Different sites of action of electrical and magnetic stimulation of the human brain

The latency of the surface recorded electromyographic response to either an electrical or magnetic stimulus applied to the scalp has been measured in the first dorsal interosseous and abductor pollicis brevis muscles of 3 subjects. In the contracting muscle the response latency to the magnetic stimulus was longer by 1.4-2.7 ms compared to the electrical stimulus. Poststimulus time histograms of the firing of single motor units of first dorsal interosseous muscle were studied in 4 subjects. The first period of increased probability of firing of the single motor units showed a similar latency difference (mean 2.8 ms) to the two modes of stimulation. It is concluded that the extra delay to magnetic stimulation is consumed in central motor pathways. This implies that the two modes of stimulation activate the brain at different sites. It is suggested that the magnetic stimulus excites the corticospinal neurones transynaptically, whereas the electrical stimulus excites these neurones directly.