EXAM 2 SG - CH 3 • • • • • • • • • The...

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CH. 3 The path of image processing from the eye to the brain Retina is furthest from processing center Contrast The difference in illumination between a figure and its background Acuity The smallest spatial detail that can be resolved To measure visual acuity: Eye doctors use distance (20/20) Vision scientists use the smallest visual angle of one cycle of a grating (.oo17 degrees is normal visual acuity for humans) 1862 - Herman Snellen invented a method for designating visual acuity Stroke of the letter was 1/5 the entire letter size Aliasing Misperception of a grating due to under-sampling. We don’t see a gray slab, but instead see b&w stripes with pink/purple dots) We perceive the cycles to be longer than they really are Spatial frequency The # of cycles of a grating per unit of visual angle (specified in degrees) Cycles per degree # of dark and bright bars per degree of visual angle Why do we use sine gratings? Patterns of stripes with fuzzy boundaries are very common The edge of any object will produce a single stripe, often blurred by a shadow, in the retinal image The visual system appears to break down images into vast # of components Each is a sine wave grating w/ particular spatial frequency Retinal cells like spots of light Each cell responds well to certain types of stripes or gratings Medium frequency have the strongest response Response of on-center RGCs – depend on ‘phase’ of grating 0 degrees = on, strong 90 degrees & 270 degrees = ½ on, ½ off 180 degrees = all dark, off The Lateral Geniculate Nucleus (LGN) Part of the thalamus Axons of retinal ganglion cells synapse here Parvocellular layer Likes stationary objects Top 4 layers of LGN, smaller than magnocellular Receive input from midget ganglion cells Magnocellular layer Likes large, fast moving objects Neurons in bottom 2 layers of LGN, larger than parvocellular Receive input from parasol ganglion cells Topographical mapping Highly organized stimuli 1 st : Left LGN receives projections from the L sides of the retinas in both eyes, then the right LGN receives projections from the R sides of the retinas. 2 nd : Each layer of the LGN receives input from one or the other eye From bottom to top, layers 1, 4, and 6 of the right LGN get input from the L eye Layers 2, 3, 5 get input from R eye Each LGN layer has a highly organized map of a complete half of the visual field
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Striate cortex (AKA: primary visual cortex or V1) Major transformation of visual info takes place here Has about 200 million cells, 6 major layers Fibers from LGN mainly project into layer 4 2 important features: Cortical topography - Topographical mapping
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EXAM 2 SG - CH 3 • • • • • • • • • The...

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