The need for connection– to form and maintain at least a minimal number of positive, stable, intimate relationships– is a fundamental need that affects our whole being, permeating our entire suite of emotions, thoughts, and behaviors. While voluntary solitude can be great fodder for creativity, and being alone doesn’t necessarily indicate loneliness, what happens when people are forced into isolation and are severely deprived of this fundamental human need?
Surprisingly, while the physical and mental health effects of loneliness are well documented, there is a lack of research on the consequences of severe forced isolation. If the need for connection really is a basic need, then its deprivation should show similar effects on the brain and behavior as the deprivation of other basic needs such as food and sleep.
The feeling of “wanting” something has repeatedly been shown to increase dopamine transmission in the brain reward circuit (see here and here). This circuit consists of the dopamingeric midbrain and the striatum. These areas are particularly active in response to images of food when hungry, to drug-related images for those who are addicted, and people with Internet Gaming Disorder who are deprived of gaming (see here, here, and here).
What about social interactions? For social animals, it would make sense that social interactions would be a primary reward. However, so far such research has primarily been conducted on mice. In 2016, Gillian Matthews and colleagues published a paper showing that after 24 hours of social isolation, dopamine neurons in the midbrain were activated when mice sought social interaction. These dopamine neurons showed similar activation patterns to other cravings. It appears that the acute social isolation in these mice led to an aversive “loneliness-like” state that increased motivation for social engagement. Nevertheless, researchers have questioned whether these findings would apply to humans, especially since it’s not possible to assess whether a mouse subjectively feels lonely.
Livia Tomova, a postdoctoral fellow in the Saxelab at MIT, was inspired by this earlier research on mice and pitched to Rebecca Saxe the idea of trying to replicate the findings in humans. The researchers had a number of methodological challenges to overcome, however.
For one, a single day of social isolation is not that long for a human, and being alone doesn’t necessarily translate to feeling social isolated. Solitude can be restorative. To address this challenge, the researchers had 40 socially-connected healthy human adults spend 10 hours (9am to 7pm) alone, with no social interaction and no other social stimulation (e.g., twitter, email, reading fiction).
Another methodological limitation is the measurement of neural responses in the relevant dopaminergic midbrain regions. This is a real technical challenge. The relevant areas are really tiny, right next to the sphenoid sinus and are prone to distortions and signal loss. To address this challenge, the researchers used an optimized imaging protocol and a newly available midbrain atlas to identify the relevant areas in the brain of each individual participant.
Finally, the researchers were unsure whether they could actually measure signals associated with craving, although they were optimistic considering that in the substania nigra part of the reward circuit, about 70% of the neurons are dopamingergic! To assess craving, the researchers had participants view images of their favorite social activities, favorite foods, and a pleasant baseline condition (flowers) to tease apart the brain responses to these differing stimuli. Undergraduate researchers spent hours scouring thorough databases of open source images from pexels to custom tailor pictures of favorite foods and activities for each individual participant.
After only ten hours of social isolation– and even despite people knowing exactly when their deprivation would end– people reported substantially more social craving, loneliness, discomfort, dislike of isolation, and decreased happiness than they did at baseline. Similarly, the same findings were seen after ten hours of food fasting.
Critically, the researchers found similar midbrain activity in response to food cues after fasting and social cues after isolation. The response was variable across participants, and those who reported more social craving after the social isolation period showed a larger brain response to the social stimuli.
Interestingly, the variability across participants was also partially explained by the variability in pre-existing chronic levels of loneliness. Participants with higher levels of chronic loneliness at baseline reported less craving for social contact after 10 hours of isolation in response to the social stimuli, and showed a muted response in their midbrain in response to the social cues after social isolation (they also showed reduced midbrain responses to food cues after fasting). This finding is consistent with prior research showing that chronic loneliness is associated with reduced motivation to engage socially with others.
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