Emergence of Internally Generated Sequences in Non-Memory Tasks

Raphael Heldman1, Yingxue Wang1

1 Max Planck Florida Institute for Neuroscience

Place cells in the hippocampus fire at distinct locations as an animal explores its environment. The same sequence of place cells is activated when an animal repeats a traversal through a familiar environment, indicating that place cell sequences may represent memory traces of previous experiences. A potential complication of this interpretation is that the reactivation of place cell sequences could be sensory responses to the identical cues present in the environment. Elimination of this confound has been achieved in behavioral tasks that incorporate a wheel-running period, in which an animal is navigating through an environment where sensory cues are held fixed. Place field like sequences have been observed during wheel running periods, and because their reactivation can be decoupled from environmental signals, are thought to be driven by internal network mechanisms. Importantly, these internally-generated sequences (IGSs) have only been observed in episodic memory tasks, and the elimination of these sequences coincides with the impairment of task performance. While these results provide evidence that IGSs are correlated with memory, the precise role that IGSs play in hippocampal memory processes remains unclear. In this study, we attempt to clarify the role of IGSs by tracking their emergence in animals performing different behavioral tasks taking place in a virtual reality environment. We show that IGSs emerge in tasks that require active engagement of the animal but that lack an explicit memory demand. This provides evidence that more generally than representing memory traces, IGSs may be utilized for encoding attended experiences.