Background: Bipolar disorder (BD) and unipolar depression (UD) can be difficult to distinguish clinically, particularly during episodes of depression. In this study we test for differences between BD, UD, and healthy control (HC) adults regarding within- and between-session changes in BOLD response during implicit emotional processing.
Methods: During fMRI, HC adults (N=19) and depressed adults with UD (N=19) and BD (N=16) performed an implicit emotion-processing task. Each participant was scanned twice, separated by 6-months, resulting in 108 scans. BOLD response and linear change in BOLD response were examined within and between sessions.
Results: We observed within-session linear decreases in BOLD signal (irrespective of group, condition, or session) in the left amygdala, a right-sided temporo-parietal region, and a right-sided fronto-insular region. Furthermore, we observed group differences in within-session BOLD signal change (p<0.05, FWE corrected) in a left-sided striatal-insular-thalamic region. Individuals with BD demonstrated a linear decrease in BOLD signal compared to HC (p<0.008, FWE corrected) across this region and compared to UD in the posterior insula portion of the region (p<0.008, FWE corrected). Finally, we observed main effects of emotional valence in bilateral visuo-spatial processing regions as well as in the left and right amygdala.
Conclusions: Individuals with BD demonstrated linear attenuation of BOLD response to emotional stimuli within left-sided striatal-insular-thalamic regions. Individuals with BD may either have experienced abnormal habituation in this region or disengaged quickly from processing the emotional stimuli, despite comparable task performance. This pattern may represent an underlying pathophysiological process associated with BD that differs from UD.
Keywords: BOLD attenuation; Bipolar Disorder; Emotion processing; Major Depressive Disorder; Whole brain analysis; fMRI.