Magnetic stimulation of the human brain: facilitation of motor responses by voluntary contraction of ipsilateral and contralateral muscles with additional observations on an amputee.

An intense rapidly changing magnetic field generated in a coil over the scalp can excite motor pathways to hand muscles. With a suprathreshold stimulus, the amplitude of the muscle action potential of the abductor digiti minimi is increased: by weak contraction of the muscle itself, by weak contraction of the ipsilateral first dorsal interosseus, by stronger contraction of the contralateral abductor digiti minimi, but not by contraction of the contralateral first dorsal interosseus, nor by the ipsilateral quadriceps muscle. This facilitation of response to brain stimulation may occur by two mechanisms, one related to the focussing of attention onto a particular hand, the second involving a rise in excitability of homologous motor pathways.