July 24, 2017Human Development
Autism spectrum disorder is a neurodevelopmental disorder characterized by impaired social interaction, verbal and non-verbal communication, and restricted and repetitive behavior. Parents usually notice signs in the first two years of their child's life. These signs often develop gradually, though some children with autism reach their developmental milestones at a normal pace and then regress. How children visually engage with others in social situations is a heritable behavior that is altered in children with autism. Reduced attention to other people's eyes and faces is a behavior associated with autism, and it is often used to screen for and help diagnose the disorder. The diagnostic criteria require that symptoms become apparent in early childhood, typically before age three.
According to an article published in Nature (20 July 2017), eye-tracking experiments were conducted in a group of 250 typically developing toddlers ages 18 to 24 months, including 82 identical twins (41 pairs), 84 non-identical twins (42 pairs) and 84 non-sibling children (42 randomized pairs). The study also evaluated 88 non-twin children diagnosed with autism.
For the study, each child watched videos that showed either an actress speaking directly to the viewer or scenes of children interacting in daycare. In all video frames, children could look at the onscreen characters' eyes, mouth, body or surrounding objects. Special software captured how often the children looked at different regions, as well as the timing and direction of eye movements. Results showed that identical twins had synchronized visual patterns, compared to non-identical twins and non-sibling pairs. There was high monozygotic twin-twin concordance and relatively low dizygotic concordance. Identical twins also tended to shift their eyes at the same times and in the same direction. They also were more likely to look at the subject's eyes or mouth at the same moments. Using a statistical measurement called the intraclass correlation coefficient (ICC), which measures how well individuals within a group resemble each other (with a value of 1 marking perfect agreement), the authors found that identical twins had an ICC of 0.91 for eye-looking and 0.86 for mouth-looking. On the other hand, non-identical twins had scores of 0.35 and 0.44, respectively, while non-sibling pairs had scores of 0.16 and 0.13. According to the authors, by comparing identical twins who share the same genes to non-identical twins, and randomly paired children who do not share the same genes, the study is one of the first to show that social visual behaviors are under genetic control.
To explore this concept further, the authors evaluated children with autism and discovered that they looked at eye and mouth regions - the most heritable visual traits - much less compared to the other groups of children. These characteristics, that are the most highly heritable, were also found to be differentially decreased in children with autism (P?<0.0001).
According to the authors, the study results implicate social visual engagement as a neurodevelopmental endophenotype not only for autism, but also for population-wide variation in social-information seeking. The authors added that with these findings, it should be possible to 1) explore which genes are involved in social visual engagement, 2) how these genes interact with a child's environment to shape his/her social engagement, and 3) how these genetic pathways are disrupted in neurodevelopmental disorders such as autism.