A near-infrared spectroscopy study of prefrontal cortex activation during a verbal fluency task and carbon dioxide inhalation in individuals with bipolar disorder

There is evidence of prefrontal cortex (PFC) dysfunction in patients with bipolar disorder (BP). Magnetic resonance and neuropathological studies show abnormalities of the brain microvasculature in patients with BP. However, the underlying biological mechanisms are not well understood. We investigated the relationship between activation of the PFC during a cognitive task and the vascular function in response to a physiological task in patients with BP. Fourteen euthymic patients with BP and 14 control subjects matched for age, sex, and education were recruited. We examined the response of the PFC during a verbal fluency task and during 5% CO2 inhalation using a 24-channel near-infrared spectroscopy imaging system to measure alteration of oxygenated and deoxygenated hemoglobin. The BP patients showed a significantly lower level of PFC activation during the cognitive task compared to the healthy controls, but the task-performance of the BP patients was not significantly different from that of the controls. The vascular response of the BP patients to CO2 was not significantly different from that of controls. This study suggests functional hypoactivation of the PFC during a cognitive load in patients with BP while they are in a euthymic state. The mechanism of this hypoactivation is different from that of vascular regulation in response to a physiological stimulus.