Temporal Characteristics of Categorical Perception of Emotional Facial Expressions

Perceptual processing of emotional facial expressions occurs very quickly, even without awareness. To determine whether the fast processing of facial expressions is categorical, we studied temporal characteristics of categorical perception (CP) of facial expressions. We investigated the effect of shortening stimulus duration on participant performance with respect to identifying and discriminating morphed facial expressions. The results of two experiments showed that CP was attenuated or even disappeared when facial stimuli were presented for as briefly as 50-75 milliseconds. These findings indicate that CP is irrelevant to the fast perceptual processing of facial expressions. Facial expressions of emotion are processed very quickly, even without awareness (Morris, Ohman, & Dolan, 1998; Whalen et al., 1998). It has been proposed that this fast emotional processing provides a ‘dirty’ image of the external world, enabling organisms to detect salient stimuli immediately (Adolphs, 2002; LeDoux, 1996). What is still not understood, however, is how this fast and dirty processing of facial expressions works. A hallmark of facial expression recognition is its categorical nature, that is, facial expressions are recognized as belonging to discrete categories of emotion, so-called basic emotions (e.g., Ekman, 1992; Izard, 1992). Reports on categorical perception (CP) of facial expressions are thought to provide strong evidence that people process facial expressions categorically (Calder et al., 1996; DeGelder, Teunisse, & Benson, 1997; Etcoff & Magee, 1992; Young et al., 1997). Previous studies have confirmed that recognizing facial expressions fulfills the following features of CP: (a) in identifying a stimulus within a continuum extending from one category to another, the rate of categorizing the stimulus changes abruptly at a boundary (category boundary); and (b) in discriminating a pair of stimuli that differ by a constant physical amount, discrimination is superior for pairs straddling the category boundary (between-category pairs), as compared to pairs falling within one category (within-category pairs). It has been argued that the perceptual system transforms the information continuously received from a given facial expression into categorical information corresponding to the most likely emotion (Etcoff & Magee, 1992). In previous research, however, facial stimuli were presented for a relatively long period (≧750 ms), so it remains unclear whether the fast perceptual processing of facial expressions is categorical in nature. Our goal was to investigate the effects of shortening stimulus duration on CP of facial expressions and to examine whether the fast perceptual processing is categorical.