MAGNETOENCEPHALOGRAPHIC CORTICAL RHYTHMS

We have characterized the magnetic 10- and 20-Hz rhythms recorded with a whole-scalp neuromagnetometer during different conditions. Sources of the posterior 10-Hz (alpha) rhythm clustered mainly around the pari eto-occipital sulcus and, to a lesser extent, around the calcarine sul ci, with several generators. Temporal Spectral Evolution (TSE) analysi s, used to follow event-related changes in the different frequency ban ds, showed strong dampening of the alpha within 200 ms after the appea rance of a visual stimulus and also during visual imagery. Suppression was often followed by a rebound above the baseline level. The rolandi c mu rhythm consisted of 10- and 20-Hz components with different react ivity and source locations. The 10-Hz component seems to be mainly som atosensory in origin whereas the 20-Hz signal also receives contributi ons from the motor cortex, and even shows 'motorotopy' in its reactivi ty: the source locations depend in a somatotopical manner on the site of the moving body part. The frequency composition of the posterior sp ontaneous activity was disturbed in patients with small infarcts of th e medial thalamus. It is shown with simulations that a surprisingly sm all number of synchronized cortical neurons could generate the major p art of the recorded oscillatory signal. Finally, some clarifications a re suggested to the terminology of brain rhythms. (C) 1997 Elsevier Sc ience B.V.