Magnetoencephalography: Applications in psychiatry

Magnetoencephalography (MEG) measures the extracranial magnetic fields prod uced by intraneuronal ionic current flow within appropriately oriented cort ical pyramidal cells. Based upon superconducting quantum interference devic e technology operating at liquid helium temperatures (4 K), MEG offers exce llent temporal and spatial resolution for selected sources, and complements information obtained from electroencephalograms and other functional imagi ng strategies, Current instrumentation permits recording up to several hund red channels simultaneously with head-shaped dewars, although the cast of s uch systems is high. The fact that magnetic fields fall off with the square of the distance from the source is both a benefit (when separating activit y in the two hemispheres) and a limitation (when attempting to record deep sources). The lack of skin contact facilitates using MEG to record direct c urrent and very high frequency (>600 Hz) brain activity. The clinical utili ty of MEG includes presurgical mapping of sensory cortical areas and locali zation of epileptiform abnormalities, and localization of areas of brain hy poperfusion in stroke patients. MEG studies in psychiatric disorders have c ontributed materially to improved understanding of anomalous brain laterali zation in the psychoses, have suggested that P50 abnormalities may reflect altered gamma band activity, and have provided evidence of hemisphere-speci fic abnormalities of short-term auditory memory function.