Excerpt from Motor Evoked Potentials

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Significant studies and landmarks on the effects of electricity on animals and human brains have been described in the past.

The most famous pioneers were Penfield and Boldrey, who made direct observations by stimulating the human brain with weak electrical shocks in conscious patients who were undergoing surgery. They constructed the homunculus, a caricature of the human form with body parts drawn in sizes that are proportional to the presumed extent of their representations.

From 1950-1970, several other studies of electrical stimulation of the exposed motor cortex (ie, during neurosurgical procedures) were performed in animals and humans to study the pyramidal pathway and other corticospinal connections. This technique was possible only in a surgical setting and not practical in a standard clinical environment.

Noninvasive elicitation of motor evoked potentials (MEPs) was made possible by Merton and Morton in 1980. They designed a high-voltage transcranial electrical stimulator that excited the motor cortex using cutaneous electrodes, which were placed over the scalp. After transcranial electrical stimulation (TES), a contraction of contralateral muscles is recorded in a conscious subject.

The usefulness of this method has remained limited by the local discomfort of the electrical currents that are applied over the scalp. An exception to this limitation is its use for intraoperative monitoring.

The development of transcranial magnetic stimulation (TMS) in 1985 opened new possibilities for MEP studies. Barker et al created a new type of cortical magnetic stimulator, based on the principle of electromagnetic induction. The device was composed of a main unit, which contains a bank of heavy-duty capacitors; the hand-held part was freely movable so that it could be placed over any part of the body. The investigator holds the stimulating coil tangentially over the motor cortex and a technician holds a digitizing pen over the stimulating coil to record its 3-dimensional position, which allows stereotactic mapping of the motor cortex. Motor evoked potentials are recorded with surface electrodes, which are placed over small hand muscles.

Although magnetic stimulation was first used to stimulate the peripheral nervous system (PNS) and muscles, cortical stimulation has become the focus of many studies.

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