Transcranial electrical and magnetic motor cortex stimulation: studies in intact man

The new method of transcranial electrical and magnetic brain stimulation was tested in 41 normal subjects. Stimulation on the scalp excites corticospinal neurones in the motor cortex while stimulation over the spine excites spinal nerve roots. The difference between EMG response latencies after both stimulations represents conduction in the central motor pathways and is called central motor latency (CML). The aim of out experiments was to investigate certain methodological aspects of the technique in order to standardize the procedure. Recordings were done from slightly contracted abductor digiti minimi and tibialis anterior muscles after electrical stimulation on the scalp and from the relaxed and contracted abductor digiti minimi and biceps brachii muscles after magnetic brain stimulation. Stimulation over the spine (C7/T1 interspace in case of upper limb muscles stimulation and T12/L1 interspace in case of tibialis anterior stimulation) was always electrical. Using a rather weak non-commercial magnetic stimulator we were not able to activate lower limb muscles, neither we succeeded to evoke responses from the relaxed arm muscles in all subjects. Electrical scalp stimulation proved successful in all cases. Muscle response after cortical stimulation in contracting muscles and shorter latencies and provided more accurate estimate of conduction time in the central motor pathways than responses in the relaxed muscles. Latencies should be measured from several superimposed responses and not from averaged ones. The intensity of stimulation over the neck did not affect CML. We nevertheless suggest that the strongest stimulus intensities should not be used in order to avoid CML overestimation.