A good starting point in a discussion of shape perception is the paper of Lettvin, et al. (1959), who recorded from the frog’s optic nerve fibers and classified the retinal ganglion cells, from which the fibers originate, with regard to their responses to visual stimulation. On the basis of these observations the authors divided the cells into four types: sustained contrast detectors, net convexity detectors, moving edge detectors, and net dimming detectors. From the descriptions it was clear that the outputs of many of these cells were more elaborate than the outputs of the corresponding cells in higher animals, but still, all told, the four cell types were not much more than detectors of moving edges and dots, or changes of illumination.
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