Magnetic mu rhythm in man.

We report detection of magnetic mu rhythm in four subjects using a large-area seven-channel first-order superconducting quantum interference device gradiometer. The polarity of this activity was opposite at the upper and lower ends of the rolandic fissure, and during the sharp transients the field patterns could be satisfactorily explained by a current dipole model. The equivalent dipoles were located close to the sources of the early somatosensory evoked field component N20m, which suggests that the mu rhythm is generated mainly at the primary somatosensory hand projection area. The frequency spectrum of the mu had major peaks around 10 and 21 Hz in all subjects. The high-frequency activity was blocked by clenching of the fist, but not by opening of the eyes, in agreement with characteristics of the electric mu rhythm.