Test 4 Review Questions Visual Pathway Visual Fields Primary Visual Cortex 1 Striate cortex How many layers What is cortical magnification What is visual crowding Where is the fovea represented How is striate cortex organized answer topographically aka retinotopically What types of stimuli do cells in striate cortex respond to Compare and contrast simple cells and complex cells What do hypercomplex cells respond to What are columns in cortex and how do the response properties of neurons in a given column compare to each other What are ocular dominance columns What is a hypercolumn The striate cortex has 6 layers all neocortex has 6 layers Topographically retinotopically organized o Whatever we are looking at is going to be reflected onto the retina map of the visual world in the retina we put that map in primary visual cortex aka striate cortex Cortical magnification More neurons in cortex to process info from fovea compared w info from peripheral retina Visual acuity decreases w eccentricity distance from the fovea Not all parts of the retina are represented equally in primary visual cortex This is why we have some visual distortion Why Or why not magnify the entire visual world like this The visual system must make a trade off High resolution requires a great of resources a dense array of photoreceptors one to one lines from photoreceptors to retinal ganglion cells a large chunk of striate cortex To see the entire visual field with such high resolution we might need eyes brains too large to fit in our heads Thus we have evolved a visual system that provides high 1 resolution in the center lower resolution in the periphery If you need to look at details on an object in the corner of your eye you can simply turn your head Right homonomous hemianopsia losing the right visual field from each eye Your representation of the world is different on each eye that is why we draw the line all the way to represent the whole visual field Bitemporal visual field defect each eye is more central neither is able to view the periphery because the fibers coming from the periphery have been damaged o Occasionally a tumor can grow on the pituitary gland that can press up on the optic chiasm sometimes on the very center of this structure this where fibers are crossing from the nasal retina bringing info from the periphery 2 Convergence in rod pathway and cortical magnification aren t only reasons we don t see well in periphery Perceptual problem of visual crowding in periphery Makes it hard to identify objects Maybe to simplify periphery to promote attention on fovea You would see the kid to the right 1st because he is your central focus not hidden behind the signs like on the opposite side Periphery anterior fovea gets lots of representation posteriorly 3 Meningioma growing inward from back of head can cause a blind spot in the center of the field of view for both eyes Why o Growing from the very back of the head pressing forward it is pressing on primary visual cortex The part of primary visual cortex that is 1st affected with this is the one that deals with central vision o Also caused by macular degeneration leads to something similar o Periphery is fine retina is fine optic nerve is fine optic tract is fine LGN is intact but the part of the cortex that tries to process that part of the visual world is damaged Receptive fields in striate cortex They are not circular they do not respond much to spots of As opposed to retinal ganglion cells that have a circular center surround receptors respond to spots of light as well as cells in LGN They are linear they respond to lines bars edges gratings For each cell in striate cortex the line has to be of a particular light angle Cell responds best to that angle hardly at all if 30 off from that angle Called orientation tuning cells have preferred stimuli based on the orientation of those lines The whole world is not composed of only lines gratings etc but even when looking at someone s face you break up that contour into little itty bitty lines 4 More cells are tuned to horizontal and vertical lines than to obliques In this picture cells have higher firing rates when responding to vertical lines as you move away to horizontal lines you can see less less firing Each line also must be a particular width cells are tuned to a line of a particular angle width Some cells respond best when that line is moving in a particular direction Based on what they respond to we call these cells simple cells or complex cells Both get a direct line from LGN Retina receptive fields based on photoreceptors LGN receptive fields based on retinal ganglion cells V1 linear receptive fields How does that work The receptive field of cortical neurons is based on the LGN cells that feed them The arrangement of those LGN cells ie in a row helps establish orientation specificity of cortex cells That s not the entire story lateral inhibition b t cortical cells 5 Each of these is a neuron in lateral geniculate nucleus in the thalamus each one has a circular receptive field but all of those neurons are feeding into a single neuron in primary visual cortex So what we end up with is 4 overlapping circular receptive fields feeding into one cell we get a linear receptive field still having on off organization but now instead of concentric circles it is linear Simple cells have simple receptive fields responds best to object of given shape and orientation Orientation specific sometimes length specific Small receptive field meaning that their preferred stimulus needs to be in a very specific part of the world Best stimuli are bars lines rectangles with definite edges Also respond to gratings tuned to spatial frequency which corresponds to line width 6 2 simple cells edge detector and stripe detector Edge detector likes light on one side of its receptive field and darkness on the other side Stripe detector likes a line of light of a particular width surrounded on both sides by darkness If grating w correct spatial frequency drifts across receptive field of this cell it will modulate its response as dark and bright bars drift across receptive field s center Complex cells specific Orientation specific movement specific sometimes length 7 Larger receptive fields than simple cells o Stimulus can occur in wider range of visual field next slide Best response to movement if stimulus is properly oriented movement is in particular direction Used to think simple cells fed into complex cells now think 2
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