Motor evoked potentials following highly frequent transcranial magnetoelectrical motor cortex stimulation: normal data and potential modulation by stimulation-dependent inhibitory and activating mechanisms.

OBJECTIVE

To determine the normal values (latency, amplitude) of motor evoked potentials (MEP) of the abductor pollicis brevis (APB) and tibialis anterior (TA) muscle after high-frequency repetitive transcranial magnetic stimulation of the motor cortex (rTCMS), and to evaluate stimulation-dependent MEP modulations.

PATIENTS AND METHODS

29 healthy volunteers underwent rTCMS with 2 and 4 stimuli. The interstimulus interval (ISI) was 2, 3, and 4 ms respectively, which corresponded to frequencies between 250 and 500 stimuli/s. The evoked potentials of the relaxed and voluntarily contracted APB and TA were registered.

RESULTS

Depending on the frequency and number of stimuli, the mean corticomuscular latency to the relaxed APB varied between 22.2 and 22.9 ms, and to the relaxed TA between 30.4 and 32.0 ms. The intra- and interindividual variability of the amplitudes was substantial. Voluntary contraction of the target muscle always led to a decrease in latency and increase in amplitude (p < 0.05).

CONCLUSION

The high variability of the amplitudes does not allow the computation of meaningful normal values. The latencies after rTCMS are close to those of normal data after single TCMS, which indicates that in awake humans identical cortical and spinal structures are similarly activated. The discrete variations of latency and amplitude after changing the frequency and stimulus number suggest that inhibitory and excitatory mechanisms on the cortical and/or spinal level modulate the muscle response.