Loneliness Stems From Neural Roots in the Back of Your Brain

Optogenetics allows scientists to pinpoint neurons linked to social isolation.

Posted Feb 16, 2016

Source: Petrol/CanStockPhoto

Homo sapiens are one of the most social animals on the planet. Because we're hardwired to be social creatures, most of us have an innate drive to connect with other humans and maintain strong social bonds. That said, some of us are perfectly content being alone for long stretches of time without feeling lonely. Obviously, many people thoroughly enjoy spending time alone or prefer the company of a favorite pet to that of a fellow human being.

Why are some of us more driven to maintain strong social bonds than others? Is there a neurobiological explanation for why some people can be alone without feeling lonely? What neural mechanisms drive the intensity, or lack of interest, in seeking out social connections with other people? A groundbreaking study by neuroscientists from the Picower Center at the Massachusetts Institute of Technology (MIT) provides some new clues for answering these questions.

The February 2016 study, “Dorsal Raphe Dopamine Neurons Represent the Experience of Social Isolation,” was published in the journal Cell. This study used optogenetics (a technique that allows researchers to control neuronal activity with light) to pinpoint specific neurons that may dictate the intensity of someone's urge to be reunited with others after periods of social isolation and feelings of loneliness.

Optogenetics Enables Neuroscientists to Pinpoint Loneliness in the Brain

In this image of the dorsal raphe nucleus (DRN) dopamine neurons are labeled in green, red, or both (appearing yellow).
Source: Gillian Matthews/The Picower Institute at MIT

Using state-of-the-art optogenetics in a study of mice, MIT neuroscientists report that they’ve pinpointed a brain region that may represent specific feelings of loneliness. The particular cluster of cells—located near the back of the brain in an area called the dorsal raphe nucleus (DRN)—also appears to drive increased urges to socialize that often occur after periods of social isolation. DRN dopamine neurons appear to be hypersensitive to acute social isolation. 

Historically, structural changes in the DRN have been associated with depression. Previous studies have suggested that people suffering from depression may have lower DRN volume. Although a wide range of studies have been conducted on how the brain seeks out and responds to rewarding social interactions; until now, very little has been known about how the neural mechanisms of isolation and loneliness motivate social behavior.

Monkey Business Images/Shutterstock
Source: Monkey Business Images/Shutterstock

Someone’s inherent motivation to seek social contact can spring from either the positive reward of connecting with others or the negative emotional states triggered by feelings of loneliness. For most of us, the urge to connect with friends and family is a combination of avoiding the dejection of feeling alone blended with the uplifting feeling created by social connections. Many of our primal urges to be social—or to be a loner—may be driven by neural mechanisms beyond the locus of our conscious control. 

In a press release, Kay Tye, a member of the Picower Institute for Learning and Memory at MIT, and one of the senior authors of the study, said, "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.”

Interestingly, Gillian Matthews, a postdoc at the Picower Institute and the paper's lead author, identified the loneliness neurons in the DRN accidentally while investigating how drugs affect dopamine neurons in the brain.

Matthews continued her research at the Imperial College London and then in Tye's lab at MIT. Her findings revealed that these neurons were responding to the state of isolation. When mice are cohabitating, their DRN neurons tend to become dormant. However, during a period of isolation, the DRN neurons become like a sponge that wants to soak up social contact. When most of the mice who had been isolated were reunited with other mice, their DRN activity levels surged. In a press release, Matthews said, 

"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. 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.”

As part of the experiment, each mouse was isolated for 24 hours. Matthews observed that in the control mice, which hadn't received any drugs, there was a strengthening of connections in the DRN following the period of isolation.

Also, the DRN neurons appear to drive the mice to be more sociable. When the researchers suppressed DRN neurons using optogenetics, they found that isolated mice didn’t exhibit the same drive to socialize when they were re-introduced to the group. In a press release, Tye said, 

"That suggested these neurons are important for the isolation-induced rebound in sociability. 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."

The researchers noticed that not every mouse reacted the same way to being socially isolated. Some mice who ranked higher on the social hierarchy appeared to have more robust DRN activation, while the mice who were lower ranking socially didn’t appear to have fine-tuned DRN responses. This suggests that DRN activity could drive feelings of loneliness and subsequent cravings for social connectivity. 

These findings open up fascinating questions as to whether differences in DRN neurons might explain why some people prefer more social contact than others. However, this study also presents a classic question of correlation vs. causation. Although there appears to be a link between DRN neurons, feelings of loneliness, and the urge to mingle with a group—it’s still too early to identify the role that environment and other factors play in the functioning of our DRN neurons.

Currently, the MIT researchers are digging deeper to identify specifically how DRN neurons interpret loneliness and drive subsequent actions in response to perceived social isolation. They're also looking into how the DRN neurons might function as part of a larger brain network that responds to social isolation.

Conclusion: Perceived Social Isolation vs. Contented Solitude

Recently, there’s been an avalanche of various studies on the health impacts of perceived social isolation, loneliness, and the importance of maintaining strong social bonds throughout your lifespan to optimize well-being.

For example, in November 2015, I wrote a Psychology Today blog post, “Loneliness: Perceived Social Isolation Is Public Enemy No. 1,” based on a study which found that feeling lonely can increase stress hormones that lead to illness and premature death. Again, in this study the perception of social isolation was the driving factor of detrimental side effects. Some people could be alone but never feel lonely, which appeared to be harmless. 

How often do you feel lonely? Are you happy and content being a loner? Personally, as a writer and endurance athlete, I've always enjoyed spending large chunks of time alone. Very rarely, do I ever feel lonely. In fact, I crave the peace and quiet of being introverted more than I usually crave being part of a social group. At this point in my life, socializing with big groups is often more draining than it is rewarding . . . That said, eventually I reach a saturation point where being alone starts to feel like solitary confinement and I crave human contact.

Now, with the optogenetic images provided by MIT, it’s easy to visualize the DRN neurons driving feelings of loneliness and the urge to socialize from a primal place deep within the brain. These findings make me feel less guilty for loving my zen-like solitude and alone time so much. It's reassuring to know that some people's DRN neurons may be hardwired to experience social isolation without feeling lonely.

In the future, it will be interesting to see if neuroscientists are able to pinpoint an individual’s propensity for feelings of social isolation early in life based on DRN activity. This could lead to revolutionary targeted interventions for anyone who is especially vulnerable to feelings of loneliness on a neurobiological level. Stay tuned for new discoveries on these fascinating DRN neurons.

To read more on this topic, check out my Psychology Today blog posts,

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