Bipolar disorder: distinct brain activity even during stable mood states

Bipolar disorder is often illustrated through emotional “roller coasters”: alternating depressive episodes and periods of elevated or irritable mood. But beyond these visible mood fluctuations lie more subtle differences in brain functioning, as shown by the study coordinated by Prof. Mircea Polosan and Dr. Ronen Sosnik, and published in 2025 in the Journal of Affective Disorders. 

To explore this, the researchers focused on event-related potentials (ERPs): tiny brain waves recorded with electroencephalography (EEG) that track, millisecond by millisecond, how the brain perceives, selects, and evaluates incoming information. Early ERP components (such as N100 and N200) are mainly related to sensory processing and stimulus detection, while later components (such as P300 and the Late Positive Potential, LPP) are linked to attention, updating of information, and emotional evaluation.

The team recorded EEG activity in several dozen individuals with bipolar disorder (type I or II, in depressive or euthymic states), in their unaffected relatives, and in healthy control participants, while they performed an emotional task.

ERP analyses showed that bipolar disorder, its subtypes (I/II), and current mood state (depressive or stable) differentially affect specific early and late components involved in cognitive and emotional processing. 

Importantly, some of these specific patterns were also found in patients in euthymic states (who were clinically stable) and unaffected relatives frequently showed intermediate profiles between patients and healthy controls.
“These findings support the idea that bipolar disorder is associated with more persistent, and potentially familial, neurophysiological traces in the way the brain processes emotional information, beyond acute mood episodes,” explains Prof. Polosan.

Towards biomarkers for bipolar disorder?

Because these electrical signatures are present in some patients even during inter-episode periods, and, for certain measures, in their unaffected relatives, the authors suggest that ERPs elicited during emotional tasks may represent promising candidate biomarkers for bipolar disorder. “They could provide objective, quantifiable indicators to support clinical assessment of the disorder, its subtypes identification, and familial vulnerability, as well as help evaluate treatment response,” notes Prof. Polosan.

Compared with MRI, EEG is rapid, non-invasive, and relatively inexpensive, making it an attractive tool for clinical research.

The authors remain cautious, however: some subgroups are small, treatment regimens are heterogeneous, and the study is cross-sectional. These results do not yet allow for an individual diagnostic test, but they represent an important step toward a more precise understanding of how bipolar disorder is reflected in the brain’s electrical activity.

In short: bipolar disorder is not only visible in mood fluctuations; it also leaves a discreet yet measurable imprint on brain dynamics, even when everything appears to be going well.

Référence : 
Event-related potential alterations in bipolar disorder subtypes and mood states: Insights from an emotional visual task
Ronen Sosnik, Antoine Bertrand, Muli Linder, Mircea Polosan. 
J Affect Disord. 2025 Dec 15;391:119970. doi: 10.1016/j.jad.2025.119970. Epub 2025 Jul 23. PMID: 40712679.