Concurrent implicit adaptation to multiple opposite perturbations.

Simultaneous adaptation to opposite visuomotor perturbations is known to be difficult. It has been shown to be possible only in situations where the two tasks are associated with different contexts, being either a different colored background, a different area of workspace, or a different follow-through movement. However, many of these elements evoke explicit mechanisms that could contribute to storing separate (modular) memories. It remains to be shown whether simultaneous adaptation to multiple perturbations is possible when they are introduced in a fully implicit manner. Here we sought to test this possibility using a visuomotor perturbation small enough to eliminate explicit awareness. Participants (N=25) performed center-out reaching movements with a joystick to 5 targets located 72° apart. Depending on the target, visual feedback of cursor position was either veridical (1 target) or could be rotated by +5 or -5° (2 targets each). After 300 trials of adaptation (60 to each target), results revealed that participants were able to fully compensate for each of the imposed rotations. Moreover, when veridical visual feedback was restored, participants exhibited after-effects that were consistent with the rotations applied at each target. Questionnaires collected immediately after the experiment confirmed that none of the participants were aware of the perturbations. These results speak for the existence of implicit processes that can smoothly handle small and opposite visual perturbations when these are associated with distinct target locations. SIGNIFICANCE STATEMENT Simultaneous adaptation of reaching movements to opposite visuomotor perturbations has been shown to be possible mainly in situations where the two tasks are associated with different contexts. However, the relative contribution of explicit and implicit mechanisms has remained unclear. Here, by introducing visuomotor rotations small enough to rule out the implication of explicit mechanisms, we show that implicit processes alone are sufficient to handle multiple (even opposite) perturbations.