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Transcript
Mind, Brain & Behavior Monday February 17, 2003 Retina Two kinds of photoreceptors (Table 22-1): Cones – wavelength specific (perceive color) and detail, see in daylight, detect fast flicker Rods – perceive motion, require less illumination and see in black and white Cones are concentrated in the fovea. Rods are concentrated on the periphery. Cones and rods send axons to ganglion cells. Types of Ganglion Cells Magnocellular (M cells) – large cells that receive input from rods. Parvocellular (P cells) – small cells that receive input from cones. Blob pathway – concerned with color perception. Interblob pathway – concerned with shape/form. Both types synapse on layers within the LGN (lateral geniculate nucleus) of the thalamus. Mapping Within the LGN Optic nerve carries information from ganglia to LGN. Crosses at optic chiasm. Separate layers are maintained for each eye and for each type of cell (M and P). Interneurons project from areas of the LGN to striate cortex. Mapping in the Striate Cortex Separate layers from LGN to striate cortex are maintained in ocular dominance columns. M and P cells enter the cortex at different levels of layer 4 of the visual cortex. Information is combined by pyramidal cells that synapse at higher levels in the striate cortex. Input from both eyes is combined at layer 3. Extrastriate Pathways Parallel processing of visual information from the striate cortex. Three pathways: Color processing – P blob cells, goes from V1 to V2, then V4, then inferior temporal cortex. Shape processing, depth perception – P interblob cells, goes from V1 to interior temporal cortex. Motion & spatial relations – M cells, V1 to V2, then MT (V5), to parietal cortex. Equiluminance Brightness is held constant – permits study of the contribution of color to perception. Results: Brightness, not color, is important to motion detection, perspective, relative sizes, depth perception, figure-ground relations, visual illusions. Motion is a cue for distinguishing among objects. Things that move together belong together. Visual Agnosias Existence of distinct agnosias for aspects of perception suggests that these abilities are localized to areas selectively damaged. Achromatopsia – good perception of form despite inability to distinguish hues. Prosopagnosia – inability to recognize faces as particular people (identity). Can recognize that it is a face, and tell the parts. Binding Mechanisms How is information from the separate, parallel pathways brought together and associated? Treisman & Julesz – combination requires attention. A pre-attentive process detects the major outline of an object. An attentive process notices, selects & highlights combinations of features. Maintained in separate global and detailed maps. Edge Detection Ganglion cells respond to contrast and change in visual input. Center-surround (on-off) receptive field. Bipolar cells also have center-surround receptive fields. Neurons in the visual cortex have rectilinear receptive fields with excitatory and inhibitory zones. Complex cells provide positional invariance. Complex Forms, Motion Processing of form occurs outside the visual cortex – inferior temporal cortex. Not organized retinotopically. 10% selective for specific images (hands, faces). Processing of motion occurs in middle temporal area (MT or V5), then parietal lobe. Used for seeing moving objects, pursuit eye movements, guidance of bodily movement