[Transcranial cortex stimulation with magnetic field pulses: methodologic and physiologic principles]

Abstract

The recently introduced method of painless transcranial brain stimulation using magnetic pulses has proved to be particularly suitable for exciting the motor cortex in conscious humans. The magnetic pulse is generated by a brief current pulse which passes through the stimulating coil, and the time varying magnetic field induces stimulating currents within the brain. Experimental evidence indicates that transcranial magnetic stimulation as opposed to electrical brain stimulation activated corticospinal neurones transsynaptically. With a circular stimulating coil centered near the vertex, upper limb muscles of the right side are preferentially activated with the inducing current clockwise viewed from above and vice versa. For the leg muscles the optimal position of the coil on the scalp is more critical and varies from subject to subject. Voluntary contraction of the target muscle reduces the threshold stimulus intensity, increases the amplitude and shortens the latency of the evoked compound muscle action potentials (CMAPs). This facilitatory effect of background muscle activity is most pronounced in the small hand muscles, where only a slight contraction is sufficient to greatly enhance the responses. In a relaxed small hand muscle facilitation of the responses can also be achieved by contraction of either the homologous contralateral or a neighbouring ipsilateral muscle. Even in relaxed state, the CMAPs show an inherent variability which is not related to the subject's motor readiness or mental state in any obvious way, provided the muscles' relaxed state is ascertained. The stimulus strength affects the amplitudes but not the latencies of the responses over a relatively wide range of suprathreshold intensities.