Hemispheric contributions to the processing of emotion in chimeric faces : behavioural and electrophysiological evidence

Abstract

The face in general and facial expressions in particular have always been a focus of sociological, biological and psychological interest, with numerous different scientific approaches and objectives, investigating the expression and perception of facial affect or the social interaction of transmitting facial information between poser and perceiver.This study, dealing with two different aspects of facial expression, namely intensity and efficiency of facial expression, focuses on hemispheric contributions to the expression and perception of facial affect. Stimuli used that were based on the FEEST stimuli (Young, Perrett, Calder, Sprengelmeyer, & Ekman, 2002), differed in terms of the type of expression (anger, disgust, fear, happiness, sadness, surprise), in terms of the level of intensity (100% vs. 50%) and in terms of the type of chimeric (either consisting of left (LL) or right (RR) hemifaces).Using six basic expressions, the first experiment of the present study (intensity task) aimed to investigate whether the left or the right side of an expressive face is perceived as being more intense, following a classical study by Sackeim et al. (1978) that was claiming a dominant role for the right hemisphere/ left hemiface. In a 2- AFC intensity judgement, subjects were simultaneously presented with two chimerics created from corresponding left (LL) and right (RR) hemifaces and had to select the more expressive one. Contrary to earlier findings, regardless of the level of intensity and regardless of the type of expression, participants judged the RR- composites as more expressive.A second experiment (efficiency task), dealing with the question of the efficiency of emotional expression, investigated the statement of Indersmitten et al. (2003), who were claiming a dissociation between intensity and efficiency of emotional expression with a dominant role of the left hemisphere/ right hemiface for efficiency of emotional expression (Indersmitten & Gur, 2003). Testing this hypothesis, a subset of the chimeric stimuli from the first experiment were used (anger, fear, happiness, sadness). Subjects performed 4- AFC responses and classified faces (either being LL- or RR- chimerics) as fast and as accurate as possible according to the type of facial expression. There was indeed a RR- composite advantage, but modulated by emotion: It was present for anger and fear, but not for happiness, whereas sadness even yielded a LL-composite advantage. During the second experiment, ERP s were recorded, supplying additional information, allowing further differentiation and interpretation of the processing of different basic emotions, hemifaces and intensity levels