Altered prefrontal lobe oxygenation in bipolar disorder: a study by near-infrared spectroscopy

Abstract

Previous studies have reported prefrontal cortex (PFC) pathophysiology in bipolar disorder. We examined the hemodynamics of the PFC during resting and cognitive tasks in 29 patients with bipolar disorder and 27 healthy controls, matched for age, verbal abilities and education. The cognitive test battery consisted of letter and category fluency (LF and CF), Sets A and B of the Raven's Colored Progressive Matrices (RCPM-A and RCPM-B) and the letter cancellation test (LCT). The tissue oxygenation index (TOI), the ratio of oxygenated hemoglobin (HbO2) concentration to total hemoglobin concentration, was measured in the bilateral PFC by spatially resolved near-infrared spectroscopy. Changes in HbO2 concentration were also measured. The bipolar group showed slight but significant impairment in performance for the non-verbal tasks (RCPM-A, RCPM-B and LCT), with no significant between-group differences for the two verbal tasks (LF and CF). A group x task x hemisphere analysis of variance (ANOVA) on the TOI revealed an abnormal pattern of prefrontal oxygenation across different types of cognitive processing in the bipolar group. Post hoc analyses following a group x task x hemisphere ANOVA on HbO2 concentration revealed that the bipolar group showed a greater increase in HbO2 concentration in the LCT and in RCPM-B, relative to controls. Both indices of cortical activation (TOI and HbO2 concentration) indicated a discrepancy in the PFC function between verbal versus non-verbal processing, indicating task-specific abnormalities in the hemodynamic control of the PFC in bipolar disorder.