Previous studies have shown that synchronized beta frequency (14 -30 Hz) oscillations in the primary motor cortex are involved in maintaining steady contractions of contralateral arm and hand muscles. However, little is known about the role of postcentral cortical areas in motor maintenance and their patterns of interaction with motor cortex. We investigated the functional relations of beta-synchronized neuronal assemblies in pre-and postcentral areas of two monkeys as they pressed a hand lever during the wait period of a visual discrimination task. By using power and coherence spectral analysis, we identified a beta-synchronized largescale network linking pre-and postcentral areas. We then used Granger causality spectra to measure directional influences among recording sites. In both monkeys, strong Granger causal influences were observed from primary somatosensory cortex to both motor cortex and inferior posterior parietal cortex, with the latter area also exerting Granger causal influences on motor cortex. Granger causal influences from motor cortex to postcentral sites, however, were weak in one monkey and not observed in the other. These results are the first, to our knowledge, to demonstrate in awake monkeys that synchronized beta oscillations bind multiple sensorimotor areas into a large-scale network during motor maintenance behavior and carry Granger causal influences from primary somatosensory and inferior posterior parietal cortices to motor cortex.cerebral cortex ͉ motor maintenance ͉ parietal ͉ local field potential ͉ coherence O scillatory activity in the beta frequency range (14-30 Hz) is widely observed in sensorimotor cortex in relation to motor behavior in both humans (1, 2) and nonhuman primates (3-6). Specifically, beta oscillations in monkeys appear in local field potential (LFP) and spiking activity during tactile exploratory forelimb movements (4, 7, 8), movement preparation (5, 6, 9), and steady-state isometric contractions (10). Beta oscillatory activity is often observed to be synchronized between different parts of sensorimotor cortex (4,5,7,(9)(10)(11), between motor cortical LFPs and descending pyramidal tract neuron discharge (10, 12), between single motor units (13,14), and between motor cortical activity and muscle activity (1,4,10,12,15,16).Although the role of beta oscillations in the outflow of activity from motor cortex to muscles is relatively well characterized for certain types of behavior, the relation of beta oscillations in postcentral areas and motor cortex remains poorly understood. This article addresses the functional relations between beta oscillations in pre-and postcentral cortical areas in premovement motor maintenance behavior. It has long been proposed that behavior of this type depends on a corticoperipheral cortical sensorimotor loop (17, 18), and more recently that this loop is supported by oscillatory neuronal activity (19). Reports of beta oscillations in both somatosensory and motor cortices during premovement maintenance behavior (3, 5, 6, 9), taken toge...
