Temporal Characteristics of Categorical Perception of Emotional Facial Expressions

Temporal Characteristics of Categorical Perception of Emotional Facial Expressions Atsunobu Suzuki (suzuki@bayes.c.u-tokyo.ac.jp) Department of Cognitive and Behavioral Science, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan Susumu Shibui (shibui@bayes.c.u-tokyo.ac.jp) Department of Cognitive and Behavioral Science, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan Kazuo Shigemasu (kshige@bayes.c.u-tokyo.ac.jp) Department of Cognitive and Behavioral Science, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan 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. Abstract 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. Experiment 1 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 In Experiment 1, participants identified facial expressions from three continua extending from anger to happiness, from happiness to fear, and from fear to anger. We investigated how shortening stimulus exposure duration (750 ms, 150 ms, 50 ms) affected the abrupt change in the rate of identification at the category boundary. Method Participants Twelve undergraduates and postgraduates participated (8 men, 4 women; 20-24 years old). Facial Stimuli Three sets of eight gray-scale images of facial expressions were used. Each set consisted of a continuum extending either from anger to happiness, from happiness to fear, or from fear to anger. The original (endpoint) facial expressions were posed photographs of a Japanese woman, and interpolated (morphed) expressions between the two continuum endpoints were created with software for facial expression processing (Information- technology Promotion Agency, Japan (IPA), 1998). The morph transformation started with the delineation of two endpoint expressions; landmarks were placed manually on critical positions of each image. There were 759 landmarks in total, 88 points of which were placed manually on corresponding positions of each image: for the head, 4 points; for the outline, 28 points; for the eyes, 5 x 2 points; for the eyebrows, 4 x 2 points; for the nose, 4 points; for the