NEURAL DISCHARGE AND LOCAL-FIELD POTENTIAL OSCILLATIONS IN PRIMATE MOTOR CORTEX DURING VOLUNTARY MOVEMENTS

The role of ''fast,'' or gamma band (20-80 Hz), local field potential (LFP) oscillations in representing neuronal activity and in encoding m otor behavior was examined in motor cortex of two alert monkeys. Using chronically implanted microwires, we simultaneously recorded LFPs and single or multiple unit (MU) discharge at a group of sites in the pre central gyrus during trained finger force or reaching movements, durin g natural reaching and grasping, and during quiet sitting. We evaluate d the coupling of oscillations with task-related firing at the same si te, the timing of oscillations with respect to the execution of traine d and untrained movement, and the temporal synchrony of oscillations a cross motor cortical sites. LFPs and neural discharge were examined fr om a total of 16 arm sites (7 sites in 1 monkey and 9 in the other), e ach showing movement-related discharge modulation and arm microstimula tion effects. In the trained tasks, fast LFP and MU oscillations occur red most often during a premovement delay period, ceasing around movem ent onset. The decrease in oscillation roughly coincided with the appe arance of firing rate modulation coupled to the motor action. During t his delay, LFP oscillations exhibited either ''overlapping'' or ''mixe d'' relationships with the simultaneously recorded neural discharge at that site. Overlap was characterized by coincident epochs of increase d neural discharge and LFP oscillations. For the mixed pattern, episod es of LFP oscillation typically coincided with periods of diminished f iring but overlap also sometimes appeared. Both patterns occurred conc urrently across motor cortex during preparation; LFP suppression with motor action was ubiquitous. Fast oscillations reappeared quickly upon transition from quiet sitting to resumption of task performance, indi cating an association with task engagement, rather than the general mo tor inaction of the delay period. In contrast to trained movements, fa st oscillations often appeared along with movement during untrained re aching, but oscillations occurred erratically and were not reliably co rrelated with elevated neural discharge. Synchronous oscillations occu rred at sites as much as 5 mm apart, suggesting widespread coupling of neurons and LFP signals in motor cortex. Widespread coupling of oscil latory signals is consistent with the concept that temporal coding pro cesses operate in motor cortex. However, because the relationship betw een neuronal discharge and the appearance of fast oscillations may be altered by behavioral condition. they must reflect a global process ac tive in conjunction with motor planning or preparatory functions, but not details of motor action encoded in neuronal firing rate.