Mechanisms for visuomotor adaptation to left-right reversed vision

Behavioral studies suggest that the adaptation of planar arm movements to rotated visual feedback is achieved by the interplay of a gradual process which slowly rotates participants' responses by up to +/-90 degrees , and a discrete process which changes the responses by means of axis inversion. The processes for adaptation to left-right reversed visual feedback are far less well understood. To clarify this issue, 12 healthy participants performed pointing movements to targets presented in eight different directions, before and during exposure to left-right reversed visual feedback. We quantified the direction of each response 150ms after movement onset and analyzed the time-course of those directions throughout the adaptation phase, separately for different targets. For targets along the axis of inversion, we only found an increase of response variability, for targets perpendicular to that axis, we observed a discrete 180 degrees change of response direction, and for diagonal targets, we found a discrete 180 degrees change followed by a gradual "backward" shift of 90 degrees . The present findings confirm that sensorimotor adaptation is based on discrete and gradual processes, that both types of processes can occur concurrently, and suggests that those processes can contribute to adaptation in a target-specific fashion.