Much of what we know about the world comes from experiences we have never had ourselves. A child observes how a parent reacts to a situation which influences their own future actions. Someone becomes more cautious after seeing another person in danger.
Learning by watching others is an important part of how social species navigate the world. Yet scientists still know relatively little about how the brain turns witnessing what happens to somebody else into our own memories. New research led by researcher Frédéric Michon Linde at Valeria Gazzola's and Christian Keysers' Social Brain Lab, provides insight into how this process may work. The researchers studied how rats learn by observing the experiences of others.
"A lot of what we learn, especially as social species, does not come from our own direct experiences, but from observing what happens to others," says Frédéric Michon Linde. "That is a very efficient way to learn. You can learn to avoid danger without having to experience it yourself."
"If you see someone slip and fall at a particular street-corner in the winter, you learn to avoid that corner. We already knew that seeing someone else in pain triggers a feeling of pain in the observer, but in humans we couldn't understand what in the brain transformed this empathic sharing into a spatial map with a label 'danger' in a particular location" explains Christian Keysers. "For that you need to know what individual neurons do. We need animal studies for that." Learning from others is not uniquely human. "Animals, such as rodents, also learn from each other," Frédéric says.
The role of the hippocampus
The researchers recorded the activity of single neurons in the hippocampus, a brain region that plays a central role in spatial memory. They found neurons there that only respond when the animal is in specific locations - so called place cells. The new finding is that these neurons also respond to seeing another animal go through salient emotional experiences, but only when the observer was in the specific location that this neuron responds to. "So we could see a map of space in the hippocampus, and how the experiences of others get anchored in that map" Christian concludes. In other words, the brain appeared to link a particular place with what happened to someone else there.
"For me, that is one of the most important findings," Frédéric says. "In a brain structure that is central to space and memory, we find neurons that combine where we are with what we perceive happening to another individual."
Memories are not like photographs: the brain needs to process the newly acquired information again to make the memory last over time. During the following rest, the same patterns of brain activity associated with the earlier observation were preferentially reactivated, a phenomenon known for contributing to the consolidation of memory. Finally, when later placed in the same environment, not only rats were more wary -confirming they had learned from the observation- but also displayed more stable neuronal representations of the space. Together, the findings point to possible neural mechanisms through which experiences we observe in others can become part of our own memory and shape our behavior in the future.
When learning from others helps or becomes a problem
Learning from others can be highly beneficial. It allows us to avoid breaking a leg on that slippery corner. In more extreme situations, the same principle could be even more important. A soldier who sees a fellow soldier seriously injured by an explosion on a particular road may later become highly alert when returning to that location, or to places that resemble it.
Normally, this is adaptive: the brain has learned that a particular environment may be dangerous without the observer having to experience the injury firsthand. But a system that links another person's distress to our own map of space could also become problematic if that association becomes too strong, persists after the danger has passed, or generalizes to many environments.
That is one reason Frédéric Michon Linde sees post-traumatic stress disorder as an interesting direction for future research. "In our experiment, the animals learned where danger looms from seeing what happens to others, and we could see how that information became linked to spatial representations in the hippocampus," Frédéric says. "An important question, which we have not tested here, is whether related mechanisms might become exaggerated in conditions such as PTSD — for example, if a memory of danger becomes too persistent or spreads to places that are actually safe and results in persistent anxiety."
The current study did not investigate PTSD. Rather, it identifies a basic mechanism that could help researchers ask, in future work, how socially witnessed danger becomes anchored to places, and how such memories remain specific or become overgeneralized in PTSD.
A fundamental part of social life
The research is fundamental rather than a direct step towards treatment. A next question for the Social Brain Lab is how information about others reaches the hippocampus. Understanding this could reveal more about how other people's experiences become part of our own memories and guide our behaviour.
Source: Nature Communications
