vision

Question 1

What two structures provide the refractive and focusing power of the eye?

Answer 1

• Cornea - 2/3
• Lens - 1/3
○ Under neural control and allows for focusin of nearby objects

Question 2

How is the pupil’s size controled?

Answer 2

• Cilliary muscles

Question 3

What are the output neurons of the retina?

Answer 3

• Retinal ganglion cells
  
  • Group together at the optic disk forming the optic nerve
  • Each optic nerve contains about a million axons

Question 4

The retina contains 5 types of neurons. What are they?

Answer 4

• Rods (work in low-light) - more of these
  
  • Cones (color vision)
  • Bipolar cells (receive electrical signal from photoreceptors)
  • Horizontal cells (receive electrical signal from photoreceptors)
  • Ganglion cells (output cells0

Question 5

Do photoreceptor cells have a larger intracellular concentration of cGMP in the dark or in the bright light?

Answer 5

• In the dark, since bright light will result in lots of phosphodiesterase activity and reduction of cGMP in the photoreceptor cell

Question 6

In terms of action potentials, which cells of the retina communicate with Aps?

Answer 6

• Only the ganglion cells make action potentials
  
  • All of the other retinal cells communicate by graded changes in the membrane potential
  • This alters the rate of exocytosis of NT in a graded fashion

Question 7

The size of the receptive field likely indicates position how?

Answer 7

• In the fovea, a ganglion cell receptive field center may be only as wide as a single cone, with an antagonistic surround not much bigger
  
  • Receptive fields are larger in the periphery of the retina

Question 8

What are the on vs. off center ganglion cells?

Answer 8

• On center ganglion cells are excited by light shining in their centers, while being inhibed by light in the periphery
  
  • The off center ganglion cells are the opposite

Question 9

How does the retina “process” information so early?

Answer 9

• It doesn’t care about absolute levels of illumination, only changes in brightness
  
  • It is the first stage in building specific detectors for edges, corners, shapes and faces
  • It cares about contrast

Question 10

what is the AP generation state of a ganglion cell at rest?

Answer 10

Ganglion cells fire action potentials spontaneously in the dark, and so are very sensitive to slight changes in both excitatory and inhibitory inputs.

Question 11

for vision sensory transduction, what are the important rules to keep in mind?

Answer 11

• Photoreceptors are hyperpolarized by light, resulting in less neurotransmitter release
• Photoreceptors release glutamate, but…
• Bipolar cells can be either excited (OFF-center) or inhibited (ON-center) by glutamate (due to different receptor types)
• Bipolar cells always make excitatory synapses on ganglion cells

Question 12

bi-polar cells and ganglion cells are matched how?

Answer 12

ON-center and OFF-center ganglion cells are determined by ON-center and OFF-center bipolar cells. ON-center bipolar cells are inhibited by glutamate while OFF-center are excited.

Question 13

a bipolar cell possesses inhibitor glutamate receptors. What is it’s classificaiton?

Answer 13

If a bipolar cell possesses inhibitory glutamate receptors, then it will be tonically inhibited in the dark, and light will relieve the inhibition, making an ON-center ganglion cell receptive field (Fig 5, left panel, ON-center pathway).

Question 14

how do horizontal cells behave (general)

Answer 14

Horizontal cells behave as though they have excitatory receptors for glutamate released from photoreceptors, and make inhibitory synapses on the neighboring photoreceptors in the field center

Question 15

if a spot of light is shone in the periphery of an ON-center ganglion cell receptive field, what will happen to the membrane potential of the cells in the pathway?

Answer 15

The illuminated photoreceptors in the surround will hyperpolarize, reducing the secretion of transmitter,

• reducing the activation of excitatory receptors on the horizontal cells, which will hyperpolarize the horizontal cells.
• That will decrease their secretion of GABA onto photoreceptors in the field center, and so will decrease inhibition of those photoreceptors in the field center, causing them to release more transmitter onto the bipolar cells.
• Since this is an ON- center 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 16

of the 4 synapses in the vision transduction pathway, 2 are ALWAYS excitatory. What are they?

Answer 16

these are the surround photoreceptor to horizontal cell synapses, and bipolar cell to ganglion cell synapses.

Question 17

2 synapses in the vision pathway are excitatory, but what are the other two?

Answer 17

• One synapse is always inhibitory – horizontal cell to photoreceptor synapses.
• One synapse may be either excitatory (OFF-center bipolar cells) or inhibitory (ON-center bipolar cells) – field center photoreceptor to bipolar cell.

Question 18

what will happen if diffuse light shines on the entire receptive field?

