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

Learning to reach with a small visuomotor rotation (VR), where visual feedback regarding hand motion is rotated relative to one's hand trajectory, has been shown to arise unconsciously (i.e., implicitly). It is unclear whether implicit processes also support learning to reach with a small mirror reversal (MR), where visual feedback regarding hand position is reflected across the body midline. To address this gap, we asked whether implicit processes contribute to learning to reach with a small MR distortion. Forty-two right-handed participants reached to targets located 10° to the left and right of the body midline using a Kinarm exoskeleton robot. Half of the participants experienced a VR distortion (VR participants), in which cursor feedback was rotated 20° clockwise or counterclockwise relative to hand motion. The remaining participants experienced a small MR distortion (MR participants) of a similar magnitude (20°), such that cursor feedback was reflected across the body midline (y-axis). Following reaches with a VR or MR distortion, participants completed assessment trials in which they reached without cursor feedback to assess implicit learning. Analysis of angular errors (AE) revealed that all VR participants learned to reach with the VR distortion, however, only 55% of MR participants learned to reach with the MR distortion (MR-L group). AEs on the no-cursor assessment trials revealed that VR participants engaged in implicit learning. However, there was no evidence of implicit learning in the MR participants, regardless of whether they learned to reach with the MR distortion or not. These findings demonstrate that even when participants learn to reach with small MR distortions, implicit learning processes are not engaged and do not support motor learning. The absence of implicit learning when reaching with a MR distortion compared to a VR distortion suggests that MR and VR learning may be distinct forms of motor learning (e.g., skill acquisition versus motor adaptation respectively).