Theta-gamma coupling as a ubiquitous brain mechanism: implications for memory, attention, dreaming, imagination, and consciousness

Brain rhythms are known to play a relevant role in many cognitive functions. In particular, coupling between theta and gamma oscillations was first observed in the hippocampus, where it is assumed to implement a code for organizing multiple items in memory. More recent advances, however, demonstrate that this mechanism is ubiquitously present in the brain and plays a role not only in working memory but also in episodic and semantic memory, attention, emotion, dreaming, and imagination. Furthermore, altered mental states and neurological disorders show profound alterations in the theta-gamma code. In this review, which summarizes the most recent experimental and theoretical evidence, we suggest that the substantial capacity to integrate information characteristic of the theta-gamma entrainment is fundamental for implementing many conscious cognitive processes.