Dynamic illusion effects in a reaching task: Evidence for separate visual representations in the planning and control of reaching

The effects of an orientation illusion on perception and 2 different action s were investigated. An 8-cm X 2-cm cylindrical bar was placed in front of participants at various orientations. A background grating was used to indu ce an orientation illusion. In a perception task, the illusion affected par ticipants' ability to align the bar with their sagittal planes. In one reac hing task, a similar effect of the illusion was found on the choice between 2 possible grasping postures. In a second reaching task involving a single grasping posture, the orientation illusion affected the orientation of the hand at the beginning of the reach but not near its end. The authors argue that reaching trajectories are planned and initiated through a context-dep endent representation but are corrected on-line through a context-independe nt representation. The relation of this model to a more general dichotomy b etween perception and action is discussed.