FOCUSING AND TARGETING OF MAGNETIC BRAIN-STIMULATION USING MULTIPLE COILS

Neurones can be excited by an externally applied time-varying electrom agnetic field. Focused magnetic brain stimulation is attained using mu ltiple small coils instead of one large coil, the resultant induced el ectric field being a superposition of the fields from each coil. In mu ltichannel magnetic brain stimulation, partial cancellation of fields from individual coils provides a significant improvement in the focusi ng of the stimulating field, and independent coil channels allow targe ting of;he stimuli on a given spot without moving the coils. The probl em of shaping the stimulating field in multichannel stimulation is ana lysed, and a method is derived that yields the driving currents requir ed to induce a field with a user-defined shape. The formulation makes use of lead fields and minimum-norm estimation from magneto-encephalog raphy. Using these methods, some properties of multichannel coil array s are examined. Computer-assisted multichannel stimulation of the cort ex will enable several new studies, including quick determination of t he cortical regions, the stimulation of which disrupts cortical proces sing required by a task.