Concurrent excitation of the opposite motor cortex during transcranial magnetic stimulation to activate the abdominal muscles

The study investigated the potential for stimulation of both motor cortices during transcranial magnetic stimulation (TMS) to evoke abdominal muscle responses. Electromyographic activity (EMG) of transversus abdominis (TrA) was recorded bilaterally in eleven healthy volunteers using fine-wire electrodes. TMS at 120% motor threshold (MT) was delivered at rest and during 10% activation at 1 cm intervals from the midline to 5 cm lateral, along a line 2 cm anterior to the vertex. The optimal site to evoke responses in TrA is located 2 cm lateral to the vertex. When bilateral abdominal responses were evoked at or lateral to this site, onset of ipsilateral motor evoked potentials (MEPs) were ∼3-4 ms longer than contralateral MEPs. The difference between latencies is consistent with activation of faster crossed-, and slower uncrossed-corticospinal pathways from one hemisphere. However, latencies of MEPs were similar between sides when stimulation was applied more medially and were consistent with concurrent activation of crossed corticospinal tracts on both sides. The findings suggest that stimulation of both motor cortices is possible when TMS is delivered less than 2 cm from midline. Concurrent stimulation of both motor cortices can be minimised if TMS is delivered at least 2 cm lateral to midline.