Adaptation to altered visual-vestibular feedback: mechanisms of maintenance and recovery.

Adaptation of perceived movement during head motion (apparent concomitant motion, ACM) and the subsequent elimination of adaptation were studied in two experiments. During the adaptation phase of both experiments, subjects performed voluntary 1-Hz head oscillations for 6 min while fixating a stimulus moving either in the same (with) direction as or the opposite (against) direction of head movements. In Experiment 1, ACM adaptation was measured following either a 1- or a 4-min delay after the adaptation phase. Results indicated some loss of adaptation during the additional 3-min delay, demonstrating a tendency of the system linking head and image to return to its preadaptation state following removal of an adaptation stimulus. In Experiment 2, subjects viewed a stimulus after adaptation that appeared to move minimally in the same manner as the adaptation stimulus during 3 min of head oscillations. No loss of adaptation was measured in these subjects between the beginning and the end of the 3-min interval. In another condition, subjects viewed a stimulus that appeared to move alternately in the same direction as and in the opposite direction of the adaptation stimulus during a similar 3-min interval following adaptation. ACM adaptation was substantially reduced during this 3-min interval. These results implicate two mechanisms that operate to either maintain or eliminate the short-term adaptation. One is passive and operates in the absence of visual feedback to eliminate the short-term adapted state, and the other responds to postadaptation visual feedback.