The hippocampus and entorhinal cortex map events across space and time

Abstract

The medial temporal lobe supports the encoding of new facts and experiences, and organizes them so that we can infer relationships and make unique associations during new encounters. Evidence from studies on humans and animals suggest that the hippocampus is specifically required for our ability to form these internal representations of the world. The mechanism by which the hippocampus performs this function remains unclear, but electrophysiological recordings in the hippocampus support a general model. One component of this model suggests that the cortex represents places, times, and events separately, and then the hippocampus generates conjunctive representations that connect the three. According to this hypothesis, the hippocampus binds places and events to an existing relational structure. This dissertation explores how item and place associations develop within cortex, and then examines the relational structure that organizes these events within the hippocampus. The first study suggests that contrary to previous models, events and places are bound together outside of the hippocampus in the entorhinal cortex and perirhinal cortex. The second study shows that this relational scaffold may be embodied by a continually changing code that permits both the association and separation of information across the continuum of time. The final study suggests that the hippocampus and entorhinal cortex contain qualitatively different time codes that may act in a complementary fashion to bind events and places and relate them across time. Overall, these studies support a theory wherein time is encoded in a range of brain regions that also contain conjunctive item and position information. In these regions, conjunctive representations of items, places, and times are organized not only by their perceptual similarity but also their temporal proximity.