Sensorimotor adaptation to rotated visual input: different mechanisms for small versus large rotations.

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.