Locating sweet memories

30 January 2018

Insights into how the brain remembers locations linked with positive and negative experiences could one day inform treatments for memory disorders.

Our brains continually create and rework spatial maps of our environment, allowing us to navigate and to form our ‘episodic’, or autobiographical, memories. Episodic memories link positive and negative experiences to specific places; for example, if you visit a restaurant and love the food, you will remember and go there again. However, the exact mechanisms within the brain that govern this process are unclear.

Now, Marian Tsanov and colleagues at Trinity College, Dublin, along with scientists in the US, have revealed how a group of neurons called ‘place cells’ gather and encode information not just on time and place, but also on the level of reward gained or lost in specific locations.

“Place cells in the hippocampus encode the brain’s spatial map, like a GPS, helping us to navigate across familiar environments,” says Tsanov. “The same brain region is also responsible for the formation of episodic memories. An intriguing question is how the place cells can also store information about positive or negative experiences that occur in a given location.”

Previous studies on rats found that place cell activity would increase in physical locations associated with reward, such as food. Tsanov’s team wanted to clarify if the place cell activity was a passive response to the reward, or if the rats were actually learning where a reward was located. They conducted a series of experiments using sugar tablets placed in different mazes to find out if place cells in the rats’ brains could encode simultaneously for spatial information and experience-dependent reward.

Twenty rats with implants that monitored neuronal activity were trained using one maze, in which both close-range and distant visual cues were placed to help the rats learn where food, which was always positioned in the same place, was located. After three days, the maze was rotated, but the rats still showed a preference for the corners of the maze where they had previously found food.

“We’ve shown that the distribution of place cells gradually changes throughout the learning process; place cell activity accumulates over time around zones where reward is located,” says Tsanov. “The level of accumulation represents the degree to which the rats have learnt to associate a place with a reward.”

The researchers demonstrated that the brain signal that regulates this learning process originates from dopamine-releasing neurons in the reward centre of the brain.

Their findings improve our understanding of episodic memory formation and could ultimately inform future treatments for memory impairment disorders.

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References

1. PLOS Biology 15, e2002365 (2017). doi: 10.1371/journal.pbio.2002365