Linear mechanisms of directional selectivity in simple cells of cat striate cortex

The role of linear spatial summation in the directional selectivity of simple cells in cat striate cortex was investigated. The experimental paradigm consisted of comparing the response to drifting grating stimuli with linear predictions based on the response to stationary contrast-reversing gratings. The spatial phase dependence of the response to contrast-reversing gratings was consistent with a high degree of linearity of spatial summation within the receptive fields. Furthermore, the preferred direction predicted from the response to stationary gratings generally agreed with the measurements made with drifting gratings. The amount of directional selectivity predicted was, on average, about half the measured value, indicating that nonlinear mechanisms act in concert with linear mechanisms in determining the overall directional selectivity.