Visual System 2

Question 2

Why do sensory systems adapt?

Answer 2

To maintain sensitivity across different background intensities and prevent saturation; adaptation occurs in the retina, ear, and olfactory system.

Question 3

What do retinal output cells (RGCs) encode about light?

Answer 3

Contrast, edges, and spatial structure via center–surround receptive fields.

Question 4

How do intermediate retinal layers transform photoreceptor signals?

Answer 4

Through integration and lateral inhibition mediated by bipolar and horizontal cells.

Question 5

What is a photoreceptor’s receptive field?

Answer 5

A specific point in the visual field where light changes the cell’s membrane potential.

Question 6

Why do RGCs have center-surround receptive fields?

Answer 6

Because limited numbers of RGCs must efficiently encode spatial contrast rather than raw light intensity.

Question 7

What are the two main types of center-surround receptive fields?

Answer 7

ON-center/OFF-surround and OFF-center/ON-surround.

Question 8

What is the purpose of center-surround receptive fields?

Answer 8

To detect edges, contrast, and boundaries—crucial for object recognition.

Question 9

How do center-surround receptive fields create brightness illusions?

Answer 9

Lateral inhibition exaggerates contrast: dark surrounds make centers appear lighter, and light surrounds make centers appear darker.

Question 10

How do RGCs differ from photoreceptors?

Answer 10

RGCs respond to contrast and edges, not absolute light levels; they fire action potentials and integrate larger areas of the retina.

Question 11

How does an ON-center RGC respond to full black or full white illumination?

Answer 11

Minimal change—uniform illumination activates center and surround equally, resulting in little output.

Question 12

What activates an ON-center RGC most strongly?

Answer 12

Light in the center with dark in the surround.

Question 13

What suppresses an ON-center RGC strongly?

Answer 13

Dark in the center with light in the surround.

Question 14

What is the response of an OFF-center RGC to uniform illumination?

Answer 14

Little activity because center and surround cancel out.

Question 15

What strongly activates an OFF-center RGC?

Answer 15

Dark in the center with light in the surround.

Question 16

What suppresses an OFF-center RGC?

Answer 16

Light in the center with dark in the surround.

Question 17

How are center-surround receptive fields built?

Answer 17

Through direct vertical pathways from photoreceptors to bipolar cells and lateral inhibition via horizontal cells.

Question 18

What type of receptors do ON bipolar cells express and how do they behave?

Answer 18

Metabotropic mGluR6 receptors; inhibited by glutamate → depolarize when photoreceptors hyperpolarize in light.

Question 19

What type of receptors do OFF bipolar cells express and how do they behave?

Answer 19

Ionotropic AMPA/kainate receptors; excited by glutamate → depolarize when photoreceptors are depolarized in darkness.

Question 20

What is the central logic behind ON vs OFF bipolar cells?

Answer 20

ON bipolar cells do the opposite of the photoreceptor; OFF bipolar cells follow the photoreceptor’s changes.

Question 21

What sets up the center response in bipolar cells?

Answer 21

Direct synapse from the photoreceptor; glutamate level determines ON/OFF response.

Question 22

What role do horizontal cells play?

Answer 22

Provide lateral inhibition by feeding back onto photoreceptors, shaping surround responses.

Question 23

How does the surround influence bipolar cell activity (general rule)?

Answer 23

Light in the surround opposes the response to center light through horizontal cell inhibition.

Question 24

What is the direct pathway for an OFF-center bipolar cell when the center is dark?

Answer 24

Center photoreceptors depolarize → release more glutamate → OFF bipolar depolarizes.

Question 25

What is the indirect pathway for an OFF-center bipolar cell when the surround is dark?

Answer 25

Surround photoreceptors depolarize → horizontal cells activated → inhibit center photoreceptors → center hyperpolarizes → OFF bipolar hyperpolarizes.

Question 26

What is the direct pathway for an ON-center bipolar cell when the center is dark?

Answer 26

Center photoreceptors depolarize → glutamate increases → ON bipolar hyperpolarizes (inhibited).

Question 27

What is the indirect pathway for an ON-center bipolar cell when the surround is dark?

Answer 27

Horizontal cells activate → inhibit center photoreceptors → center depolarizes → ON bipolar depolarizes.

Question 28

What is the impact of horizontal cell feedback on photoreceptors?

Answer 28

It keeps photoreceptors releasing glutamate even when hyperpolarized, sharpening contrast signals.

Question 29

How does lateral inhibition affect color processing?

Answer 29

Center and surround receive different cone inputs, producing color opponency.

Question 30

What cone inputs do bipolar cells receive?

Answer 30

Center: from one cone type (e.g., blue or a specific red or green cone). Surround: pooled from many cones (mostly red+green → yellow).

Question 31

What are the major combinations of color opponency?

Answer 31

Yellow center + Yellow surround → luminance opponency; Blue center + Yellow surround → blue-yellow opponency; Red center + Green surround or vice versa → red-green opponency.

Question 32

Why do 'reddish-green' or 'greenish-red' not exist?

Answer 32

Red and green cones form opponent pairs; activation of one suppresses the other, so mixed perception is impossible.

Question 33

How does color opponency lead to afterimages?

Answer 33

Prolonged stimulation adapts (fatigues) one opponent side so the opposite color appears when looking away.

Question 34

What is a luminance-opponent RGC?

Answer 34

A cell receiving mixed center and surround cone input (e.g., yellow/yellow), encoding brightness not color.

Question 35

What is a color-opponent RGC?

Answer 35

A cell whose center and surround receive different cone inputs, producing contrast signals for color perception.

Question 36

What combinations do color-opponent cells process?

Answer 36

Blue vs Yellow; Red vs Green.

Question 37

What is parallel processing in vision?

Answer 37

Different visual features (light/dark, red/green, blue/yellow, motion, depth) are processed simultaneously in separate pathways.

Question 38

What is processed in parallel in the retina?

Answer 38

Left vs right eye input, ON vs OFF pathways, and color opponency channels.

Question 39

What is the overall role of horizontal cells in shaping the RGC output?

Answer 39

They create surround inhibition, enabling contrast sensitivity and edge detection.

Question 40

How do intermediate retinal layers shape the visual output?

Answer 40

"Through integration, contrast enhancement, and lateral inhibition by horizontal cells.