of the ganglion cell.

Question 14

4 synapses of photorecep pathway. Which are excitatory and which are inhibitory?

Answer 14

2 are excitatory (NT depol cell): surround photoreceptor to horizontal cell synapses; and bipolar to ganglion cell synapses.

One is always inhibitory: horizontal cell to photoreceptor synapse

One is either excitatory (OFF center bipolar cells) or inhibitory (ON center bipolar cells) field center photoreceptor to bipolar cell.

Question 15

What is the response to light in inhibitory area (center or surround) being turned off?

Answer 15

rebound response when light is turned off. (abrupt removal of inhibition)

Question 16

What do ganglion cells care about?

Answer 16

CONTRAST

Question 17

Pathway of optic nerve

Answer 17

Retinal ganglion cells form the optic nerve. Optic nerves cross at optic chiasm. Half of the axons from each eye cross to other side and continue in optic tract to lateral geniculate nucleus (LGN) of the thalamus.
At the chiasm, axons from the NASAL half of each retina cross over to the opposite side (decussate). So: right optic tract contains axons from RIGHT of each retina, which see the LEFT side of the world. The LGN represents the contralateral visual field.
After LGN, axons fan out in optic radiations to visual cortex in back of brain.

Question 18

LGN layers

Answer 18

6 layers, no direct interaction between eyes at level of LGN (not binocular).
Layers 1, 4, 6 receive inputs from contralateral eye (decussates)
Layers 2, 3, 5 receive inputs from the ipsilateral
eye

Layers 1-2: inputs from magnocellular ganglion cells
Magnocellular System
Spatial Vision – Motion and depth
1. Low acuity (crude form)
2. Large receptive fields
3. Responsive to motion
4. No color vision (input from rods) 

Layers 3-6: inputs from parvocellular ganglion cells
Parvocellular System
Object Vision – color, form, detail
1. High acuity (fine detail)
2. Small receptive fields
3. Not responsive to motion
4. Color vision (input from cones)

Question 19

Explain the meaning of “ocular dominance” in cells of the visual cortex and indicate its physical basis.

Answer 19

LGN axons rad to V1 (primary visual cortex) in cortex (forms hypercolumns)
Layers 1 (top) thru 6.
LGN axons terminate in layer 4.
Columns divided in two parts, one half for each eye; these are called
ocular dominance columns.
About half of the cortical cells (near border b/t two eyes in hypercolumn) become BINOCULAR–receive inputs from both eyes.
All cells in a vertical column are sensitive to the same ORIENTATION of lines/light (different rays of pinwheels).
Color information is handled in central regions of hypercolumns called “BLOBS”.

Question 20

Discuss the changes in ocular dominance caused by monocular deprivation, binocular deprivation, and alternating monocular deprivation.

Answer 20

f

Question 21

Binocular cells

Answer 21

Half of the cells in V1 receive input from LGN from both eyes (binocular). The receptive fields are virtually identical. Sensitive to and mediate depth perception

Question 22

Complex cells

Answer 22

Have receptive fields like simple cells BUT they abstract for spatial POSITION. The line or edge can be anywhere within the RF.
Created by convergence of several simple cells whose positions are slightly offset. Any simple cell can then cause complex cell to fire.

Simple and complex cells are located together in the same hypercolumns.
(LGNs into layer 4 creating simple cells. Simple cells send axons up and down in same hypercolumn to higher and lower cortical layers, creating complex cells. Output of hypercolumn from 3-6 to higher order visual areas.

Question 23

How many cone types do humans have?

Answer 23

3: blue (short), green (middle), red (long)
A single cone cannot encode color info (need relative activities of 3 types, and NS uses info to create cells in cortex to respond only to particular colors).

