Synchrony and amplitude modulation of cortical activity in humans performing manipulative visuomotor tasks

Synchrony of oscillatory brain activity has been postulated to be a binding mechanism for cognitive and motor functions. Spectral analysis of human electrocorticogram (ECoG) in sensorimotor cortex has shown that power density of gamma band activity (30-60 Hz) increased and that of alpha-beta band activity (10-20 Hz) decreased during performance of manipulative visuomotor tasks, indicating that amplitude modulation of the gamma band activity occurred in relation to the task performance. Amplitude modulation may provide evidence for synchrony of local neuronal assembly. However, it does not implement the binding mechanisms for distributed networks that are necessary for cognitive and motor functions. To prove that oscillatory activity mediates a binding mechanism, phase modulation of oscillatory activity in a wide range area should be shown. We performed coherence analysis of the ECoG signals in sensorimotor cortex to study if synchrony of the gamma band activity between these areas occurs in relation to manipulative task performance. The ECoGs were recorded from 14 sites in sensorimotor cortex including hand-arm areas with subdural grid electrodes in four subjects. Coherence estimates in all pair-wise sites were calculated in different frequency bands with 10 Hz widths from 10 to 80 Hz. In all subjects, coherence estimates increased in the lower gamma band (20-50 Hz) during the performance of the manipulative tasks. But coherence in the alpha-beta band (10-20 Hz) also increased even though amplitude modulation did not occur in this frequency band. Coherence estimates increased in site pairs within and between sensory and motor areas, many separated by intervening sites. This interregional synchrony of the alpha-beta and the lower gamma activities may play a role in integration of sensorimotor information. Task-dependent increases in coherence estimates, i.e., greater increases during performance of the manipulative tasks than during the simple tasks, suggest another role of synchrony in attention mechanism. Time-series coherence analysis showed that phase modulation occurred in different timings for activities in the alpha-beta and the lower gamma bands. For the activity in higher gamma band (50-80 Hz), power density increased but coherence estimates decreased. Thus, only amplitude modulation occurred in this frequency band. Altogether these results suggest that oscillatory activities in different frequency bands may reflect different functional roles by modulating neural activity in different ways.