It might surprise you what kind of people are most prone to loneliness.
As social animals, humans have a fundamental need for contact with other beings. In fact, studies on the effects of isolation have found that spending just hours in isolation and darkness can drive humans to the brink of insanity.
This ingrained need for social contact actually helps us to survive — finding food, shelter, and other necessities is easier to do when it’s a group effort rather than an individual one.
Now, in a study published in the journal Cell, neuroscientists from MIT have pinpointed the brain region that represents these feelings of loneliness. In a study of mice, the researchers found that a cluster of cells in an area located near the back of the brain, called the dorsal raphe nucleus (DRN), plays a key role in generating the increased sociability that usually occurs after a period of social isolation.
"To our knowledge, this is the first time anyone has pinned down a loneliness-like state to a cellular substrate. Now we have a starting point for really starting to study this," says one of the senior study authors Kay Tye, a professor of brain and cognitive sciences and member of MIT's Picower Institute for Learning and Memory.
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Previous research has explored how the brain seeks out and responds to rewarding social interactions, but we know very little about how loneliness and isolation motivate social behavior by their effects on the brain.
"There are many studies from human psychology describing how we have this need for social connection, which is particularly strong in people who feel lonely,” lead author Gillian Matthews, a postdoc at the Picower Institute, said in a press release. “But our understanding of the neural mechanisms underlying that state is pretty slim at the moment. It certainly seems like a useful, adaptive response, but we don't really know how that's brought about.”
Interestingly, Matthews hadn’t planned to study the effects of loneliness on the brain — she serendipitously noticed these “loneliness neurons” during a study on how drugs affect the brain’s dopamine neurons. As part of the experiment, each mouse was isolated for 24 hours, and Matthews noticed strengthening connections in the DRN following this period of isolation in the control mice who hadn’t received any drugs.
Further research revealed that these neurons were responding to loneliness. When the animals were together, the DRN neurons weren’t very active. However, during isolation, these neurons became sensitized to the lack of social contact and DRN activity surged once the animals were reunited with others.
"That suggested these neurons are important for the isolation-induced rebound in sociability," Tye says. "When people are isolated for a long time and then they're reunited with other people, they're very excited, there's a surge of social interaction.”
“We think that this adaptive and evolutionarily conserved trait is what we are modeling in mice, and these neurons could play a role in that increased motivation to socialize,” she continued.
Interestingly, the researchers found that social dominance played into which animals were most susceptible to feelings of loneliness. While it may seem most logical that the animals who were lowest in the social hierarchy would be most prone to loneliness, it’s actually the animals with a higher rank in the social hierarchy who were most responsive to changes in DRN activity.
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"The social experience of every animal is not the same in a group," Tye says. "If you're the dominant mouse, maybe you love your social environment. And if you're the subordinate mouse, and you're being beat up every day, maybe it's not so fun. Maybe you feel socially excluded already."
The next step in the research is to determine whether these neurons actually detect loneliness in the brain or if they’re responsible for triggering the response to the lonely feelings. Additionally, the researchers want to study whether differences in these “loneliness” neurons can explain why some people enjoy being more social than others, and if these differences in social preference are innate or formed by our experiences.
"There's probably some part that could very well be determined by innate brain features, but I think probably an equal, if not greater, contribution would be from the environment in which individuals have developed," Tye says.
At least every once in awhile, dealing with loneliness is inevitable, so hopefully future research will provide some more insight into such a complex aspect of our human experience.
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