On the results of causal optogenetic engram manipulations

Hippocampal neurons encoding memories discharge in sparsely-distributed patterns; they can be optogenetically tagged, then photostimulated to elicit conditioned behavior, potentially generating a synthetic memory trace, "engram". We investigated mouse hippocampal population responses to the photostimulation of memory-tagged neurons to determine if memory-associated discharge is mimicked in hippocampus, a network with non-linear and homeostatic interactions. Both memory-tagged and not-tagged CA1 cells adjusted firing during photostimulation without altering place cell firing fields. Cell-pair cofiring relationships also maintain during photostimulation, indicating a low-dimensional, dynamical structure-preserving, homeostatic network response instead of the photostimulated pattern. Photostimulating neurons that were tagged during place-avoidance memory elicits similar place-avoidance memory in control conditions. Thus artificial photostimulation elicits natural, stored, homeostatic neuronal network cofiring patterns to elicit memory, establishing mimicry evidence for the engram.