How does our sense of smell play a role in autism?

Our noses serves as a subliminal pathway of communication that experiments show gets mixed up in people on the autism spectrum
CC BY 2.0 Sho Hashimoto

Impaired social interaction serves as one of the main diagnostic criteria for autism. This is often explained by an inability to read the facial signals that serve as an unspoken language between neurotypical people. But there is another secret language: smell.

Scientists reported yesterday in Nature Neuroscience that volunteers on the autistic spectrum react differently to the cues sent by odors. In fact, they react exactly opposite to the "smell of fear" harvested from skydiver sweat as well as to a chemical associated normally with a calming effect.

The team of scientists at the Weizmann Institute started by exposing control participants (not on the austism spectrum) and participants on the high end of the autism spectrum (what doctors used to, and many people still do, refer to as Asperger's) to various smells, and proved that the Asperger's syndrome volunteers could detect the odors as well as the control volunteers.

Then they moved to testing the volunteers' reactions when the odor communication signals were sent during a typical interaction situation. The test subjects were smelling either the sweat of a skydiver, or the sweat of the same person taken when they were merely exercising (sweating without fear).

Here is where it gets interesting

The scientists measured the physical reactions of the test subjects, for example by measuring skin conductivity. They found the expected response when control subjects were exposed to the smell of fear: their own fear response increased. But the men with Asperger's reacted in the opposite manner: fear response increased in the case of the normal sweat odor, and decreased when the odor used was the smell of fear.

They confirmed this reverse reaction in experiments with mannequins. The mannequins gave participants tasks that would indicate the level of trust, which normally goes down when people sense fear in the air. Again, they got the expected response in the control group, but the behavior in the Asperger's syndrome group indicated that trust levels increased in the cases where the fear odor was in use.

In a final set of experiments, the researchers exposed subjects to a chemical that is usually calming at the same time they were bombarded with sudden, loud noises. Measuring the blinking response suggested that control volunteers are calmed by the calming chemical, while the autistic group blinked more in the presence of what should have been a calming signal than they blinked in the absence of the calming chemical.

In summary, the researchers established that volunteers on the autism spectrum react exactly opposite to how neurotypical people perceive both arousing and calming odors and that this difference does not arise from a difference in ability to recognize the presence of the odors.

How is this important?

This research has two potentially significant contributions. First, the physical reaction that can be tested may serve as a useful diagnostic tool in this notoriously difficult-to-pin-down syndrome.

But more importantly, this research can be combined with other studies indicating that there are cells similar to the smell receptors in our noses all over our bodies. This research will lead to studies attempting to learn if confused signals from smell receptor cells could be a factor causing autism. Or conversely, could environmental factors or pollution be affecting smell receptors throughout the developing fetus negatively, and leading to developmental disorders? Better understanding of what causes autism, and what doesn't, will bring us closer to a cure for autism (a questionable quest for those on the high functioning end of the spectrum, but certainly a goal for improving treatment options in any event.)

As is so often the case, this study appears to have focused only on male test subjects. Given the vastly different presentation of autism spectrum disorders in females, a follow-up assessing the opposite sex seems in order as well.

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