Internal operations in the hippocampus: single cell and ensemble temporal coding

Abstract

Most of our cognitive life depends on our brain's ability to generate internal representations of the external world. The hippocampus is a brain structure that supports the formation of internal representations of the spatial environment (O'Keefe and Nadel, 1978) as well as the formation (Scoville and Milner, 1957) and consolidation (Squire and Alvarez, 1995) of episodic memories. In rodents, hippocampal pyramidal cells are active at discrete places along the trajectory of the animal in linear and two-dimensional spatial environments, and therefore are called place cells (O'Keefe and Dostrovsky, 1971). During exploratory behavior, the firing rates of individual place cells are thought to encode the moment-to-moment location of the animal in space (O'Keefe and Dostrovsky, 1971; Wilson and McNaughton, 1993). With reference to the background local field potential theta oscillation (~8 Hz), individual place cells oscillate at slightly faster frequency (~10 Hz) and fire at more advanced theta phases the further the animal travels through the cell's place field, a phenomenon called phase precession (O'Keefe and Recce, 1993; Skaggs et al., 1996; Huxter et al., 2008). Since most place cells go through almost a full 360° cycle of precession from the beginning to the end of their place field (O'Keefe and Recce, 1993), the theta phase of firing is thought to encode the distance of the animal relative to the beginning of the place field (Huxter et al., 2003).