Desynchronization and synchronization of central 20-Hz rhythms associated with voluntary muscle relaxation: a magnetoencephalographic study

To clarify the cortical mechanisms involved in motor inhibition, modulation of cortical rhythms around 20 Hz during voluntary muscle relaxation was co mpared to that during muscle contraction in ten normal volunteers, using a whole head type neuromagnetometer. Each subject relaxed or contracted the r ight forearm muscles, from which electromyograms were recorded. After bandp ass filtering magnetoencephalographic signals into frequency bands of 6-10, 10-14, 14-18, 18-22, 22-26, and 26-30 Hz, the signals of each frequency ba nd were rectified and averaged with respect to the onset of motor trial. Th e relaxation task showed movement-related 20-Hz desynchronization over bila teral central areas beginning a few seconds before the termination of muscl e contraction. Twenty-hertz desynchronization was present also for the cont raction task at the same location within each subject. The two tasks had a positive correlation among subjects in both the percent decrease (r(2)=0.76 , P<0.01) and the peak time (r(2)=0.61, P<0.05) for the contralateral 20-Hz desynchronization. After the muscle relaxation, desynchronization was foll owed by conspicuous movement-related synchronization of the 20-Hz passband in the contralateral central areas, which was significantly larger than tha t after the contraction (P<0.001). The results suggest that the voluntary m uscle relaxation involves the modulation of central rhythms starting a few seconds before the actual event, and the 20-Hz desynchronization has a simi lar temporal property in the muscle relaxation and contraction. The 20-Hz s ynchronization in the contralateral central area after the muscle relaxatio n may be associated with the temporally arrayed termination of the ongoing muscle contraction.