COHERENT OSCILLATIONS IN MONKEY MOTOR CORTEX AND HAND MUSCLE EMG SHOWTASK-DEPENDENT MODULATION

1. Recordings were made of local field potential (slow waves) and pyra midal tract neurone (PTN) discharge from pairs of sites separated by a horizontal distance of up to 1.5 mm in the primary motor cortex of tw o conscious macaque monkeys performing a precision grip task. 2. In bo th monkeys, the slow wave recordings showed bursts of oscillations in the 20-30 Hz range. Spectral analysis revealed that the oscillations m ere coherent between the two simultaneously recorded cortical sites. I n the monkey from which most data were recorded, the mean frequency of peak coherence was 23.4 Hz. 3. Coherence in this frequency range was also seen between cortical slow wave recordings and rectified EMG of h and and forearm muscles active during the task, and between pairs of r ectified EMGs. 4. The dynamics of the coherence were investigated by a nalysing short, quasi-stationary data segments aligned relative to tas k performance. This revealed that the 20-30 Hz coherent oscillations w ere present mainly during the hold phase of the precision grip task. 5 . The spikes of identified PTNs were used to compile spike-triggered a verages of the slow wave recordings. Oscillations were seen in 11/17 a verages of the slow wave recorded on the same electrode as the trigger ing spike, and 11/17 averages of the slow wave recorded on the distant electrode. The mean period of these oscillations was 45.8 ms. 6. It i s concluded that oscillations in the range 20-30 Hz are present in mon key motor cortex, are coherent between spatially separated cortical si tes, and encompass the pyramidal tract output neurones. They are disce rnable in the EMG of active muscles, and show a consistent task-depend ent modulation.