The human somatosensory cortex contributes to the encoding of newly learned movements.

Recent studies have indicated somatosensory cortex involvement in motor learning and retention. However, the nature of its contribution is unknown. One possibility is that the somatosensory cortex is transiently engaged during movement. Alternatively, there may be durable learning-related changes which would indicate sensory participation in the encoding of learned movements. These possibilities are dissociated by disrupting the somatosensory cortex following learning, thus targeting learning-related changes which may have occurred. If changes to the somatosensory cortex contribute to retention, which, in effect, means aspects of newly learned movements are encoded there, disruption of this area once learning is complete should lead to an impairment. Participants were trained to make movements while receiving rotated visual feedback. The primary motor cortex (M1) and the primary somatosensory cortex (S1) were targeted for continuous theta-burst stimulation, while stimulation over the occipital cortex served as a control. Retention was assessed using active movement reproduction, or recognition testing, which involved passive movements produced by a robot. Disruption of the somatosensory cortex resulted in impaired motor memory in both tests. Suppression of the motor cortex had no impact on retention as indicated by comparable retention levels in control and motor cortex conditions. The effects were learning specific. When stimulation was applied to S1 following training with unrotated feedback, movement direction, the main dependent variable, was unaltered. Thus, the somatosensory cortex is part of a circuit that contributes to retention, consistent with the idea that aspects of newly learned movements, possibly learning-updated sensory states (new sensory targets) which serve to guide movement, may be encoded there.

The Consolidation of Newly Learned Movements Depends upon the Somatosensory Cortex in Humans.

Studies using magnetic brain stimulation indicate the involvement of somatosensory regions in the acquisition and retention of newly learned movements. Recent work found an impairment in motor memory when retention was tested shortly after the application of continuous theta-burst stimulation (cTBS) to the primary somatosensory cortex, compared with stimulation of the primary motor cortex or a control zone. This finding that the somatosensory cortex is involved in motor memory retention whereas the motor cortex is not, if confirmed, could alter our understanding of human motor learning. It would indicate that plasticity in sensory systems underlies newly learned movements, which is different than the commonly held view that adaptation learning involves updates to a motor controller. Here we test this idea. Participants were trained in a visuomotor adaptation task, with visual feedback gradually shifted. Following adaptation, cTBS was applied either to M1, S1, or an occipital cortex control area. Participants were tested for retention 24 h later. It was observed that S1 stimulation led to reduced retention of prior learning, compared with stimulation of M1 or the control area (with no significant difference between M1 and control). In a further control, cTBS was applied to S1 following training with unrotated feedback, in which no learning occurred. This had no effect on movement in the retention test indicating the effects of S1 stimulation on movement are learning specific. The findings are consistent with the S1 participation in the encoding of learning-related changes to movements and in the retention of human motor memory.

Persistence of adaptation following visuomotor training.

Retention tests conducted after sensorimotor adaptation frequently exhibit a rapid return to baseline performance once the altered sensory feedback is removed. This so-called washout of learning stands in contrast with other demonstrations of retention, such as savings on re-learning and anterograde interference effects of initial learning on new learning. In the present study, we tested the hypothesis that washout occurs when there is a detectable discrepancy in retention tests between visual information on the target position and somatosensory information on the position of the limb. Participants were tested following adaptation to gradually rotated visual feedback (15° or 30°). Two different types of targets were used for retention testing, a point target in which a perceptual mismatch is possible, and an arc-target that eliminated the mismatch. It was found that, except when point targets were used, retention test movements were stable throughout aftereffect trials, indicating little loss of information. Substantial washout was only observed in tests with a single point target, following adaptation to a large amplitude 30° rotation. In control studies designed to minimize the use of explicit strategies during learning, we observed similar patterns of decay when participants moved to point targets that suggests that the effects observed here relate primarily to implicit learning. The results suggest that washout in aftereffect trials following visuomotor adaptation is due to a detectable mismatch between vision and somatosensation. When the mismatch is removed experimentally, there is little evidence of loss of information.NEW & NOTEWORTHY Aftereffects following sensorimotor adaptation are important because they bear on the understanding of the mechanisms that subserve forgetting. We present evidence that information loss previously reported during retention testing occurs only when there is a detectable discrepancy between vision and somatosensation and, if this mismatch is removed, the persistence of adaptation is observed. This suggests that washout during aftereffect trials is a consequence of the experimental design rather than a property of the memory system itself.