Sustained looking at faces at 5 months of age is associated with socio-communicative skills in the second year of life.

Efficiently processing information from faces in infancy is foundational for nonverbal communication. We studied individual differences in 5-month-old infants' (N = 517) sustained attention to faces and preference for emotional faces. We assessed the contribution of genetic and environmental influences to individual differences in these gaze behaviors, and the association between these traits and other concurrent and later phenotypes. We found an association between the mean duration of looking at a face (before looking away from it) at 5 months and socio-communicative abilities at 14 months (ß = 0.17, 95% CI: 0.08; 0.26, p < 0.001). Sustained attention to faces predicted socio-communicative abilities over and above variance captured by mean fixation duration. We also found a statistically significant but weak tendency to prefer looking at smiling faces (relative to neutral faces), but no indication that variability in this behavior was explained by genetic effects. Moderate heritability was found for sustained attention to faces (A = 0.23, CI: 0.06; 0.38), while shared environmental influences were non-significant for both phenotypes. These findings suggest that sustained looking at individual faces before looking away is a developmentally significant 'social attention' phenotype in infancy, characterized by moderate heritability and a specific relation to later socio-communicative abilities.

Genetic and environmental contributions to gaze lateralization across social and non-social stimuli in human infants.

A tendency to look at the left side of faces from the observer's point of view has been found in older children and adults, but it is not known when this face-specific left gaze bias develops and what factors may influence individual differences in gaze lateralization. Therefore, the aims of this study were to estimate gaze lateralization during face observation and to more broadly estimate lateralization tendencies across a wider set of social and non-social stimuli, in early infancy. In addition, we aimed to estimate the influence of genetic and environmental factors on lateralization of gaze. We studied gaze lateralization in 592 5-month-old twins (282 females, 330 monozygotic twins) by recording their gaze while viewing faces and two other types of stimuli that consisted of either collections of dots (non-social stimuli) or faces interspersed with objects (mixed stimuli). A right gaze bias was found when viewing faces, and this measure was moderately heritable (A = 0.38, 95% CI 0.24; 0.50). A left gaze bias was observed in the non-social condition, while a right gaze bias was found in the mixed condition, suggesting that there is no general left gaze bias at this age. Genetic influence on individual differences in gaze lateralization was only found for the tendency to look at the right versus left side of faces, suggesting genetic specificity of lateralized gaze when viewing faces.

A field test of computer-vision-based gaze estimation in psychology.

Computer-vision-based gaze estimation refers to techniques that estimate gaze direction directly from video recordings of the eyes or face without the need for an eye tracker. Although many such methods exist, their validation is often found in the technical literature (e.g., computer science conference papers). We aimed to (1) identify which computer-vision-based gaze estimation methods are usable by the average researcher in fields such as psychology or education, and (2) evaluate these methods. We searched for methods that do not require calibration and have clear documentation. Two toolkits, OpenFace and OpenGaze, were found to fulfill these criteria. First, we present an experiment where adult participants fixated on nine stimulus points on a computer screen. We filmed their face with a camera and processed the recorded videos with OpenFace and OpenGaze. We conclude that OpenGaze is accurate and precise enough to be used in screen-based experiments with stimuli separated by at least 11 degrees of gaze angle. OpenFace was not sufficiently accurate for such situations but can potentially be used in sparser environments. We then examined whether OpenFace could be used with horizontally separated stimuli in a sparse environment with infant participants. We compared dwell measures based on OpenFace estimates to the same measures based on manual coding. We conclude that OpenFace gaze estimates may potentially be used with measures such as relative total dwell time to sparse, horizontally separated areas of interest, but should not be used to draw conclusions about measures such as dwell duration.

Infants' looking preferences for social versus non-social objects reflect genetic variation.

To what extent do individual differences in infants' early preference for faces versus non-facial objects reflect genetic and environmental factors? Here in a sample of 536 5-month-old same-sex twins, we assessed attention to faces using eye tracking in two ways: initial orienting to faces at the start of the trial (thought to reflect subcortical processing) and sustained face preference throughout the trial (thought to reflect emerging attention control). Twin model fitting suggested an influence of genetic and unique environmental effects, but there was no evidence for an effect of shared environment. The heritability of face orienting and preference were 0.19 (95% confidence interval (CI) 0.04 to 0.33) and 0.46 (95% CI 0.33 to 0.57), respectively. Face preference was associated positively with later parent-reported verbal competence (ß = 0.14, 95% CI 0.03 to 0.25, P = 0.014, R2 = 0.018, N = 420). This study suggests that individual differences in young infants' selection of perceptual input-social versus non-social-are heritable, providing a developmental perspective on gene-environment interplay occurring at the level of eye movements.

