Association of intracortical myelin and cognitive function in bipolar I disorder.

OBJECTIVE: Although cognitive dysfunction persists through affective and euthymic states in bipolar disorder (BD), its neurobiological correlates remain undetermined. We explore whole-cortex intracortical myelin (ICM) and cognition in BD-I and controls. METHODS: T1 -weighted images (3T) optimized for ICM measurement were analyzed using a surface-based approach. MRI signal was sampled at cortical mid-depth. Cognitive performance was measured via standardized computerized battery and paper-and-pencil Trails B. RESULTS: ICM was associated with verbal memory (VM) in BD throughout a cortical network identified with pertinence to VM function, with strongest effects in left caudal middle temporal cortex and left dorsolateral prefrontal cortex (Pcorrected  < 0.05). Subanalyses revealed specific association with correct word recognition, without delay. Processing speed, executive function, and reaction time were also predicted by ICM in BD, but not controls, although this did not survive Bonferroni correction. CONCLUSION: This is the first study to show VM association with ICM in BD. ICM has been implicated in the integrity of neural connections and neural synchrony. VM dysfunction is one of the most replicated cognitive abnormalities in BD. Therefore, these results provide a novel mechanism for understanding cognitive dysfunction in BD, which can aid in the development of targeted therapeutics to improve cognitive outcomes in BD.

The association between biological rhythms, depression, and functioning in bipolar disorder: a large multi-center study.

OBJECTIVE: We examined the relationship between biological rhythms and severity of depressive symptoms in subjects with bipolar disorder and the effects of biological rhythms alterations on functional impairment. METHOD: Bipolar patients (n = 260) and healthy controls (n = 191) were recruited from mood disorders programs in three sites (Spain, Brazil, and Canada). Parameters of biological rhythms were measured using the Biological Rhythms Assessment in Neuropsychiatry (BRIAN), an interviewer administered questionnaire that assesses disruptions in sleep, eating patterns, social rhythms, and general activity. RESULTS: Multivariate analyses of covariance showed significant intergroup differences after controlling for potential confounders (Pillai's F = 49.367; df = 2, P < 0.001). Depressed patients had the greatest biological rhythms disturbance, followed by patients with subsyndromal symptoms, euthymic patients, and healthy controls. Biological rhythms and HAMD scores were independent predictors of poor functioning (F = 12.841, df = 6, P < 0.001, R2  = 0.443). CONCLUSION: Our study shows a dose-dependent association between the severity of depressive symptoms and degree of biological rhythms disturbance. Biological rhythms disturbance was also an independent predictor of functional impairment. Although the directionality of this relationship remains unknown, our results suggest that stability of biological rhythms should be an important target of acute and long-term management of bipolar disorder and may aid in the improvement of functioning.