The present study investigates if sensorimotor adaptation to large visual rotations is achieved by a continuous angular change of the internal representation of space. Human subjects performed manual tracking movements under rotated visual feedback in two sessions; the magnitude of rotation in the second session was 45 degrees larger or smaller than in the first. We found mostly a facilitatory effect of the first adaptation on the second, which supports the view that the internal representation can gradually shift from one angular transformation to another. However, no facilitation was found for visual rotations in the 80-120 degrees range, suggesting that the internal model changes gradually only up to a limiting angle. A subsidiary experiment, employing small stepwise changes of visual rotation throughout a testing session, confirmed this view and placed the limiting angle near 120 degrees for an increasing, and near 70 degrees for a decreasing visual rotation. We conclude that adaptation to large-magnitude rotations may be achieved in two stages: a polarity inversion of both axes (=180 degrees rotation), followed by a "backward" shift toward somewhat smaller angles.

The mechanisms for adaptation to visual rotation were investigated by exposing subjects to different rotation angles in a stepwise fashion. We found that response direction continuously changed to compensate for the imposed rotation, but this change was limited to 90 deg. Larger changes were accomplished by inverting both spatial axes (which is equivalent to a 180 deg rotation), and then gradually changing response direction "backwards" to the prescribed value. The angle of 0 deg had no such limiting value like 90 deg: Response direction could continuously change through 0 deg and beyond. Our data provided no evidence that adaptation to opposite-directed visual rotations results in interference, due to competition in working memory; instead subjects' performance under such conditions is fully explained by the said continuous changes of response direction. We conclude that adaptation is achieved by a coordinated interplay of continuous (gradual rotation between +/-90 deg) and discrete (sign reversal) processes.

It is well known that sensorimotor adaptation will transfer from the practiced to the unpracticed arm, which has been taken as evidence that adaptation is located in the brain before the divergence point for left and right arm control. We now explore whether adaptation will transfer between different movement categories as well. Subjects were exposed to a 60-deg visual rotation first in a tracking and then in a pointing task, or vice versa. We found a substantial transfer of adaptation between tasks, but its magnitude was larger from pointing to tracking than from tracking to pointing. This benefit of pointing persisted when the use of cognitive strategies was minimized by a concurrent, attention-demanding task, but it was lost when pointing amplitudes were very small. We conclude that adaptation is located in the brain before the divergence point for different movement categories, and that movements with a large ballistic component facilitate adaptation transfer.

Using the multiple-exposure approach, we investigated sensorimotor adaptation by exposing human subjects to different angles of visual rotation in a tracking task. Generally, the tracking error was high at the onset of the visual rotation and gradually declined towards the baseline level during the exposure period. In experiment A, we confirmed that the initial tracking error increases more than proportionally with the angle of rotation. In experiment C, we were unable to confirm intermanual transfer, and attribute this discrepancy with previous literature to details of the experimental tasks. In our main experiment, B, we found that pre-exposure to 45 degrees or 60 degrees of visual rotation facilitated the subsequent adaptation to a 90 degrees rotation, with the facilitatory effect being more pronounced following the 60 degrees rotation. We interpret this finding as evidence that adaptation is achieved by a gradual process, which progresses from small angles of output transformation through intermediate values up to the prescribed angle of rotation.