درک قطعی ابراز، رنگ و حالات چهره: مطالعه تکاملی

Abstract The aim of the present paper was to compare the development of perceptual categorization of voicing, colors and facial expressions in French-speaking children (from 6 to 8 years) and adults. Differences in both categorical perception, i.e. the correspondence between identification and discrimination performances, and in boundary precision, indexed by the steepness of the identification slope, were investigated. Whereas there was no significant effect of age on categorical perception, boundary precision increased with age, both for voicing and facial expressions though not for colors. These results suggest that the development of boundary precision arises from a general cognitive maturation across different perceptual domains. However, this is not without domain specific effects since we found (1) a correlation between the development of voicing perception and some reading performances and (2) an earlier maturation of boundary precision for colors compared to voicing and facial expressions. These comparative data indicate that whereas general cognitive maturation has some influence on the development of perceptual categorization, this is not without domain-specific effects, the structural complexity of the categories being one of them.

. Introduction Since its first theorization by Liberman et al. (1957) categorical perception has been a central notion in psychophysics and cognitive psychology. Psychophysicists have begun to study the relation between the physical parameters of the stimulus and its concomitant sensation and psychologists have emphasized the fact that by transforming physical sensations into discrete representations, categorical perception constitutes an economical way to process the flow of information present in the environment (Snowdon, 1987). According to the original definition, there is categorical perception (CP) when discrimination between stimuli depends on their identification into different categories (Liberman et al., 1957). Another categorical property resides in the occurrence of a response non-linearity around a category boundary. The degree of nonlinearity, or boundary precision (BP), can be evaluated either by measuring the steepness of the identification function around the perceptual boundary (the steeper the identification function, the greater the precision, Simon and Fourcin, 1978) or by measuring the size of the discrimination peak around the phoneme boundary (the larger the peak, the greater the precision, Wood, 1976). Although some minimal amount of response non-linearity is a prerequisite for CP (Schouten et al., 2003), these two properties are fairly independent (for a review see: Damper and Harnad, 2000). Fig. 1 schematically illustrates how the independent variation of CP and BP can lead to various levels of categorical performance.1 In addition to CP and BP measures, the location of the boundary is also of interest in a developmental context. When only discrimination responses are collected, which is often the case in pre-linguistic children, the location of the discrimination peak indicates whether their categorical responses are organized around the adult boundaries in the environmental language. Categorical perception properties. Boundary precision (BP) is larger when the ... Fig. 1. Categorical perception properties. Boundary precision (BP) is larger when the discrimination peaks are larger (compare a with c and b with d). Categorical perception (CP) is larger when the match between observed and expected discrimination curves is better (compare a with b and c with d). Figure options 1.1. Categorical perception development Different studies have claimed an effect of age on the categorical perception of speech sounds. However, a closer look at the available evidence makes it clear that it does not concern CP but rather the location and the precision of the perceptual boundaries (see Table 1 for a summary). It is now accepted that during the first year of life, infants move from a language-general to a language-specific mode of perception and that the location of the perceptual boundaries changes accordingly (for a review see: Hoonhorst et al., 2009a). Whereas during the first months of life babies discriminate voicing contrasts according to the −30 and +30 ms language-general VOT boundaries (Aslin et al., 1981), they soon become specialists in their native language(s) by adopting the phonological boundary/ies relevant to the language(s) spoken in their environment (e.g. 0 ms VOT in French: Serniclaes, 1987). It is also fairly clear from different studies that there is an increase in BP which extends well after one year of age and is still not complete at the end of childhood. Contrary to the location and the precision of perceptual boundaries, CP does not progress during this period (Medina and Serniclaes, 2009) and there even seems to be a decrease in CP between childhood and adulthood ( Elliott, Longinotti, Meyer, Raz & Zucker, 1981; see Table 1). Table 1. Summary of the main studies dealing with the development of categorical perception (CP) and boundary precision (BP). Study Age groups (from/to) Stimuli Tasks Results: effect of age Implications Zlatin and Koeningsknecht (1975) 2/6 yrs; adults Synthetic voicing contrasts Identification Slope becomes steeper BP increases Streeter and Landauer (1976) 7/15 yrs Synthetic voicing contrasts Discrimination Correct discrimination increases BP increases Simon and Fourcin (1978) 2/14 yrs Synthetic voicing continuum Identification Slope becomes steeper. Boundary location changes BP increases Elliott et al. (1981) 6/10 yrs; adults Synthetic place continuum Identification discrimination JND size becomes smaller. Difference between identification and discrimination boundaries becomes larger BP increases CP decreases Krause (1982) 3/6 yrs; adults Synthetic voicing continua Identification Slope becomes steeper. Boundary location changes BP increases Elliott et al. (1986) 8/11 yrs Synthetic voicing continuum Discrimination JND size becomes smaller BP increases Burnham et al. (1991) 2/6 yrs; adults Synthetic voicing