People who have siblings with bipolar disorder but have not developed the illness themselves have unusually high connectivity within a brain network that is most active during wakeful rest, according to a study reported in the American Journal of Psychiatry. Such connectivity may prevent or delay onset of illness in people whose genetics put them at risk.

Genetics play a strong role in determining who develops bipolar disorder. People who have a brother or sister with the illness are about 10 times more likely to develop it themselves than someone without an affected sibling. Still, only a fraction of these individuals actually develop the illness. (The rate in the general population is estimated at 2.4%)

In the new study, researchers led by Sophia Frangou, M.D., Ph.D., a 2008 Independent Investigator and 2002 Young Investigator at the Icahn School of Medicine at Mount Sinai, used functional magnetic resonance imaging (MRI) to compare brain activity in people with bipolar disorder to that of their unaffected siblings, as well as to a control group of people without the disorder who had no affected siblings. David C. Glahn, Ph.D., a 2014 Independent Investigator and 2005 and 2003 Young Investigator at Yale School of Medicine, was also a member of the research team.

The researchers found that the unaffected siblings shared some patterns of brain activity with their affected brothers or sisters: both groups had poor connectivity within brain circuits dedicated to processing sensory information and generating appropriate motor responses.

However, the unaffected siblings had high degrees of connectivity within the brain's default mode network, a network involving many different brain regions that is most active when a person is not focused on the outside world or involved in any particular task. High connectivity within the default mode network may offset other deficiencies, effectively protecting against inherited brain changes that put people at risk of developing bipolar disorder, the researchers speculate.