Oscillations in the premotor cortex: single-unit activity from awake, behaving monkeys

We examined single-unit activity in the dorsal premotor cortex for evidence of fast neuronal oscillations. Four rhesus monkeys performed a task in whi ch visuospatial instruction stimuli indicated the direction of forelimb mov ement to be executed on each trial. After an instructed delay period of 1.5 -3 s, movements to either the right or left of a central origin were trigge red by a second visuospatial stimulus. From a database of 579 single units, 78 units (13%) contained periodic peaks in their autocorrelation histogram s (ACHs), with oscillation frequencies typically 20-30 Hz (mean 27 Hz). An additional 26 units (5%) had oscillatory features that were identified in j oint interspike-interval (ISI) plots. Three observations, taken together, s uggest entrainment by rhythmic drive extrinsic to these neurons: shuffling ISIs attenuated ACH peaks, indicating a dependency on serial-order effects; oscillation frequency did not change during either increases or decreases in firing rate; and joint ISI plots contained features consistent with a rh ythmicity interrupted by intervening discharges. In some cells, oscillation s occurred for only one of the two directions of movement. During the delay period, such directional selectivity was observed in 37 units (60% of dela y-period oscillators). For at least 17 of these units, we could exclude the possibility that oscillatory directional selectivity resulted from the dif ficulty in detecting oscillations due to low discharge rates (for one of th e two movement directions). Directional selectivity in fast oscillations sh ows that they can reflect specific aspects of an intended action.