Lecture 2 - visual pathways 1

Question 1

What are the three types of anatomically defined ganglion cells?

Answer 1

• form around 90% of all optic nerve fibres
• midget, parasol, and bistratified ganglion cells

Question 2

Midget cells

Answer 2

Input area: small
polarity: ON and OFF
Number: 70%
Inputs: L and M cones

Question 3

Parasol cells

Answer 3

Input area: large
Polarity: ON and OFF
Number: 10%
Inputs: L and M cones

Question 4

Bistratified cells

Answer 4

Input area: from 2 areas
Polarity: ON
Number: 5-10%
Inputs: S cones (and L and M)

Question 5

what are the types of physiologically defined ganglion cells

Answer 5

• magnocellular cells
• parvocellular cells
• koniocellular cells

Question 6

magnocellular cells

Answer 6

• 10% in total
• have a large receptive field (similar to parasol cells)
• produce fast responses
• not sensitive to colour of light (just care about difference between center and surround)
• some are sensitive to directions of visual motion
• sensitive to low contrasts but they saturate when the contrast is high

Question 7

Parvocellular cells

Answer 7

• 70% in total
• smaller receptive field than magno (similar to midget cells)
• more sensitive to the form and fine details of visual stimuli
• respond poorly to low contrast but do not saturate at high contrasts
• sensitive to differences in wavelength of light
• most have colour opponent center-surround receptive field for red-green

Question 8

koniocellular cells

Answer 8

• 8-10%
• similar to bistratified cells
• they are the only cell class known to carry the S-cone signal
• low contrast sensitivity

Question 9

Rod pathways

Answer 9

• pathway starts with many rods converging information to the rod bipolar cell
• rod bipolar cells do not contact ganglion cells directly
• they contact amacrine cells which spread out the rod information before conveying on ganglion cells

Question 10

what does the optic nerve do

Answer 10

• transmits visual information to the brain
• exits the eye at optic disk
  the two optic nerves come together at the optic chiasm

Question 11

what is the thalamus

Answer 11

• an essential link in transfer of sensory information to the cerebral cortices
• the main visual component of the thalamus is the dorsal lateral geniculate nucleus

Question 12

what are the layers of the lateral geniculate nucleus

Answer 12

• 6 layers
• Layers 1 and 2 = magnocellular (large cells)
• Layers 3 to 6 = parvocellular (small cells)
  koniocellular cells in between layers

Question 13

General info about LGN

Answer 13

• each layer contains excitatory and inhibitory neurons
• each cell receives most of its retinal input from single retinal ganglion cell
• 90% of retinal outputs terminate in the LGN

Question 14

where do the magno-, parvo- and konio-cellular cells project inputs to in V1

Answer 14

magnocellular to 4Ca
parvocellular to 4Cb
koniocellular to CO blobs in V1

Question 15

cerebral cortex

Answer 15

• the outer layer of neural tissue in humans
• its divided into two cortices which are highly convoluted

Question 16

the primary visual cortex is also known as the…

Answer 16

striate cortex

Question 17

where does the information leaving the lateral geniculate nucleus go to

Answer 17

the primary visual cortex

Question 18

what are the layers of the cortex

Answer 18

• 6 layers
• main input is layer 4
• main projection is layer 2/3
• main feedback layer is layer 6

Question 19

what are the output layers of the visual cortex

Answer 19

1) extrastriate cortex - different pathways predominantly from layers 2/3 and 4b

2) LGN - feedback comes primarily from layer 6

3) superior colliculus- predominantly from layer 5

Question 20

what are the basic organisations of V1

Answer 20

1. retinotopic organisation
2. ocular dominance
3. orientation selectivity
4. direction selectivity
5. depth selectivity
6. colour selectivity

Question 21

retinotopic organisation

Answer 21

• nearby cells in V1 represent nearby locations in the visual field in other words
• neighbouring regions of the retina are represented by neighbouring in v1

Question 22

Ocular dominance

Answer 22

• Axons from the different layers of the LGN (eyes) go to separate “zones” in the cortex, forming “ocular dominance columns”
• responsible for integration of visual information from both eyes. they play a crucial role in binocular vision and depth perception.

Question 23

orientation selectivity

Answer 23

• certain cells are more responsive to horizontal, vertical or diagonal orientations.

Question 24

what are simple cells

Answer 24

1) elongated antagonistic RFs
2) highly selective for size of stimulus (SF) and orientation
3) small RFs -> likes high SFs
4) large RFs -> likes low SFs
5) RFs sizes increase with eccentricity
6) sensitive to spatial phase

Question 25

Complex cells

Answer 25

• complex cells sum up multiple simple cells to produce a receptive field that has overlapping ON and OFF responses
• larger RF size
• insensitive to phase
• usually more transient than simple cell

Question 26

Hypercomplex cells

Answer 26

• Hypercomplex cells are length tuned – they respond maximally to a defined length and are inhibited if the stimulus is longer than this

Question 27

direction selectivity

Answer 27

• Direction selectivity mostly found in layer 4B in V1.
• This layer projects out of V1 to V2 and also directly to MT – two regions involved in motion processing