Visual Processing (Dr. Bianco)

Question 1

What is receptive field (RF) ?

Answer 1

A RF is the region in space (part of body, visual field or auditory space) in which the presence of a stimulus will affect the electrical activity (change in MP or firing rate) of a neuron.

Question 2

How are visual RFs measured ?

Answer 2

In degrees angle subtended by incident light that stimulates the neuron.
θ=2arctan(h/2D)

Question 3

What discovery about GCs did Stephen Kuffler make in 1953 ?

How are their RFs referred to ?

Answer 3

That there were “ON centre” and “OFF centre” GCs.
The center and surround have opposite ON vs OFF responses.
Ganglion cell RFs are therefore referred to as Centre Surround Antagonistic Receptive Fields (CSARFs)

Question 4

How are centre-surround receptive fields formed ?

Answer 4

By lateral inhibition from HCs.

Question 5

How does lateral inhibition work ?

When does this system work best ?

Answer 5

GABA release from horizontals cells increases the hyperpolarization of the photoreceptor that it connects to, enhancing the response of the postsynaptic bipolar cells to light.
GABA release is max when surround region is DARK.

Question 6

When is RGC activity maximal ?

Why is this the case ?

Answer 6

When the difference in light levels in centre compared to surround is greatest. This is contrast enhancement.

Question 7

What are the 2 major population of RGCs ?

What are there properties ?

Answer 7

Parvocellular/midget RGCs:
1. Can be either ON or OFF centre
2. Have sustained responses to light
3. Small physical size and small receptive fields
4. Occupy mostly the centre of the visual field (fovea)
5. Carry information about colour
6. Center and surround sum linearly

Magnocellular/parasol GGCs:

1. Can be either ON or OFF centre
2. Have transient responses to light
3. Large physical size and large receptive fields
4. Are distributed across the whole retina
5. Center and surround do not always sum linearly
6. Receives info from many photoreceptors (via BPs)

Question 8

Which type of RGCs exhibit color opponency ?

Answer 8

Parvocellular RGCs exhibit color opponency (red-green or M-L, and blue-yellow or S (L+M).
These is no such opponency in magocellular RGCs: M, L (and rare S) are mixed only to detect difference in luminance levels

Question 9

What is the opponent “color opponent” theory of color vision ?

Answer 9

The opponent-process theory of color vision suggests that our ability to perceive color is controlled by three receptor complexes with opposing actions. These three receptors complexes are red-green, blue-yellow and black-white.
According to the opponent-process theory, these cells can only detect the presence of one color at a time because the two colors oppose one another. Hence we cannot see greenish-red because the opponent cells can only detect one of these colors at a time

Question 10

Are RGCs uniformly distributed across the retina ?

Answer 10

No, they from mosaics.

Question 11

What are bistratified RGCs ?

Answer 11

Bistratified retinal ganglion cells have been identified only relatively recently. About 10% of all retinal ganglion cells are bistratified cells, and these cells go through the koniocellular pathway. They receive inputs from intermediate numbers of rods and cones. They have moderate spatial resolution, moderate conduction velocity, and can respond to moderate-contrast stimuli. They may be involved in color vision. They have very large receptive fields that only have centers (no surrounds) and are always ON to the blue cone and OFF to both the red and green cone.

Question 12

Where do the axons of RGCs directly project to ?

Answer 12

Superior Colliculus: orienting and avoidance responses, eye movements
Pretectum: pupillary reflex
Lateral Geniculate Nucleus (90% of axons in primate), the main route to visual cortex: visual perception

Question 13

Which RGCs from the retina cross the optic chiasm to go the the contralateral visual cortex ?

Answer 13

RGCs on the nasal retina cross the optic chiasm and project to the contralateral visual cortex while the RGCs on the temporal retina project to the ipsilateral visual cortex.

Question 14

How are the optic pathways to the visual cortex organized ?

Answer 14

All functionally parallel pathways are kept separate on their path to the visual cortex and are anatomically organized by R-eye/L-eye origin, by RGC type and by their location on the retina.

Question 15

Where to RGCs synapse ?

How well is info transmitted ?

Answer 15

RGCs synapse in the LGN w/ LGN neurons (the only synapse in this pathway).
LGN neurons inherit their receptive field properties directly from their ganglion cell inputs, they are thus good relay cells.

Question 16

How is the right LGN structured ?

Answer 16

6 layers:

1: L-eye, magnocellular
2: R-eye, magnocellular
3: R-eye, parvocellular
4: L-eye, parvocellular
5: R-eye, parvocellular
6: L-eye, parvocellular

Question 17

In which cortical layer do most LGN neurons project ?

Answer 17

Layer IV in V1 (striate cortex or area 17 or primary visual cortex).

Question 18

What are ocular dominance columns ?

How can this be proved ?

Answer 18

Ocular dominance columns refers to the axons from thalamic relay cells innervating layer IV in patches that are segregated according to eye origin. Right and left eye inputs alternate.
This can be proved by injective of radio-labelled proline into one eye and observing an autoradiograph obtained from a slice through layer IV of V1. We can observe that bright stripes are innervated by LGN axons from the injected eye.

Question 19

If there are ocular dominance columns, where and how are inputs from both eyes mixed ?

Answer 19

Above and below layer IV via intracortical circuitry.

Question 20

What is retinotopy ?

Answer 20

Retinotopy is retinal mapping, and refers to the fact that adjacent areas of visual space are mapped to adjacent physical areas on the retina and cortex (can be seen in monkey by activity dependent labeling of V1 while a visual stimulus is presented).

Question 21

What is “orientation selectivity” in V1 ?

Answer 21

Orientation selectivity in V1 was a phenomenon discovered by Hubel and Wiesel in 1959-61. This refers to regions in V1 that respond selectively to the orientation of an edge or light bar. These regions have “elongated” ON and OFF sub-regions.
These can form orientation columns, w/ each column having its preferred orientation.

Question 22

What 2 main types of cells have been discovered in V1 and what are their properties ?

Answer 22

Simple cells:
1. Respond best to edges and bars
2. Orientation matters
3. Can show direction selectivity
4. Precise location of stimulus is important
Complex cells:
1.  Respond well to bars, edges
2.  Have orientation preference but location of light
stimulus not important
3.  Cannot be mapped into fixed ON and OFF regions 
4.  Can be direction selective
5.  Formed by inputs from simple cells?

Question 23

How is orientation selectivity in cortical neurons generated if the inputs from retina via LGN are not orientation tuned ?

Answer 23

That’s a an idea for your PhD if you’re really into vision.

Question 24

What model did Hubel and Wiesel propose to explain visual cortex RF properties ?

Answer 24

That there was a convergence of the ON and OFF patterns in V1.
Overlapping ON and OFF centre RGC-LGN inputs could thus contribute to the RF properties of a cortical simple cell.

Question 25

What is direction selectivity ?

Answer 25

Direction selectivity refers to the property of certain neurons in V1 who fire more AP when an object/image is moved in a particular direction. This can lead to the creation of complex direction maps.

Question 26

What are muliple columnar representations ?

Answer 26

These are maps of V1 representating:

• orientation selectivity
• direction selectivity
• ocular dominance
• visual space (retinotopy)