continuum Identification Native contrast: precision increases. Foreign contrast: precision decreases BP increases Hazan and Barrett (2000) 6/12 yrs Synthetic different continua Identification Slope becomes steeper BP increases Medina and Serniclaes (2009) 2/12 yrs Synthetic voicing and place continua Identification discrimination Identification peak increases and discrimination peak increases. Difference between identification and discrimination does not change BP increases CP does not change Table options To summarize, whereas it has been demonstrated that both the location and precision of the boundaries change with age, little is known about the development of CP because most of the previous studies did not compare discrimination and identification. The first purpose of the present study was to collect further evidence on the possible effect of age on CP. 1.2. Influence of reading on categorical development The second question raised in this paper concerned the influence of reading on the late development of speech perception. According to the reading hypothesis ( Burnham et al., 1991 and Burnham, 2003), “the intensification of language speech perception between two and 6 years is related to the onset of reading instruction” ( Burnham, 2003, p. 573). This hypothesis is supported by data on the identification of the −30 ms prevoiced/voiced contrast, which is phonemic in Thai but not in English. English-speaking children who started learning to read at 6 years are more sensitive to this foreign contrast than those starting to read earlier, at the age of 5 years ( Burnham, 2003). These results suggest that reading experience sharpens the identification of native contrasts at the expense of the non-native ones, i.e., that reading experience improves BP. However, as sensitivity was only assessed with identification data, it cannot be inferred from these results that reading experience affects CP because the latter depends on the relationship between identification and discrimination. Another hypothesis has been proposed to account for perceptual development; the general cognitive hypothesis defended by Lalonde and Werker (1995). In their study ( Lalonde and Werker, 1995), 8–10-month-old babies showed a synchronous change in a native vs. non native linguistic discrimination task, a visual categorization task and an object search task, a result which led Lalonde and Werker to conclude that speech perception development is underlined by a maturation of general cognitive competences. This hypothesis led us to explore the perception of different perceptual continua, namely those of voicing, color and facial expression continua. Data on color perception mimic those found with VOT. Non-linearity in color perception has been demonstrated in adults whatever their spoken language ( Franklin et al., 2005), in 4-month-old infants ( Bornstein et al., 1976) and animals (bees: Von Frisch, 1964; pigeons: Wright and Cumming, 1971; monkeys: Sandell et al., 1979). However, these results have to be interpreted with caution because other data show that color categories are not genetically determined but emerge through cultural constraints ( Roberson and O’Hanlon, 2005). This top-down influence has also been demonstrated in studies using the perceptual learning paradigm and showing, as for voicing, a flexibility in boundary location (see Kraljic and Samuel, 2006 for voicing and Mitterer and de Ruiter, 2008 for colors). Concerning the perceptual development of categorical color perception during childhood, Raskin et al. (1983) showed that whereas the location of the categorical boundary was stable, BP improved with age (from children aged between 3 and 4 years to adults tested on blue-green and green–yellow continua). The perception of facial expressions has been less well documented but studies by Ekman, 1992 and Beale and Keil, 1995 and Campanella et al. (2001) showed non-linearity in the perception of facial expressions, facial identities and facial genders by human adults. Recently, Martin-Malivel and Okada (2007) expanded these results to chimpanzees who perceived pictures of human faces categorically. With human infants, Kotsoni et al. (2001) demonstrated that 7-month-old infants perceive happiness and fear categorically. Concerning childhood, a recent paper ( Gao and Maurer, 2009) provides evidence for the gradual improvement of BP between 5 years and adulthood. Children aged 5, 7 and 10 years, and adults did not differ when identifying happiness and sadness but children were less precise than adults. To summarize, these studies on the perceptual development of visual categories, be it colors or facial expressions, reveal several similarities with the development of speech categories. In each case, non-linear perception is present both in animals and in human children before one year of age, and improvements in BP take place during later development in humans. Another common point between all these studies is that they did not provide evidence for changes in CP, mainly because most of these studies did not compare identification with discrimination. The second purpose of the present study was to examine whether the same patterns of categorical development hold for voicing, color and facial expression continua.

. Conclusion The present study confirmed that only BP, not CP, improves with age. The results presented in this study suggest that (1) the development of BP extends over a long period of time during childhood, suggesting that the development of BP arises from a general cognitive development across different perceptual domains; and (2) this general development does not exclude domain-specific effects. In Experiment 1, the correlation between the development of voicing perception and some reading performances suggests a relationship between reading and speech perception. In addition, in Experiment 2, the earlier maturation of BP for colors – compared to voicing and facial expressions – appears to be due to a difference in the structural complexity of the categories, the perception of both facial expressions and voicing involving the integration of multiple cues.