The findings also add to the growing body of evidence that depression and bipolar may fall on a spectrum of mood disorders, rather than being completely unrelated. This new information could transform the way these disorders are diagnosed and treated.
For the study, researchers from the University of Michigan Medical School and Depression Center recruited 612 women, more than two-thirds of whom had experienced either major depression or bipolar disorder. They presented their final data from detailed brain scans of 52 of the women, who took tests while the scans were conducted.
Seen as groups, women with depression or bipolar disorder did equally poorly on the test, which required sustained concentration. They were asked to react quickly when certain letters flashed briefly on a screen, amid a random sequence of other letters.
Compared with the group with no mental health problems, the groups with either diagnosis significantly fell behind on this standard test of cognitive control.
And while many individual women with depression or bipolar disorder were able to score as well on the test as healthy participants, nearly all the test-takers in the bottom five percent of performers had one of the two mood disorders.
According to the brain scans, the participants with depression or bipolar disorder had different levels of activity than healthy women in a particular area of the brain called the right posterior parietal cortex. This region helps control “executive function:” activities such as working memory, problem solving, and reasoning.
In those with depression, the activity in this area was higher than in healthy individuals, while in those with bipolar disorder it was lower.
“In all, we show a shared cognitive dysfunction in women with mood disorders, which were pronounced in the cognitive control tests and more nuanced in scans,” said Kelly Ryan, Ph.D., a University of Michigan neuropsychologist and lead author of the study.
“These findings support the idea of seeing mood disorders dimensionally, as a continuum of function to dysfunction across illnesses that are more alike than distinct,” said Ryan, who sees patients as a clinical assistant professor in the University of Michigan Department of Psychiatry.
“Traditionally in psychiatry we look at a specific diagnosis, or category. But the neurobiology is not categorical — we’re not finding huge differences between what clinicians see as categories of disease. This raises questions about traditional diagnoses.”
The researchers note that this view of mood disorders is growing in favor, and even the National Institute of Mental Health has put it forth as an area of research focus.