Identifying cortical sources of incoherent fields of the human brain

The extracranial spontaneous magnetic field of the human brain changes over time. Unlike the stationary fields of evoked brain responses, these fields cannot be sampled at different places at different times. However, other things being equal, field power averaged over time is stationary. Sources of these stationary field power patterns, and of changes of level within these patterns, can be identified. Changes in levels of this ongoing activity are related to cognitive processes, and also to pathology. It is shown that mathematically unique solutions to this inverse problem can be obtained given information as to the underlying cortical geometry, thus permitting the identification of regions involved in cognitive processes and those that reflect some pathological states.