Memory entrainment by visually evoked theta-gamma coupling

Abstract

It is an integral function of the human brain to sample novel information from the environment and to update the internal representation of the external world. The formation of new memories is assumed to be orchestrated by neuronal oscillations, the rhythmic synchronization of neuronal activity within and across cell assemblies. Specifically, successful encoding of novel information is associated with increased theta oscillations (3-8Hz) and theta coupled gamma activity (40-120Hz), and a decrease in alpha oscillations (8-12Hz). However, given the correlative nature of neurophysiological recordings, the causal role of neuronal rhythms in human memory encoding is still unclear. Here, we experimentally enhance the formation of novel memories by a visual brain stimulation at an individually adjusted theta frequency, in contrast to the stimulation at an individual alpha frequency. Critically, the memory entrainment effect by the theta stimulation was not explained by theta power per se, but was driven by visually evoked theta-gamma coupling in wide spread cortical networks. These findings provide first evidence for a functional role of the theta rhythm and the theta-gamma neuronal code in human episodic memory. Yet more strikingly, the entrainment of mnemonic network mechanisms by a simplistic visual stimulation technique provides a proof of concept that internal rhythms align with visual pacemakers, which can entrain complex cognitive functions in the wake human brain.