Psychology 215b-- second half

Psychology 215b-- second half - Sensation and Perception,...

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Psychology 215b—Sense and Perception Tuesday and Thursday 8:30-10:30am February 12, 2008 Chapter 4: Continuation from last lecture Retinotopy The retina contains a representation of the visual world Brain structures, such as the LGN, SC and V1, contain maps of the retina Therefore these structures contain maps of the visual world Retinotopy is the expression used to describe how the retina is mapped in a given structure The maintenance of Retinotopy is that it will be continued from structure to structure—be all interconnected and adjacent to things that represent themselves Organization of the Striate Cortex Each point in the cortex corresponds to a point on the retina. The area of cortex that represents the fovea is much larger than would be expected based on the small size of the fovea. The fovea accounts for 0.01% of the retina’s area. The fovea representation in cortex represents 8-10% of the area of primary visual cortex. This phenomenon is called cortical magnification factor Cortical Magnification Factor Retina Fovea—50,000 ganglion cells/mm^2 Peripheral—1,000 ganglion cells/mm^2 Cortex—neuron packing density is uniform Therefore, more cortical space is allotted to parts of the retina that send more ganglion cell signals to cortex. Cortical Location Columns Perpendicular electrode penetrations find receptive fields to be in about the same location (significant overlap), regardless of the cortical layer Orientation Columns Perpendicular penetrations show that neurons have receptive fields with the same location on the retina, and they all prefer the same orientations Ocular Dominance Columns 80% of all V1 neurons respond to input from both eyes. However, most respond better to one eye. This preferential response is called ocular dominance Cells with the same ocular dominance are organized into columns Ocular dominance columns alternate across the cortex Autoradiology Inject radioactive amino acids into the eye and it goes up to cortex, then process the eye like an x-ray. The part with the radioactive elements turn white when developed—this gives you alternating strips of ocular dominance. If you lose vision early in life in one Sensation and Perception, prep for 2 nd exam, page 1
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eye, your cortex flips over and almost exclusively processes information from the remaining eye, whereas you use both eyes equally otherwise. Hypercolumns: The Ice Cube Tray Theory The combining of all three types of columns: 1. Location 2. Orientation 3. Ocular Dominance Representation of an image in striate cortex The cortical representation of a stimulus does not have to resemble the stimulus; it just has to contain the features that represent the stimulus. Ungerleider and Mishkin (1982)
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Psychology 215b-- second half - Sensation and Perception,...

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