Magnetic stimulation: motor evoked potentials

Although the electrical excitability of the cerebral cortex was discovered more than 100 years ago by Fritz and Hitzig and Ferrier, it has proved dif®cult until recently to perform effective stimulation without removing the scalp and skull. The reason is that both these structures have a high resistance to the passage of electrical current. Thus, if stimulating electrodes are placed over an area of interest, then much of the applied stimulating current will ̄ow along the skin and subcutaneous tissues rather than penetrating into the brain. This causes contraction of nearby scalp muscles and cutaneous pain, and is considered to be rather uncomfortable by most individuals. Because of this, electrical stimulation of the brain through the skull, as introduced by Merton and Morton (1980), is only used infrequently. In 1985, Barker and colleagues introduced the technique of transcranial magnetic stimulation (Barker et al. 1985). In this method, a large electrical capacitance is charged to a high voltage and then discharged (short circuited) through a coil of insulated copper wire held on the scalp. A very large transient current ̄ows through the coil and produces a large magnetic ®eld at right angles to the coil. The skull and scalp presents little or no impedance to the passage of magnetic ®eld of this type and therefore it penetrates readily into the brain. Because the magnetic ®eld lasts only for a very short time reaching a peak of 1±2 T in about 50 ms and decaying to zero in the following half millisecond or so, it induces an electric current in any conductive structures nearby, including the brain. This induced electrical current can then activate neural tissue in the normal manner. The stimulation is not unpleasant since the electrical current induced on the scalp is very similar in magnitude to that induced in the brain, and therefore causes little local sensation. Ever since its introduction, there has been debate over which structures within the cerebral cortex are activated by transcranial magnetic stimulation. In animal experiments, direct electrical stimulation of the surface of the cortex 97 Recommendations for the Practice of Clinical Neurophysiology: Guidelines of the International Federation of Clinical Neurophysiology (EEG Suppl. 52) Editors: G. Deuschl and A. Eisen q 1999 International Federation of Clinical Neurophysiology. All rights reserved. Published by Elsevier Science B.V.