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Lec 15-17 Part 2 - Vision

Lec 15-17 Part 2 - Vision - How do antagonistic...

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1 How do antagonistic center- surround receptive fields influence visual perception? Herring grid illusion: brain only knows about intensity of light at a point in space from the frequency of action potentials in the ganglion cells whose receptive fields coincide with that point.
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2 The perceived gray spot disappears when you gaze directly at a particular intersection because the receptive fields of ganglion cells in the central part of the visual field are much smaller, so both the center and surround fall within the white area without overlapping the black zones. Lateral inhibition enhances the perceived contrast in brightness at the border between light and dark regions of the visual field
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3 The dark stripe at the dim side of the transition and the bright stripe on the bright side are called Mach bands. These dark and light stripes are not really there in the stimulus! Here, the gray background is actually of uniform luminance. The gradient in the blue areas is real, and it induces illusory brightness changes in the gray.
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4 Color vision: We cannot distinguish colors (wavelength of light) based on the output of a single type of photoreceptor. Consider the response of a rod photo- receptor.
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5 Color vision: three types of cone photoreceptors in humans allow color discrimination. Cone signals are carried to subsets of ganglion cells by special types of bipolar cells. Color-sensitive ganglion cells have center- surround receptive fields whose center and surround arise from different types of cones. This example shows a red/ green opponent ganglion cell , with red excitatory center and green inhibitory surround (arising from L cones and M cones, respectively). Other red/ green ganglion cells have green excitatory centers, red inhibitory surround. There are also blue/yellow opponent ganglion cells , whose centers arise from S cones and surrounds from M cones, or vice versa.
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6 Note that this type of cell is also strongly excited by a white light that covers only the center of the receptive field. So, a small white light stimulated this cell the same way as a diffuse red light.
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