Cortical Stimulation

Single-pulse TMS was performed using a hand-held figure-of-eight coil (90 mm outer diameter) connected to a magnetic stimulator (CCY-II, YIRUIDE Medical Equipment Co. Ltd., Wuhan, China) with maximal output of 2.2 T. The stimulation procedures for all participants were conducted by the same trained physician.

Each patient wore an elasticated head cap. The experimenter marked the nasion–inion and interaural lines, and then located the vertex (C _z) at the intersection of the above two lines according to the international 10–20 system. A grid of 1 cm squares covering a 10 × 10 cm area was then drawn on each side of the vertex by marking the scalp with an indelible marker. The anteroposterior rows of this grid were aligned parallel to the sagittal plane, while the mediolateral rows were aligned 90° to the sagittal plane.

TMS was then applied separately on random hemisphere in an anteroposterior direction, with the plane of the coil parallel to the scalp surface and the handle of the coil approximately 45° to the mid-sagittal line. The optimal sites for evoking the maximum submental MEPs from both hemispheres were identified as hotspots. According to Hamdy et al. [36] and Plowman-Prine et al. [37], a hot spot was positioned 2–4 cm anteriorly and 4–6 cm laterally.

We moved the coil one grid at a time in this area at an intensity of 100% of the stimulator output to obtain a response. Once identified, the location axis of the hotspots related to C _z was recorded and marked as “x” on the cap to ensure that the site could be reproducibly obtained. A series of cortical stimuli over this position was then performed, gradually decreasing by 5% stimulator output steps until threshold intensity was found that evoked pharyngeal EMG responses of 50 μV in at least 5 of 10 consecutive trials. Ten repeated single-pulse stimuli from both hemispheres were then applied at an intensity of 110% threshold with an interval of 5 s. Cortical stimulation was always performed between swallows.