Directional selectivity and its use in early visual processing.

The construction of directionally selective units, and their use in the processing of visual motion, are considered. The zero crossings of delta 2G (x,y) * I(x,y) are located, as in Marr & Hildreth (1980). That is, the image is filtered through centre-surround receptive fields, and the zero values in the output are found. In addition, the time derivative delta[delta 2G(x,y) * I(x,y)]/delta t is measured at the zero crossings, and serves to constrain the local direction of motion to within 180 degrees. The direction of motion can be determined in a second stage, for example by combining the local constraints. The second part of the paper suggests a specific model of the information processing by the X and Y cells of the retina and lateral geniculate nucleus, and certain classes of cortical simple cells. A number of psychophysical and neurophysiological predictions are derived from the theory.