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Motor cortical (M1) transcranial magnetic stimulation (TMS) interventions increase corticospinal output and improve motor learning when delivered during sensorimotor mu rhythm trough but not peak phases, suggesting that the mechanisms supporting motor learning may be most active during mu trough phases. Based on these findings, we predicted that motor sequence learning-related corticospinal plasticity would be most evident when measured during mu trough phases.

Healthy adults were assigned to either a sequence or no-sequence group. Participants in the sequence group practiced the implicit serial reaction time task (SRTT), which contained an embedded, repeating 12-item sequence. Participants in the no-sequence group practiced a version of the SRTT that contained no sequence. We measured mu phase-independent and mu phase-dependent MEP amplitudes using EEG-informed single-pulse TMS before, immediately after, and 30 minutes after the SRTT in both groups. All participants performed a retention test one hour after SRTT acquisition.

In both groups, mu phase-independent MEP amplitudes increased following SRTT acquisition, but the pattern of mu phase-dependent MEP amplitude changes after SRTT acquisition differed between groups. Relative to the no-sequence group, the sequence group showed greater peak-specific MEP amplitude increases 30 minutes after SRTT acquisition. Further, the magnitude of these peak-specific MEP amplitude increases was negatively associated with the magnitude of sequence-specific learning.

Contrary to our original hypothesis, results revealed that motor sequence-specific learning elicits peak specific corticospinal plasticity. Findings provide first direct evidence for the presence of a mu phase-dependent motor learning mechanism in the human brain.

Tharan Suresh is a PhD candidate in the Movement and Cognitive Rehabilitation Science (MCRS) program at the University of Texas at Austin. Here, he is investigating the neurophysiological mechanisms of human motor learning. He received his bachelor’s degree in Clinical Neurophysiology from Christian Medical College, Vellore - India, where he developed cost-effective techniques to diagnose meralgia paresthetica and diabetic peripheral neuropathy. He then got a master’s degree in Cognitive Science from the Indian Institute of Technology, Gandhinagar - India, where he investigated motor memory reconsolidation. His long-term career goal is to develop a deeper understanding of the neurophysiological mechanisms that support the acquisition and retention of human motor memories and use this information to develop effective treatments for post-stroke motor impairments.

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