Objective: Studies have begun to identify neural changes in patients with mood disorders. Brain regions known to be involved in emotional circuitry have been repeatedly activated using emotional stimuli and network dysfunction has been implicated in both unipolar and bipolar disorder. Comparisons have been conducted in relation to healthy subjects and few studies have attempted to distinguish unipolar and bipolar patients. The present study aimed to differentiate unipolar and bipolar patients on the basis of neural responsivity.

Method: Studies were conducted on a dedicated 3-T Siemens scanner (ARCHI). An affective visual word paradigm known to reliably elicit emotional changes was used to induce positive, negative and neutral mood change. Concurrent eye-movement tracking was used to ensure engagement with the visual stimuli. Seventeen patients with bipolar I disorder (BP) and fifteen with unipolar (major) depression (UP) were recruited through the CADE Clinic and advertisement and compared with 21 age and gender matched controls. Patients provided informed consent in accordance with Ethics and were euthymic at the time of scanning.

Results: Robust fMRI activations in response to emotion especially negative mood were observed in all three groups. Activations involved prefrontal and subcortical structures implicated in emotional networks and a marked difference in responsivity and connectivity emerged contrasting unipolar and bipolar patients and healthy controls. Overall UP patients had more robust responses than healthy controls who in turn showed greater responsivity as compared to BP patients.

Conclusions: The differential pattern and intensity of responsivity on fMRI across phenotypes of mood disorders is a promising preliminary finding. The different patterns of activation on fMRI in unipolar and bipolar subjects, suggests that there are functional differences in emotion processing between these phenotypes that could potentially serve diagnostic differentiation.