When you're depressed, it is easy to doubt your abilities. So self-reported “fuzzy thinking” during bouts of depression is treated with suspicion. However, a new study suggests the description is accurate, concentration and decision making really are affected by mood disorders.

University of Michigan researchers tested the idea of fuzzy thinking in two ways, publishing their findings in Brain. 

The first experiment compared 260 women with major depressive disorder (MDD), 202 with one of three types of bipolar disorder (BD) and 150 demographically similar controls with no diagnosed mental health issues. Participants were shown a sequence of mostly random letters on a screen for three minutes and asked to respond when certain letters appeared to test their cognitive control and attention.

A second test took a small subsample of each group (19, 16 and 17 respectively) and had them perform a similar test in an MRI machine.

Almost three quarters of the participants with MDD were depressed at the time, while half of those with bipolar disorder were experiencing depression when the test was done. None were in a manic state. Education levels were virtually identical across the three groups, as were scores on a pretest for intelligence.

The results demonstrated that depression does not necessarily interfere with snap decision making, but it can. The controls were, on average, both more accurate and faster in responding than either of the groups with mood disorders.

Some of the women with either MDD or bipolar were able to perform well above the average for the controls. However, those who struggled most, both in accuracy and timing, overwhelmingly had mood disorders. Controls were very much underrepresented among both the bottom 5% and bottom 16% for performance on the test.

When comparing the MRI responses the authors concluded, “These attention difficulties appear to be localized in a performance-specific and disease-shared way to the right posterior parietal cortex.”

^{Credit: Ryan et al. The image shows the differences in activation between those who performed well and badly on the task allowing for group status.}

The right posterior parietal cortex is considered important in the brain's executive decision making However, rather than both disorders showing a similar effect, the control group's level of activation in the region fell was below those with MDD and higher than those with bipolar disorder, suggesting that either too much, or too little activation can interfere with responses. Nevertheless, the small sample size in the MRI component of the study led the authors to warn that these findings should be treated with care.

All three groups of participants performed more poorly in the MRI machines than outside, which the authors attribute to anxiety, but the drop was several times larger for those with bipolar disorder.

“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,” says lead author Dr Kelly Ryan.

Potentially the difference in images could be used as an indicator of people at risk of developing a mood disorder, however senior author Dr Scott Langenecker said the work, “Is not meant to replace [existing] diagnostic systems.”