Answer 18

The answer: not much, if the center and surround exactly cancel each other

Question 19

Trace the axons from the retina to the brain

Answer 19

The retinal ganglion cells group together at the optic disk forming the optic nerve
*The optic nerves
from the two eyes merge at the optic chiasm, where about half of the axons from each eye cross to the other side, and then continue on as the optic tract to the lateral geniculate nucleus (LGN) of the thalamus
*At the chiasm, the axons from the nasal half of each retina cross over to the opposite side (decussate).
*The result is that, for example, the right optic tract contains axons
from the right side of each retina, which ‘see’ the left side of the visual world.
*In this way, if you look straight ahead, all of the information to the right of your center of view goes first to your left visual cortex.
*Beyond the LGN, axons involved in visual processing fan out in the optic radiations to the visual cortex at the back of the brain.

Question 20

The left LGN is representative of what visual field?

Answer 20

• the right visual field

* In other words, the LGN represents the contralateral visual field.

Question 21

Give an example of the orderly path of axons from the retina to the cortex

Answer 21

For example, the lower half of each retina (which ‘sees’ the upper half of the visual world) projects to the lower half of each visual cortex (the lower bank of the calcarine sulcus). Clinically, it is important to understand how the retina is connected (via the lateral geniculate nucleus) to the primary visual cortex, since blindness in specific parts of visual space (visual field defects) can be useful in diagnosing the location of brain damage due to injury, strokes or tumors

Question 22

describe the organization of the lateral geniculate nucleus

Answer 22

The ganglion cell axons end in the LGN.

• LGN is 6 layers
• Information from each eye projects to separate layers so that there is no direct interaction between the eyes at the level of the LGN (that is, LGN cells are NOT binocular).
• Layers 1, 4, and 6 receive inputs from the contralateral eye (these inputs decussate at the chiasm)
• while layers 2, 3, and 5 receive inputs from the ipsilateral eye.
• There is also a secondary segregation of information in these 6 layers:
• Layers 1 and 2 receive inputs from the so-called magnocellular ganglion cells while
• layers 3-6 receive inputs from the so-called parvocellular ganglion cells.

Question 23

What is the parvocellular system for?

Answer 23

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 24

what is the magnocellular system for?

Answer 24

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)

Question 25

Describe the micro-region architecture of V1

Answer 25

V1 - primary visual cortex

• Each micro-region of V1, called a hypercolumn, is about 1 mm on a side, receives about 10,000 LGN axons, and possesses the same basic structure
• LGN axons terminate in layer 4

Question 26

how is color information encoded?

Answer 26

color information, which is separated out from spatial information in the retina, is handled in central regions of the hypercolumns called blobs.

Question 27

Using the visual bar example, explain what a simple cell in the visual cortex is

Answer 27

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
*(imagine a convergence of LGN cells with slightly different on-center or off-center RFs that allow for orientation determination)

• Such a cell is best stimulated by a narrow line of light covering all of the center ON area
• diffuse light is ineffective
• the spatial position and orientation of the line is crucial

Question 28

Summarize the simple cell receptive field

Answer 28

Visual cortex discussion:
*In summary, simple cells have receptive fields with antagonistic flanking regions; the shape of the field is a straight line (the longer the line the better, up to a maximum length, beyond which no further change occurs), and the orientation of the line is crucial.

Question 29

Describe the synaptic circuitry of a simple cell that detects the orientation of a bar

Answer 29

Imagine several overlapping ON-center LGN (and ganglion cell) receptive fields, lined up along a diagonal.

• Suppose three of those cells converge on one cortical cell in area V1, and that each input is excitatory.
• The cortical cell will then have a receptive field that is the sum of the LGN cells’ receptive fields
• In this example, its ON-center will be a diagonal line, with flanking OFF regions.

Question 30

Convergence of several LGN processes on one simple cell is called what?

Answer 30

This is called hierarchical processing: Several cells with similar but spatially offset receptive fields converge on a higher order cell to create an altogether new type of receptive field

Question 31

What’s up with binocular cells in the visual cortex?

Answer 31

In about half of the cells recorded from V1, the cell receives input from the LGN on one side (monocularly driven by contra or ipsi eye, Fig 9). The other half receive inputs from the LGN from both eyes (binocular), and the receptive fields of the two eyes are virtually identical - same orientation, same region of retina, same width of line, same on-off organization. Binocular cells tend to be found at the boarders of the occular dominance columns (Fig 9). Binocular cell are sensitive to and mediate depth perception.

Question 32

if you orient several simple cells in series that have overlapping receptive fields you can create what?

Answer 32

“Complex” cells have receptive fields like simple cells, with one big exception: they abstract for position.

• That is, while simple cells require a line or edge (of a specific orientation, with specific ON and OFF regions) at a particular position in the visual field, complex cells are not so finicky about the position.
• The line or edge can be anywhere within the RF, and these cells especially like to see lines or edges moving across the field.

Question 33

*How are cells with complex receptive fields created?

