High Gamma Power Is Phase-Locked to Theta Oscillations in Human Neocortex

Abstract

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 (1), influence reaction time (2), correlate with attention (3) and perceptual binding (4), and are proposed to play a role in transient, longrange coordination of distinct brain regions (5). 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 (1), with spatial coherence between these oscillations extending from several centimeters for the corticospinal tract (6) to the micrometer scale for subthreshold membrane oscillations in a single neuron (7). Exactly how these transient oscillations influence each other and coordinate processing at both the single-neuron and population levels remains unknown.Evidence for cross-frequency coupling, where one frequency band modulates the activity of a different frequency band, is more abundant in animal than human data. For example, the theta rhythm can modulate the firing rate and spike timing of a single neuron (8-11) as well as the gamma power of the intracortical local field potential (8,12,13). Task-related changes in theta power have been observed in humans (14-16), and cross-frequency coupling at frequencies up to 40 Hz has been detected at the scalp (17,18). However, given the difficulty in localizing electrical sources from scalp recordings alone (19), subdural electrodes that record directly from the human cortex are needed to address this question. Furthermore, subdural electrodes are ideal for studying activity in the recently described human high gamma band (HG) at 80 to 150 Hz. HG activity is modulated by sensory, motor, and cognitive events (20), is

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