Fig. 1. Model of theta–gamma coupling during the N-back task.

a High theta–gamma coupling scenario: letters are presented in a sequential order and to answer correctly to a target trial (e.g., that the second A letter matches two letters back on a 2-back task), the participant needs to remember the order of this sequence correctly. In response to the presentation of these letters, each individual letter (one item of information) is represented by a unique pattern of activation of a neuronal assembly (small ellipse) of pyramidal and interneurons (black triangles and white circles). This activation results in a unique gamma oscillation that represents this letter. A different letter is then represented by a different pattern of activation of a different set of neurons, resulting in a different gamma oscillation. These different neuronal assemblies are intertwined with a common and larger neuronal assembly (large ellipse) the activation of which results in theta oscillations. This common and larger neuronal assembly, through its connections with the individual items neuronal assemblies, sets the order for sequential activation of the latter and thus, codes for the order in which the letters were presented during the N-back task. This larger assembly driven sequential activation of the smaller assemblies representing the letters results in coupling the unique gamma oscillations to specific phases of the theta oscillations, which, in turn, results in modulating gamma oscillations amplitudes by theta oscillation phase. The behavioral outcome of these intertwined activations of small and larger neuronal assemblies, and the corresponding coupling of gamma oscillations to theta oscillations, results in accurate recollection of the letters in their correct sequence. b Low theta–gamma coupling scenario: the neuronal assemblies representing the individual letters are not strongly intertwined with a common and larger assembly. Thus, their individual activations are not precisely ordered in a sequence that represents the sequence in which the letters were presented. In turn, the corresponding gamma oscillations are not coupled to an underlying oscillation generated by a network oscillating in activity at a slower frequency. i.e., theta. And consequently, the amplitudes of the gamma oscillations are not modulated by the phase of these theta oscillation, and the theta–gamma coupling index is low.