Visual System

Question 1

How does light activate photoreceptors?

Answer 1

Light induces a conformational change in retinal, which binds rhodopsin, a transmembrane receptor in the photoreceptor membrane; binding of retinal to rhodopsin activates a G-protein which activates a PDE that degrades cGMP; this causes closing of cGMP-gated cation channels which causes membrane hyperpolarization

Question 2

How do different bipolar cells respond to glutamate?

Answer 2

ON-center bipolar cells are inhibited by glutamate

OFF-center bipolar cells are excited by glutamate

Question 3

What is the role of horizontal cells in the retina?

Answer 3

Horizontal cells are excited by glutamate released from surround photoreceptors; they send inhibitory axons to center photoreceptors

Question 4

What region of space does the LGN process?

Answer 4

LGN processes information from the contralateral visual field

Question 5

Which layers of the LGN receive information from the contralateral eye?

Answer 5

2, 3, 5

Question 6

Which layers of the LGN receive information from the ipsilateral eye?

Answer 6

1, 4, 6

Question 7

Which layers of the LGN receive input from the magnocellular ganglion cells?

Answer 7

1 and 2

Question 8

Which layers of the LGN receive input from the parvocellular ganglion cells?

Answer 8

3 through 6

Question 9

What information does the magnocellular system carry?

Answer 9

Spatial visual information related to motion and depth

Low acuity, no color vision (input from rods only)

Question 10

What information does the parvocellular system carry?

Answer 10

Object vision - high acuity color vision (input from cones)

Not responsive to motion

Question 11

Which layer of V1 do LGN axons terminate in?

Answer 11

Layer 4

Question 12

What is a hypercolumn?

Answer 12

A vertically stratified micro-region of V1 which receives input from the LGN; each hypercolumn is divided into two ocular dominance columns, each of which receives input from the corresponding region of the retina of the two eyes

Question 13

Simple cells

Answer 13

These are the cells found in layer 4 of primary visual cortex; respond to receptive fields that are shaped like narrow lines at some preferred orientation, flanked on each side by OFF surround area; do not respond to diffuse light

Created by the convergence of several LGN cells with overlapping receptive fields; found in a ‘pinwheel’ orientation

Question 14

Role of binocular cells in the visual cortex

Answer 14

About half of V1 cells receive input from both LGNs and the receptive fields of the two eyes are identical

Binocular cells are found at the borders of the ocular dominance columns; they mediate depth perception

Question 15

Complex cells

Answer 15

Created by the hierarchical convergence of several simple cells with slightly off-set positions of their receptive fields; have receptive fields like those of simple cells except that they detect position and movement

Question 16

Which hypercolumn layers send output from V1?

Answer 16

3 and 6

Question 17

How is color information encoded?

Answer 17

Any given wavelength of light is absorbed to a differing extent by each cone type; the relative activities of the 3 cone types to a single wavelength of light encodes the information about color

Question 18

Where is color discrimination best in the retina?

Answer 18

In the fovea, where all photoreceptors are cones

Question 19

Color-opponent ganglion cells

Answer 19

Receive input from bipolar cells which are connected directly to one kind of cone in the field center and indirectly, via horizontal cells, to cones of a different color preference in the field surround

Question 20

Dorsal visual pathway

Answer 20

Carries visual information from the magnocellular system (visuospatial) from V1 to the parietal lobe; responsible for motion and depth perception

Question 21

Ventral visual pathway

Answer 21

Carries visual information from the parvocellular system (color, shape) from V1 to the temporal lobe; responsible for object vision

Question 22

Where does dorsal stream information travel from V1?

Answer 22

Magnocellular information courses from V1 through the “thick stripe” region of V2, to area V5 (MT, middle temporal)

MT is sensitive to direction of visual motion and visual depth; lesions of area MT result in impaired motion and depth perception

Question 23

Where does ventral stream information travel from V1?

Answer 23

Parvocellular information travels from V1 through the “stripe” and “interstripe” regions of V2, to area V4; the stripe regions of V2 receive input from the color-sensitive blob cells of V1

Lesions of V4 can cause impairment of color discrimination

Question 24

What is the critical / sensitive period?

Answer 24

The period time when connections between the retina and cortical cells can be altered by visual experience

In humans, the sensitive period is 2-3 years; as little as one week of monocular deprivation in infants can cause reduced acuity in adult life

Question 25

What is the effect of monocular deprivation?

Answer 25

Loss of connections between cortical cells and the deprived eye; i.e. these cortical cells cannot be driven by visual stimuli and the connections, once lost, do not recover

Question 26

What happens during binocular deprivation?

Answer 26

Primary visual cortex cells are largely normal (most remain binocularly driven); however, the animal is functionally blind, meaning that disruption of vision is occurring at higher visual processing centers

Question 27

What was learned from the strabismus experiment?

Answer 27

Severing the medial rectus muscle of one eye causes visual input from the same point in space to fall on non-corresponding regions of the retina; therefore, cortical neurons do not receive synchronous input from corresponding retinal ganglion cells of the two eyes and so cortical cells develop to respond monocularly rather than binocularly Demonstrates that cortical cells rely on synchronous excitement from both eyes in order to develop binocularly

Question 28

How does the pupillary reflex work?

Answer 28

Light information is carried by the afferent optic nerve, which diverges to synapse bilaterally on the Edinger-Westphal nucleus, which houses preganglionic parasympathetic nerve fibers; these fibers innervate postganglionic parasympathetic fibers in the ciliary ganglion, which mediate pupillary contraction Light shined in one eye produces both constriction of the stimulated eye (the direct response) and the non-stimulated eye (the consensual response)

Question 29

What is the ocular dominance pattern of V1 cortical cells?

Answer 29

7 categories of cortical cells exist Category 1 cortical cells are driven only by the contralateral eye; category 7 cells are driven only by the ipsilateral eye; category 4 cells are driven equally by both eyes