Bipolar disorder and major depression share more than clinicians have traditionally recognized. People living with either condition frequently describe a sense that their thinking has become fuzzy or less sharp since their symptoms began, a complaint that has sometimes been dismissed as subjective. A large scale study from the University of Michigan Medical School and Depression Center has now confirmed that this cognitive impairment is real, measurable, and visible on advanced brain imaging, and that it points to a shared neurobiological foundation between bipolar disorder and depression that challenges longstanding assumptions about how these conditions should be classified and treated.
How the Bipolar Disorder and Depression Study Was Conducted
The study, published in the journal BRAIN, analyzed results from 612 women, more than two thirds of whom had experienced either major depression or bipolar disorder. To control for gender related differences in cognition, the researchers focused exclusively on female participants. Participants completed a standardized cognitive control test requiring sustained attention and rapid response, specifically pressing a button when target letters appeared briefly on a screen amid a random sequence of other letters.
The sample was drawn from multiple University of Michigan studies including the Prechter Longitudinal Study of Bipolar Disorder. The scale of the study, 612 participants, is notably large for this type of psychiatric research, which strengthens the reliability of the findings. An additional subset of 52 women completed the same cognitive test while undergoing functional MRI brain scanning at U M’s neuroimaging laboratory.
What the Cognitive Tests Revealed About Bipolar Disorder and Depression
Seen as groups, women with bipolar disorder and women with major depression performed equally poorly on the cognitive control test compared to the 150 healthy women in the study. Both diagnostic groups lagged noticeably behind healthy participants on measures of sustained attention and rapid response. While many individual women with either condition performed within the normal range, nearly all participants in the bottom five percent of test performance had a diagnosis of bipolar disorder or depression.
This concentration of poor performance at the lowest end of the distribution is a meaningful statistical signal. It suggests that severe cognitive control impairment is not randomly distributed across the population but is specifically concentrated in people with mood disorders, regardless of whether the diagnosis is bipolar disorder or depression.
Brain Scan Findings in Bipolar Disorder Versus Depression
The fMRI data revealed distinct but related patterns of abnormal brain activity in both groups. Women with depression showed higher than normal activity in the right posterior parietal cortex during the cognitive test, while women with bipolar disorder showed lower than normal activity in the same region. This brain area is centrally involved in executive function, including working memory, problem solving, and reasoning.
The fact that both conditions showed abnormalities in the same brain region, rather than in entirely different neural circuits, supports the hypothesis that depression and bipolar disorder share an underlying neurobiological substrate. The direction of the difference between the two groups, higher activity in depression versus lower in bipolar disorder, suggests they may represent different points on a continuum of dysfunction in the same system rather than categorically separate diseases.
What This Means for How We Understand Bipolar Disorder
Lead author Kelly Ryan, PhD, a neuropsychologist and clinical assistant professor at the University of Michigan Department of Psychiatry, described the findings as supporting a dimensional view of mood disorders, one that sees conditions like bipolar disorder and depression as points along a continuum of function to dysfunction rather than as entirely distinct categories. This perspective is gaining traction in psychiatric research and has been formally embraced by the National Institute of Mental Health through its Research Domain Criteria initiative, which aims to develop classification systems for mental health disorders based on neurobiology and genetics rather than purely on clinical symptom groupings.
Senior author Scott Langenecker emphasized that RDoC is not intended to replace existing diagnostic systems but to extend understanding of the neurobiological signatures underlying mental illness, which often overlap more than clinical categories suggest.
The practical application proposed by the research team is to use the cognitive control test as an intermediate phenotype, a way of stratifying participants in future research by biology rather than diagnosis. Researchers could administer the inexpensive cognitive test broadly and reserve the costly brain scanning for those who perform poorly, improving the efficiency and precision of psychiatric research studies.
FOMAT conducts CNS and psychiatric clinical trials at sites across the United States. To learn more about active studies, visit FOMAT’s patient studies page. To read more about psychiatric research, visit the FOMAT blog.
For the full source, see the original article at Bioscience Technology.