When does action resist visual illusions?

Over the past decade, many studies of non-pathological individuals have rep orted functional dissociations between perceptual judgments and motor respo nses. These results suggested an interpretation of the ventral and dorsal s treams in the primate visual system as independent modules for visual aware ness and the visual guidance of actions. However, recent comparisons of per ception and grasping responses in size-contrast displays have been widely r eported to show that apparent dissociations are actually experimental artif acts. An overview of the literature suggests that the issue of visuomotor d issociations in healthy individuals is far from settled. Some results sugge st that immunity from visual illusions might be found when task requirement s emphasize observer-relative reference frames. These results suggest that the functional specialization of the two visual subsystems might be less ri gid than originally posited.