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

The developing visual system: A building block on the path to autism

Longitudinal neuroimaging studies conducted over the past decade provide evidence of atypical visual system development in the first years of life in autism spectrum disorder (ASD). Findings from genomic analyses, family studies, and postmortem investigations suggest that changes in the visual system in ASD are linked to genetic factors, making the visual system an important neural phenotype along the path from genes to behavior that deserves further study. This article reviews what is known about the developing visual system in ASD in the first years of life; it also explores the potential canalizing role that atypical visual system maturation may have in the emergence of ASD by placing findings in the context of developmental cascades involving brain development, attention, and social and cognitive development. Critical gaps in our understanding of human visual system development are discussed, and future research directions are proposed to improve our understanding of ASD as a complex neurodevelopmental disorder with origins in early brain development.

Genetic and environmental contributions to individual differences in visual attention and oculomotor control in early infancy

Infants differ in their level of eye movement control, which at the extreme could be linked to autism. We assessed eye movements in 450 twins (225 pairs, 57% monozygotic, 46% female, aged 5-6 months) using the gap-overlap eye-tracking task. Shorter latency in the gap condition was associated with having more parent-rated autistic traits at 2 years. Latency across the task's three conditions was primarily explained by one highly heritable latent factor likely representing individual differences in basic oculomotor efficiency and/or in visual information processing. Additionally, disengagement of attention was linked to unique genetic factors, suggesting that genetic factors involved in visual attention are different from those involved in basic visual information processing and oculomotor efficiency.

Early selective attention to the articulating mouth as a potential female-specific marker of better language development in autism: a review

Autism is a neurodevelopmental condition with early onset, usually entailing language differences compared to neurotypical peers. Females are four times less likely than males to be diagnosed with autism, and the language features associated with this condition are less frequent in females than in males. However, the developmental mechanisms underlying these sex differences remain unclear. In neurotypical populations, sex differences in language development are also observable from early on, with females outperforming males. One mechanism underlying these sex differences may be early differences in selective attention to talking faces. During the first year, more mouth-looking generally predicts better language development, but sex differences exist. Female infants look at the mouth of a talking face more than males without penalizing looking to the eyes, and reduced mouth-looking in early infancy relates to better vocabulary in toddlerhood only in females. In this hypothesis and theory article, we propose that unique female gaze patterns to the mouth may constitute an early female-specific candidate marker that acts as a protective marker for language development also in autism. Since autism is highly heritable, investigating infants at elevated likelihood for autism offers the opportunity to search for sex-specific markers operating early in life before autistic features and language differences emerge. We argue that, as in neurotypical female infants, mouth-looking may also protect female infants-at-elevated-likelihood-for-autism population from potential later differences in language skills. If so, then sex-specific early behavioral markers, potentially acting as protective markers of language, may compensate for some genetic risk markers affecting this population. Here we gather evidence from neurotypical infants and those with elevated likelihood of autism to uncover why biological sex, the development of selective attention to the mouth, and language acquisition could be intimately related in both populations. We also propose hypotheses regarding potential sex-differentiated neurodevelopmental pathways. We end discussing future research challenges: how generalizable mouth-looking could be as a potential female-specific early language marker across contexts (experimental vs. real life), countries, and developmental time. Ultimately, we aim to target a novel protective candidate of language acquisition, informing tailored interventions that consider sex as an important source of individual variability.

Dissociable genetic influences on eye movements during abstract versus naturalistic social scene viewing in infancy

Eye-movement metrics like fixation location and duration are increasingly being used in infancy research. We tested whether fixation durations during meaningful social stimulus viewing involve common or different familial influences than fixation durations during viewing of abstract stimulus. We analysed the duration of fixations, and the allocation of fixations to face and motion, from 536 dizygotic and monozygotic 5-month-old twins in: naturalistic scenes including low- and high-level social features, and abstract scenes only having low-level features. We observed significant genetic influences in both conditions (h2naturalistic = 0.30, 95% confidence interval (CI) 0.14 to 0.44; h2abstract = 0.25, 95% CI 0.09 to 0.39), while shared environmental influences were negligible. Although some genetic influences were shared between the two conditions, unique genetic factors were linked to naturalistic scene viewing, indicating that fixation durations index different phenomena dependent on the context. Heritability for face looking was moderate (h2 = 0.19, 95% CI 0.03 to 0.34), and no familial influences were found for motion looking. Exploratory polygenic score analyses revealed no significant associations with fixation measures. This study underscores the dissociable genetic influences on infants' visual exploration of abstract versus naturalistic stimuli and the importance of considering context when interpreting eye-tracking data.

Sensitivity to temporal synchrony and selective attention in audiovisual speech in infants at elevated likelihood for autism: A preliminary longitudinal study

Autism Spectrum Disorder is a highly heritable condition characterized by sociocommunicative difficulties, frequently entailing language atypicalities that extend to infants with a familial history of autism. The developmental mechanisms underlying these difficulties remain unknown. Detecting temporal synchrony between the lip movements and the auditory speech of a talking face and selectively attending to the mouth support typical early language acquisition. This preliminary eye-tracking study investigated whether these two fundamental mechanisms atypically function in infant siblings. We longitudinally tracked the trajectories of infants at elevated and low-likelihood for autism in these two abilities at 4, 8, and 12 months (n = 29). We presented two talking faces (synchronous and asynchronous) while recording infants' gaze to the talker's eyes and mouth. We found that infants detected temporal asynchronies in talking faces at 12 months regardless of group. However, compared to their typically developing peers, infants with an elevated likelihood of autism showed reduced attention to the mouth at the end of the first year and no variations in their interest to this area across time. Our findings provide preliminary evidence on a potentially atypical trajectory of reduced mouth-looking in audiovisual speech during the first year in infant siblings, with potential cascading consequences for language development, thus contributing to domain-general accounts of emerging autism.

The breakdown of social looking

Individual differences in social looking are commonly believed to reflect one single heritable dimension tightly linked to autism. Yet, recent data suggest that in human infants, looking to eyes (rather than mouth) and preference for faces (versus non-social objects) reflect distinct genetic influences, and neither appear to have a clear-cut relation to autism.

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 (β = 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.

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 (β = 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.

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).