primary visual pathway & cortex Flashcards
what are the different inputs to the LGN?
retinal
brainstem
primary visual cortex
inhibitory feedback
what do inhibitory inputs do to the LGN via feedback when the illumination increases across the whole receptive field (ie the size of the spot increases)
that as the illumination increases across the receptive field of the retinal ganglion cell, it still produces excitation, which is then sent to the cells of LGN. However, the cells of LGN have a stronger surround mechanism, such that they will receive greater inhibitory inputs as the size of the illumination increases beyond the centre.
this surrounding mechanism, or stronger inhibition than normal, is due to hyperpolarization due to opening of chloride channels and release of GABA neurotransmitter from inhibitory LGN interneurons
(so LGN isn’t only receiving inputs from on center retinal ganglion cells but also inhibitory LGN interneurons, thus producing a larger inhibitory response)
This is in response to a poor contrast in illumination between the centre and surround
which input layers in the primary visual cortex do magno and parvo inputs synapse onto (and add which layer is the output layer)
-magno is layer 4ca
-parvo is layer 4cb
-layer 6 is output layer where it can feedback onot LGN
what are neurons of V1 fussy about what they are receptive to and list their preferred stimulations (5) and what is it not responsive to (1)
-direction tuning
-orientation tuning
-elongated stimuli
-velocity tuned
-length tuned
not responsive to flashing lights or spots
what are the three different cortical cells that Hubel and Wiesel discovered and what do they respond to and what layers are they found in
simple cells - on and off cells, that are arranged sided by side and elongated bars of light.
(mainly in layer 4 but also seen in layer 2-3 o V1)
complex cells - has loads of on- and off-cells within that cell (multiple simple cells in one complex cell), thus responding to both a decrease and an increase in illumination because they’re both on- and off-cells in that one cell.
(mainly in 2-3 but also seen in layer 4)
hypercomplex cells - these are length-tuned or end-stop cells whereby they respond greatly to short-lengthened bars of light, such that if you increase the length of the bar, it will respond less.
(mainly in 2-3 but also seen in layer 4)
What involvement do the inhibitory interneurons have in v1 cells in terms of their selectivity and what happens if you block GABA?
It enhances and sharpens direction and orientation selectivity.
if you block GABA, you would lose that selectivity
what happens when cortex feedback to LGN from layer 6
it will provide LGN with cortical-like properties such as length tuning and generate sensitivity to differences (discontinuities) in the image (generating sensitivity to contrasting features in the visual stimuli, ie antagonistic centre surround receptive fields), as well as orientation discontinuity by simple cells such that it will be sensitive to a preferred orientation in the centre and respond best with opposing orientations or gratings in surround
all this provide LGN cells with greater selectivity and sensitivity
what happens if you silence inputs from layer 6 that are feedbacking from end-stopped/ hypercomplex cells
the LGN cells will lose their length-tuned sensitivity
what will the response be with a larger or smaller receptive field of grating
if the grating covered large RF, it will generate poorer response then if it covers a smaller RF with an antagonistic surround
what are ocular dominance columns and which layer are they found in
each column has a preferred visual input from one eye or the other
its found in layer 4 of the cortex
what are hypercolumns
composed of ocular dominance and orientation columns (they receive visual inputs from either eye that are receptive to preferred orientation within the visual stimuli)
what are pinwheels in a hypercolumn concerning orientation columns and what stain was used to visualize this
pin wheels are the pinpoint of all the orientation columns intercepting that become non-orientation-specific and therefore respond to all orientations
the stain used was cytochrome oxidase
what is stereopsis
the visual ability to view the world in three dimensions
we all have that
perceive the world in length, width and depth
what are the perceptual benefits of binocular visual fields?
allows to see in three dimensions, allows for good depth perception, ie what’s in the foreground and what’s in the background and provides cue for distance between objects based on what’s in the foreground and in the background
make question on fixation point in binocular vision
help
