Beyond motor cortex: A novel relationship between associative parietal cortex and ipsilateral silent period

Objectives: The ipsilateral silent period (iSP) is used to study interhemispheric control of voluntary motor output. It involves a brief suppression of electromyographic (EMG) activity in a muscle during isometric contraction, triggered by Transcranial Magnetic Stimulation (TMS) over the ipsilateral primary motor cortex (M1). The aim of this study was to investigate cortical activity associated with iSP. Methods: The study involved 28 healthy right-handed volunteers who performed maximal contractions of the left hand (unimanual) or both hands (bimanual) across conditions with and without TMS. We combined TMS to left M1 with functional Near-Infrared Spectroscopy (fNIRS) over the right hemisphere. EMG was recorded from the hand muscles to quantify the iSP. Results: A significantly greater normalized iSP area was found in the TMS bimanual condition compared to the unimanual one, with a strong correlation between the two. fNIRS revealed higher oxyhemoglobin (HbO) concentration changes in the No TMS condition than in the TMS conditions. Specifically, TMS induced HbO decreases in motor, prefrontal, premotor, parietal, and temporal areas. Reductions in premotor and parietal areas correlated with M1 activity decrease only in unimanual condition. In TMS conditions, a positive correlation was found between fNIRS HbO values in associative parietal cortex and the normalized iSP area: the higher the HbO concentration changes, the greater the inhibition. Conclusions: These findings show reduced M1 activity and demonstrate that a broader frontoparietal network is influenced by the transcallosal motor output. Significance: These findings contribute to a better understanding of interhemispheric inhibition and may have implications for the study of neurological disorders.