How 18-month-olds with Later Autism Look at Other Children Interacting: The Timing of Gaze Allocation.

When observing other people during naturally paced and dynamic interactions, it is essential to look at specific locations at the right time to extract a maximum of socially informative content. In this study, we aimed to investigate the looking behavior of typically developing toddlers and toddlers later diagnosed with autism when observing other children interact. The sample consisted of 98 toddlers; 22 in a low-likelihood of autism group, 60 in an elevated likelihood of autism group who did not receive a subsequent diagnosis, and 16 in an elevated likelihood group who did receive an autism diagnosis. Participants performed an eye tracking task at 18 months of age and were assessed for diagnostic outcome at 36 months. The video stimuli consisted of two children interacting, where a boy reaches out for a toy and a girl refuses to give it to him. The low likelihood group showed an expected increase in ratio of looking at the girl's face after the boy requested the toy, as compared to before (t(21) = -3.337, p = .003). Toddlers with later autism showed a significantly lower ratio of looking at the girl's face during this time window, as compared to the other groups (F(2,91) = 3.698, p = .029). These findings provide new leads on how social gaze may be different in children with autism in everyday life (e.g., kindergarten), and highlight the need of studying the dynamics of gaze on short time scales.

Stable eye versus mouth preference in a live speech-processing task.

Looking at the mouth region is thought to be a useful strategy for speech-perception tasks. The tendency to look at the eyes versus the mouth of another person during speech processing has thus far mainly been studied using screen-based paradigms. In this study, we estimated the eye-mouth-index (EMI) of 38 adult participants in a live setting. Participants were seated across the table from an experimenter, who read sentences out loud for the participant to remember in both a familiar (English) and unfamiliar (Finnish) language. No statistically significant difference in the EMI between the familiar and the unfamiliar languages was observed. Total relative looking time at the mouth also did not predict the number of correctly identified sentences. Instead, we found that the EMI was higher during an instruction phase than during the speech-processing task. Moreover, we observed high intra-individual correlations in the EMI across the languages and different phases of the experiment. We conclude that there are stable individual differences in looking at the eyes versus the mouth of another person. Furthermore, this behavior appears to be flexible and dependent on the requirements of the situation (speech processing or not).

Preferential looking to eyes versus mouth in early infancy: heritability and link to concurrent and later development.

BACKGROUND: From birth, infants orient preferentially to faces, and when looking at the face, they attend primarily to eyes and mouth. These areas convey different types of information, and earlier research suggests that genetic factors influence the preference for one or the other in young children. METHODS: In a sample of 535 5-month-old infant twins, we assessed eye (relative to mouth) preference in early infancy, i.e., before neural systems for social communication and language are fully developed. We investigated the contribution of genetic and environmental factors to the preference for looking at eyes, and the association with concurrent traits and follow-up measures. RESULTS: Eye preference was independent from all other concurrent traits measured, and had a moderate-to-high contribution from genetic influences (A = 0.57; 95% CI: 0.45, 0.66). Preference for eyes at 5 months was associated with higher parent ratings of receptive vocabulary at 14 months. No statistically significant association with later autistic traits was found. Preference for eyes was strikingly stable across different stimulus types (e.g., dynamic vs. still), suggesting that infants' preference at this age does not reflect sensitivity to low-level visual cues. CONCLUSIONS: These results suggest that individual differences in infants' preferential looking to eyes versus mouth to a substantial degree reflect genetic variation. The findings provide new leads on both the perceptual basis and the developmental consequences of these attentional biases.

Infants' sense of approximate numerosity: Heritability and link to other concurrent traits.

