A New Role For Hippocampal Neurons Encoding Memories

By James R. Howe VI ‘17

This spatial receptive field of a neuron in the entorhinal cortex represents sequences encoded in memories. (Source: Wikimedia Commons)

A recent study published in Nature challenges a long-held orthodoxy in neuroscience: do neural networks that map physical spaces in the brain actually represent space, or do they represent something more general? David W. Tank, a neuroscientist at Princeton and the study’s corresponding author, found that these networks do not represent space itself, but continuous sequences of stimuli in general (Aronov et al., 2017). These networks are composed of a wide variety of cells previously shown to encode diverse spatial phenomena including head direction, velocity, location, and boundaries. These diverse cells form circuits within the hippocampus, a structure that encodes memories, and the entorhinal cortex, which gates input into the hippocampus. However, similar circuits also represent many other memory systems and regulate memory-dependent behaviors, pointing to a potential general function not just space (Squire, 1992).

The authors tested this hypothesis by designing a sound manipulation task (SMT) where a rat would hold down a lever, producing a series of tones with increasing frequency. If the rat released the lever within a specific range of tones, it would be given a reward. This allowed the rat to explore and recognize features of a sonic environment, instead of a spatial one. Throughout the SMT, an array of electrodes measured activity from many cells in both the hippocampus and the entorhinal cortex. Data showed that the activity patterns of these cells were similar to those of cells encoding spatial information.

Next, the group compared the activity of these SMT-active cells in a spatial task, to figure out whether these cells mapped space as well. Here, they used the electrode array to measure the activity of the same cells during a foraging task, a spatial memory test in which rats attempt to find food hidden in various locations in an environment. Researchers found that these cells displayed the same pattern of activity during both the sonic and spatial tasks

These results clearly show cells in the hippocampal-entorhinal circuit previously thought to encode only spatial information also encode non-spatial information, like sound, in the same manner. This discovery changes our understanding of both the hippocampal-entorhinal circuit and the cells within it, which brings with it a range of new questions. Do cells thought to represent more specialized aspects of space, like head direction or boundaries, really represent these alone, or do they encode more general patterns? Does this more general role for these cells hold true across all sequences in memory, or are there cases where it does not apply? These questions will surely provide fertile ground for future research programs, and will surely yield interesting results.

References:

Aronov, D., Nevers, R., and Tank, D.W. (2017). Mapping of a non-spatial dimension by the hippocampal–entorhinal circuit. Nature 543 , 719-722.

Squire, L.R. (1992). Memory and the hippocampus: a synthesis from findings with rats, monkeys, and humans. Psychological review 99 , 195-231.

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