Control of Vision
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.