The brains of people who are experiencing loneliness have their own neural signature, a new study has revealed, findings which might help us better understand what is a growing mental health problem in the age of COVID-19.

Through an analysis of 38,701 middle-aged and older volunteers taking part in the UK Biobank health database project, which combined data from MRI scans with self-assessments on feelings of loneliness, the researchers identified several differences in the brains of those who often felt lonely with those who didn't.

These differences centred around the default network in the brain, a group of regions linked to mental exercises like reminiscing, planning for the future, thinking about others, and imagining scenarios.

"In the absence of desired social experiences, lonely individuals may be biased towards internally-directed thoughts such as reminiscing or imagining social experiences," says neurologist Nathan Spreng from McGill University in Canada.

"We know these cognitive abilities are mediated by the default network brain regions, so this heightened focus on self-reflection, and possibly imagined social experiences, would naturally engage the memory-based functions of the default network."

The researchers looked at what they called 'trait loneliness', the enduring negative impacts a person subjectively experiences when socially isolated, rather than simply time spent alone or the number of social contacts (people can have lots of friends and still feel lonely, and vice versa).

In the brains of people who often felt lonely, the default network was more strongly wired together, and more grey matter was observed, suggesting more activity or capacity in these particular neural regions.

Loneliness was also linked to a more complete fornix, the bundle of nerve fibres that connects the default network to the hippocampus (which is very important for forming memories). The brain activity seems to be compensating for a social void, propose the researchers.

"In the context of previous research, we speculate that in the absence of desired social experiences, lonely individuals may be biased towards internally directed cognitions mediated by default network brain regions," write the researchers in their published paper.

Even before the global pandemic arrived, loneliness had been linked to health problems including increased blood pressure, a higher risk of developing Alzheimer's, and premature death. Humans aren't built to be isolated.

It's therefore important for scientists and medical professionals to understand how loneliness develops and how it affects the brain – areas of research that we've still got a lot of learning to do in.

According to this large sample of volunteers, brain changes due to loneliness are more to do with what you might call an inner voice rather than in how the brain processes external information, which should provide a good foundation for future studies.

"We are just beginning to understand the impact of loneliness on the brain," says biomedical engineer Danilo Bzdok, from McGill University. "Expanding our knowledge in this area will help us to better appreciate the urgency of reducing loneliness in today's society."

The research has been published in Nature Communications.