The primary visual pathway and cortex

Question 1

describe Visual processing in the dorsal lateral geniculate nucleus (dLGN)

Answer 1

-dLGN receptive fields strongly resemble those of RGCs
-Usrey et al sought retinal cells with RFs in the same position as per the dLGN cell already recorded (n = 205)
-and used correlation of responses to tell if they were directly connected (n=12)

Question 2

why do dLGN receptive fields strongly resemble those of RGCs

Answer 2

• RGC axons make big, powerful, ionotropic synapses
• predominately on proximal dendrites
• single RGCs make multiple connections with single LGN cells
  
  • which will be activated synchronously each time the RGC fires

Question 3

What are the inputs to the LGN?

Answer 3

• Retinal GCs ~7%
  
  • strength of connections
• Brainstem inputs (e.g. ACh) ~30%
  
  • sleep/wake cycle, attention
• Local inhibitory cells ~30%
• Primary visual cortex ~30%
  
  • target for relay LGN cells

Question 4

What do the inhibitory inputs do?

Answer 4

-‘on/off’ response
-activating LGN cell; strong connection
-Reversal potentials indicates chloride channels are open ⇒ GABA-A: fast inhibitory inputs ⇒ strong inhibitation

Question 5

What do the cortical inputs do?

Answer 5

see loop

Question 6

Receptive fields of the primary visual cortex

Answer 6

-layer 4 simple cell
-single eye preference
-short summation without end-stopping
-moderate velocity sensitivity

Question 7

where do P and M cells terminate in the visual cortex of a primate

Answer 7

m: 4C-alpha
p: 4C-beta

Question 8

describe Cortical receptive fields

Answer 8

-discovered by Hubel & Wiesel

Question 9

what are V1 cells?

Answer 9

-unresponsive to flashing spots
-prefer elongate stimuli
-orientation tuned
-direction tuned
-velocity tuned
-length tuned
-have varying degrees of eye preference

Question 10

what are simple cells?

Answer 10

-have separate ‘on’ and ‘off’ zones, found in LGN-receptive layers

Question 11

what are complex cells?

Answer 11

-respond to ‘on’ and ‘off’ throughout their fields, found on other layers

Question 12

what are hypercomplex cells?

Answer 12

-have complex fields with inhibitory and zones, found esp in layer 2-3

Question 13

how are simple cells made?

Answer 13

LGN cells (nb cat)

Question 14

how are complex cells made?

Answer 14

simple cells +

Question 15

how are hypercomplex cells made?

Answer 15

complex cells +

Question 16

Layer 4 simple cells?

Answer 16

Layer 4 simple cells DO inherit their zones from the LGN

Question 17

summarise the connective data

Answer 17

LGN -> VC pairs

Question 18

where are simple cells more common?

Answer 18

in LGN-recipient layer but both types can be found in all layers

Question 19

what cells can be ‘end-stopped’

Answer 19

-simple and complex and many in LGN-recipient layers

Question 20

what % of cortical cells are inhibitory? and what does that mean?

Answer 20

25%
-inhibition has shown to create/enhance a wide range of visual selective

Question 21

what do cortical cells respond best to?

Answer 21

discontinuities in the stimulus = orientation more different

Question 22

what do Cortical inputs to dLGN create?

Answer 22

‘cortex-like’ properties

Question 23

what is one function of corticofugal feedback?

Answer 23

generates length tuning in the dLGN

Question 24

what is a more global function of corticofugal feedback?

Answer 24

generate sensitivity to discontinuities in the image

Question 25

describe columnar organisation

Answer 25

cortical surface; 1-6 layers
R and L eye axons

Question 26

describe the Sophisticated hypercolumn

Answer 26

-‘blobs’ stain for cytochrome oxidase
-they are strung along the centres of the OD columns - cells tend to be monocular
-they form the pinwheels in the orientation columnar system - cells tend to be broadly oriented
-they receive koniocellular LGN input - these sites have a role in colour vision

Question 27

describe stereopsis

Answer 27

the computation of depth information from views acquired simultaneously from different points in space
-binocular visual fields
-‘near’ and ‘far’ tuned cell