Noninvasive Stimulation of Contralateral Primary Motor Cortex Reduces the Amount of Skill Generalization to the Untrained Arm: Primary Motor Cortex and Intermanual Skill Generalization.

Successfully learned motor skills can generalize or transfer to the untrained arm. The neural substrate underlying such intermanual/interlimb generalization of newly acquired skill memory is unclear. Here, we focused on contralateral primary motor cortex (cM1), which is considered a key brain area for skill learning and memory consolidation. We probed the causal role of cM1 in intermanual skill generalization in a 2-day study involving right-handed young individuals (n = 31) who learned a novel motor skill reaching task. Immediately following (right-arm) learning, we delivered low-frequency (1 Hz, 1800 pulses) repetitive transcranial magnetic stimulation (rTMS) to target left cM1 in one group of individuals (n = 15), whereas another group (n = 16) served as an active control in which ipsilateral M1 (iM1) was targeted. On the same day, we measured corticospinal excitability (CSE) to assess learning-induced and rTMS-induced neuroplastic changes occurring in the targeted M1s. Next day after 24-h, both groups were tested for intermanual skill generalization (left arm), followed by a brief test of intralimb retention (right arm). Our results show that stimulating cM1, versus iM1, reduced the amount of generalization to the untrained arm on the next day, without affecting its (re)learning ability or the follow-up retention performance of the trained arm. Further, low frequency rTMS stimulation in our study surprisingly induced a net facilitation in CSE-with higher facilitation tending to correlate to lower generalization in a subset of high learners in the cM1 group. Taken together, this study highlights the role of cM1 in skill generalization such that it seems to mediate the early transfer of learning to the untrained arm.