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class+6+lecture_posted - Sensation and Perception Class VI:...

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Sensation and Perception Class VI: Visual Processing
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Figure 3.11 The primate lateral geniculate nucleus
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Figure 3.14 The mapping of objects in space onto the visual cortex
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Striate Cortex Striate cortex : Also known as primary visual cortex, area 17, or V1 A major transformation of visual information takes place in striate cortex Circular receptive fields found in retina and LGN are replaced with elongated “stripe” receptive fields in cortex It has about 200 million cells!
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Striate Cortex Two important features of striate cortex: Topographical mapping: Neighboring points in physical space are represented in neighboring points in the brain Cortical magnification : Dramatic scaling of information from different parts of visual field The amount of cortex devoted to processing the fovea is proportionally much more than the amount of cortex devoted to processing the periphery
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Center-surround receptive field organization in retina and LGN. What about in the visual cortex?
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Receptive Fields in Striate Cortex Cells in striate cortex respond best to bars of light rather than to spots of light Some cells prefer bars of light, some prefer bars of dark (simple cells) Some cells respond to both bars of light and dark (complex cells) Orientation tuning : Tendency of neurons in striate cortex to respond more to bars of certain orientations and less to others Response rate falls off with angular difference of bar from preferred orientation
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Figure 3.16 Orientation tuning function of a cortical cell
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Receptive Fields in Striate Cortex How are the circular receptive fields in the LGN transformed into the elongated receptive fields in striate cortex? Hubel and Wiesel (Nobel laureates) : A cortical neuron that responds to oriented bars of light might receive input from several retinal ganglion cells If you string several retinal ganglion cells together, they can form an oriented bar A cell that is tuned to any orientation you want could be created in cortex by connecting it up with the appropriate retinal ganglion cells
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Figure 3.17 Hubel and Wiesel’s model of how cortical simple cells get their orientation tuning
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Receptive Fields in Striate Cortex Many cortical cells respond especially well to: Moving lines Bars Edges Gratings Direction of motion
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Receptive Fields in Striate Cortex Each LGN cell responds to one eye or the other, never to both Each striate cortex cell can respond to input from both eyes By the time information gets to primary visual cortex, inputs from both eyes have been combined BUT…Cortical neurons tend to have a preferred eye, however. They tend respond more vigorously to input from one eye or the other
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Simple cells versus complex cells Complex cells are a category of cells that prefer specific stimuli more complicated than just oriented lines. Can you imagine how you would line up
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class+6+lecture_posted - Sensation and Perception Class VI:...

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