How sleeping neonates smile

Infants over one month of age tend to produce two types of smiling during especially positive social interactions, Duchenne smiles involving cheek raising and open-mouth smiles. Little is known, however, about the prevalence, frequency, duration and organization of these smiles among neonates. Twenty-five full-term, healthy neonates (12 female) were videotaped during six minutes of sleep. Smiles were identified and analysed using an anatomically based coding system (FACS/Baby FACS). One-half of the neonates showed bilateral Duchenne smiles. One-quarter of the neonates showed bilateral Duchenne smiles at a mature level of intensity whose median duration was 1 1 3 s. By contrast, open-mouth bilateral smiles occurred in less than one-tenth of the sample. The contrast between the more frequent bilateral Duchenne smiles and the less frequent open-mouth smile is discussed in terms of the early synergistic functioning of facial muscles and contrasted with the smiling patterns of older infants. Duchenne smiling is associated with self-reported positive emotion, but this is not the case for non-Duchenne smiling (Ekman et al., 1990). Among 10-month-old infants, Duchenne smiles are associated with mother’s smiling approach while non-Duchenne smiles are associated with the approach of an impassive stranger (Fox & Davidson, 1988). Among infants between one and 6 months of age, Duchenne smiling was more prevalent than non-Duchenne smiling when mother was smiling (Messinger, Fogel & Dickson, 2001). Ekman (1992, 1994) has argued that Duchenne smiles are a unique index of positive emotion. Yet the literature now suggests that open-mouth smiles may also index positive emotion in infancy. Messinger et al. (2001) found that open-mouth smiling involving a lowered jaw was more likely when infants were gazing at mother’s face (Messinger et al., 2001). Combined open-mouth Duchenne smiling was more likely both when infants were gazing at their mothers and when their mothers were smiling. These combination smiles also tended to occur during physical play with fathers (Dickson et al., 1997) and at the peak of tickle games with mother (Fogel et al., 2000). Investigation of Duchenne and open-mouth Smiling is an early social and emotional behavior. However, smiles often first occur during sleeping states without obvious external stimulation, creating what are known as endogenous smiles. This puzzle has long attracted the attention of developmental researchers (Harmon & Emde, 1972; Fogel & Thelen, 1987; Spitz, 1946; Sroufe & Waters, 1976; Wolff, 1963). More recent research indicates that in older individuals, smiles involving raising of the upper part of the cheeks (Duchenne smiles) and smiles in which the jaw is lowered (openmouth smiles) are more likely during positive periods of interaction than are smiles without these features (Dickson, Walker & Fogel, 1997; Ekman, Davidson & Friesen, 1990; Fogel, Nelson-Goens, Hsu & Shapiro, 2000; Fox & Davidson, 1988; Messinger, Fogel & Dickson, 1997). The current research used anatomically based coding to address whether and how frequently sleeping neonates exhibit Duchenne and open-mouth smiles, smiles that are frequently regarded as indices of positive affect in older, alert individuals. Duchenne and non-Duchenne smiles are distinguished, respectively, by the presence or absence of cheek raising caused by the muscle circling the eye. Among adults, DSC08 2/1/02, 11:58 AM 48 © Blackwell Publishers Ltd. 2002 How sleeping neonates smile 49 smiles in sleeping neonates will shed light on similarities and differences with smiles that occur during emotioneliciting social interaction at older ages. Researchers have also examined muscular dynamics involved in the organization of Duchenne smiles (Messinger et al., 1997). Duchenne smiles involve simultaneous lip corner raising (zygomatic major) and cheek raising caused by the action of the muscle circling the eye (orbicularis oculi, pars lateralis). Because the muscles have an overlapping function in raising the cheek and may operate synergistically (Williams, Warick, Dyson & Bannister, 1989), Messinger et al. hypothesized that stronger lip corner raising would be more likely to be associated with the presence of cheek raising than weaker lip corner raising. If the muscles function synergistically, it is also likely that stronger levels of lip corner raising should be associated with stronger levels of cheek raising when both actions occur simultaneously. Messinger et al. also found that smiles with cheek raising (Duchenne) had longer durations than smiles without (non-Duchenne) (Messinger, Fogel & Dickson, 1999). They suggested that cheek raising might stabilize lip corner raising, creating a longer lasting expression. If similar patterns of association and duration emerged in neonates, muscular synergies would be a possible explanation. Although smiling in neonates has been documented, little is known about the form of the smiling. Emde and his colleagues first observed that smiling in the first days of life predominated during REM states, occurring once (Emde & Koenig, 1969) to twice (Emde, McCartney & Harmon, 1971) every 10 minutes. We are not aware of descriptions or photographs of neonates with smiles involving a dropped jaw in the literature. There is photographic evidence (Oster, 1978) and several unambiguous descriptions of the occurrence of Duchenne smiles in full-term and pre-term neonates (Emde et al., 1971; Wolff, 1987). However, quantitative descriptions of the prevalence, frequency and duration of Duchenne smiles are lacking. Emde and his colleagues (Emde & Koenig, 1969a; Emde et al., 1971), for example, measured the intensity of smiles in real-time on a 5-point descriptive scale. Unfortunately, the scale was not anatomically based and slow-motion review of smiles was impossible without video records. The researchers noted, for example, that 20% of the smiles observed involved movement of the cheeks. However, their scale did not distinguish smiles in which strong contractions of the zygomatic (lip corner raising) raised the cheeks from Duchenne smiles in which orbicularis oculi (pars lateralis) raised the cheeks. In addition, the highest rating possible on the scale involved intense contraction of the muscles responsible both for lip corner raising and cheek raising; but the rating could only be made if the smile lasted at least two seconds. Brief Duchenne smiles could not, by definition, be observed. The overall goal of this study was to document different forms of smiling in sleeping neonates. Specifically, we investigated the prevalence, frequency, duration and organization of neonatal Duchenne and openmouth smiles. The presence of mature open-mouth and Duchenne smiles of substantial duration in sleeping neonates would suggest continuities with the interactive smiles of older infants. If stronger lip corner raises were associated with (stronger) cheek raises, and if the resulting Duchenne smiles had longer durations than nonDuchenne smiles, this would suggest a synergistic association between the muscular constituents of Duchenne smiles. Relatively infrequent open-mouth smiling and a higher frequency of Duchenne smiling, might lend support to the view that neonatal Duchenne smiling is dependent on neuromuscular synergies specific to Duchenne smiling.