EFFECT OF COIL POSITION AND STIMULUS-INTENSITY IN TRANSCRANIAL MAGNETIC STIMULATION ON HUMAN BRAIN

Evoked spinal cord potentials (ESCPs) from the cervical and high thora cic epidural space following transcranial magnetic stimulation were re corded from eight subjects in awake and anesthetized condition. Motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM) a nd rectus femoris (RF) muscles were simultaneously recorded Juring vol untary contraction. The stimulus intensity was at 30% above the MEPs t hreshold of the ADM when the coil center was fixed on 10-20 internatio nal Cz position. In awake condition, multiple ESCP components (greater than 3) were recorded from the cervical epidural space but no or mini mal components were recorded from the upper thoracic epidural space. W hen the coil was moved anteriorly so that the posterior edge of the co il was positioned on Cz, the amplitude of the first ESCP component was significantly increased (P<0.02) and shortened (not significant) at c ervical levels. In addition, several ESCP components were more evident at high thoracic levels. Although the amplitude of the ADM was not en hanced, that of the RF was enhanced. During general anesthesia with vo latile anesthetics (sevoflurane), only the first component of the ESCP s (D-wave) was elicited. Its amplitude was enhanced (P<0.02 when the c oil edge was fixed on Cz, similar to results in awake condition. This enhancement of the first ESCP component was accompanied by enhancement of those recorded from the high thoracic epidural space. However the amplitude of D-wave was the same in the two different coil positions w hen the stimulus intensity was set at 100% of the output. These result s suggest that at low stimulus intensity, positioning the coil edge on Cz is optimal in inducing D-wave effectively but at high stimulus int ensity, D-wave generation can be achieved in either if the two differe nt coil position. (C) 1997 Elsevier Science B.V.