Exam 2 - Problem Set Questions

Question 1

Much of our visual perception is organized by higher cortical structures, but visual processing begins prior to reaching the cortex. Please list the structures in the pathway from the light entering the eye to the primary visual cortex, V1/striate cortex/area 17.

Answer 1

Retina (photoreceptors [rods and cones], bipolar cells and ganglion cells), optic nerve, optic chiasm, optic tract, dorsal lateral geniculate nucleus in the thalamus, optic radiations to V1

Question 2

What types of cells are in the retina?

Answer 2

M-type ganglion cells
P-type ganglion cells
non-M-non-P ganglion cells

Question 3

What are the layers that the LGN projects to in the magnocellular pathway, parvocellular interblob pathway, and blob pathway?

Answer 3

M-type ganglion cells project to the magnocelullar layers 1 and 2 of the LGN

P-type ganglion cells project to the parvocellular layers 3, 4, 5, 6

non-M-non-P ganglion cells project to the koniocellular layers that lie in between each layer of the LGN

Question 4

LGN projects to what areas in what pathways?

Answer 4

LGN project to the magnocellular layers 1,2
and
parvocellular layers 3,4,5,6
and
koniocellular layers

Question 5

Where does the V1 project to in the magnocellular, parvocellular-interblob pathway, and blob pathway?

Answer 5

V1 projects to the layer IVC-alpha and layer IVB (magnocellular pathway)

V1 projects to the layer IVC-beta and interblob

V1 projects to blobs

Question 6

What areas do the extrastriate areas project to in the magnocellular pathway, parvocellular interblob pathway, and blob pathway?

Answer 6

In the magnocellular pathway, extrastriate areas project to the V2, V3, MT, MST and posterior parietal lobe

In the parvocellular-interblob pathway, extrastriate areas project to V2, V3, V4, IT

In the blob pathway, extrastriate areas project to the V2, V3, V4

Question 7

What is the functional specialization of the magnocellular pathway, parvocellular-interblob pathway, and the blob pathway?

Answer 7

In the magnocellular pathway:

1. Motion processing (where is it?)
2. directing eye movements
3. navigation

In the parvocellular pathway:

1. Shape perception (what is it?)
2. Face recognition

In the blob pathway:

1. color perception

Question 8

Retinal ganglion cells, cells of the LGN, cells in the primary visual cortex and cells in the dorsal and ventral visual processing streams all have receptive fields. What are receptive fields?

Answer 8

That area of the retina, that, when stimulated by light energy, changes the membrane potential or AP firing pattern of a cell in the visual system.

Question 9

What are orientation columns and where are they found?

Answer 9

Cells oriented in columns that are most responsive to bars of light at a specific angle presented to their receptive fields—all cells in each vertical column are optimally sensitive to the same angle of orientation. Found in V1 (or striate cortex, or primary visual cortex).

Question 10

Both simple and complex cells respond to changes in light in their receptive fields. How are simple cells and complex cells different? What do they process

Answer 10

Simple cells have specific “on” and “off” areas in their receptive fields. Edge detectors (for simple cells)

complex cells do not have “on” and “off” areas. Coding for more complex shape (one possible difference).

Question 11

From the Sacks article, Dr. P is a patient who has a unique problem processing visual information. What area of Dr. P’s visual system do you think is malfunctioning and why? Do you think there may be more than one area affected, why or why not?

Answer 11

In light of the symptomatology of being unable to recognize faces, it is most likely that the ventral extra-striate areas for face recognition were affected. It is also possible that dorsal extrastriate areas for visuospatial processing were affected as he made some mistakes locating objects in space and had trouble processing more than one object at a time, as evidenced by his inability to perceive scenes.

Question 12

The ____ is the region of the retina with the greatest acuity, but relatively poor light sensitivity in comparison to the _____ retina

Answer 12

The __fovea__ is the region of the retina with the greatest acuity, but relatively poor light sensitivity in comparison to the __peripheral___ retina

Question 13

Which cells in the retina can fire action potentials?

a) ganglion cells
b) rod photoreceptors
c) bipolar cells
d) horizontal cells

Answer 13

a) ganglion cells

Question 14

Which cells mediate the center-surround antagonism characteristic of most bipolar cells?

a) ganglion cells
b) rod photoreceptors
c) cone photoreceptors
d) horizontal cells

Answer 14

d) horizontal cells

Question 15

With regards to the cell highlighted in bold letters, write “depolarization” or “hyperpolarization” in the following blanks (for example, an increase in light striking a photoreceptor causes a hyperpolarization of the photoreceptor, so the answer for (a) is “hyperpolarization”):

a) an increase in light striking a _________
b) a decrease in glutamate released onto an _________
c) an increase in glutamate released onto a _________
d) a decrease in glutamate released onto an _________

Answer 15

4. With regards to the cell highlighted in bold letters, write “depolarization” or “hyperpolarization” in the following blanks (for example, an increase in light striking a photoreceptor causes a hyperpolarization of the photoreceptor, so the answer for (a) is “hyperpolarization”):
   a) an increase in light striking a photoreceptor hyperpolarization___\_
   b) a decrease in glutamate released onto an ON bipolar cell depolarization___\_
   c) an increase in glutamate released onto a ganglion cell _depolarization__[and increased AP firing rates]
   d) a decrease in glutamate released onto an OFF bipolar cell __\_hyperpolarization____

Question 16

A typical human’s peripheral vision has a greater sensitivity to low levels of light than their foveal vision. Offer a brief explanation that can explain this difference.

