Research paperPrefrontal activation during a working memory task differs between patients with unipolar and bipolar depression: A preliminary exploratory study

- Prefrontal activation differs between unipolar (UD) and bipolar depression (BD).
- BD exhibited less deactivation in the left frontopolar and Broca's area than UD.
- NIRS measurement may aid in differentiating BD from UD at an early stage.

BackgroundTo identify bipolar disorder during the initial stages of a depressive episode has always been a great clinical challenge. Patterns of functional brain activity may underlie the differences in the neural mechanisms of bipolar depression (BD) and unipolar depression (UD). This study aimed to investigate the differences in neural activity between BD and UD patients during executive task.MethodsWe performed a 52-channel near-infrared spectroscopy (NIRS) scan in 39 patients with BD, 35 patients with UD, and 36 healthy controls (HCs). The relative concentration changes in oxygenated hemoglobin ([oxy-Hb]) and deoxygenated hemoglobin ([deoxy-Hb]) during a 1-back working memory task were measured for each channel. Clinical characteristics including current mood were evaluated within one week prior to NIRS examination.ResultsCompared to HCs, BD (CH34: Z = −2.354, P = 0.019) and UD patients (CH18: Z = −2.358, P = 0.018; CH30: Z = −2.174, P = 0.030; CH34: Z = −1.990, P = 0.047) showed reduced activation of [oxy-Hb] in the inferior prefrontal region. Compared to patients with UD, patients with BD showed less decreased [oxy-Hb] changes in the left frontopolar cortex (FPC) (CH18: Z = −2.366, P = 0.018), left pars opercularis and pars triangularis (POPE/PTRI) regions (Broca's area) (CH30: Z = −2.333, P = 0.020). No correlation existed between clinical characteristics and NIRS measurements.LimitationsThe effect of medication could not be excluded, and behavioral data was not systematically collected.ConclusionThe results from this preliminary exploratory study suggest distinct prefrontal activation patterns underlie BD and UD, especially in the left frontopolar region and Broca's area. The NIRS-based prefrontal activation measurement may serve as a potential marker to aid in differentiating bipolar from unipolar depression.