Working memory network dysfunction in bipolar I vs. bipolar II disorder: a systematic review of task-fMRI evidence

Area of Science:

• Neuroscience
• Cognitive Psychology
• Psychiatry

Background:

• Working memory impairment is a key cognitive feature in bipolar disorder.
• Understanding the neural basis of working memory across bipolar disorder subtypes (BD-I and BD-II) is crucial but remains unclear.
• This review focuses on task-based functional magnetic resonance imaging (fMRI) evidence to compare working memory mechanisms in BD-II versus BD-I.

Purpose of the Study:

• To systematically review task-based fMRI studies on working memory in bipolar disorder.
• To characterize the neural correlates of working memory in BD-II.
• To compare these findings with those reported for BD-I.

Main Methods:

• Systematic literature search of PubMed, Embase, and Web of Science (Jan 2011 - June 2025).
• Included studies used task-based fMRI with working memory paradigms in adults, reporting separate results for BD-I and BD-II.
• A total of 22 studies were included for qualitative synthesis, adhering to PRISMA 2020 guidelines.

Main Results:

• Evidence suggests potentially different working memory network engagement patterns between BD-I and BD-II.
• BD-I studies often show altered executive control region recruitment and reduced default mode network suppression.
• Limited BD-II data indicate preserved function in euthymia but attenuated upregulation in depressive states or under higher cognitive load, with potential context-specific fronto-limbic coupling differences.

Conclusions:

• Current task-fMRI literature provides preliminary indications that BD-I and BD-II may not be explained by a simple severity continuum.
• Firm subtype-specific conclusions are limited by the scarcity of BD-II studies and direct comparisons.
• Larger, harmonized studies with mood-state stratification are necessary to validate these provisional patterns.