Implicit processes do not contribute to learning to reach in small mirror reversed environments

Learning to reach with a small visuomotor rotation (VR; a rotation of visual feedback relative to hand motion) has been shown to arise unconsciously (i.e., implicitly). Whether the same processes support learning in a small mirror reversal (MR), where feedback is reflected across the body midline, remains unknown. To address this gap, we asked whether implicit processes contribute to learning in a small MR. Forty-two right-handed participants reached to targets located 10° to the left and right of body midline using a Kinarm exoskeleton robot. Half of the participants experienced a VR distortion (VR group), which consisted of a 20° clockwise or counterclockwise cursor rotation. The remaining participants experienced a 20° MR distortion (MR group), where cursor feedback was reflected across body midline (y-axis). Following reaches with a VR or MR distortion, participants completed assessment trials in which they reached in the absence of cursor feedback to assess implicit learning. Analysis of angular errors (AE) revealed that all participants in the VR group learned to reach with the VR distortion, however, only 55% of MR participants learned to reach with the MR distortion. AEs on the no-cursor trials revealed that only the VR group engaged in implicit learning. These findings demonstrate that MR learning, even when small MR distortions are introduced, is not supported by implicit learning. The absence of implicit learning in MR provides evidence that MR is a different form of learning (i.e., skill acquisition) compared to VR learning (i.e., motor adaptation).