High Gamma Power Is Phase-Locked to Theta Oscillations in Human Neocortex
We observed robust coupling between the high- and low-frequency bands of ongoing electrical activity in the human brain. In particular, the phase of the low-frequency theta (4 to 8 hertz) rhythm modulates power in the high gamma (80 to 150 hertz) band of the electrocorticogram, with stronger modulation occurring at higher theta amplitudes. Furthermore, different behavioral tasks evoke distinct patterns of theta/high gamma coupling across the cortex. The results indicate that transient coupling between low- and high-frequency brain rhythms coordinates activity in distributed cortical areas, providing a mechanism for effective communication during cognitive processing in humans.

Neuronal oscillations facilitate synaptic plasticity, influence reaction time, correlate with attention and perceptual binding, and are proposed to play a role in transient, long-range coordination of distinct brain regions. Direct cortical recordings reveal that ongoing rhythms encompass a wide range of spatial and temporal scales — ultraslow rhythms less than 0.05 Hz coexist with fast transient oscillations 500 Hz or greater, with spatial coherence between these os-
cillations extending from several centimeters for the corticospinal tract to the micrometre scale for subthreshold membrane oscillations in a single neuron. Exactly how these transient oscillations influence each other and coordinate processing at both the single-neuron and population levels remains unknown.

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