Temporal dynamics of colour and motion perception

When a stimulus oscillates in both colour and direction of motion, changes in colour must lag behind changes in direction if they are to be seen as concurrent1. It has been argued that this perceptual lag arises as a consequence of asynchronous neural activity1,2 with changes in colour being processed more rapidly than those in direction. This is a contentious proposal3, not least because an asynchrony of this form appears to contradict physiological evidence that suggests motion should be processed faster than colour. Here, we demonstrate that the extent of the perceptual lag is influenced by 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. We believe that this pattern of results suggests that asynchronous neural activity arises as a consequence of differential levels of inhibition within relatively independent cortical structures. A further implication of this data is that the time course of neural processing may correlate directly with that of perceptual experience without any necessary mediation by subsequent interpretive processing. Of course this proposal does not explain how the time course of activity in distinct cortical structures might be introspected and contrasted so that temporal judgments can be made. To do so seems to require self-awareness and, while at this point we do not know how this might arise, assuming a singular temporal processing system does not necessarily clarify the issue