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 poten tial (MEP) pattern depended on the interstimulus interval (ISI), the s timulus intensity and the type of coil employed. At low intensity, usi ng either an and shaped or a circular coil, there was a test MEP inhib ition at ISIs of 50-150 msec. During trains of stimuli, this inhibitio n showed a periodic trend with an interval of 250-300 msec. At high st imulus intensity we observed a progressive disappearance of test MEP i nhibition which was incomplete with an 8-shaped coil and complete, rea ching an MEP facilitation, with a circular coil. The inhibition observ ed at low intensity might be due to cortical inhibitory mechanisms. Th e effect found at high intensity and with the circular coil could depe nd on the activation of deeper and at higher threshold cortico-spinal structures. This hypothesis, however, does not explain the simultaneou s delay of the test MEP latency which might depend on the activation o f different cortico-spinal pathways.