Visuo-motor adaptation: evidence for a distributed amplitude control system.

We investigated the constraints for visuo-motor adaptation in human pointing movements. Subjects pointed at sequentially presented visual targets while visual feedback about their finger position was either absent (pre- and post-period), or was manipulated such as to require a gradual reduction of response amplitude (per-period). We found that response amplitudes were smaller during the post- than during the pre-period, which documents the existence of adaptation to distorted visual feedback. We further found that adaptation can transfer fully to untrained amplitudes (Exp. 1), although the amount of transfer may be reduced if trained and untrained amplitudes are substantially different (Exp. 2). However, selective adaptation of one amplitude but not another can also be yielded if the paradigm explicitly asks for it (Exp. 3), and if the two amplitudes differ by more than about 10 cm (Exp. 4). We conclude from these findings that the adapted mechanism consists of amplitude-specific elements, tuned to amplitude spans of some 10 cm.