Data from: Theta-phase-specific modulation of dentate gyrus memory neurons

The theta rhythm, a quasi-periodic 4-10 Hz oscillation, is observed during
memory processing in the hippocampus, with different phases of theta
hypothesized to separate independent streams of information related to the
encoding and recall of memories. At the cellular level, the discovery of
hippocampal memory cells (engram neurons), as well as the modulation of
memory recall through optogenetic activation of these cells, has provided
evidence that certain memories are stored, in part, in a sparse ensemble
of neurons in the hippocampus. In previous research, however, engram
reactivation has been carried out using open loop stimulation at fixed
frequencies; the relationship between engram neuron reactivation and
ongoing network oscillations has not been taken into consideration. To
address this concern, we implemented a closed-loop reactivation of engram
neurons that enabled phase-specific stimulation relative to theta
oscillations in the local field potential. Using this real-time approach,
we tested the impact of activating engram neurons during the peak
(encoding phase) and trough (recall phase) of theta oscillations.
Consistent with previously hypothesized functions of theta oscillations in
memory function, we show that stimulating engram neurons at the trough of
theta is more effective in eliciting behavioral recall than either fixed
frequency stimulation or stimulation at the peak of theta. Moreover,
phase-specific trough stimulation is accompanied by an increase in the
coupling between gamma and theta oscillations in CA1 hippocampus. Oure
results provide a causal link between phasespecific activation of engram
cells and the behavioral expression of memory.