Pattern of motor evoked response to repetitive transcranial magnetic stimulation

The changes in motor pathway excitability, induced by pairs or trains of transcranial magnetic stimuli, were studied. The motor evoked potential (MEP) pattern depended on the interstimulus interval (ISI), the stimulus intensity and the type of coil employed. At low intensity, using either an 8-shaped or a circular coil, there was a test MEP inhibition at ISIs of 50-150 msec. During trains of stimuli, this inhibition showed a periodic trend with an interval of 250-300 msec. At high stimulus intensity we observed a progressive disappearance of test MEP inhibition which was incomplete with an 8-shaped coil and complete, reaching an MEP facilitation, with a circular coil. The inhibition observed at low intensity might be due to cortical inhibitory mechanisms. The effect found at high intensity and with the circular coil could depend on the activation of deeper and at higher threshold cortico-spinal structures. This hypothesis, however, does not explain the simultaneous delay of the test MEP latency which might depend on the activation of different cortico-spinal pathways.