The construction of directionally selective units, and their use in the processing of visual motion, are considered. The zero crossings of ∇
2
G(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 ∂[∇
2
G(x, y)
∗
l(x, y)
]/∂
t
is measured at the zero crossings, and serves to constrain the local direction of motion to within 180°. 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.
Directional selectivity and its use in early visual processing
