Concurrent adaptations of left and right arms to opposite visual distortions.

Previous research has shown that subjects can adapt with either arm to an opposite visual distortion, and the two adaptive states can then be used in sequence to control the respective arm. To extend this finding, we exposed the left and right arms of our subjects to opposite-directed rotations of the visual field alternately for 20 s each, and determined the time-course of adaptation, as well as aftereffects without visual feedback under uni- and bimanual conditions. Our data confirm that two adaptive states can co-exist in the sensorimotor system, one for each arm. We further found that the time-course of adaptive improvement was similar for both arms, that the improvement was present as early as the first movement after a change of arm and discordance, and that the magnitude of adaptation was similar to control data yielded by a single arm and discordance. Taken together, these findings suggest that the two adaptive states were formed concurrently, and without mutual interference. We also observed significant aftereffects. They were smaller but still appreciable under bimanual conditions; the two arms moved at the same time in different directions even though they were aimed at a common visual target. This outcome indicates that the two adaptive states were not merely of a strategic nature, but rather changed the rules by which sensory information was transformed into motor outputs; it also suggests that the two states not only co-exist, but can also be engaged concurrently in movement control. The reduced aftereffects observed under bimanual conditions can be attributed to the well-known phenomenon of bimanual coupling, which is unrelated to adaptation.