The eyes may be the mirror of the soul, but for those with autism, the mouth will have to do.
Researchers at Cedars-Sinai Medical Center isolated neurons in the brain's amygdala that respond to facial expressions, and tested patients with autism against those without. Both groups could correctly identify a "happy" or "fearful" face, a function long associated with the amygdala.
But when the researchers examined which neurons fired in relation to areas of the face, they found that those with autism "read" the information from the mouth area more than from the eyes and seemed to be lacking a population of nerve cells that respond only to images of eyes.
Eye contact has been shown to be crucial for social interaction. People who are on the autism spectrum tend to exhibit deficits in social behavior, including avoiding eye contact and focusing on the mouth.
The researchers from Cedars, Caltech and Huntington Memorial Hospital examined patients who already had electronic probes inserted into their brains as part of diagnostic procedures for surgery to treat epilepsy -- a disorder that also is centered in the region of the brain that includes the amygdala.
Two of those patients also had been diagnosed with autism spectrum disorder. Their data were compared with that of others who had no autism symptoms.
Both groups viewed a variety of whole or partial face images for about a half-second. In both, a population of neurons in the amygdala that preferentially respond to whole faces seemed to fire in equal proportion and strength.
"There was no difference that we could identify," said Ueli Rutishauser, a Cedars-Sinai neuroscientist who was lead author of the study published online Wednesday in the journal Neuron.
"They definitely have them, and as far as we can tell they respond with equal strength, and there are equal numbers of whole face neurons," Rutishauser added. "This is very important because it tells you that in principal the amygdala is functional and the responses are very similar. It's not that in some way their amygdala doesn't work or doesn't have neurons, or something simple like this."
Both groups also had similar populations of neurons that fired preferentially for facial parts.
"They existed -- it was not that they were absent; they were present in similar proportion and similar response strength -- but very noticeably, the neurons recorded from the people with autism avoided the eyes," Rutishauser said. "They have part-sensitive responses mostly around the mouth and other parts of the face, but very focused on the mouth.
"You can see this very clear distinction, in that the eyes were just not important for the autistics, meaning they did not use the information presented by the eyes. They just used a different strategy."
The findings strongly suggest that there is no trouble with the nerves themselves, but with how populations of neurons specialize. Searching for genetic drivers of that difference could be a promising route toward treatment, Rutishauser said.
Researchers believe the speed of the neuron response -- on the order of milliseconds -- and the uniformity of response times regardless of where stimuli appeared rule out issues such as expectation, or attentional signals from elsewhere in the brain.
The study, however, addresses only one type of behavioral deficit among autistics, and one suspected source.
"Is this abnormality caused by some dysfunction in the amygdala? That's possible. But could it be caused by a dysfunction somewhere in the cortex? Yes, that's possible." Rutishauser said.
Opportunities to test the notion further will be scarce. A dual diagnosis of autism occurs in only 20% of the epileptic population, Rutishauser said.