Evoked alpha- and mu-rhythm in humans: a neuromagnetic study.

We present the results of a neuromagnetic study on the spatial structure of brain rhythms enhanced by photic and somatosensory stimulation, as measured on the occipital, rolandic and frontorolandic regions in humans. It emerges that, while it is always possible to drive the cerebral activity during sustained stimulation at any given frequency, only certain specific frequencies can produce prolonged synchronization (i.e. the oscillating activity elicited by the repetitive stimulation continues well beyond its termination). In both studied modalities we were able to localize equivalent sources for the synchronized responses; their relationship with the known evoked responses is discussed. In the visual modality the synchronization was characterized by a potentiation of the subjects' alpha-rhythm. In the somatosensory modality synchronization was reflecting two different activities: one probably related to the rolandic mu-rhythm, the second suggesting the presence of two widely separated and time correlated sources possibly driven by a unique, deep clock. Possible implications for other studies of the dominant brain rhythms, or experimental checks on specific brain models, as well as of the visual and somatosensory evoked responses are discussed.