The common symptom of "fuzzy thinking" reported among people with depression or bipolar disorder has been confirmed for the first time in a large study of brain scans. The finding also adds to growing evidence that these mood disorders exist as points on a spectrum, rather than being "completely separate" conditions.

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In the study, women with depression or bipolar disorder exhibited lower levels of activity than healthy women in the right posterior parietal cortex.

For the new study, researchers at the University of Michigan (U-M) in Ann Arbor recruited 150 healthy women, 266 women who had been diagnosed with major depression, and 202 women with bipolar disorder but who were not in a manic state during the study. The researchers only used female participants because they did not want the results to be confounded by gender differences.

All participants were required to take a test that assessed how well they were able to sustain attention and respond quickly. In this test, participants had to respond rapidly when certain letters flashed up on a screen, amid a random sequence of other letters.

The team found that women with depression and bipolar disorder performed about as well as each other, but both groups' performance was significantly worse than the women with no mental health problems. Despite some individual participants with bipolar disorder or depression scoring as well as healthy participants, nearly all of the participants with scores in the bottom 5% had bipolar or depression.

Because of the expense of functional magnetic resonance imaging (fMRI) scans, only 17 of the healthy women, 19 of the depressed women and 16 of the bipolar women took part in the next phase of the study, where they retook the test while inside the scanner at U-M.

Analyzing the brain scans, the researchers found that women with depression or bipolar disorder exhibited lower levels of activity than healthy women in the right posterior parietal cortex. This brain region is responsible for working memory, problem-solving and reasoning.

Findings raise questions about traditional diagnoses of psychiatric disorders

"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," says lead author Kelly Ryan, PhD, a neuropsychologist at U-M and lead author of the new paper, which is published in the journal Brain.

"Traditionally in psychiatry we look at a specific diagnosis, or category," Ryan continues. "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."

Ryan notes that as scientists are increasingly moving away from this perception of psychiatric disorders as discrete conditions, the National Institute of Mental Health has now given this area of research added focus under the name Research Domain Criteria (RDoC).

Senior author Scott Langenecker, PhD, explains the reasoning behind the initiative:

"The RDoC initiative is not meant to replace this clinical diagnostic system. It is designed to extend ways of understanding neurobiology, performance, and genetics at natures' seams, rather than at groupings of clinical symptoms based on a patient's current memory of recent symptoms and clinician observations of signs. The RDoC initiative is a recognition that we need to rely more heavily on what these neurobiological results are telling us - mental diseases have more overlap in the basic brain and genetic signatures."

Rather than clinicians using brain scans to diagnose a mood disorder, the U-M researchers suggest that other scientists might find their approach useful as a way of dividing up participants in future studies of mood disorders. For instance, by first giving participants the cognitive control test and then using the expensive fMRI option only on participants with poor performance.