Magnetic Stimulation of the Nervous System

In 1985 Barker and his colleagues (Barker et al., 1985a) produced the first practical magnetic stimulator which, when placed over the skull, was capable of exciting neurons within the human brain. The advance for the study of neural function in intact man was enormous in practical terms because the technique was non-invasive, painless and well tolerated by subjects, even children. In contrast, electrical stimulation via an electrode placed on the skull is quite painful and not well tolerated even by highly motivated subjects such as members of a research team. The application of electrical current to the scalp for stimulation of brain cells is limited by the fact that current flow is attenuated by skin and bone. The intensity of surface current needed to achieve excitation of nerve cells at a depth within the brain needs to be so high that it excites small myelinated axons and unmyelinated axons of free nerve endings in the skin of the scalp and the meninges - hence the pain. The rapidly changing magnetic field produced by a brief current pulse in a wire coil placed over the head is not attenuated by tissues of the head. Although there is a rapid decrease in intensity of the magnetic field with distance from the coil, the magnetic stimulators currently available commercially are able to stimulate neurons within the grey matter of the cerebral cortex but do not appear capable of exciting axons deep within the white matter or nuclei below the cortex.