Spring 1995 Volume 4, Number 1

Examing the motorcortex without surgery

The human motor system can suffer afflictions ranging from traumatic damage to degenerative disease. It can be stimulated and evaluated directly during neurosurgery, but, in the clinic, examination has depended on putting patients through simple tasks - resisting the examiner's pull/push, squeezing fingers, etc. - that yield little for objective analysis of central pathways.

Now, new technology is making precise evaluation possible. "Transcranial magnetic stimulation (TCMS)," developed in England in the 1980's, uses a magnetic field to activate the cerebral cortex through the intact skull. Several groups in the US, including that of Keith Chiappa and Didier Cros at the Massachusetts General Hospital, are using TCMS to study cortical functions, not only of the motor system, but also of sensory and cognitive processes.

The motor cortex is activated by holding the stimulating coil (2-6 inches in diameter) over the appropriate scalp area. Stimulation causes muscles in the hands or feet to twitch, and the researchers record the associated electrical signals. They make similar measurements with the coil positioned over the neck or low back where the nerve roots are stimulated. At the MGH, the team has been able to evaluate patients with amyotrophic lateral sclerosis (Lou Gehrig's disease), multiple sclerosis, hysteria/malingering, cervical spondylosis and other conditions even if their motor problems are almost indiscernible clinically. They are also using TCMS to study normal motor system functions as well as language and short-term visual memory function.

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