People with autism find it difficult to read social cues
Watching how a toddler responds to animations could help diagnose autism, research has suggested.
Babies usually start paying attention to movement soon after birth, and pick up information from the cues they see but children with autism often do not.
A study, published in Nature, where two-year-olds were shown manipulated animations found those with autism focussed on movement linked to sound.
UK experts said a test of this kind could help pick up autism early.
In the Yale study, researchers created five versions of animated children's games such as 'peek-a-boo' and 'pat-a-cake' where points of light marked movement, each with sound.
On the other half of the screen, the same animation was presented upside down and in reverse, but with the same audio as the upright version.
Previous studies have shown that, normally, children's attention is drawn to such changes from around eight months old.
Twenty-one toddlers with autistic-spectrum disorders (ASD), 39 who were developing normally and 16 who had developmental problems but did not have autism were studied.
Both the toddlers who were developing normally and those with developmental problems showed a clear preference for looking at the upright animations. However the toddlers with ASD showed no preference and looked backwards and forwards between the two halves of the screen.
But when the toddlers were shown the 'pat-a-cake' animation - where the figure repeatedly and audibly claps his hands - those with ASD showed a marked preference for the upright animation, where the sounds were in time with the movement, choosing it 66% of the time.
The other children continued to prefer the upright version.
'Grabbing their attention'
Dr Ami Klin, of the Yale Child Study Center, who worked on the research, said: "Our results suggest that, in autism, genetic predispositions are exacerbated by atypical experience from a very early age, altering brain development.
"Attention to biological motion is a fundamental mechanism of social engagement, and in the future, we need to understand how this process is derailed in autism, starting still earlier, in the first weeks and months of life."
Thomas Insel, director of the National Institute of Mental Health - which helped fund the study, said: "For the first time, this study has pinpointed what grabs the attention of toddlers with ASDs.
"In addition to potential uses in screening for early diagnosis, this line of research holds promise for development of new therapies based on redirecting visual attention in children with these disorders."
A spokeswoman for the National Autistic Society said: "This is a really interesting study which suggests that children with autism are on a different learning pathway from other children from a very early age.
"We warmly welcome all research which helps us further our understanding of autism, and how best to help and support those with the condition."