MAGNETOENCEPHALOGRAPHY - THEORY, INSTRUMENTATION, AND APPLICATIONS TONONINVASIVE STUDIES OF THE WORKING HUMAN BRAIN

Magnetoencephalography (MEG) is a noninvasive technique for investigat ing neuronal activity in the living human brain. The time resolution o f the method is better than 1 ms and the spatial discrimination is, un der favorable circumstances, 2-3 mm for sources in the cerebral cortex . In MEG studies, the weak 10 ff-1 pT magnetic fields produced by elec tric currents flowing in neurons are measured with multichannel SQUID (superconducting quantum interference device) gradiometers. The sites in the cerebral cortex that are activated by a stimulus can be found f rom the detected magnetic-field distribution, provided that appropriat e assumptions about the source render the solution of the inverse prob lem unique. Many interesting properties of the working human brain can be studied, including spontaneous activity and signal processing foll owing external stimuli. For clinical purposes, determination of the lo cations of epileptic foci is of interest. The authors begin with a gen eral introduction and a short discussion of the neural basis of MEG. T he mathematical theory of the method is then explained in detail, foll owed by a thorough description of MEG instrumentation, data analysis, and practical construction of multi-SQUID devices. Finally, several ME G experiments performed in the authors' laboratory are described, cove ring studies of evoked responses and of spontaneous activity in both h ealthy and diseased brains. Many MEG studies by other groups are discu ssed briefly as well.