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When cortex speaks to the muscleStephan Salenius
Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, 02015 HUT, Espoo, Finland
Although it is well known that the motor cortex gives rise to rhythmic activity and that activity of the spinal motor units is rhythmic, the connection between these phenomena has been highlighted only recently. Conway et al.  showed that the electromyogram (EMG) measured from the isometrically contracting first dorsal interrosseus muscle is coherent with the magnetoencephalogram (MEG) recorded approximately above the contralateral motor cortex, with the strongest coherence peak at about 15 - 30 Hz. In an independent experiment  we demonstrated similar results for voluntary self-paced finger-lifts. In a follow-up experiment  we extended the results of Conway et al.  by showing that during weak isometric contraction the coherent MEG activity is roughly somatotopically organized. In ad-dition, we observed that the latency between coherent cortical and muscular activity increased systematically with increasing cortico-muscular distance, always with the MEG signal leading the EMG signal. The results obtained with MEG in humans have been nicely complemented and confirmed by findings obtained by direct recordings from the exposed monkey cortex . We conclude that the 20 - 30 Hz os-cillatory activity paces the descending motor commands, which may increase the ef-fectiveness of motor commands in upholding a steady contraction.
Recently, Brown et al.  added a new dimension to the field of cortico-muscular coherence by showing that although the coherence occurs at 15 - 30 Hz during strong to moderate contractions, the strongest coherence during maximal contractions is found between 35 - 60 Hz. This fast 'Piper'  rhythm can be recorded during all grades of muscle contraction and may play a fundamental role in the control of such contractions.
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