The subjective experience of loneliness may be caused by activity in a brain region called the dorsal raphe nucleus

Interaction with others is vital for the survival of social animals such as humans, as the establishment of relationships allows for the sharing of food and other resources, while also providing safety in numbers. As such, social isolation brings a range of obvious negative consequences, although scientists may have now figured out what causes the subjective experience of loneliness, revealing how certain neurological processes drive feelings of pain and a desire for companionship.

After placing mice in solitary confinement for a period of 24 hours and later examining slices of their removed brains, researchers noted that the connections between neurons in a region called the dorsal raphe nucleus (DRN) were significantly stronger than those of non-isolated mice. This led them to speculate that the neurological response to loneliness may be mediated via the DRN.

To test this hypothesis, they repeated the experiment, although rather than removing the rodents’ brains, they allowed them to return to their social groups after the 24 hours were up, and used a brain implant to measure activity in their DRNs. Reporting their findings in the journal Cell, they explain how these mice displayed a much greater desire to socialize than would normally be expected, and that this appeared to correlate with an increase in DRN activity.

Interestingly, this response was much more pronounced in mice that were socially dominant, indicating that the effect is not simply caused by the removal of certain objective stimuli, but is greatly influenced by the nature of an individual’s social life. This, in turn, suggests that the subjective experience of loneliness was more intense for dominant mice than subordinate mice.

Mice placed in solitary confinement displayed a greater desire for companionship. Ciotu Cosmin via Wikimedia Commons

To verify this conclusion, the study authors conducted another test using mice that had been engineered to possess certain light-activated genes, enabling the researchers to either block or stimulate activity in the DRN by using a flashing light – a technique known as optogenetics. They found that when this activity was subdued, the increased tendency to socialize following a period of isolation vanished. Similarly, stimulating DRN activity in mice resulted in a greater drive to interact with others, even among mice that had not been placed in solitary confinement.

"That suggested these neurons are important for the isolation-induced rebound in sociability," Kay Tye, one of the senior study authors, said in a statement. "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."

Finally, the researchers placed these mice in an environment containing multiple chambers, one of which was fitted with a flashing light that caused activity in the rodents’ DRNs to surge. Noting that the mice consistently avoided entering this chamber, the study authors conclude that increased activity in the DRN generates a painful experience, and may therefore be the key brain region involved in creating subjective feelings of loneliness.

While it’s not clear at this stage whether DRN neurons are actually detecting social isolation or drive responses to this stage, studies are currently underway to explore this further. 

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