Predominant activation of I1-waves from the leg motor area by transcranialmagnetic stimulation

We performed transcranial magnetic stimulation (TMS) to elucidate the D- an d I-wave components comprising the motor evoked potentials (MEPs) elicited from the leg motor area, especially at near-threshold intensity. Recordings were made from the tibialis anterior muscle using needle electrodes. A fig ure-of-eight coil was placed so as to induce current in the brain in eight different directions, starting from the posterior-to-anterior direction and rotating it in 45 degrees steps. The latencies were compared with those ev oked by transcranial electrical stimulation (TES) and TMS using a double co ne coil. Although the latencies of MEPs ranged from D to I3 waves, the most prominent component evoked by TMS at near-threshold intensity represented the I1 wave. With the double cone coil, the elicited peaks always represent ed I1 waves, and D waves were evoked only at very high stimulus intensities , suggesting a high effectiveness of this coil in inducing I1 waves. Using the figure-of-eight coil, current flowing anteriorly or toward the hemisphe re contralateral to the recorded muscle was more effective in eliciting lar ge responses than current flowing posteriorly or toward the ipsilateral hem isphere. The effective directions induced I1 waves with the lowest threshol d, whereas the less effective directions elicited I1 and I2 waves with a si milar frequency. Higher stimulus intensities resulted in concomitant activa tion of D through I3 waves with increasing amount of D waves, but still the predominance of I1 waves was apparent. The amount of I waves, especially o f I1 waves, was greater than predicted by the hypothesis that TMS over the leg motor area activates the output cells directly, but rather suggests pre dominant transsynaptic activation. The results accord with those of recent human epidural recordings. (C) 2000 Elsevier Science B.V. All rights reserv ed.