The primary visual pathway and cortex Flashcards
describe Visual processing in the dorsal lateral geniculate nucleus (dLGN)
-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)
why do dLGN receptive fields strongly resemble those of RGCs
- 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
What are the inputs to the LGN?
- 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
What do the inhibitory inputs do?
-‘on/off’ response
-activating LGN cell; strong connection
-Reversal potentials indicates chloride channels are open ⇒ GABA-A: fast inhibitory inputs ⇒ strong inhibitation
What do the cortical inputs do?
see loop
Receptive fields of the primary visual cortex
-layer 4 simple cell
-single eye preference
-short summation without end-stopping
-moderate velocity sensitivity
where do P and M cells terminate in the visual cortex of a primate
m: 4C-alpha
p: 4C-beta
describe Cortical receptive fields
-discovered by Hubel & Wiesel
what are V1 cells?
-unresponsive to flashing spots
-prefer elongate stimuli
-orientation tuned
-direction tuned
-velocity tuned
-length tuned
-have varying degrees of eye preference
what are simple cells?
-have separate ‘on’ and ‘off’ zones, found in LGN-receptive layers
what are complex cells?
-respond to ‘on’ and ‘off’ throughout their fields, found on other layers
what are hypercomplex cells?
-have complex fields with inhibitory and zones, found esp in layer 2-3
how are simple cells made?
LGN cells (nb cat)
how are complex cells made?
simple cells +
how are hypercomplex cells made?
complex cells +
Layer 4 simple cells?
Layer 4 simple cells DO inherit their zones from the LGN
summarise the connective data
LGN -> VC pairs
where are simple cells more common?
in LGN-recipient layer but both types can be found in all layers
what cells can be ‘end-stopped’
-simple and complex and many in LGN-recipient layers
what % of cortical cells are inhibitory? and what does that mean?
25%
-inhibition has shown to create/enhance a wide range of visual selective
what do cortical cells respond best to?
discontinuities in the stimulus = orientation more different
what do Cortical inputs to dLGN create?
‘cortex-like’ properties
what is one function of corticofugal feedback?
generates length tuning in the dLGN
what is a more global function of corticofugal feedback?
generate sensitivity to discontinuities in the image
describe columnar organisation
cortical surface; 1-6 layers
R and L eye axons
describe the Sophisticated hypercolumn
-‘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
describe stereopsis
the computation of depth information from views acquired simultaneously from different points in space
-binocular visual fields
-‘near’ and ‘far’ tuned cell
