Triple transcranial static magnetic stimulation of a deep cortical area: Findings from the leg motor cortex

Transcranial static magnetic stimulation (tSMS) has been shown to reduce cortical excitability using a small but strong neodymium magnet. Recently, a triple tSMS system combining three magnets has been developed to generate stronger static magnetic fields. However, whether this system can effectively modulate neural activity in deeper cortical regions remains unexplored. To address this open question, we applied triple tSMS over the primary motor cortex (M1) leg area, which is located deep within the interhemispheric fissure. Healthy young adults received conventional single tSMS over the M1 leg area for 20 min in the first experiment and triple tSMS in the second experiment. Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation were recorded from the tibialis anterior (TA) and rectus femoris (RF) muscles before (pre), immediately after (post), and 10 min after stimulation (post10) in both experiments. The primary analysis demonstrated no differential effect of either single or triple tSMS relative to sham. However, secondary exploratory analyses revealed temporal changes in MEP amplitudes following triple tSMS, characterized by a transient decrease in TA MEP amplitude and a subsequent increase in RF MEP amplitude. These exploratory findings provide a rationale for future studies to test the hypothesis that triple tSMS can modulate corticospinal excitability in deep cortical regions.