Categorical perception of facial expressions: Evidence for a category adjustment model

1814 The extent to which the phenomenon of categorical perception represents underlying perceptual asymmetry, or merely the engagement of category labels has been the subject of much recent debate (Bornstein & Korda, 1984; Calder, Young, Perrett, Etcoff, & Rowland, 1996; Harnad, 1987; Munnich & Landau, 2003; Pilling, Wiggett, Özgen, & Davies, 2003; Roberson & Davidoff, 2000; Young et al., 1997). Categorical Perception (CP) is the name given to the increased sensitivity to a physical change found when that change crosses the boundary between two perceptual categories (Harnad, 1987). It has been demonstrated for simple perceptual continua such as phoneme blends (Pastore, 1987) or color categories (Roberson, Davies, & Davidoff, 2000; Pilling et al., 2003), and a number of recent studies have examined the phenomenon with regard to face processing. A steady change between two faces (differing in either identity or facial expression) can be created using morphing software (Beale & Keil, 1995; Calder et al., 1996). However, rather than being perceived as a monotonic linear progression, the continuum of identity or expression is perceived as an abrupt discontinuity at the boundary between two categories (e.g., happy–sad). Demonstrations of CP for morphed facial expressions (Etcoff & Magee, 1992; Calder et al., 1996; Young et al., 1997) as well as for, facial identity (Beale & Keil, 1995; Campanella, Hanoteau, Seron, Joassin, & Bruyer, 2003; Stevenage, 1998) have shown that it is relatively hard to make discriminations near the center of a category and easy close to a boundary (Calder et al., 1996). Young et al., (1997) used an X–AB two-alternative forced-choice (2-AFC) memory paradigm in which a series of morphed facial expressions were created, with an equal amount of physical difference between each successive pair in the continuum (see Figure 1). Participants viewed one of the morphs as a target expression, followed by a test pair of images (target distractor), differing only in morphed expression. Their task was to identify the position of the target face in the test pair by keypress. Participants showed superior identification of expressions from pairs that crossed a category boundary (e.g., happy–fearful) than from pairs where both members belonged to the same category (e.g., both happy). The finding is robust, but the locus of the effect remains unclear. Some researchers have suggested that CP arises at an early perceptual level, or even that, for domains such as color, it might be innate and hardwired into the visual system (Bornstein, Kessen, & Weiskopf, 1976; Franklin & Davies, 2004). Young et al. (1997) argued for perceptually (rather than verbally) mediated CP for facial expressions, because labeling emotional facial expressions is actually more difficult at boundaries than at the center of categories, where a “prototypical” emotional face gives a more consistent and rapid response. Thus it seems unlikely that verbal labeling could underlie the superior discrimination of emotional expressions at category boundaries. Categorical perception of facial expressions: Evidence for a “category adjustment” model