Harvard University researchers say they've identified a particular chemical in the human brain that shows evidence of being linked with autism.
An interruption in the signaling pathways that a significant inhibitory neurotransmitter known as GABA uses is associated with autistic behavior, they report.
"This is the first time, in humans, that a neurotransmitter in the brain has been linked to autistic behavior," says study leader Caroline Robertson.
Animal models have previously suggested a role the GABA signaling pathways take in autism, she noted, "but until now we never had evidence for it actually causing autistic differences in humans."
For the study, brain imaging was performed on people diagnosed with autism and on a control group without the disorder while they were given visual tests known to trigger dissimilar reactions in the autistic brain and normal brains.
They focused on a phenomenon known as binocular rivalry, in which two conflicting images are presented at the same time, one to each eye.
In order to focus in on one image or the other, the brain must push one of the images out of its awareness, accomplishing this by using inhibiting neural signals.
People suffering from autism have trouble suppressing visual images in this way, the researchers explain.
"Autism is often described as a disorder in which all the sensory input comes flooding in at once, so the idea that an inhibitory neurotransmitter was important fit with the clinical observations," Robertson says.
A strong link between binocular rivalry dynamics and levels of GABA was noted in study participants without autism, while those with autism displayed dysfunctional GABA activity, with any connection between perception and GABA brain chemistry completely absent, the researchers reported in the journal Current Biology.
People with autism often exhibit detail-oriented visual perception, Robertson points out, showing remarkable attention to small details in their sensory environment but having great difficulty in filtering out or suppressing irrelevant sensory information.
"It's long been thought this might have something to do with inhibition in the brain, and our findings lend support to this notion," she says.
However, the findings about GABA failing to dampen neural activity are likely just one piece in a complex puzzle, she acknowledges.
"I'm excited about this study, but there are many other molecules in the brain, and many of them may be associated with autism in some form," she says.