NEW YORK – A team of autism researchers has found that patterns of social-visual engagement are markedly more similar among identical twin toddlers than among fraternal twins.
Social-visual engagement (SVE), which can be measured using eye-tracking technology, is how humans give preferential attention to social stimuli – in particular, people’s eyes and mouths, which provide important information for communication.
Lower levels of SVE have been shown to be associated with the later development of autism, even in children just a few months old (Nature. 2013 Dec 19;504:427-31). “But what hasn’t been shown until now is that this measure relates to genetics,” said Natasha Marrus, MD, PhD, of the department of psychiatry at Washington University in St. Louis.
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The identical twins, who share 100% of their genes, “showed much more similar levels of social-visual engagement than fraternal twins,” Dr. Marrus said, with an intraclass correlation coefficient (ICC) of 0.91 (95% confidence interval, 0.85-0.95) for time spent looking at eyes, compared with 0.35 (95% CI, 0.07-0.59) for fraternal twins. Similar results were obtained for the caregiver questionnaire, suggesting strong genetic influences on both early reciprocal social behavior and SVE, said Dr. Marrus, also of Washington University.
At 36 months, 69 of the twin pairs were reevaluated. The investigators again found significantly greater SVE concordance for the identical twins: ICC, 0.93 (95% CI, 0.75-0.98), compared with ICC, 0.25 (95% CI, 0.0-0.60) for fraternal twins. They also found SVE patterns strongly correlated between 21 and 36 months for individual twins, indicating traitlike stability of this behavior over time.
“These two measures that are heritable, like autism, can be measured in a general population sample, which means they show good variability – potentially allowing the detection of subtle differences that may correspond to levels of risk for autism,” Dr. Marrus said. “By 18-21 months, the risk markers for later autism are already there – if you use a nuanced enough measure to detect them.”
Dr. Marrus said that while some practitioners have been able to reliably diagnose autism in children younger than 24 months, “it’s usually with the most severe cases,” she said. “But 18 months is a big time for social as well as language development, which becomes easier to measure at that point.”
A future direction for study, she said, “would be to go earlier. If we’re seeing this at 18 months, maybe we’d see it at 12.”
With autism, “early intervention is key, and even 6 months could make a difference,” Dr. Marrus said. “These two measures stand a really good chance of telling us important things about autism – which at early ages means better diagnostic prediction, measurement of severity and risk, and the potential to monitor responses to interventions.”
The National Institutes of Health supported the study through a grant to Dr. Constantino, and Dr. Marrus’s work was supported with a postdoctoral fellowship from the Autism Science Foundation. The investigators declared no relevant financial conflicts.