Researchers have long known that people diagnosed with autism spectrum disorders (ASDs) are not neurologically typical in terms of the structure and function of their brain. Now scientists have started to connect two unusual sense- and perception-related neurological differences to autism: prosopagnosia and synesthesia.
Up to two-thirds of people with ASD diagnoses appear to also have some form of prosopagnosia, or face-blindness-the inability to recognize even familiar faces. And a recent study found that the rate of synesthesia-an apparent cross-talk among the senses in which a person might taste colors or see sounds- in adults with autism was three times greater than in those who are considered "neurotypical." Learning more about why these two conditions occur at higher rates in people with autism may yield clues to the nature of the brain differences seen in autism as well as how brains typically process such information.
About four percent of the population experience synesthesia, the most common form being when words or sounds trigger an experience of color. Neuroimaging studies of the brains of people who experience synesthesia show a greater degree of connectivity in the brain compared with controls. And it's possible that hyper-connectivity may be part of the neurological differences in autism spectrum disorders as well: One hypothesis is that people with autism have fewer long-range connections and greater short-range connections, which might help explain symptoms such as hyperfocus on details and obsessions.
In November 2013, Simon Baron-Cohen and his colleagues at the Autism Research Centre at the University of Cambridge published the results of a study to assess whether synesthesia occurs more frequently in people with ASDs. Of a sample of roughly 300 people total, the researchers found the rate of synesthesia to be three times higher in those with ASDs compared with neurotypical controls. To make sure that it was true synesthesia that was reported and not more transitory associations, they used something called the Test of Genuineness-Revised, developed by Baron-Cohen, which assesses whether the sense associations stay true over time.
While preliminary, the results suggest that there is indeed a significantly higher prevalence of synesthesia in autism. Why might that be?
"There are two main hypotheses as to why synesthesia occurs," says Roi Cohen Kadosh, a psychologist at the University of Oxford who studies the phenomenon. "One is that there are more connections between different brain regions. The other is that synesthesia might occur because of some imbalance in brain excitation and inhibition, and this may eventually lead to greater connectivity, as a byproduct of the different levels of excitation and inhibition that exist."
It's estimated that as many as 1 in 50 people worldwide have developmental prosopagnosia, a condition occurring since childhood where they are unable to recognize faces of relatives and friends they have seen hundreds or thousands of times. They can perceive parts of faces and describe them, and they can identify emotional expressions on faces, but they can't assemble the perception of a face into a recognizable whole, the information that would tell them who a particular person is.
"Many of the early patients with developmental prosopagnosia were first seen in autism clinics," says Jason Barton, a neuro-ophthalmologist and director of the Human Vision and Eye Movement Laboratory at the University of British Columbia. "It turned out that they didn't have autism at all. It was just that their ability to recognize people was severely impaired."
So while not all people with developmental prosopagnosia have autism, many people with autism-as many as two-thirds according to a study Barton and colleagues conducted-have trouble with processing information about faces. Researchers have suggested a few theories: Does being born with face-blindness skew social development enough to create the symptoms of ASDs? Might it be that people with autism, due to their social difficulties, are less interested in faces, and this leads to the inability to recognize faces? Or is it an underlying neurological difference that those with prosopagnosia and those with ASDs share?
On the prosopagnosia-first-then-autism theory, "Given the number of people we've seen who have developmental prosopagnosia and not autism, we can probably say that is not the case," says Barton. On the other hand, because it's suspected that autism is not truly a single, discrete, disorder, it's possible that face-processing difficulties contribute to autism in some people with it, but not in others.
Barton and colleagues have published several studies on people with autism-in 2004 in Brain, in 2007 in Neurology, and in 2005 in Neurology-that pursue answers to these questions. What they found is that about one-third of people with autism seem to perceive and recognize faces just as do controls without autism. "They were fine no matter which way we cut it-memory, perception or recognition," says Barton. "This makes a very solid case that a substantial fraction of people with autism do have normal face processing."
In the two-thirds that do not, the type and degree of the impairment look similar to people with developmental prosopagnosia. But, significantly, "as far as we can tell, we can't predict which people will have face-processing problems and which won't, from the type of autism diagnosis they have," says Barton. So whether people have what used to be called Asperger's or are somewhere further along the ASD spectrum does not seem to correlate with their ability to recognize faces.
Such clues are tantalizing, but as with all first-stage research, there are more questions than answers. The hunt for answers to how the brain processes sensory information-our world-continues.