~ Pergamon 0306-4522(94)E0020-5 Neuroscience Vol. 60, No. 2, pp. 537-550, 1994 Elsevier ScienceLtd Copyright © 1994IBRO Printed in Great Britain.All rights reserved 0306-4522/94$7.00+ 0.00 SPATIOTEMPORAL CHARACTERISTICS OF SENSORIMOTOR NEUROMAGNETIC RHYTHMS RELATED TO THUMB MOVEMENT R. SALMELIN* and R. HARI Low Temperature Laboratory, Helsinki University of Technology, Otakaari 3A, SF-02150 Espoo, Finland Abstract--To assess the spatial extent and temporal behavior of rolandic rhythms we recorded neuromagnetic signals from four healthy subjects with a 24-channel magnetometer. The subjects performed self-paced thumb movements or the motions were triggered by electrical stimulation of the median nerve at the wrist. The main frequency components of the magnetic mu rhythm signals centered at 10 and 20 Hz. Both components were completely suppressed during the movement and increased substantially 0.5-2.5 s after it; the 20-Hz component reacted about 300 ms faster. The rebound was stronger after self-paced than after stimulated motion, and after contra- than after ipsilateral movement. The reactive source areas were identified for both frequency ranges, and they clustered on partly overlapping cortical areas of 6-8 cm 2 wide along the course of the central sulcus. The 10-Hz rhythmic oscillations occurred predominantly at the primary somatosensory hand cortex; the sources of the 20-Hz signals were slightly more anterior. We hypothesize that the 10-Hz signal is a true somatosensory rhythm whereas the 20-Hz activity is essentially somatomotor in origin. Awake human and animal brains exhibit a wide variety of rhythmic signals in the 5-15 Hz (generally denoted as "alpha"), 13-35 Hz ("beta"), and higher ("gamma") frequency ranges. Although commonly referred to as "spontaneous", those signals are closely connected to external events. The somatomotor activity in the 10-Hz range is strongest during missing or monotonous peripheral input, whereas activity around 20 Hz emerges in situations requiring high vigilance and attention. 4's'~s The rolandic mu rhythm was described in detail by Gestaut. 6 This "rhythme en arceau", so denoted owing to its arch-like shape, is centered in the medial parts of the primary somatomotor cortex (SmI). Mu rhythm's close connection to the somatomotor system is further supported by its reactivity: move- ments of the contralateral hand or--to a lesser extent, of foot or of ipsilateral limbs--block it. Suppression of the rolandic 20-Hz activity due to somatosensory stimulation had been observed earlier) ° Preparation for a movement or even imagining it, as well as tactile stimulation, also block the rolandic 10- and 20-Hz *To whom correspondence should be addressed. Abbreviations: CFD, correlated-frequency-domain analysis; ECD, equivalent current dipole; EEG, electro- encephalography; EMG, electromyogram; EOG, electro-oculogram; FFT, fast Fourier transform; MEG, magnetoencephalography; MRI, magnetic resonance imaging; PPI, probe position indicator; SEF, somato- sensory evoked field; SII, second somatosensory cortex; SmI, primary somatomotor cortex; SQUID, supercon- ducting quantum interference device; TSE, temporal- spectral~volution analysis. activities. The effect caused by a voluntary movement is usually more dramatic than that brought about by stimulation of the peripheral nerves: Rhythmic mu oscillations reappear in a couple of seconds after the motion or, if the tension persists, even during the action. 5,2°-22 Immobilization of the whole body often enhances somatosensory rhythms. 28 Spectral analysis, combined with appropriate func- tional tests, has revealed the existence of reactive sensorimotor activity in most people. 14,24 Magnetoencephalographic (MEG) mu rhythm, similar to its electric counterpart in shape and reac- tivity, has also been observed) ° It originated close to the hand projection area in the SmI cortex, identified by evoked responses to median nerve stimulation. MEG is particularly suited for source localization, since the skull and the scalp do not blur the magnetic fields. 7 We have investigated neuromagnetic rhythmic activity of the human somatomotor cortex. Our aims were (i) to follow the dynamics of the main frequency components of the activity related to thumb move- ment, and (ii) to find the spatial spread of cortical generators involved in the process. This type of characterization lays the necessary basis for under- standing the functional significance of human cerebral rhythms. EXPERIMENTAL PROCEDURES Subjects and magnetoencephalographic recording Four healthy male members of laboratory personnel volunteered as subjects (Ss 1-4; age 29-43 years). During 537