Answer 16

Peripheral vision represents responses of areas of the retina outside of the fovea, an area dominated by rod photoreceptors. Rods are more sensitive to light than cones, requiring just a single photon to cause a change in Vm, in comparison to cones that require at least 100 photons [one reason for this is that rods have more rhodopsin per cell than cones].

OTHER equally acceptable response: Receptive fields of ganglion cells in the peripheral retina are larger, meaning that they incorporate the responses from many more photoreceptors. Thus, a single ganglion cell in the peripheral retina can integrate (summate) the responses from many times more photoreceptors in comparison to a ganglion cell in the fovea, allowing for a greater sensitivity to low levels of light.

Question 17

How does a rod respond to an increase in light? Your answer should include the terms “rhodopsin”, “cGMP”, “membrane potential”, and “glutamate”.

Answer 17

Light activates rhodopsin, which then activates a g-protein. This activated g-protein (transducin) activates a phosphodiesterase (PDE) which reduces the amount of cGMP in the photoreceptor. Reduced levels of cGMP result in decreased conductances of sodium channels, resulting in a relatively hyperpolarizing membrane potential. This hyperpolarized membrane potential results in the release of less glutamate from the photoreceptor.

Question 18

How does an OFF bipolar cell respond to an increase in light presented to its receptive field center? Your answer should include the terms “photoreceptor”, “membrane potential”, and “glutamate”.

Answer 18

An increase in light in the center of a bipolar cell’s receptor field causes decreases in the membrane potentials of the respective presynaptic photoreceptors, resulting in less glutamate released by the photoreceptors. The OFF bipolar cell responds to glutamate with excitatory, depolarizing responses (via AMPA_Rs), and thus this reduction in glutamate from presynaptic photoreceptors results in a relative hyperpolarization of the bipolar cell.

Question 19

Offer two explanations for why visual acuity is higher in the fovea than in the periphery of the retina.

Answer 19

1) The photoreceptors in the fovea have a one-to-one relationship to the ganglion output cells; ie, the ganglion cells are only summating the input from one photoreceptor in their receptive field center, and are therefore not averaging the response over a large area of photoreceptors. In other words, the receptive fields of ganglion cells are much smaller in the fovea, allowing greater discrimination between very small regions of the visual field.
2) At the fovea, the axons, blood vessels, and cellular layers of bipolar and ganglion cells are “pushed away”, so that light is less scattered by these structures before it reaches the photoreceptors. Less scattered light means that the image is less blurred when it reaches the photoreceptor layer.

Question 20

Why is color vision lost in scotopic conditions? (Definition of scotopic: relatively low light levels, night-time darkness).

Answer 20

Cones are nearly unresponsive in such low light conditions. Cones are required for color vision, and without their input there is no color vision.

Question 21

Much of our visual perception is organized by higher cortical structures, but visual processing begins prior to reaching the cortex. Please list the structures in the pathway from the light entering the eye to the primary visual cortex, V1/striate cortex/area 17.

Answer 21

Retina (photoreceptors [rods and cones], bipolar cells and ganglion cells), optic nerve, optic chiasm, optic tract, dorsal lateral geniculate nucleus in the thalamus, optic radiations to V1.

Question 22

Describe parallel processing through the M, P, and nonM-nonP pathways beginning with functionally distinct types of retinal ganglion cells, through V1 and into extrastriate cortex. What aspects of visual information processing are analyzed by these different parallel pathways? (fill in the blanks in the table below)

Answer 22

Magnocellular Pathway ==> M-type ==> magnocellular layers 1,2 ==> Layer IVC-alpha, Layer 1VB ==> V2, V3, MT, MST, posterior parietal

motion processing, directing eye movements, navigation

Parvocellular Pathway ==> P-type ==> parvocellular layers 3,4,5,6 ==> Layer IVC-beta and interblob ==> V2, V3, V4, IT

shape perception, facial recognition

blob pathway ==> non-M and non-P ==> koniocellular layers ==> blobs ==> V2, V3, V5

color perception

Question 23

What are the blobs in v1?

Answer 23

Blobs are sections of the visual cortex where groups of neurons that are sensitive to color assemble in cylindrical shapes

Question 24

Retinal ganglion cells, cells of the LGN, cells in the primary visual cortex and cells in the dorsal and ventral visual processing streams all have receptive fields. What are receptive fields?

Question 25

What are orientation columns and where are they found?

Answer 25

Cells oriented in columns that are most responsive to bars of light at a specific angle presented to their receptive fields—all cells in each vertical column are optimally sensitive to the same angle of orientation. Found in V1 (or striate cortex, or primary visual cortex).

Question 26

Both simple and complex cells respond to changes in light in their receptive fields. How are simple cells and complex cells different?

Answer 26

Simple cells have specific “on” and “off” areas in their receptive fields, while complex cells do not.

Question 27

What do simple and complex cells process?

Answer 27

Edge detectors (for simple cells) versus coding for more complex shape (one possible difference).

Question 28

From the Sacks article, Dr. P is a patient who has a unique problem processing visual information. What area of Dr. P’s visual system do you think is malfunctioning and why? Do you think there may be more than one area affected, why or why not?

Answer 28

In light of the symptomatology of being unable to recognize faces, it is most likely that the ventral extra-striate areas for face recognition were affected. It is also possible that dorsal extra-striate areas for visuospatial processing were affected as he made some mistakes locating objects in space and had trouble processing more than one object at a time, as evidenced by his inability to perceive scenes.