Central Processing

Question 2

What is the functional role of horizontal cells in retinal circuits?

Answer 2

They integrate input from many photoreceptors and provide lateral inhibition, making active photoreceptors less active when neighbors are active, and more active when neighbors are suppressed.

Question 3

How can a single photoreceptor be center for one RGC and surround for another?

Answer 3

Because each bipolar/RGC circuit samples different spatial pools of photoreceptors; receptive field organization depends on wiring rather than location alone.

Question 4

What pathway do retinal ganglion cell axons follow leaving the eye?

Answer 4

They exit through the optic nerve, travel to the optic chiasm, and then reach the lateral geniculate nucleus (LGN) or other subcortical targets.

Question 5

What happens at the optic chiasm?

Answer 5

Axons from the nasal retina cross to the opposite side; axons from the temporal retina remain uncrossed.

Question 6

What is partial decussation?

Answer 6

Only nasal retinal fibers cross, allowing each hemisphere to receive information from the opposite visual hemifield.

Question 7

What visual information does each LGN receive?

Answer 7

The left LGN receives the right visual field; the right LGN receives the left visual field.

Question 8

What happens if the optic chiasm is cut?

Answer 8

Loss of peripheral vision in both eyes (bitemporal hemianopia), resulting in tunnel vision—often caused by pituitary tumors pressing on the chiasm.

Question 9

What are the main targets of RGC axons?

Answer 9

LGN (conscious vision), Superior Colliculus (eye movements), Pretectum (pupillary reflex), Hypothalamus (circadian rhythms).

Question 10

What is the LGN?

Answer 10

A six-layered thalamic relay nucleus for vision that preserves retinotopy and sends structured output to V1.

Question 11

How does retinotopic mapping work in the LGN?

Answer 11

Adjacent positions in the visual field are represented by physically adjacent neurons, preserving spatial organization.

Question 12

What kinds of receptive fields do LGN neurons have?

Answer 12

Center-surround receptive fields similar to retinal ganglion cells, including luminance and color opponency.

Question 13

What is top-down modulation in the LGN?

Answer 13

Input from cortex, attention systems, and brainstem that gates or filters bottom-up retinal signals.

Question 14

What is the primary visual cortex (V1)?

Answer 14

A layered cortical region (striate cortex) in the occipital lobe responsible for the first major stage of cortical visual processing.

Question 15

What is special about V1’s structure?

Answer 15

It contains more layers (9 total sublayers), heavy granularity in layer 4, and a visually striking stripe pattern (stria of Gennari).

Question 16

What are the main cell types in V1?

Answer 16

Pyramidal neurons (output projection cells), spiny stellate neurons (local excitatory cells), and various inhibitory interneurons.

Question 17

What V1 layer receives most LGN input?

Answer 17

Layer 4 (specifically layer 4C).

Question 18

What are the main connection patterns in V1?

Answer 18

Vertical connections dominate, but horizontal connections in layer 3 and 2 integrate information across space.

Question 19

What is retinotopic mapping in V1?

Answer 19

Spatial arrangement of the visual world is preserved; neighboring cortical neurons represent neighboring locations in visual space.

Question 20

What is cortical magnification?

Answer 20

Foveal inputs occupy disproportionately large areas of V1, giving high resolution for central vision.

Question 21

What are ocular dominance columns?

Answer 21

Stripes in V1 where neurons preferentially respond to one eye; fully segregated in layer 4 and gradually integrated in layers above.

Question 22

When does binocularity first emerge in V1?

Answer 22

Layer 3, where input from the two eyes begins to converge.

Question 23

How do ocular dominance columns develop?

Answer 23

Through activity-dependent plasticity: inputs that fire together wire together; asynchronous inputs weaken.

Question 24

What happens if an infant has strabismus?

Answer 24

Lack of coordinated input during the critical period prevents normal binocular fusion, impairing depth perception.

Question 25

What is a critical period for binocular development?

Answer 25

A time window (first ~1 year in humans) when input competition shapes ocular dominance and stereovision.

Question 26

What do receptive fields in V1 look like?

Answer 26

Many V1 neurons respond not to spots but to bars, edges, or gratings at specific orientations.

Question 27

What are simple cells in V1?

Answer 27

Neurons with spatially segregated ON and OFF subregions that respond best to bars of light at a specific orientation and position.

Question 28

What do simple cell receptive fields represent?

Answer 28

Oriented edges created by aligned LGN center-surround inputs.

Question 29

What is an example of a simple cell response?

Answer 29

A light bar in the correct orientation and location produces strong firing; incorrect orientation produces weak firing.

Question 30

What are complex cells in V1?

Answer 30

Neurons that respond to oriented edges regardless of exact position within the receptive field; integrate across many simple cells.

Question 31

What does it mean that V1 cells are selective for orientation?

Answer 31

They prefer edges of a certain angle, forming the basis of shape and contour perception.

Question 32

What is the role of adaptation in sensory systems (vision, hearing, smell)?

Answer 32

To adjust sensitivity to background levels so changes become more detectable; prevents overstimulation and increases dynamic range.

Question 33

What happens when horizontal cells inhibit photoreceptors strongly?

Answer 33

Photoreceptors release more glutamate despite hyperpolarization, strengthening center–surround contrast.

Question 34

What is lateral inhibition?

Answer 34

A processing mechanism where active neurons suppress neighboring neurons to enhance contrast and edge detection.

Question 35

What is the role of horizontal cells in lateral inhibition?

Answer 35

They gather input from many photoreceptors and send inhibitory feedback to the center photoreceptor.

Question 36

How does lateral inhibition create depth and edge perception?

Answer 36

It sharpens boundaries and produces differential activation on each side of an edge, aiding stereopsis and texture discrimination.

Question 37

What is the role of the superior colliculus in visual processing?

Answer 37

Coordinates rapid eye movements (saccades) and orienting behaviors toward stimuli.

Question 38

What is the role of the pretectum in visual processing?

Answer 38

Controls pupillary light reflex and adjusts pupil size subconsciously.

Question 39

What is the role of the hypothalamus in visual processing?

Answer 39

Receives light input to regulate circadian rhythms via the suprachiasmatic nucleus.

Question 40

What distinguishes V1 from V2 and other extrastriate areas?

Answer 40

V1 performs initial feature extraction (orientation, spatial frequency), while higher areas analyze motion, color, depth, and object identity.

Question 41

What is the significance of LGN–V1 parallel pathways?

Answer 41

Different streams (parvocellular, magnocellular, koniocellular) handle color, form, or motion separately before integration.

Question 42

What is the parvocellular pathway specialized for?

Answer 42

High-resolution form, color, and detail from cone-rich regions.

Question 43

What is the magnocellular pathway specialized for?

Answer 43

Motion, flicker detection, and low-contrast sensitivity.

Question 44

What is the koniocellular pathway specialized for?

Answer 44

Blue–yellow color signals and other specialized functions.