Science —

Neurons recall place-related memories if you just get close

Get anxious every time you drive by your high school? Blame hippocampal neurons.

Fear is the mind killer.
Enlarge / Fear is the mind killer.

Place cells in the hippocampus are known to encode memories associated with a specific location. However, the precise role that these cells play in location-based memory retrieval is somewhat unclear. A recent study using rats was published in Nature Neuroscience, and it finds that these cells can be active even when a rat is near a location associated with a fear memory. This result shows that place cells may play a role in avoidance behavior without requiring that an animal be in the place it's trying to avoid.

In this study, the researchers used an IRB-approved fear-conditioning protocol in which rats were conditioned to be afraid of a specific part of a long linear cage. When they walked over to that area, they received a small electric shock. The researchers knew that the mice learned to be afraid of the shock zone of the cages because, when they were placed in the cages, they avoided the shock zone, but wandered freely in the other parts of the space.

After the rats were conditioned to be afraid of the shock zone, the researchers lesioned the part of the hippocampus (called the dorsal CA1) associated with this fear memory in some of the rats. These lesioned rats no longer showed fear of the shock zone, which demonstrated that the place-fear-linked memories were encoded in the neurons of this one specific brain region.

Then, having confirmed which neurons were encoding the fear memories, the researchers watched the rats’ behavior. Rats that did not receive brain lesions showed avoidance of the shock zone—when they walked toward the shock zone, they paused and then turned away without entering. According to the scientists, this behavior shows that the rats’ fear-conditioned memories were being accessed before they entered the shock zone.

The researchers then looked at the firing of neurons in a specific area of the hippocampus (the brain region where the fear memory was encoded) as the rats approached the shock zone. These neurons showed increased firing as the rats approached the shock zone, even if they did not enter it. This observation clearly indicates that the place cells were accessing the memory before the rats entered the shock zone. It also indicates that the brain was using that information to direct the rats away from this region of the cage.

As people, we have the capacity for metacognition—thinking about how we think. This means we shouldn’t be surprised that being near a place where something bad happened to you would remind you of that bad experience. However, we don’t know if other animals experience memories the same way. And we didn’t previously know that place cells, which were identified based on their activity when a rat was in a specific location, were responsible for this type of memory recall.

This study shows that rats can associate place-related memories with proximity to the place where the memory happened and that their dorsal CA1 hippocampal cells play a critical role in encoding that memory. Further directions for this type of work include learning about how other types of cognition interact with these existing memories—for example, if the rats were hungry, could that hunger overcome their fear-related memory of the shock zone? It may be slow going, but this is the only way we’re going to piece together the neural circuits that manage our behavior.

Nature Neuroscience, 2017. DOI: 10.1038/nn.4507 (About DOIs)

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