The Neural Control of Vision

M. The Processing of Shape

There is ample evidence to show that single neurons in various parts of the visual system respond selectively to shape. Already in the retina, the retinal ganglion cells respond more vigorously to small spots of light that impinge on the center of their receptive fields than large spots. In V1 cells are selective to the orientation of line segments. In inferotemporal cortex numerous studies have shown cells that respond well to objects and to faces. Just how selective these cells are is still debated. It has variously been proposed that some of these cells are extremely selective and, alternatively, that they respond to a broad range of stimuli in which case the differential response of a large aggregate of neurons produces the specificity required for object identification.

Several theories have been proposed as to how shape is analyzed. According to one of these, based on the remarkable finding that neurons in V1 are selective to the orientation of line segments, these neurons form the building blocks for object recognition. According to another theory, based on the fact that neurons in V1 are also spatial-frequency selective, shape perception is achieved by spatial-frequency analysis. Models have convincingly shown that such analysis is feasible and can yield the pattern analysis of which we are capable.