Answer 33

The answer is: by convergence of several simple cells whose positions are slightly offset (Fig 11). The converging simple cells make excitatory synapses on the complex cell, and any single simple cell can cause the complex cell to fire.
*Simple cells send axons up and down in the same hypercolumn to higher and to lower cortical layers, creating complex cells.

Question 34

what is the output of the hypercolumns?

Answer 34

The output of each hypercolumn, which contains about five times more axons that does the LGN input, exits from layers 3 and 6 to higher order visual areas.

Question 35

Of all the types of neurons involved in vision transduction, which are orientation selective?

Answer 35

simple and complex cells

Question 36

Of all the types of neurons involved in vision transduction, which are binocularly driven?

Answer 36

simple and complex cells

Question 37

Of all the types of neurons involved in vision transduction, which are position sensitive?

Answer 37

photoreceptor
ganglion cell
simple cell
NOT complex cell

Question 38

What is the Receptive field shape of the different visual cells?

Answer 38

Photoreceptor - tiny spot
Ganglion cell - donut
Simple cell - bar
Complex - edge

Question 39

one cone cannot see color. How then can humans see color?

Answer 39

It is from the relative activities of the 3 cone types by single wavelengths of light that the nervous system extracts the information it needs to create cells in the cortex that respond only to particular colors.

Question 40

what are the different type of opponent cells?

Answer 40

In addition to red-green opponents, there are blue-yellow opponent cells, thereby spanning the entire spectrum (the yellow selectivity is created by converging both red and green cones).

Question 41

What’s up with opponent cells?

Answer 41

In the fovea, where color discrimination is best because all of the photoreceptors there are cones, most of the bipolar cells are connected directly to one kind of cone in the field center (e.g., a red cone) and indirectly (via horizontal cells) to cones with a different color preference (e.g., green cones) in the field surround, thereby creating a RED ON-center and GREEN OFF-surround receptive field, which is passed along to the ganglion cells.

Question 42

of the two primary parallel pathways/streams through the ascending visual systems what does the dorsal pathway do?

Answer 42

The Dorsal Pathway travels from V1 dorsally to the parietal lobe and is generally believed to be responsible for spatial vision, including motion and depth perception.

Question 43

What are the names of the two streams of ascending visual systems?

Answer 43

Dorsal and Ventral Pathway

• dorsal - spatial, motion, depth
• ventral - object, color, form, pattern
• Although these two pathways are separated out spatially in area V2, they have their beginnings in the retina – the magnocellular and parvocellular pathways project to separate layers in LGN and from LGN project to different layers in V1
• These pathways remain segregated in the output pathways from V1 to V2 (Fig 13).

Question 44

What does the ventral ascending visual system do?

Answer 44

The Ventral Pathway travels ventrally from V1 to the temporal lobe and is generally believed to be responsible for object vision, including color, form, and pattern vision.

Question 45

The dorsal ascending pathway in the visual system ends up at V5 after going through the thick stripe region of V2. What is V5?

Answer 45

V5 is often called MT (middle temporal). 95% of cells in MT are selective for the direction of visual motion, and many of those neurons are also sensitive to visual depth.

• Lesions of area MT result in impaired motion and depth perception.
• Recent studies have shown that the activity of neurons in MT correlates directly with the perception of visual motion and/or depth.

Question 46

Part of the job of the ventral pathway is color perception. On it’s way to V4 it gets general color information from what cells?

Answer 46

Blobs in V1. cells in the blob areas do not care about shape, only color.
*These cells do not have center-surround anatomy, but are simpler: their receptive fields comprise a uniform area of retina within which light of one color excites the cell and light of another color inhibits the cells.

Question 47

Give an example of how a blob cell can carry information from several lower-order cells

Answer 47

Color- only blob cells receive input from many color-opponent neurons. For example, a red-ON green- OFF cell gets excitatory synaptic inputs from both red ON-center green OFF-surround cells and from green OFF-center red ON-surround cells, as well as inhibitory synaptic inputs from green ON-center red OFF-surround cells and red OFF-center green ON-surround cells.
*The positions of all of these fields overlap entirely, so there is no spatial information in the color-only blob cell’s response.

Question 48

What goes on in V4?

Answer 48

V4, anterior and inferior to the primary visual cortex (V1), cells have relatively large receptive fields in the central areas of the retina (where cones predominate) and respond only to fairly narrow bands of wavelengths over the visible spectrum, some as narrow as 10 nm. Given a little lateral inhibition at a higher level of color processing, color cells with bandwidth selectivities as narrow as our perceptions are (~3 nm) could easily exist. Lesions in V4 can result in impairments in color discrimination.

Question 49

What is proposagnosia?

Answer 49

cases have been reported in which patients lose the ability to recognize specific faces, , while retaining the ability to recognize parts of a face - nose, lips, and so forth. (proposagnosia

Question 50

Of the seven categories of ocular dominance, which are monocularly or binocularly driven?

