How do social interactions conjure up memories and emotional responses in the brain?

Photo courtesy of The Ramirez Lab

What did you aim to study with your research? What made you want to examine this issue?

Our research studies what memory looks like in the brain (i.e. its physical basis) as well as how to artificially manipulate memories to restore the brain. Memory is one of the most fundamental processes of the brain and we can even artificially tinker with the brain to restore memories, enhance positive memories or suppress negative ones. Our work could one day be harnessed for future therapeutic techniques to treat a variety of conditions including depression, anxiety, and post-traumatic stress disorder (PTSD).

Walk us through your experiments? What did the findings reveal?

Our experiments examined the effect of different social and non-social experiences on an individual’s memory of a past negative event. These social experiences can include having an animal, which we call the “observer” or “stressed familiar individual,” interact with its cage mates either directly, behind an opaque wall, or behind a one-way mirror. The non-social experiences can include access to the opaque wall itself without a cage-mate on the other side. The behavioral experiments revealed that two types of socially-transmitted stressors enhanced fear memory, while non-socially transmitted stressors and our control experiences had no effect on fear memory strength.

What was the most surprising finding?

We were surprised by the counterintuitive finding that direct physical interaction with a stressed familiar individual did not have a fear recall effect on the mice — while experiencing ambient stress cues indirectly (presumably through their vocalizations and scents, for instance) from the stressed familiar individual had such a strong enhancing effect on fear memory in the mice. The team brainstormed possible explanations, and tested the hypothesis that this phenomenon might be due to the physical interaction impacting not only the cage-mates, but actually reducing the distress signals put out by the stressed familiar individual. We put a “social buffering” mouse on the same side of the wall as the stressed mouse and were able to show that this direct interaction did indeed block the fear memory enhancing signals — and therefore blocked the fear memory recall effect on the entire group.

Photo courtesy of The Ramirez Lab

What is the significance of these findings?

We show that individually acquired memories can be enhanced by another animal’s stress more so than by one’s own, revealing a new intersection of individual history and sociality.

Could this new understanding of socially transmitted fear memories inform treatment approaches for individuals/groups experiencing trauma or post-traumatic stress disorder?

Our work suggests that in addition to the social transmission of fear memory discovered in recent years, exposure to stressed individuals can amp up one’s own, independently learned, fearful memories. This rather surprising finding highlights the importance of focusing on the social environment of someone recovering from trauma or PTSD, as our work in mice suggests that even mild social stressors can reactivate and strengthen fear memory.

What do you hope to study next?

We would love to study how memories are socially transmitted among rodents — specifically, which sights and sounds and smells are the most important to trigger and tap into previously held memories? Moreover, a single memory does not exist in a single X-Y-Z coordinate point in a brain; rather, it is distributed throughout the brain in three-dimensions (of which the hippocampus is just one key region). With this in mind, we’re excited to map out this three-dimensional diagram of memories in the brain as well.

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