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30 F. Z. Sai stranger’s face from 4, 12, 72 and 78 h, respectively. Using a high amplitude sucking (HAS) technique, Walton et al. (1992) reported that infants from 12 h of age produced more sucking responses to the videotaped image of their mother’s face than that of a female stranger. Not only do neonates prefer to look at a composite of previously seen faces than at a composite of previously unseen faces (Walton and Bower, 1993), but can recognize a learned face over transformations (Walton et al., 1997), and even learn an image similar to a composite of the faces they have seen in the first hours of birth (Slater et al., 1998). Altogether these data suggest a rapid learning about faces within the first hours of birth, and the establishment of}at least}a rudimentary representation of faces. Nonetheless the basis of such learning and what is being learned remains obscure. There has been more than one interpretation offered to explain the data suggesting early preference, such that there exists a configurational response in the neonatal period. Goren et al. (1975) and Dziurawiec (1987) reported that infants only minutes old oriented their heads to follow a two-dimensional schematic face-like pattern than either of two patterns consisting of the same facial features in different arrangements. Johnson and Morton (1991) who replicated Goren et al.’s study proposed the existence of ‘Conspec’. Consisting of three dots, the Conspec serves to direct the newborn’s visual attention to faces. While this proposition seems plausible, there are some data that are not easily explained (e.g. Turati et al.’s (2002) results). Preference for the mother’s live face vanished with the slightest orientation, half-profile of that face (Sai, 1990), and when hair-face line information was masked (Pascalis et al., 1995). Recognition of the half-profile pose of the mother’s face was, however, possible only at 4–5 weeks from birth (Sai and Bushnell, 1988), and it was only at 10–12 weeks that infants demonstrated preference for the mother’s face shown in profile (Sai, 1990). Together, these findings seem to favour the learning hypothesis; it Is possible that the infants had not been sufficiently exposed to the half-profile and profile poses of their mother’s face. If recognition is based solely on facial configuration, then infants should have been able to show preference for the mother’s face since at least some information about facial configuration was still available, at least in the half-profile pose. It is fairly clear in retrospect that there were some methodological shortcomings with the studies that used real faces. For example Field et al.’s (1984) study failed to take the olfactory information into account. Those which controlled for discrimination on the basis of olfactory information, implemented a control over the mother’s and stranger’s odours only during the testing (Sai, 1990; Bushnell et al., 1989) and, so was the mother’s voice. Nevertheless, exposure to the mother’s voice–face from birth to the time of testing might have facilitated learning the mother’s visual features. Evidence suggests learning of the mother’s voice at birth. For example, in a study by DeCasper and Fifer (1980), 2–4-day-old neonates preferred their mother’s voice. Moon et al. (1991) demonstrated that with both Spanish and English speaking women, 2-day-old newborns preferred their mother’s language. Therefore, the question of controlling for the mother’s voice is, particularly critical for understanding the mechanism underlying preference for the mother’s face at birth. The mother’s face, like many other stimuli we experience is intermodal in that it provides information to more than one sensory modality. For example, during interaction with the mother, infants are provided with tactile stimulation, warmth and exposure to the mother’s voice, odours, taste of her milk and even to the rhythms to her heart beats. This wide range of information Copyright # 2004 John Wiley & Sons, Ltd. Inf. Child Dev. 14: 29–50 (2005)
The Role of the Mother’s Voice 31 may not only play a role in determining the mother’s face recognition, but is likely to lay ground to early cross-modal learning. Research has now clearly revealed many interesting intermodal capabilities in infancy (for review see, Bushnell and Boudreau, 1991, 1993; Lewkowicz and Lickliter, 1995; Rose and Ruff, 1987). Some of the intermodal relations are amodal and redundant across the senses. For example, the synchrony of voice and mouth when a person speaks. Other cues are modality-specific and arbitrary-related across the senses, such as the pairing of a face and the sound of the voice. From birth, newborn infants perceive a wide range of invariant amodal relations. For example, they reliably co-ordinate visual and auditory space, in that they turn their heads and eyes in the direction of a sound source, suggesting that knowledge about spatial location is provided by both visual and auditory information (e.g. Wertheimer, 1961; Muir and Field, 1979; Clifton et al., 1981; Butterworth, 1983; Muir and Clifton, 1985). At 5 weeks, infants can recognize a visual shape (a pacifier) that had been experienced tactually by sucking, suggesting that the pacifier is coded both tactually and visually (Meltzoff and Borton, 1979). Twenty week old infants show a sensitivity to temporal synchrony specified intermodally in that they detect the common rhythm and duration of tones and flashing lights (Lewkowicz, 1986). Around this age, infants can also match appropriately the sounds made either by a single unitary element or by a cluster of smaller elements (Bahrick, 1987, 1988). Together, these findings indicate that infants from birth perceive amodal relations. On the other hand, recent evidence suggests that arbitrary intermodal relations are detected and learned in infancy too. Two-day-old infants can learn arbitrary visual–auditory associations when familiarized to two alternating visual stimuli differing in colour and orientation, and when each association was accompanied by its appropriate sound (Slater et al., 1997). By 14 weeks, infants were able to learn to associate the sound of their mother’s voice with the sight of her face (Spelke and Owsley, 1979), and 24 week olds learned the association between the colour of a container and the taste of the food it contained (Reardon and Bushnell, 1988). Prior to 12 weeks, infants were not able to detect the arbitrary pitch colour/shape relations (Bahrick et al., 1996). Only infants of 28 weeks demonstrated learning of the arbitrary relations (Bahrick, 1994). If young infants are able to learn the synchrony of voice and mouth when the person speaks (amodal information), and can learn the pairing of a stimulus and the sound that stimulus produces (arbitrary information), then learning about faces would be most likely the outcome of an early intermodal processing. It is, however, possible that there exists a specific mechanism for learning the visual features of the mother’s face. If this were the case, then newborn infants would still prefer their mother’s face to that of a female stranger even when the mother’s voice is not available to the infants from birth through testing. On the other hand, preference for the mother’s face might disappear with the unavailability of a postnatal auditory experience to the mother’s voice. In this case, the conclusion that would be drawn is that face preference does not solely depend on visual processing. Instead, we would assume that face preference is the product of intermodal learning, especially since 2-day-old newborn infants are able to learn arbitrary visual–auditory associations (Slater et al., 1997) as mentioned earlier. However, we do not have evidence concerning whether infants are able to perceive the mother’s face–voice associations and later use such information in the recognition test. The experiments described here were designed to determine the role of the mother’s voice in facilitating preference for Copyright # 2004 John Wiley & Sons, Ltd. Inf. Child Dev. 14: 29–50 (2005)
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