Motor evoked potentials obtained with double magnetic stimulation of the motor cortex.

The technique of motor evoked potentials (MEP) obtained with single and dou ble magnetic stimulation of the motor cortex in man has considerably improv ed over the past decade. We present the techniques and parameters involved in double magnetic stimulation for clinical purposes. Method - The conditioning-test design is used to study modifications in the amplitudes of the muscular responses to the "test" a shock, recorded on th e first dorsal interosseus muscle. Enhanced amplitudes of conditioned respo nses indicate facilitation. reduced response inhibition. Results - The effects vary according to the shock intensity, the delay betw een shocks and the position of the conditioning coil The latter may be loca ted at the same place as the rest shock (to rest interneural circuitry rela ted to pyramidal tract), on the hand area opposite the test shock (to test interhemispheric influences), or over the cerebellar area contralateral to the test sine (to test the effect of cerebellar stimulations over the motor cortex). When the coils were located on the same conical hand area there w as facilitation when the intensities rr ere both set at the threshold with an interstimulus interval (ISI) between 1 and 2.5-3 ms. At conditioning sho ck intensities below the threshold and the test shock 150% above, inhibitio n occurred at ISI 1-5 ms followed by facilitation at ISI 15-35 ms. When the intensities of both shocks rr ere 150% above threshold there were two clea r cut individual responses at ISI above 10 ms; facilitation was recorded at ISI 15-35 ms, and inhibition between 55 and 255 ms. When the conditioning coil was located on the opposite hand area from the test shock (conditionin g shock intensity supramaximal. rest shock intensity above the threshold), ISI 1-5 ms facilitation occurred followed by inhibition up to ISI 30 ms. Wh en the conditioning shock (intensity supramaximal) was located on the cereb ellar area contralateral to the rest sine (intensity above the threshold), inhibition occurred at ISI 5 ms. Conclusions - Double magnetic stimulations delivered over the same cortical area reflect facilitatory and inhibitory influences over the pyramidal tra ct controlled by interneurons, i.e.. these tests investigate the intrinsic circuitry of the motor strip. Double magnetic stimulations delivered on eac h motor area study interhemispheric influences mediated by the corpus callo sum which are facilitatory and inhibitory Double magnetic stimulations deli vered on the cerebellar area demonstrates inhibitory influences over the co ntralateral cerebral motor cortex. (C) Elsevier, Paris.