Answer 50

• 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.
• Thus, categories 1 and 7 contain monocularly driven cells, while all other categories contain binocular cells of varying eye preference.

Question 51

Is ocular dominance primarily genetically determined or determined by visual experience?

Answer 51

• uh, both. Newborns have ocular dominance pathways in place from birth
• however, depriving the experience of binocular vision rewires these cortical cells

Question 52

We know that monocular deprivation during the sensitive period of a young mammal means there is rewiring of the cortex to favor one eye. What about recovery? If one eye is closed for 3-6 days, and then reopened for the duration of the sensitive period, does the cortex recover?

Answer 52

No, it does not recover. The connections, once lost, are gone for good.

Question 53

we can think of the sensitive period as a kind of competition. Why is that?

Answer 53

*imagine deprived eye is reopened, the other eye is deprived (still during the critical period)

• the reopened eye recovers, and the newly deprived eye loses its connections with the cortex.
• This experiment further illustrates the competition - active suppression by the active eye - that occurs between converging inputs from each eye in the cortex.

Question 54

How does the strabismus experiment further reinforce the competition theory?

Answer 54

• during the sensitive period kittens had one eye’s medial rectus muscle cut
• the end result is cortical cells only monocularly driven, by one eye or the other
• neurons that fire together wire together
• normally there is synchronous stimulation in eyes with overlapping visual fields, but not in this case so one eye wins everything in the competition

Question 55

What is the anatomical correlate of plasticity?

Answer 55

At birth, LGN axon terminals commingle, and a broad white stripe marks layer 4.
*Over time, the LGN terminals sort themselves into discreet bands characteristic of the adult animal, and layer 4 shows a banded appearance

*These are the ocular dominance columns of the adult animal, and the technique provides striking confirmation of the electrophysiological experiments.

Question 56

how does the tetrodotoxin experiment further confirm “no sync, no link”?

Answer 56

• TTX block of optic chiasm - no stimulus and thus cortical columns reflect blindness
• TTX block with upstream synchronous stimulation - results in normal cortical map
• TTX block with asynchronous upstream stimulation - results in strabismus profile of all cortical columns monocularly driven

Question 57

What is the biochemical basis of “no sync no link”?

Answer 57

Indeed, blocking NMDA receptors chronically during the critical period interferes with the normal emergence of ocular dominance. The coincidence of pre- and postsynaptic firing activates NMDA receptors, and calcium ions enter the postsynaptic cell, where they trigger processes that lead to strengthening of those synapses. Evidence suggests that the postsynaptic cell releases a trophic factor that diffuses back across the synapse to the presynaptic terminal.
The presynaptic terminal needs the trophic factor to survive, but evidently can take it up only if it has been recently active. Thus, terminals that do not fire in synchrony miss out on the opportunity, wither, and die.

Question 58

Describe the pupillary light reflex

Answer 58

When bright light shines on the retina, the muscles in the iris contract, reducing the size of the pupil and therefore reducing the amount of light entering the eye.
*Action potentials in ganglion cells excite CNS neurons that diverge to and excite preganglionic parasympathetic motor neurons on both sides of the brain, which leads to excitation of the muscles in the iris.

Question 59

If the right optic nerve is fully cut, what is the resulting deficit in the visual field?

Answer 59

• right eye visual field is fully lost

* left eye visual field is maintained

Question 60

if the optic chiasm is cut in the midline, what is the resulting deficit in the visual field?

Answer 60

the peripheral vision of both eyes is lost

*only the midline vision of each eye is retained

Question 61

If the right optic tract (distal to the optic chiasm decussation) is cut, what is the resulting visual field deficit?

Answer 61

• right eye visual field loses its medial field but retains everything lateral
• the opposite for the left eye, which loses lateral vision but maintains medial vision

Question 62

If the right optic radiation is partially severed (dorsal fibers only), what is the resulting visual field deficit?

Answer 62

Here you are starting to get to higher order cells, so it’s not that simple

• example given was the left eye loses the top-lateral field and the right eye loses the top medial field
• the point is that there is segregation of the visual system even to higher order structures
• in the optic radiation information is from one certain area of the entire shared visual field (in this case persons top left field)

Question 63

What is the resulting visual field deficit if the entire optic radiation on one side (right) is cut before the striate cortex?

Answer 63

• a “hemisphere” of the entire shared visual field is lost

* the left eye loses the lateral vision field, the right eye loses the medial visual field

Question 64

Why, when the optic nerve is cut on the right side, do you retain the right eye’s lateral vision?

Answer 64

It has to do with what fibers cross the midline

• in the right eye, the lateral field of vision shines on the medial retina
• the medial retina fibers cross the midline at the optic chiasm to join the lateral retinal fibers of the left eye
• the brain wants to preserve the visual fields to higher order vision centers