Activation and suppression of the sternocleidomastoid muscle induced by transcranial magnetic stimulation

Responses in the stemocleidomastoid muscle (SCM) induced by transcranial magnetic stimulation (TMS) were investigated in 10 healthy subjects. Stimuli were given with the Dantec MagLiteTM magnetic stimulator using a 12.5 cm circular coil with counter-clockwise current direction. Monopolar needle electrodes with isolated shafts were used for simultaneous bilateral electromyographic (EMG) recordings of the SCM. TMS given on either side invariably induced an ipsilateral motor evoked potential of the SCM (SCM-MEP), whereas a contralateral SCM-MEP was just seen in 25% of the performed stimulation series also when maximal intensity was used. The SCM-MEPs recorded ipsilaterally to the side of stimulation had significantly higher maximal amplitudes (P < 0.05) compared to the SCM-MEPs recorded contralaterally (mean +/- S.D.: 1.0 +/- 0.5 and 0.2 +/- 0.1 mV, respectively). These results support the concept of a predominantly ipsilateral control of SCM activation. The contralateral SCM-MEPs tended to have a shorter latency than the ipsilateral SCM-MEPs. The thresholds of the ipsilateral SCM-MEPs were significantly higher (P < 0.01) on the left side than on the right side (mean +/- S.D.: 78 +/- 18 and 60 +/- 16 A/microseconds, respectively), which could be due to the consistent use of counter-clockwise coil current direction. TMS given on either side induced suppression of voluntary SCM activity in all investigated subjects. Responses induced by TMS given ipsilaterally and contralaterally to the voluntary activated muscle did not differ significantly (P > or = 0.05) regarding threshold or latency of the suppression.