Vision I-III - Filley

Question 1

Identify where color processed in the cortex.

Answer 1

The blobs detect and sort the color, but “processing” seems to occur in V4.

[In area 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 2

What is meant by “dark current”? What abolishes the dark current?

Answer 2

In the dark, a nonspecific cation channel (Na, really) is held open by high levels of cGMP, creating a resting potential of around -40mV. Activation of rhodopsin (Vitamin A, cis/trans retinoic acid) activates transducin, which binds a phosphodiesterase which hydrolyzes cGMP, forcing the gate shut.

A separate K+ channel remains open, and K+ leaves the cell, thus hyperpolarizing it to around -70mV.

Question 3

Of the 5 cell types in the eye (photoreceptors, bipolar cells, horizontal cells, amacrine cells, ganglion cells) which make APs? How do the others communicate?

Answer 3

Gangion cells are the only cells to create APs.

All others use graded potentials.

Question 4

For an ON-center cell, the receptors of the bipolar cell are?

Answer 4

Inhibited by glutamate.

Question 5

For an OFF-center cell, the receptors of the bipolar cell are?

Answer 5

Excited by glutamate.

Question 6

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 6

Peripheral photoreceptors hyperpolarize–> reduced secretion of Glu onto the horizontal cell–> decreased secretion of GABA by the horizontal cell–> increased release of Glu by the ON center photoreceptor–> bipolar cell is further inhibited by Glu–> decreased AP fired by the ganglion cell.

Question 7

Horizontal cells have what type of receptors to glutamate (excitatory/inhibitory)?

Answer 7

Excitatory. So decreased release of Glu from a hyperpolarization will lead to less excitation, and less GABA release.

Question 8

Rhodopsin is located in the ____ of rods and the ____ of cones.

Answer 8

internal membranous sacks; surface membrane infoldings

Question 9

The parvocellular system accounts for ____, and arises from layers ____ at the level of the LGN.

Answer 9

Object Vision – color, form, detail; 3-6

Question 10

The magnocellular system accounts for _____, and arises from layers ____ at the level of the LGN.

Answer 10

Spatial Vision – Motion and depth, 1 and 2

Question 11

Damage to the right Meyer’s loop leads to what type of visual field damage?

Answer 11

Upper left quadrant of visual field is lost (bilateral).

Question 12

Cleavage of the optic nerve on the right will lead to?

Answer 12

Unilateral loss of vision in the right eye.

Question 13

Cleavage of the optic tract at the chiasm will lead to?

Answer 13

Loss of lateral visual fields bilaterally

Question 14

Cleavage in the L optic tract will lead to?

Answer 14

Loss of the right visual fields bilaterally

Question 15

The primary visual cortex is located where? What Broadmann area?

Answer 15

Surrounding the calcarine fissure/sulcus.

broadmann area 17

Question 16

Differentiate hierarchical and parallel processing.

Answer 16

hierarchical processing–using successive synaptic integrations of highly specific synaptic inputs to construct higher and higher levels of representation of the retinal image until eventually we have cells that respond only to the complete form of an object (e.g., “face” cells.).

Parallel processing is simply the requirement that dissimilar dimensions (e.g. color and form) must be analyzed by separate, but parallel, neural systems.

Question 17

Dorsal Pathway travels from_____ and is responsible for____

The Ventral Pathway travels from ____ and is responsible for ____.

Answer 17

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

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

[Dorsal: V1–V2–MT (aka V5, aka “middle temporal)]
[Ventral: V1–V2–V4]