People with autism were almost three times as likely to have some type of synesthesia, the researchers report online this week in Molecular Autism. Of the 164 adults with autism, 31, or 18.9 percent, met the criteria for synesthesia, compared with only seven (7.2 percent) of the 97 “typical” respondents. The most common types of synesthesia reported were “grapheme-color” synesthesia, in which black letters appear in color, and “sound-color,” in which sounds evoke colors.
“We were surprised at the size of the difference - almost a fifth of people with autism had synesthesia as well. It is remarkable that this link hasn’t been documented before,” Baron-Cohen says. One explanation may be that the study was limited to people with autism capable of answering online questionnaires, he surmises; many people with autism may not be able to understand or explain their experiences.
According to Baron-Cohen, the mixed senses in each condition may result from extra neural connections that are usually pruned away in infancy, as the brain’s wiring develops. In autism and synesthesia, he explains, this pruning may not occur in the typical way, so the interconnections persist even into adulthood.
Baron-Cohen says that brain imaging protocols used to study synesthesia could now also be used for autism. He also suggests that genetic investigations into the overlap between the two conditions might help the hunt for “autism genes.”
“This is a very clever study that provides important new information,” says neuroscientist David Amaral, the research director of the Medical Investigation of Neurodevelopmental Disorders Institute at the University of California, Davis.
He feels it’s too early to draw conclusions about the neurobiological basis of autism from responses given by a small group of high-functioning people with autism. But, he says, “it will be interesting to see whether the individuals with autism and synesthesia differ in some fundamental way from those with either condition alone — whether they have different brain organization, for example.”
Johnson adds that the work may point to ways of helping caregivers to figure out which colors and stimuli may be distracting or soothing, resulting in more autism-friendly environments.
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