Somatosensory evoked magnetic fields: relation to pre-stimulus mu rhythm

Objectives: Brain responses to auditory and visual stimuli have been previously shown to depend on the level of spontaneous brain activity in the 8‐13 Hz range. Our aim was to determine whether somatosensory evoked responses are influenced by ongoing rhythmic activity in the 8‐13 Hz frequency range originating in the sensorimotor cortex (mu rhythm). Methods: We used a whole-head 122 channel magnetoencephalography (MEG) system to record somatosensory evoked fields (SEFs) in response to median nerve stimulation in 11 subjects. Spontaneous oscillations in the 8‐13 Hz band over the contralateral sensorimotor cortex were evaluated in 3 different pre-stimulus time intervals using wavelet analysis. Results: The N20m SEF deflection did not depend on pre-stimulus activity, while the amplitude of the P35m deflection, and to a lesser extent that of the P60m deflection, showed a small positive correlation with the amplitude of the pre-stimulus mu rhythm. Although the amplitude of the mu rhythm varied by a factor of 2.3‐5, the maximum variations in P35m and P60m amplitude were only 21 and 12%, respectively. The latencies of the peaks were not affected by the strength of the pre-stimulus mu rhythm. Conclusions: It appears that the first excitatory cortical response (N20m) is independent of the oscillatory state (8‐13 Hz frequency range) of the sensorimotor cortex. Later parts of the response (P35m and P60m) are also relatively stable compared with the large variations in mu rhythm. q 2000 Elsevier Science Ireland Ltd. All rights reserved.