# Differences in event-related potentials between unipolar depression and bipolar II disorder during depressive episodes: a retrospective case-control study

## TL;DR

This study finds differences in brain activity between people with unipolar depression and bipolar II disorder during depressive episodes, which could help in accurate diagnosis.

## Contribution

The study identifies specific neurophysiological markers, such as prolonged S2-P50 latency, that distinguish bipolar II disorder from unipolar depression.

## Key findings

- Both unipolar depression and bipolar II disorder groups had longer reaction times and larger P2-N2 complex amplitudes compared to healthy controls.
- The bipolar II disorder group showed significantly longer S2-P50 latency compared to the unipolar depression group.
- Bipolar II disorder patients had prolonged N2 latency compared to healthy controls.

## Abstract

Bipolar II disorder (BD II) is a chronic and severe mental illness frequently misdiagnosed as major depressive disorder (MDD) due to symptom overlap and the absence of objective diagnostic tools. Consequently, establishing pathophysiological markers to differentiate BD II from MDD is critical.

A total of 180 patients were enrolled in the study and allocated to three groups: patients with unipolar depression (UD group; MDD currently experiencing a major depressive episode, n = 60), patients with bipolar II disorder during depressive episodes (BD II group; n = 60), and age- and sex- matched healthy controls (HC; n = 60). Sociodemographic data were collected, and all participants underwent psychological assessments using the 7-item Generalized Anxiety Disorder (GAD-7), Patient Health Questionnaire-9 (PHQ-9), and 32-item Hypomania Checklist (HCL-32). Additionally, all participants passed auditory brain stem response (ABR) test and subsequently underwent event-related potential (ERP) examinations.

No significant differences were observed in demographic characteristics between the three groups, including age, sex, educational level, marital status, and socioeconomic status (all P > 0.05). Compared with HC, patients in both the UD and BD II groups showed significantly longer reaction time (HC: 254.4 ± 43.8 ms; UD: 297.7 ± 72.2 ms; BD II: 300.3 ± 70.0 ms; P = 0.028) and larger amplitude of P2-N2 complex (HC: 5.7 ± 4.4 μV; UD: 8.1 ± 4.8 μV; BD II: 8.6 ± 5.6 μV; P = 0.001) in P300 paradigm. The BD II group exhibited longer S2-P50 latency than the UD group (UD: 50.4 ± 11.1 ms vs. BD II: 63.2 ± 11.5 ms; P = 0.025). Additionally, the BD II group had prolonged N2 latency compared to HC (BD II: 216.2 ± 22.1 ms vs. HC: 205.2 ± 16.5 ms; P = 0.044).

This study may identify neurophysiological distinctions between BD II and UD depression, notably a prolonged S2-P50 latency in BD II.