The Hippocampal Ensemble Code for Spatial Navigation and Episodic Memory.

In our minds we can vividly re-experience a series of past events or episodes that occurred along space and time. In 1957, a highly influential clinical investigation published by Scoville and Milner suggested that episodic memory retrieval is severely impaired after physical damage occurs to the hippocampus. In fact, loss of the hippocampus can lead to a profound disturbance to spatial navigation. Within the hippocampus reside place cells: excitatory pyramidal cells that maximally fire at a particular location. The presence of hippocampal place cells across several mammalian species, including mice, rats, bats, and primates, has led to a theory that the hippocampus is a locus of the 'where' element of episodic memory. However, growing evidence suggests that place cell's firings do not merely encode spatial information, but also represent a series of experienced events that occur along the spatiotemporal continuum. This suggestion inspires the following questions: is the ensemble activity of place cells a neuronal substrate for forming or recalling episodic memory? And, why does the hippocampus enable flexible spatial navigation?To address these questions, in this chapter I review several compelling investigations on the firing pattern of place cells, and the temporal flow and reactivation of their ensemble activity. I argue that the sequential activity pattern of hippocampal place cells is capable of encoding not only future trajectory but also future events that occur along space and time. Furthermore, their activity can be reactivated in a temporally compressed fashion while the animal briefly pauses. These pivotal reports strongly support a view that the hippocampal neuronal ensemble enables flexible spatial navigation and contributes to the capacities of episodic memory: mental time travel and foreseeing future situations.