J Neurophysiol. 2020 Oct 1;124(4):1122-1130. doi: 10.1152/jn.00242.2020. Epub 2020 Sep 9.
Sensorimotor learning is thought to entail multiple learning processes, some volitional and others automatic. A new method to isolate implicit learning involves the use of a “clamped” visual perturbation in which, during a reaching movement, visual feedback is limited to a cursor that follows an invariant trajectory offset from the target by a fixed angle. Despite full awareness that the cursor movement is not contingent on their behavior, as well as explicit instructions to ignore the cursor, systematic changes in motor behavior are observed, and these changes have the signatures of motor adaptation observed in studies using classic visuomotor perturbations. Although it is clear that the response to clamped feedback occurs automatically, it remains unknown whether participants are sensitive to the large deviations in hand position that occur during adaptation. To address this question, we used the clamp method and asked participants to report their hand position after each reach. As expected, we observed robust deviations in hand angle away from the target (average of ∼18°). The hand reports also showed systematic deviations over the course of adaptation, initially attracted toward the visual feedback and then in the opposite direction, paralleling the shift in hand position. However, the shift in perceived hand position was subtle, reaching only ∼2° at asymptote. These results confirm that participants have limited awareness of the behavioral changes that occur during sensorimotor adaptation while revealing the impact of feedforward and feedback signals on their subjective experience.NEW & NOTEWORTHY Sensorimotor adaptation operates in an obligatory manner. Qualitatively, subjective reports obtained after adaptation demonstrate that, in many conditions, participants are unaware of significant changes in behavior. In the present study, we quantified participants’ sensitivity to these adaptive changes by obtaining reports of hand position on a trial-by-trial basis. The results confirm that participants are largely unaware of adaptation but also reveal the subtle influence of feedback on their subjective experience.