Neural mechanisms underlying the changes in ipsilateral primary motor cortex excitability during unilateral rhythmic muscle contraction.

The aim of this study was to investigate the neural mechanisms underlying the changes in the ipsilateral primary motor cortex (ipsi-M1) excitability induced during the unilateral rhythmic muscle contraction of the first dorsal interosseous (FDI) (rhythmic contraction) muscle with three different frequencies of auditory cues (1, 2, and 3 Hz). The effect of different frequencies of unilateral rhythmic contraction on changes in the ipsi-M1 excitability was assessed using a single-pulse transcranial magnetic stimulation (TMS) technique when subjects were performing the unilateral rhythmic contractions according to each auditory cue frequency. After that, the changes in short intracortical inhibition (SICI)/facilitation (ICF), long intracortical inhibition (LICI) within the ipsi-M1, and interhemispheric inhibition (IHI), as well as dorsal premotor cortex to M1 (PMd-M1), and dorsolateral prefrontal cortex to M1 (DLPFC-M1) connectivity from the contralateral hemisphere to the ipsi-M1 were assessed using paired-pulse TMS techniques. The motor evoked potentials (MEP) induced in the right FDI were recorded. In the results, the ipsi-M1 excitability induced in response to single-pulse TMS was significantly decreased in the 2 Hz conditions, compared with the 1Hz and 3Hz conditions. Furthermore, PMd-M1 connectivity and LICI were significantly modulated depending on the frequency of the unilateral rhythmic contraction. In contrast, the changes in the SICI, ICF, IHI, and DLPFC-M1 were not directly associated with the rhythm frequency. These results suggest that PMd-M1 connectivity and LICI within the ipsi-M1 are likely to preferentially operate to modulate ipsi-M1 excitability during the performance of unilateral rhythmic contraction with different frequencies.