A valence-specific lateral bias for discriminating emotional facial expressions in free field

Findings from subjects with unilateral brain damage, as well as from normal subjects studied with tachistoscopic paradigms, argue that emotion is proc essed differently by each brain hemisphere. An open question concerns the e xtent to which such lateralised processing might occur under natural, free- viewing conditions. To explore this issue, we asked 28 normal subjects to d iscriminate emotions expressed by pairs of faces shown side-by-side, with n o time or viewing constraints. Images of neutral expressions were shown pai red with morphed images of very faint emotional expressions (happiness, sur prise, disgust, fear, anger, or sadness). We found a surprising and robust laterality effect: When discriminating negative emotional expressions, subj ects performed significantly better when the emotional face was to the left of the neutral face; conversely, when discriminating positive expressions, subjects performed better when the emotional face was to the right. We int erpret this valence-specific laterality effect as consistent with the idea that the right hemisphere is specialised to process negative emotions, wher eas the left is specialised to process positive emotions. The findings have important implications for how humans perceive facial emotion under natura l conditions.