Some people with a family history of mood disorder have increased activity in a part of the brain that processes emotions even before they become unwell, a study has found.
The discovery could help early diagnosis of the conditions, allowing people with depression to be identified before the onset of illness.
The brain scan study saw scientists at the University of Edinburgh examine a group of at-risk individuals - healthy people with a family history - and a group with no such risk.
Both groups were examined again two years later and researchers found that a fifth of the at-risk group had developed major depression.
Scientists found that the original scans of those with depression showed abnormal levels of activity in the insula cortex region of the brain, which helps regulate mood, even before they were diagnosed.
This could help predict who will become mentally ill in later life and shed light on the source of mood disorders.
The findings show that dysfunction of the insula in high-risk people is one the major causes of depression.
The study builds on previous research which shows that the insula, located in the deep folds of the brain, is involved in the onset of mood disorder.
Dr Heather Whalley, of the university's division of psychiatry, said: "These findings advance our understanding of the biological processes involved in the development of mood disorders. They show that increased activation in this part of the brain differentiates individuals at high risk of bipolar disorder who later develop depression from healthy people and those at familial risk who remain well."
The study, published in the PLOS ONE online journal, was funded by a Royal Society Dorothy Hodgkin fellowship and The Health Foundation.