Repetitive magnetic stimuli over the motor cortex impair consolidation of a balance task by suppressing up-regulation of intracortical inhibition

Low-frequency repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex (M1) was shown to impair short-term consolidation of a balance task, emphasizing the importance of M1 in balance skill consolidation. However, the disruptive mechanisms of rTMS on neural consolidation processes and their persistence across multiple balance acquisition sessions remain unclear. GABAergic processes are crucial for motor consolidation and, at the same time, are up-regulated when learning balance skills. Therefore, this study investigated the impact of rTMS on GABA-mediated short-interval intracortical inhibition (SICI) and consolidation of balance performance. Participants (n=31) underwent six balance acquisition sessions on a rocker board, each followed by rTMS (n=15) or sham-rTMS (n=16). In the PRE-measurement, SICI was assessed at baseline and after balance acquisition with subsequent rTMS/sham-rTMS. In the POST-measurement, this procedure was repeated to assess the influence of motor memory reactivation on SICI. In addition, SICI-PRE and SICI-POST were compared to assess long-term processes. Both groups achieved similar improvements within the balance acquisition sessions. However, they did not consolidate equally well indicated by significant declines in performance for the rTMS group (p = 0.006) in the subsequent sessions. Both short-(p = 0.014) and long-term (p = 0.038) adaptations in SICI were affected by rTMS: while the sham-rTMS group up-regulated SICI, rTMS led to reductions in inhibition. The interfering effect of rTMS on both balance consolidation and up-regulation of SICI suggests that increased intracortical inhibition is an important factor to protect and consolidate the newly acquired motor memory.