Heart-rate deceleration and smiling in 3-month-old infants

Estimating the cardiac signals of chimpanzees using a digital camera: validation and application of a novel non-invasive method for primate research

Cardiac measures such as heart rate measurements are important indicators of both physiological and psychological states. However, despite their extraordinary potential, their use is restricted in comparative psychology because traditionally cardiac measures involved the attachment of sensors to the participant's body, which, in the case of undomesticated animals such as nonhuman primates, is usually only possible during anesthesia or after extensive training. Here, we validate and apply a camera-based system that enables contact-free detection of animals' heart rates. The system automatically detects and estimates the cardiac signals from cyclic change in the hue of the facial area of a chimpanzee. In Study 1, we recorded the heart rate of chimpanzees using the new technology, while simultaneously measuring heart rate using classic PPG (photoplethysmography) finger sensors. We found that both methods were in good agreement. In Study 2, we applied our new method to measure chimpanzees' heart rate in response to seeing different types of video scenes (groupmates in an agonistic interaction, conspecific strangers feeding, nature videos, etc.). Heart rates changed during video presentation, depending on the video content: Agonistic interactions and conspecific strangers feeding lead to accelerated heart rate relative to baseline, indicating increased emotional arousal. Nature videos lead to decelerated heart rate relative to baseline, indicating a relaxing effect or heightened attention caused by these stimuli. Our results show that the new contact-free technology can reliably assess the heart rate of unrestrained chimpanzees, and most likely other primates. Furthermore, our technique opens up new avenues of research within comparative psychology and facilitates the health management of captive individuals.

Social, cognitive, and physiological aspects of humour perception from 4 to 8 months: Two longitudinal studies

Infants laugh by 4 months, but whether they understand humour based on social or cognitive factors is unclear. We conducted two longitudinal studies of 4-, 6-, and 8-month-olds (N = 60), and 5-, 6-, and 7-month-olds (N = 53) to pinpoint the onset of independent humour perception and determine when social and cognitive factors are most salient. Infants were shown six events in randomized repeated-measures designs: two ordinary events and two absurd iterations of those events, with parents' affect manipulated (laugh or neutral) during the latter. Four-month-olds did not smile/laugh more at absurd events, but exhibited a significant heart rate deceleration. Five-month-olds independently appraised absurd events as humorous, smiling/laughing despite their parents' neutrality. Parent laughter did not influence infants of any age to smile more, but captured 4-month-olds' attention. Results suggest that 4-month-olds laugh in response to social cues, while 5-month-olds' can laugh in response to cognitive features. Statement of contribution What is already known on this subject? By 6 months, infants can independently appraise absurd events as humorous, but it is not known whether younger infants can. What does this study add? This study replicated the finding on younger infants, showing that 5-month-olds are similarly capable of independent humour appraisal. These studies also found that although 4-month-olds do not respond to absurd events with positive affect, they do exhibit a heart rate decrease that is unrelated to looking. These studies help delineate when social and cognitive factors contribute to infant humour perception.

The Emergence of Social Smiling

The study addresses the emergence of the social smile in two different sociocultural contexts during the first 12 postnatal weeks. We examined different eliciting mechanisms like mutual gazing, maternal smile during mutual gazing, and reciprocal maternal and infant imitation of smiling. In co-constructivist theories of emotional development, all of them are considered social mechanisms that foster the emergence of social smile in early infancy around the 2-month shift. During the 6th postnatal week, we assumed that mutual gazing and the accompanying maternal smiles are the primary mechanisms that correspond with first infants’ social smile. From the 2-month shift onward, thus during the 8th, 10th, and 12th postnatal weeks, we assumed maternal imitation of infant smile moderates the positive relationship between infant imitation of maternal smile and the duration of infant social smile. We compared face-to-face interactions between 20 mother–child dyads from an independent sociocultural context (urban middle-class families from Münster, Germany) and 24 mother–child dyads from an interdependent sociocultural context (rural Nso families, Cameroon) when the infants were 6, 8, 10, and 12 weeks old. The first hypothesis could be corroborated for both cultural contexts, the second hypothesis only partly for the independent cultural contexts and staggered for the interdependent context. The consequences of culture-specific developmental pathways of social smile are discussed.

A cross-cultural comparison of the development of the social smile: a longitudinal study of maternal and infant imitation in 6- and 12-week-old infants

Social smiling is universally regarded as being an infant's first facial expression of pleasure. Underlying co-constructivist emotion theories are the assumptions that the emergence of social smiling is bound to experiences of face-to-face interactions with caregivers and the impact of two developmental mechanisms--maternal and infant imitation. We analyzed mother-infant interactions from two different socio-cultural contexts and hypothesized that cross-cultural differences in face-to-face interactions determine the occurrence of both of these mechanisms and of the frequency of social smiling by 12-week-old infants. Twenty mother-infant dyads from a socio-cultural community with many face-to-face interactions (German families, Münster) were compared with 24 mother-infant dyads from a socio-cultural community with few such interactions (rural Nso families, Cameroon) when the infants were aged 6 and 12 weeks. When infants were 6 weeks old, mothers and their infants from both cultural communities smiled at each other for similar (albeit very short) amounts of time and used imitated each other's smiling similarly rarely. In contrast, when infants were 12 weeks old, mothers and their infants from Münster smiled at and imitated each other more often than did Nso mothers and their infants.

Effects of normal and perturbed social play on the duration and amplitude of different types of infant smiles

Different types of smiling varying in amplitude of lip corner retraction were investigated during 2 mother-infant games--peekaboo and tickle--at 6 and 12 months and during normally occurring and perturbed games. Using Facial Action Coding System (FACS), infant smiles were coded as simple (lip corner retraction only), Duchenne (simple plus cheek raising), play (simple plus jaw drop), and duplay (Duchenne plus jaw drop). In addition, again using FACS, the amplitude of lip corner retraction was coded on a 5-point scale. Rather than a single smile expression that differs only in amplitude, the authors found a complex family of different smile expressions differing in their duration and amplitude as a function of game, setup versus climax of the game, and perturbation. Both type of smiling and amplitude of smiling appear to be controlled independently by the infant in relation to the context. These findings reveal systematic and context-specific nuances in infant smiles in the 2nd half of the first year.