How Kids See the World Depends a Lot on Genetics

Posted on July 25, 2017 by Dr. Francis Collins

Caption: Child watches video while researchers track his eye movements.
Credit: Washington University School of Medicine, St. Louis

From the time we are born, most of us humans closely watch the world around us, paying special attention to people’s faces and expressions. Now, for the first time, an NIH-funded team has shown that the ways in which children look at faces and many other things are strongly influenced by the genes they’ve inherited from their parents.

The findings come from experiments that tracked the eye movements of toddlers watching videos of other kids or adult caregivers. The experiments showed that identical twins—who share the same genes and the same home environment—spend almost precisely the same proportion of time looking at faces, even when watching different videos. And when identical twins watched the same video, they tended to look at the same thing at almost exactly the same time! In contrast, fraternal twins—who shared the same home environment, but, on average, shared just half of their genes—had patterns of eye movement that were far less similar.

Interestingly, the researchers also found that the visual behaviors most affected in children with autism spectrum disorder (ASD)—attention to another person’s eyes and mouth—were those that also appeared to be the most heavily influenced by genetics. The discovery makes an important connection between two well-known features of ASD: a strong hereditary component and poor eye contact with other people.

The new study was led by Warren Jones, Emory University School of Medicine, Atlanta, and John Constantino, Washington University School of Medicine, St. Louis. It follows up on an earlier study, out of the Emory laboratories of Jones and Ami Klin, tracking the early development of infants at high risk of autism. That study showed that kids who would later receive an ASD diagnosis paid less attention to the eyes of other people as early as 6 months of age [1].

Because poor eye contact emerges so early in development, Constantino, Klin, and Jones wondered whether it might be possible to connect such behavioral traits to the genetics of ASD. They decided to investigate by conducting a classic twin study. By comparing the degree of similarity for particular traits or behaviors in identical versus fraternal twins, twin studies make it possible to tease out the proportion of variation that’s explained by genetic versus environmental factors.

The researchers enrolled 338 toddlers between the ages of 18 and 24 months. The study included 41 pairs of identical twins and 42 pairs of fraternal twins. It also included a large group of unrelated children, including some diagnosed with autism.To capture the way kids engaged visually, each child was buckled into a car seat by a parent to watch a series of 27 short videos lasting a total of less than 20 minutes. As they watched, an eye-tracking device mounted beneath the screen and hidden from view captured their every eye movement.

The team then analyzed the eye-tracking data to see how similarly identical twins watched the videos compared to non-identical twins. The results were remarkable. Overall, the patterns of eye- and mouth-looking for identical twins matched by over 80 percent. That’s compared to just a 10 percent match for the fraternal twin pairs. Unrelated children in the study showed essentially no similarities in eye movement patterns.

The researchers also found that identical twins were more likely to move their eyes in the same direction at the same time to look at precisely the same images on-screen. In fact, they matched each other’s eye movements within as little as 17 milliseconds!

To further confirm the findings, the researchers tested the twins again 15 months later. For each of the children in the study, the proportion of time spent looking at facial features in the videos remained largely unchanged. And once again, identical twins watched the videos in almost precisely the same way, while fraternal twins showed many more differences.

The researchers also compared their findings in the twins to the eye-tracking behaviors of the kids with ASD. Those comparisons reveal that the highly heritable eye-and mouth-looking behaviors are markedly reduced in kids with ASD. In fact, the researchers could accurately differentiate from the eye-tracking data alone which children in the study had ASD.

All in all, the findings suggest that the way each and every one of us views the world from childhood onward is strongly dependent on the precise combination of genes we happen to carry. The findings also indicate that for the estimated 1 in 68 children born with ASD, the condition will change the way they see the world and interact with the people around them.

In fact, NIH is actively considering the initiation of a study to look at eye movements in even younger children to see whether this would allow detection of ASD at the earliest possible point, where intervention might be most effective. The more we learn about the genes that influence social behaviors and the underlying biology that leads to those differences, the better we’ll be at identifying new and better ways to help such kids.

References:

[1] Attention to eyes is present but in decline in 2-6-month-old infants later diagnosed with autism. Jones W, Klin A. Nature. 2013 Dec 19;504(7480):427-31.

[2] Infant viewing of social scenes is under genetic control and is atypical in autism. Constantino JN, Kennon-McGill S, Weichselbaum C, Marrus N, Haider A, Glowinski AL, Gillespie S, Klaiman C, Klin A, Jones W. Nature. 2017 Jul 12.

Links:

Autism Spectrum Disorder (Eunice Kennedy Shriver National Institute of Child Health and Human Development/NIH)

Warren Jones (Marcus Autism Center, Atlanta)

John Constantino (Washington University School of Medicine, St. Louis)

NIH Support: Eunice Kennedy Shriver National Institute of Child Health and Human Development; National Institute of Mental Health