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NROC64 : Lec 3 : Central Visual System (nearly word-for-word what was said in lec)

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NROC64Lec 3 The Central Visual System Slide 3 y Short answer question Bipolar cells in the fovea and elsewhere differ as to how they are activatedinhibited by light Please describe y Answer There are 2 types of bipolar cellsone is transmitter glutamate gated Na influx and the other type is GPCR o ON bipolar cellsthese cells are activated by lightmeans that theyre directly connected to photoreceptor these have GPCRsglutamate activates cells in this case opens the Na channels and activates the bipolar cells the glutamate is only available bc its dark o OFF bipolar cellsare activated when the photoreceptor doesnt receive any light where theres a direct cxn o Bipolar cells are arranged in CENTRE and SURROUNDantagonistic mechanism o If ONbipolar cells in centre the surround is inhibitory when light shines on it o If OFFbiploar cells the surround activates the cell when light is shining on it o Surround has the opposite effect compared to the centre for ON bipolar cells the SURROUND is inhibitory for OFF bipolar cells the SURROUND is activatedin terms of light o what is the reasoning for the SURROUND having the opposite effect Its bc the bipolar cells arent directly connected to the photoreceptors in the SURROUND they are connected via interneurons which reverse the effect y Slide 4 o Todays lecneurons in the visual system past the LGN in the CORTEX the system that gives rise to conscious perception o Parallel pathwaysFrom retina to LGN is what was discussed last weekToday well focus on primary visual cortex and higher order visual areas o Look at individual neurons and see what kind of receptive fields they have windows in which they show responses depending on the light present diff colors and shapes o Look at range of diff frequenciesy Slide 5 o OverviewAnatomy of the striate cortex aka V1 first visual area Area 17 y Striate cortexclear structure in terms of laminates mostly bc of the communication back to the thalamus LGN get very large cell bodies there which makes stripesPhysiology of striate cortex y Look at inputs crosscxns across diff layers for the diff systems outputsGo beyond V1 so many other visual areasextrastriate cortexlook at functionality in terms of computations how perception could potentially be achievedvisual deficits damage in select areaswhat kind of effect does that have in terms of perceptiony Slide 7o Left side see macaque brain lateral and medial o See V1 V1 takes up a very large portion of the monkey brain o Human V1 laterally theres barely anything there over evolution humans ended up having most of V1 on medial side mostly tucked into CALCARINE FISSURE o Human V1 does have more neurons than macaque V1 area bc human brainlarger but relative to the brain itself the V1 area is less o Crosssection of macaque brain striped structure 6 layers w some sublayers th 4 layer split into a b c y csplit into alpha and beta y Slide 8 o RetinotopyTopography is something thats found in all modalitiesRetinotopysome kind of map of the visual field projected onto some kind of structure depends on what kind of structure youre talking aboutWhen you look at this in term s of neurons photoreceptorsin the fovea you have lines of neuronsoverrepresentation of the central visual field and this pattern is whats continued throughout the visual pathways true for LGN and V1Youll see more inputs from fovea to V1 and youll see overrepresentation of fovea to V1 large part of the lateral surface of V1 is actually foveaDistorted map bc there is overrepresentation o Another important factor of retinotopyAny discrete point of light will always activate more than one cellIn the fovea and other parts of the retina you have some limit limit is the outer surface of a photoreceptor if light falls on that photoreceptor is stimulated if falls somewhere else its not stimulatedand its just this particular photoreceptorBUT this isnt true for later structurestheres always overlap in receptive fields
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