Theta-gamma coupling during episodic retrieval in the human EEG.

Recent findings indicate that phase-amplitude coupling between neuronal oscillations in the theta- (3-6 Hz) and the gamma-band (30-100 Hz) plays a functional role in memory processes. Here, using electroencephalography, we provide further evidence for coupling between prefrontal theta and parietal gamma during successful memory retrieval in the human brain. In a pictorial recognition task, the coupling between prefrontal theta phase and parietal gamma amplitude was quantified using the modulation index, 100-1500 ms after stimulus onset. Results show an increased coupling for remembered, as opposed to forgotten and new stimuli (i.e. a "recognition effect" and an "old/new effect"). Phase-amplitude coupling between the prefrontal theta phase and posterior gamma amplitudes is hypothesized to reflect long range communication between prefrontal control processes and the activation of posterior object representations accompanying mnemonic processing.