Determinants of asynchronous processing in vision

When a stimulus oscillates in both colour and direction of motion, changes in colour must lag behind those in direction if they are to be seen as concurrent. It has been argued that this lag is the consequence of asynchronous visual processing, with colour being processed more rapidly than motion. This proposal is contentious: it has been criticized on the basis that the time-course of cortical activity may not correlate directly with that of perceptual experience. Here, we demonstrate that the extent of the apparent asynchrony can vary according to the prevailing stimulus conditions. The apparent asynchrony is greatest if the stimulus is composed of opponent directions of motion and is reduced if the angular difference between the directions is reduced. This pattern of results suggests that asynchronous neural activity arises, in part, as a consequence of differential levels of inhibition within relatively independent cortical structures.