Background. Judgements made on chimeric faces elicit reliably a perceptual bias to the left hemispace, presumed to be due to right hemisphere dominance for emotional processes. Major depressive illness has been shown to attenuate this bias. The aim of this work was to examine lateral perceptual bias in bipolar I and II patients in a hypomanic state and unipolar depressed patients and those with unilateral hemisphere damage following stroke.

Method. Sixty patients with DSM-IV affective disorder (30 bipolar I or II, currently hypomanic, 30 unipolar depressives), 30 right brain-damaged patients, 30 left brain-damaged patients and 30 healthy controls were given the Happy–Sad Chimeric Faces Test.

Results. Right hemisphere damaged and unipolar depressed patients both showed a significantly reduced left hemispatial bias (LHB) compared to controls, bipolars and left brain-damaged patients. No significant difference in mean LHB between controls and both hypomanics and left brain-damaged patients was found. There was no significant association between LHB and clinical variables.

Conclusions. The results suggest a physiological distinction between bipolar and unipolar depression. The significantly diminished left hemifacial bias in depressed patients suggests right hemisphere dysfunction.