Problems processing visual information may stop those with autism interpreting body language, harming their ability to gauge others' emotions, a study says.
Researchers say people with autism have problems recognising physical displays of emotion, but also general difficulty perceiving certain sorts of motion.
They suggest in Neuropsychologia this may contribute to problems with social interaction, characteristic of autism.
The National Autistic Society said the UK study was an interesting one.
A team from the University of Durham studied 13 adults with autism and found the patients had difficulty identifying emotions such as anger or happiness when shown short animated video clips.
The characters had no faces, nor did they speak, so the participants were asked to judge the emotion based on the body language of the figure alone.
Along with 16 adults with no autism diagnosis, they were also shown a number of dots on a computer screen and asked which way they were moving. A proportion of dots moved noticeably to the left or right, while the others moved randomly.
The way people move their bodies tells us a lot about their feelings or intentions, and we use this information on a daily basis to communicate with each other
Anthony Atkinson Lead author
The performance of the autism group was significantly below that of the others in both tests, leading researchers to speculate that there may be serious differences between the ability to process visual information.
They point to an area of the brain needed for the perception of motion called the superior temporal sulcus, and cite previous research which has found that this area responds differently in people with autism.
"The way people move their bodies tells us a lot about their feelings or intentions, and we use this information on a daily basis to communicate with each other.
"We use others' body movements and postures, as well as people's faces and voices, to gauge their feelings," said Anthony Atkinson, who led the research.
"People with autism are less able to use these cues to make accurate judgements about how others are feeling.
"We now need to look further to see how exactly this happens and how this may combine with potential difficulties in attention."
It is thought as many as half a million people in the UK have a form of autism, a lifelong developmental disability which can severely affect how a person makes sense of the world around them.
Gina Gomez De La Cuesta, of the National Autistic Society, said the study was an interesting one.
"It certainly takes us on. We know of these problems with emotion recognition but to start to unpick the reasons why is helpful. There appear to be difficulties at the very basic processing level.
"But we really need to see this repeated in more people and then we can start thinking about how we act on it."
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