New findings suggest that people with autism have a higher than average chance of also having synesthesia, the condition in which the senses are mixed.

Professor Simon Baron-Cohen of Cambridge University and colleagues explain that synesthesia involves input to one sense triggering a response in a different sense.

For example, a person with “colored hearing” synesthesia sees colors after hearing sounds. Most mixed responses are visual, although synesthesia can involve any pair of senses, such as tasting flavors when hearing sounds. Autism is a condition involving “social-communication disability, alongside resistance to change and unusually narrow interests or activities,” explain the researchers.

They tested 164 adults with a condition on the autistic spectrum and 97 adults without autism. Synesthesia was diagnosed in almost three times as many participants with autism (19 percent) as those without (7 percent).

Of the 31 people who had both autism and synesthesia, the most common types were those known as “sound-color” (when a sound triggers a visual experience of color) and “grapheme-color” (seeing black and white letters as colored). Many also reported experiencing color when exposed to tastes or smells. Details of the study are published in the journal Molecular Autism.

“I have studied both autism and synesthesia for over 25 years and I had assumed that one had nothing to do with the other,” said Baron-Cohen. “These findings will re-focus research to examine common factors that drive brain development in these traditionally very separate conditions.”

At the level of the brain, synesthesia involves abnormal connections between brain areas that are not usually wired together, and autism may also involve over-connectivity of neurons (so that the person over-focuses on small details but struggles to keep track of the big picture).

This is linked to the mechanism called “apoptosis,” Baron-Cohen said. Apoptosis is “the natural pruning that occurs in early development, where we are programmed to lose many of our infant neural connections. In both autism and synesthesia apoptosis may not occur at the same rate, so that these connections are retained beyond infancy.”

Co-author Professor Simon Fisher of the Max Planck Institute in Germany adds, “Genes play a substantial role in autism and scientists have begun to pinpoint some of the individual genes involved. Synesthesia is also thought to be strongly genetic, but the specific genes underlying this are still unknown.

“This new research gives us an exciting new lead, encouraging us to search for genes which are shared between these two conditions, and which might play a role in how the brain forms or loses neural connections.”

The study was carried out as part of a master’s degree program by student Donielle Johnson in Cambridge, UK.

She comments, “People with autism report high levels of sensory hyper-sensitivity. This new study goes one step further in identifying synesthesia as a sensory issue that has been overlooked in this population. This has major implications for educators and clinicians designing autism-friendly learning environments.”

“Having synesthesia alongside autism may be due to excessive serotonin levels in early childhood,” says Dr. Berit Brogaard of the University of Missouri – St. Louis.

She suggests that high serotonin could lead to “decreased extracellular levels of serotonin in one brain hemisphere and compensatory increased levels in the other hemisphere.”

Evidence for this idea has been found in PET scans of people with high-functioning autism. These showed that for many participants, serotonin synthesis is suppressed in the left hemisphere and increased in the right.

“The evidence that serotonin plays a crucial role in autism is overwhelming,” Dr. Brogaard writes in the journal Frontiers in Human Neuroscience.

High blood levels of serotonin are very common in autistic people, which may indicate that there were high brain levels of serotonin as a young child, when the blood-brain barrier is not fully developed. This can hinder the development of serotonin neurons.

She states, “The expected high frequency of synesthesia in autistic individuals together with the lateralization hypothesis point to the possibility that increased extracellular levels of serotonin in the autistic brain may be a causal influence on the genesis of synesthesia.”

High serotonin levels in very young children with autism can also trigger altered multisensory processing. “If this hypothesis is correct, then we should expect to find evidence of synesthesia in autistic individuals at a very young age,” Dr. Brogaard writes. “However, we do not yet have enough data to draw any firm conclusions.”