The ability to perceive approximate numerosity is present in many animal species, and emerges early in human infants. Later in life, it is moderately heritable and associated with mathematical abilities, but the etiology of the Approximate Number System (ANS) and its degree of independence from other cognitive abilities in infancy is unknown. Here, we assessed the phenotypic specificity as well as the influence of genetic and environmental factors on the ANS in a sample of 5-month-old twins (N = 514). We found a small-to-moderate but statistically significant effect of genetic factors on ANS acuity (heritability = 0.18, 95% CI: 0.02, 0.33), but only when differences in numerosity were relatively large (1:4 ratio). Non-verbal ability assessed with the Mullen Scales of Early Learning (MSEL) was found to be heritable (0.47; 95% CI: 0.34, 0.57) and the phenotypic association between ANS acuity and non-verbal ability performance was close to zero. Similarly, we found no association between ANS acuity and general attention during the task. An unexpected weak but statistically significant negative association between ANS acuity and scores on the receptive language scale of the MSEL was found. These results suggest that early ANS function may be largely independent from other aspects of non-verbal development. Further, variability in ANS in infancy seems to, to some extent, reflect genotypic differences in the population. HIGHLIGHTS: Assessing 514 infant twins with eye tracking, we found that infants' sense of approximate numerosity is heritable and not positively associated with concurrent attentional, cognitive or motor abilities. These results have implications for our understanding of development of mathematical ability and the link between cognitive abilities early in postnatal life.

Pupil size and pupillary light reflex in early infancy: heritability and link to genetic liability to schizophrenia.

BACKGROUND: Measures based on pupillometry, such as the pupillary light reflex (PLR) and baseline pupil size, reflect physiological responses linked to specific neural circuits that have been implicated as atypical in some psychiatric and neurodevelopmental conditions. METHODS: We investigated the contribution of genetic and environmental factors to the baseline pupil size and the PLR in 510 infant twins assessed at 5 months of age (281 monozygotic and 229 dizygotic pairs), and its associations with common genetic variants associated with neurodevelopmental (autism spectrum disorder and attention deficit hyperactivity disorder) and mental health (bipolar disorder, major depressive disorder and schizophrenia) conditions using genome-wide polygenic scores (GPSs). RESULTS: Univariate twin modelling showed high heritability at 5 months for both pupil size (h2 = .64) and constriction in response to light (h2 = .62), and bivariate twin modeling indicated substantial independence between the genetic factors influencing each (rG = .38). A statistically significant positive association between infant tonic pupil size and the GPS for schizophrenia was found (ß = .15, p = .024), while there was no significant association with the GPS for autism or any other GPSs. CONCLUSIONS: This study shows that some pupil measures are highly heritable in early infancy, although substantially independent in their genetic etiologies, and associated with common genetic variants linked to schizophrenia. It illustrates how genetically informed studies of infants may help us understand early physiological responses associated with psychiatric disorders which emerge much later in life.

The Babytwins Study Sweden (BATSS): A Multi-Method Infant Twin Study of Genetic and Environmental Factors Influencing Infant Brain and Behavioral Development.

Twin studies can help us understand the relative contributions of genes and environment to phenotypic trait variation, including attentional and brain activation measures. In terms of applying methodologies such as electroencephalography (EEG) and eye tracking, which are key methods in developmental neuroscience, infant twin studies are almost nonexistent. Here, we describe the Babytwins Study Sweden (BATSS), a multi-method longitudinal twin study of 177 MZ and 134 DZ twin pairs (i.e., 622 individual infants) covering the 5-36 month time period. The study includes EEG, eye tracking and genetics, together with more traditional measures based on in-person testing, direct observation and questionnaires. The results show that interest in participation in research among twin parents is high, despite the comprehensive protocol. DNA analysis from saliva samples was possible in virtually all participants, allowing for both zygosity confirmation and polygenic score analyses. Combining a longitudinal twin design with advanced technologies in developmental cognitive neuroscience and genomics, BATSS represents a new approach in infancy research, which we hope to have impact across multiple disciplines in the coming years.

Eye tracking in human interaction: Possibilities and limitations.

There is a long history of interest in looking behavior during human interaction. With the advance of (wearable) video-based eye trackers, it has become possible to measure gaze during many different interactions. We outline the different types of eye-tracking setups that currently exist to investigate gaze during interaction. The setups differ mainly with regard to the nature of the eye-tracking signal (head- or world-centered) and the freedom of movement allowed for the participants. These features place constraints on the research questions that can be answered about human interaction. We end with a decision tree to help researchers judge the appropriateness of specific setups.