SMC-M1 Connectivity and Motor Sequence Learning: A TMS Study

Abstract

We experience various types of motor learning throughout our lives. As children, we learned to walk unconsciously. As we grow older, we continue to learn skills like sports or musical instruments. In this study, the primary motor cortex (M1) and the supplementary motor complex (SMC), which are brain regions involved in motor sequence learning but where many aspects still remain unclear, were mainly investigated, and the connectivity of these two regions was measured using the paired-pulse transcranial magnetic stimulation (ppTMS) technique. In Experiment I, sixty-three right-handed undergraduate students participated. Conditioning stimulus (CS) was administered at SMC (i.e., 4 cm anterior to Cz), and test stimulus (TS) was applied at M1. As a result, consistent with the previous ppTMS studies, a facilitatory influence was observed between SMC and M1. In Experiment II, fifty-one right-handed undergraduate students participated. After measuring baseline SMC-M1 connectivity in the same way as in Experiment I, individuals practiced one of three motor sequential tasks (i.e., implicit, explicit, or random sequence task). After practice, ppTMS as post-training stimulation was administered in the same way as the baseline stimulation to investigate the changes in the connectivity between SMC and M1. As a result, it was found that the facilitatory influence decreased after the explicit sequence task that involved motor chunking. This may be because SMC plays a role in motor chunking. In Experiment III, instead of a motor task, intermittent theta-burst stimulation (iTBS) was administered at SMC between two ppTMS sessions, and the connectivity changes were investigated. Similar to the influence of the explicit sequence task in Experiment II, the facilitatory influence decreased in the iTBS group. Although the mechanisms are different from each other, iTBS appears to induce post-synaptic plasticity in the cortico-basal ganglia-thalamic network. After post-stimulation of ppTMS, participants performed the explicit sequence task used in Experiment II twice (i.e., practice and retention test) to reveal the effect of iTBS on motor sequence learning. In the iTBS group, the offline improvement was disturbed at concatenation points compared to the Rest group. In the future, follow-up research that dissociates motor chunking conditions using neuroimaging techniques seems necessary