Beta oscillations in a large-scale sensorimotor cortical network: directional influences revealed by Granger causality

Abstract

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 large-scale 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.

Fig. 1.

Mean power spectra computed over all sites, and mean coherence and mean Granger causality spectra computed over all significant site pairs. Results are from monkeys GE (black curves) and LU (gray curves) during the wait period of the task (Methods). The overall dominance of activity near 20 Hz in the power (A), coherence (B), and Granger causality (C) spectra indicates that a beta-synchronized network of neuronal assemblies supported directionally specific Granger causal influences in the sensorimotor cortex.

Fig. 2.

Beta oscillatory networks in monkey sensorimotor cortex. Coherence (Left) and Granger causality graphs (Right) are depicted in monkeys GE (A) and LU (B). The coherence and Granger causality values are coded by the thickness of the lines between recording sites (Center). The lines in the Granger causality graphs have arrowheads, indicating the direction of Granger causal influence. Lines between site pairs not reaching significance in the coherence and Granger causality measures are not shown. Cs, central sulcus; IPs, intraparietal sulcus; STs, superior temporal sulcus; Ls, lateral sulcus; As, arcuate sulcus.