Cerebral magnetic fields.

A critical presentation is given about the state-of-the-art of neuromagnetism, the study of neural functions by the measurement of magnetic fields. First, an introduction is given about the neural origin of electromagnetic fields. The forward and inverse problems are then studied. The forward problem can be stated as follows: given electrical currents in the brain, calculate the magnetic field. Correspondingly, the inverse problem is to calculate currents in the brain from the magnetic field measured outside the head. The instrumentation necessary for successful neuromagnetic measurements is discussed, and the emphasis is on new developments. The review of experimental neuromagnetic results discusses studies on spontaneous cerebral magnetic fields in normal subjects and in epileptic patients. Furthermore, evoked-field studies of the various sensory modalities as well as studies of event-related fields are reviewed. The relevance of various efforts in neuromagnetism and prospects for the future are discussed.