Cortical correlate of the piper rhythm in humans

The electromyogram (EMG) of healthy humans demonstrates a tendency to rhyth mic oscillations at around 40 Hz (the Piper rhythm) during strong voluntary contraction. Why motor units should discharge synchronously locked to such a high-frequency is unclear. We recorded whole scalp magnetoencephalograph ic (MEG) signals simultaneously with surface EMG from 10 healthy subjects. In eight subjects, coherence and time domain analyses demonstrated correspo ndence between the MEG signal, originating near or in the hand region of th e motor cortex, and the 35- to 60-Hz EMG recorded during repeated maximal i sometric contractions of the contralateral forearm extensor muscles. Three of these subjects also showed similar coherence during isometric contractio ns of moderate strength and slow extension movements of the wrist. In addit ion, coherence and time domain analyses demonstrated correspondence between the MEG signals originating near or in the foot area of the motor cortex a nd EMG recorded during repeated maximal isometric contractions of the contr alateral tibialis anterior muscle in the 30- to 60-Hz range. Most important , the frequency at the peak of the coherence spectrum differed between fore arm and leg by as much as 10 Hz in the same subject. In contrast, the peak of the coherence spectrum occurred during sustained weak contraction in the 20- to 30-Hz range similarly for both forearm and foot. The lag between EM G and MEG activity in the leg was similar to 15 ms greater than that seen i n the forearm, an interval appropriate for conduction in fast pyramidal pat hways. It is concluded that the Piper rhythm in muscle may be driven by a c omparable oscillatory activity in the contralateral motor cortex. This cort ical rhythmicity can be picked up in several types of movement and seems di stinct from the 20- to 30-Hz rhythmicity recorded during weak sustained con tractions.