Somatosensory information drives modification of a motor memory

The role of somatosensory information in motor memory modification has remained controversial. One view holds that somatosensory information plays only a supportive role, such as feedback and sensory calibration, and is not directly involved in memory modification itself. An alternative view is that afferent somatosensory information itself can drive modification of motor memory. Here we resolve this controversy by isolating somatosensory information from voluntary motor execution using a finger-exoskeleton robot. Participants first voluntarily performed and learned a sequential finger movement task. On subsequent days, they underwent either exoskeleton-based reinstatement, which reproduced their own learned movements without voluntary execution, or voluntary reinstatement, in which they executed the same task as in the initial training. Exoskeleton-based reinstatement led to subsequent performance gain in the sequence task to a degree comparable to voluntary reinstatement. This gain was not explained by reproduction of visual information, mental rehearsal, enhanced arousal, or warm-up effects induced by exoskeleton-driven finger movements during the reinstatement sessions. Rather, the results support the idea that reinstatement of somatosensory information alone is sufficient to drive the performance gain. We further found that exoskeleton-based exposure to a sequence that was only partially matched with the trained sequence yielded performance gains, whereas voluntary execution of the same partially-matched sequence did not. This dissociation suggests that a relevant motor skill memory is broadly activated by somatosensory information, while voluntary execution activates memory in a sequence-specific manner. Together, these findings resolve a long-standing controversy in motor learning research by showing that afferent somatosensory information enables the modification of sequential motor memory through a route distinct from voluntary motor execution.