Specialists have created an optical illusion that could help diagnose autism.
The condition is notoriously difficult to diagnose, especially in young children, but pupil movement has been found to reveal differences closely linked to autistic traits.
The illusion is a video showing a pattern of black and white dots moving against a plain grey background.
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People who see the illusion as a three-dimensional rotating cylinder are less likely to have Autism Spectrum Disorder (ASD). Those who interpret it as two separate moving sheets of dots tend to have a more detail-oriented view, which is a key sign of ASD.
If someone's pupils dilate when watching the video, it means they are seeing two sheets rather than a cylinder, and are more likely to have ASD.
Those who see a cylinder will not experience dilated pupils, as they will not be focused on the individual colours of the moving dots.
Researchers from the University of Pisa in Italy developed the illusion and tested it on 50 adults, who had not been diagnosed with ASD. Participants' pupils were tracked, and they were then asked to complete a self-reporting questionnaire.
The researchers found that those whose pupils dilated while watching the illusion scored higher on the questionnaire, indicating a link between pupil dilation and being more detail-oriented - one of the key markers of ASD.
They say it's the first evidence that changes in the diameter of the pupil can reliably track how different people perceive the same visual stimuli.
"Future studies are needed to see if similar pupil fluctuations occur in people diagnosed with ASD," researchers wrote.
"[We] predict that pupil changes will be even more dramatic in people with ASD. If this is the case, these pupil measurements could be used to help diagnose ASD or determine the severity of symptoms."