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 Experiments with the retinal cells of cats revealed that retinal ganglion cells have receptive fields composed of two concentrically arranged subregions that have the opposite preference for stimulus contrast. An increase in luminance (for example, the introduction of a bright spot) within an ON subregion evokes spikes. Likewise, a decrease in luminance within an OFF subregion evokes spikes. Furthermore, stimuli of the opposite contrast evoke responses of the opposite sign. Thus, whereas a bright spot falling in an ON subregion increases spike rate, a dark spot falling in an ON subregions reduces spike rate.  Two key interpretations can be made from these findings. First, the early visual system is designed to reduce redundancy in the neural signals sent downstream to the brain. This is evident in the fact that diffuse stimuli that cover the entire receptive field evoke far fewer spikes than stimuli confined to the center or the surround. Second, the early visual system is designed to detect contrast borders. This is evident in the fact that the best stimuli are those for which the luminance in the center is very different from that in the surround. In composing a visual image, the retina and thalamus create a mosaic of gray levels, as seen in the images on your right. The cortex, in turn, creates sketches of edges, as seen in the images on your right.
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