People with autism don't sniff odors the same way other people do, say researchers who suggest that finding could yield a nonverbal test useful to spot early indicators of autism spectrum disorder.
When normal people detect a pleasant odor, they tend to sniff strongly, but do just the opposite — sniff as shallowly as possible — if the odor is an unpleasant one, the researchers explain.
However, people with autism spectrum disorder make no such natural adjustment, they report in the journal Current Biology.
Previous research has suggested people with autism have impairments in what are known as "internal action models," templates in the brain that help seamlessly coordinate our senses and our actions.
Researchers at the Weismann Institute of Science wanted to find out if that impairment would show up in the sniff response in someone with autism.
The result was unmistakable, they say.
"The difference in sniffing pattern between the typically developing children and children with autism was simply overwhelming," says institute scientist Noam Sobel.
The researchers presented 36 children — 18 with ASD and 18 without — with both pleasant and unpleasant odors to gauge their sniff response.
Pleasant aromas included such things as roses or shampoo scents, while unpleasant smells like rotten fish or sour milk were presented.
Normally developing children quickly modified their sniffing behavior based on the character of the odor — often within 305 milliseconds — while children on the autism spectrum displayed no such response, the researchers found.
Using just the response to smells, the researchers were able to correctly identify 17 of the 19 normally developing children and 12 of the 18 diagnosed as having autism spectrum disorder.
The results were somewhat surprising and not necessarily what was expected, other expert said.
"Since we know that many children with autism are hypersensitive to touch, sound, taste and visual stimuli, it is especially interesting that they seem not to be responsive to odor in the same fashion," says Dr. Glen Elliott, chief psychiatrist and medical director of Children's Health Council in Palo Alto, California, who was not involved in the study.
The study researchers suggested their findings could shine some light on why social difficulties feature so strongly in children with autism.
"The sense of smell is in fact a major component of human social interaction," says lead author Liron Rozenkrantz, a doctoral student at the Israeli institute. "Given that olfaction is probably altered in autism, could it be that this is a part of the social challenge in autism?"