The hippocampus: Going the distance beyond space and time
Anyone who’s spent a day exploring the streets of New York City will understand the importance of integrating accurate representations of space, time and distance. That hipster dive-bar you’re dying to check out is twelve streets from tonight’s dinner spot, whereas that hot new rooftop bar is just four avenues away. Which do you choose to save the most time and energy? A tiring day of shopping has taught you that a Manhattan block is not just a block; those four avenues are in fact equidistant to the twelve streets. But distance and time don’t always perfectly correlate; the tourists and congestion near the rooftop bar will certainly cost you a few minutes … dive-bar it is.
How do our brains represent and bind together the temporal and spatial features of our experiences? Since the discovery of place cells in the 1970’s we’ve known that the hippocampus is important for signaling where an animal is in its environment (O’Keefe, 1976). These hippocampal neurons possess distinct place fields, regions of space in which the cell preferentially fires with maximum frequency.
Neuroscientists have recently discovered that the hippocampus not only codes spatial information, but also represents information about when particular events occur. These neurons, aptly named “time cells”, selectively fire during particular moments of an experience. For example, when a rat runs on a treadmill, neuron A might fire one minute into its jog, whereas neuron B will remain silent until ten minutes in. But in such paradigms, time and distance are correlated, such that the longer a rat runs, the further he’s gone. This confound between time and distance makes it difficult to know whether a purported time cell indeed codes temporal information, or instead might be tuned to distance, or the integration of both spatial and temporal features.
Benjamin Kraus and colleagues set out to disentangle these confounding effects of time and distance in their study recently published in Neuron (Kraus et al., 2013). As in prior experiments, they trained rats to run on a treadmill while they recorded from hippocampal pyramidal cells, the region’s primary excitatory neurons. Many of the neurons (43%) were active when the rats ran, with most showing time cell-like behavior, demonstrating peak firing at particular moments during the run.
Critically, to dissociate spiking locked to time versus distance they next varied the rats’ running speed, and examined hippocampal activity referenced either to running time or running distance. Thus, a neuron that consistently fired after five minutes of running, regardless of distance covered, would be labeled a time cell. Likewise, a neuron that fired after running two meters would be considered a distance cell. They found cells of both types, indicating that the hippocampus not only represents elapsed time, but also distance covered. Next, they measured how much space, time or distance influenced a given cell’s response. Although they found a small portion of cells that were only tuned to time or distance, most wore many hats, simultaneously performing the roles of time cell, distance cell and place cell.
So maybe the hippocampus isn’t simply a neural map, clock or ruler, but serves more as the brain’s GPS. Its integrated spatial and temporal code could serve the obvious function of helping us navigate the space-time continuum in which our world is embedded. But does its role end there, or might it serve a higher purpose? The hippocampus was once thought to have a Jekyll-and-Hyde personality, with alter-egos subserving both spatial navigation and memory for past experiences. But scientists are now unraveling how these distinct roles interactively support one another. Episodic memories aren’t one-dimensional snapshots, but rather, integrate details about events with the spatial and temporal context in which they’re experienced. While there’s strong evidence that the hippocampus subserves memory by binding such contextual features together, the neural computations by which it accomplishes this feat are unclear. Kraus et al’s findings contribute a critical piece to this puzzle, identifying hippocampal neurons that simultaneously represent the “what”, “where” and “when” of an experience, whose coordinated activity just might hold the code for a rich, multi-dimensional memory landscape.
** This article has been cross-posted from Emilie’s personal blog. The original post can be found here.
Emilie Reas is a fifth year UCSD Neurosciences PhD student working with Dr. James Brewer. She has an unhealthy obsession with the hippocampus and uses fMRI to study human memory.
Kraus BJ et al. 2013. Hippocampal “time cells”: Time versus path integration. Neuron. 78:1090-101.
O’Keefe J. 1976. Place units in the hippocampus of the freely moving rat. Exp Neurol. 51:78-109