Thirty-two patients with bipolar disorder or major depressive disorder in remission showed similarly abnormal neural responses on functional brain imaging when presented with negative emotional stimuli, according to a report in NeuroImage.
Patients with bipolar disorder, however, showed different responses from those with major depressive disorder (MDD) when presented with positive emotional stimuli, reported Dr. Toshio Matsubara of the division of neuropsychiatry, Yamaguchi (Japan) University, and his associates.
"These findings indicate that bipolar disorder and major depressive disorder may have different neural circuits for emotional processing," compared with no mood disorder. They also suggest that abnormal neural responses might be a trait characteristic of both bipolar and major depressive disorder, the investigators said.
Dr. Matsubara and his colleagues used a noninvasive technique, functional near-infrared spectroscopy (fNIRS), to assess neural activity near the brain’s surface while study subjects were engaged in a cognitive task. The task was a Stroop test in which subjects had to read a word flashed on a video screen and identify the color of the typeface but ignore the emotional meaning of the word.
The displayed words were categorized as having happy, sad, threat, or neutral emotional impact and were shown in red, green, blue, or yellow typeface against a black background. Examples of happy words were "kindness" and "health," of sad words were "retirement" and "heartbreak," of threat words were "war" and "violence," and of neutral words were "name" and "iron."
The participants were 16 patients with bipolar disorder in remission (mean age, 44 years), 16 with MDD in remission (mean age, 45 years), and 20 healthy control subjects (mean age, 41 years). The control subjects were matched to case subjects for age, sex, handedness, and years of education.
In contrast to healthy control subjects, those with bipolar disorder or MDD showed hyperactivation of the prefrontal region during the threat stimuli. For happy words, patients with MDD showed prefrontal hyperactivation, but those with bipolar disorder showed prefrontal hypoactivation, Dr. Matsubara and his colleagues reported.
There were no significant differences among the three study groups in activity patterns during the sad or neutral stimuli, the investigators said (Neuroimage 2014;85:489-97).
This study was limited in that the small sample size likely restricted its statistical power. In addition, all of the participants with mood disorders were taking medications during testing, which might have blunted or otherwise altered their brain activity.
Still, Dr. Matsubara and his colleagues said that they hope their findings "may help to elucidate the different pathophysiologies underlying these two diseases."
One of Dr. Matsubara’s associates reported ties to numerous industry sources.