People with autism experience a more extreme version of the world than the rest of us. For more than 90 percent, sounds are louder, colors are brighter, and touch can be a disturbing intrusion. The reason, according to a new study, may be that many autistic people also have synesthesia, a condition of intertwined perception in which one sense stimulates another.
Most people with synesthesia don't find the condition disturbing; many enjoy it. It certainly makes the world a more interesting place. Synesthetes may see the sound of a symphony as a skein of rippling lines, for example, or a black letter "A" as bright red. People with synesthesia say their experience is not the same as imagination, but they also realize their perceptions are in their own mind and not part of objects in the outside world. "Their experience is somewhere in between, neither imaginary not external, an extra layer in the mind," says cognitive neuroscientist Simon Baron-Cohen of the University of Cambridge in the United Kingdom, who led the study.
Synesthesia has been linked to autism in a few isolated instances. But Baron-Cohen, who studies both conditions, felt that the underlying similarities warranted closer scrutiny. Scientists believe synesthesia is caused by an overabundance of connections between neurons. Curiously, a similar surfeit has been proposed as the cause of autism. While these interconnections may provide synesthetes with a rich intermingling of sensory experience, people with autism might find the blending of the senses distracting - sometimes to the point where they become withdrawn or soothe themselves with monotonous movements, such as rocking.
So Baron-Cohen and Donielle Johnson, who was studying autism for her graduate work at Cambridge, along with a few colleagues, sent online questionnaires to a group of adults, with and without autism or Asperger syndrome. Because autistic people often have trouble relating to others, respondents' ratings of their social skills - like the ability to empathize with a fictional character or judge from another person's tone when it was their turn to speak in a phone conversation - were used as a gauge of the disorder. Tests for synesthesia included questions like, "Do you see colors when you hear musical notes, or read individual (black) letters?"
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.