Sensory Coding and Vision

Question 1

What does vision start with

Answer 1

Light

Question 2

Light

Answer 2

Energy wave and particles (photons)

Question 3

Light that reaches the retina passes through:

Answer 3

Retinal Ganglion Cells (RGCs)
Amacrine and Horizontal Cells
Bipolar Cells
Photoreceptors (Rods and Cones)

Question 4

Retinal Ganglion Cells (RGCs)

Answer 4

Send information to the brain

Question 5

Amacrine and Horizontal Cells

Answer 5

Lateral interactions within the retina

Question 6

Bipolar Cells

Answer 6

Carry information from photoreceptors to retinal ganglion cells

Question 7

Photoreceptors (rods and cones)

Answer 7

Transduce light signals

Question 8

Visual Processing in the Retina

Answer 8

Signals from photoreceptors are transmitted through horizontal, bipolar, and amacrine cells to retinal ganglion cells
The RGCs send information down the visual pathway

Question 9

Phototransduction

Answer 9

Light strikes a light-absorbing pigment molecule (rhodopsin) in disc of photoreceptor
Rhodopsin breaks into retinal and opsin
Opsin closes the Na+ gates, hyperpolarizing the photoreceptor
Stops glutamate release
-Glutamate inhibits some bipolar cells and excites other bipolar cells
-Depolarization/hyperpolarization sent onto ganglion cells

Question 10

Rods

Answer 10

Highly sensitive to light- ideal for vision in dim environments
Respond similarly to different light wavelength
Can be viewed as sensitive Black and White sensors

Question 11

Cones

Answer 11

Much less light sensitive, need more light to be activated

Come in 3 types, each sensitive to either red, blue, or green wavelengths

Question 12

Color Blindness

Answer 12

A lack in one or more of the cone pigments causes color blindness

Question 13

Vision in rats and raccoons (Nocturnal)

Answer 13

Very poor color vision- (few cones)

Can’t see red

Question 14

Vision in dogs, cats, squirrels

Answer 14

Dichromatic (2 cone pigments)

Question 15

Vision in primates

Answer 15

Mostly trichromatic

Question 16

Fovea

Answer 16

Vision is the sharpest, corresponds to the center of the gaze
Non-photoreceptor cells pushed aside
Highest density of cones, few rods
Therefore, our focus of gaze is optimized for the day
At night, we are “blind” in the fovea. Look at stars slightly off

Question 17

Receptive fields of RGCs

Answer 17

Light–> photoreceptor releases glutamate

1. Excites some bipolar cells
2. Inhibits other bipolar cells
   - Excited BP cells excite some RGCs
   - These RGCs are excited by light at the center- “on cells”
   - Inhibited bipolar cells inhibit other RGCs
   - These RGCs are inhibited by light at center- “off-cells”

Light hyperpolarized photoreceptor=less glutamate
Light on neighboring photoreceptor depolarizes photoreceptor=more glutamate
Due to lateral inhibition by horizontal and amacrine cells
If all light or all dark, RGCs are silent
Maximal response by RGC is if center opposite surround
1. “On-center off surround”
2. “Off-center on surround”

Question 18

On-center Off-surround

Answer 18

Light in center excites

Light in surround inhibits

Question 19

Off-center On-surround

Answer 19

Light in center inhibits

Light in surround excites

Question 20

On-center Off-surround and Off-center On-surround receptive fields

Answer 20

Enhance sensitivity to edges