Impaired recruitment of the dorsolateral prefrontal cortex and hippocampus during encoding in bipolar disorder

Background: The aim of the present study was to examine the functional neuroanatomy of episodic memory impairment in euthymic subjects with bipolar I disorder. There is evidence that individuals with bipolar disorder have cognitive impairments not only during mood episodes but also when they are euthymic. The most consistently reported cognitive difficulty in euthymic subjects with bipolar disorder is impairment in verbal episodic memory (i.e., the ability to learn new verbal information). Methods: The current study examined verbal learning in eight euthymic, remitted subjects with bipolar I disorder (BP-I; seven nonmedicated) and eight control subjects matched for age, gender, education, and intelligence. Subjects underwent ¹⁵O-CO ₂ positron emission tomography scanning while completing a verbal learning paradigm that consisted of encoding (learning) several lists of words. Results: The BP-I subjects had more difficulties learning the lists of words compared with the control subjects. Compared with control subjects, BP-I subjects exhibited blunted regional cerebral blood flow (rCBF) increases in the left dorsolateral prefrontal cortex (Brodmann's area 9/46) during encoding. Conclusions: Consistent with previous studies, subjects with BP-I were impaired in learning new verbal information. This was associated with rCBF abnormalities in brain regions involved in learning and episodic memory.