Question 24

Parallel processing

Hierarchical processing

Answer 24

Parallel: Dissimilar dimensions (eg color and form) must be analyzed by separate but parallel neural systems.
Hierarchical: As we ascend the visual system, higher order cells survey lower order cells and abstract the collective properties. Eventually, cells that respond only to complete form (eg “face” cells)

Question 25

Two primary parallel pathways

Answer 25

Dorsal pathway: V1 dorsally to parietal lobe, responsible for spatial vision (motion, depth perception)
Ventral pathway: ventrally from V1 to temporal lobe; object vision (color, form, pattern)
(Segregated in V2–dorsal in thick tripe)
Dorsal through V2 and onto V4 or MT (middle temporal).–>lesion in MT: impaired motion and depth perception.

Ventral through stripe and interstripe of V2, then to area V4.
Stripe regions of V2 receive inputs from blobs in V1–cells in blobs only care about COLOR not shape.

Lesions in V4 can result in impaired color discrimination.

Question 26

Ocular dominance

Answer 26

a measure of the relative synaptic input to a cell from each
eye
category 1 cells
are driven only by the eye that is contralateral to the cortical cell that is being recorded from;
category 4 cells are driven equally by both eyes; category 7 cells are driven only by the eye that
is ipsilateral to the cell being studied

Question 27

Monocular deprivation (ocular dominance)

Answer 27

if one eye of a kitten is closed for even a few days, the cortical cells lose virtually all
connections to the deprived eye (no monocular cells for that eye, no binocular cells); if same deprivation done to adult–no effect.

The deprived eye recovers by subsequently covering the opposite eye, during the critical period. But the newly covered eye loses its connections with the cortex.

Question 28

Critical or sensitive period

Answer 28

The period of time when the connections can be altered by visual experience.
In kittens: lasts 3 months, peak sensitivity 4-6 weeks (single day of deprivation can have impact).
Humans: 2-3 years (as little as one week of deprivation can have impact)

Question 29

Binocular deprivation

Answer 29

Should have produced a nearly silent cortex with few synapses from either eye. BUT primary visual cortex was for the most part normal (most cells binocular, but a few more non-responsive cells). Half of the cells had normal receptive fields. The cats were behaviorally BLIND, meaning higher order visual cells were disrupted.
So perhaps a competition between the two eyes exists.

Question 30

Strabismus

Answer 30

One extraocular muscle (medial rectus ) was cut in one eye of a kitten. So eye deviates laterally. Result in the cortex: very FEW BINOCULAR cells. Almost all cortical cells driven exclusively by one eye or the other.
Cells that fire together wire together.

Question 31

More recent experiments

Answer 31

Radiolabeled aa injected into one eye, incorp into proteins, transported to cortex. See LGN axons go to layer 4 of visual cortex. Early: one band, later alternating bands showing ocular dominance/segregation.

TTX on optic nerves.–>No ocular dominance columns. But if stimulation of tracts electrically leads to normal ocular dom pattern (synchronous) or monocular pattern (asynchrnous stim). No sync no link.

Blocking NMDA receptors during critical period interferes with normal ocular dominance. (Needs coincident pre and post syn firing. Release of trophic factor by post syn to pre syn to survive)

Question 32

Monocular deprivation

short

Answer 32

suggests disuse atrophy (use it or lose it)

Question 33

binocular deprivation

short

Answer 33

shows competition between converging synaptic input from the two eyes, not disuse atrophy

Question 34

Artificial strabismus (short)

Answer 34

Cells that fire together wire together

Question 35

–Questions–

Answer 35

1. What are the major steps in phototransduction in the outer segments of rods?
2. What are the receptive field properties of retinal ganglion cells?
3. What are color-opponent ganglion cells?
4. Where is color processed in the cortex?
5. What are the receptive field characteristics of cortical simple and complex cells? How are
   these receptive field properties achieved by synaptic inputs from lower order cells?
6. Draw the major features of a hypercolumn.
7. Describe the meaning of a sensitive period in the development of the cortex. Indicate the
   importance of this concept in diagnosing and treating abnormal development.
8. Explain the meaning of “ocular dominance” in cells of the visual cortex and indicate its
   physical basis. Discuss the changes in ocular dominance caused by monocular deprivation,
   binocular deprivation, and alternating monocular deprivation.
9. Indicate the conditions under which the effects of abnormal developmental experiences can be
   reversed.