Coherent oscillations in monkey motor cortex and hand muscle EMG show task‐dependent modulation

Abstract

Recordings were made of local field potential (slow waves) and pyramidal tract neurone (PTN) discharge from pairs of sites separated by a horizontal distance of up to 1.5 m m in the primary motor cortex of two conscious macaque monkeys performing a precision grip task. In both monkeys, the slow wave recordings showed bursts of oscillations in the 20–30 Hz range. Spectral analysis revealed that the oscillations were coherent between the two simultaneously recorded cortical sites. In the monkey from which most data were recorded, the mean frequency of peak coherence was 23.4 Hz. Coherence in this frequency range was also seen between cortical slow wave recordings and rectified EMG of hand and forearm muscles active during the task, and between pairs of rectified EMGs. The dynamics of the coherence were investigated by analysing short, quasi‐stationary data segments aligned relative to task performance. This revealed that the 20–30 Hz coherent oscillations were present mainly during the hold phase of the precision grip task. The spikes of identified PTNs were used to compile spike‐triggered averages of the slow wave recordings. Oscillations were seen in 11/17 averages of the slow wave recorded on the same electrode as the triggering spike, and 11/17 averages of the slow wave recorded on the distant electrode. The mean period of these oscillations was 45.8 ms. It is concluded that oscillations in the range 20–30 Hz are present in monkey motor cortex, are coherent between spatially separated cortical sites, and encompass the pyramidal tract output neurones. They are discernable in the EMG of active muscles, and show a consistent task‐dependent modulation.