Synesthesia is a condition in which one sense involuntarily triggers another sense. For instance, a person who is synesthetic might see certain colors when he hears certain sounds or taste certain flavors when he sees certain numbers.

This phenomenon occurs in 4 percent of the population, and recent research has shown that a sizable part of this population is autistic. This suggests that those with autism might be more likely to have the ability of multi-sensory experience than synesthetes not on the autistic spectrum.

In a 2013 study, researchers asked 164 adults with autism and 97 adults without it to answer questionnaires that measured traits of synesthesia and autism. Of the adults with autism, 19 percent were identified as synesthetes. Of the participants without autism, only 7 percent were diagnosed with synesthesia. This finding suggests that synesthesia and autism may share some similarities.

Some autistic participants did not report any sensory crossing, but given the way particular individuals reacted during the study, the researchers judged these individuals as having synesthesia. The autistic individuals who reported no signs of synesthesia were not included in the percentage of individuals who gave a definitive affirmation on their own, but if the researchers' observations prove valid, the correlation between autism and synesthesia could be higher. The study was published in the journal Molecular Autism.

One similarity between synesthesia and autism is that the conditions generate strong hyper-connectivity between neurons. In the case of autism, many neurons are over-connected, resulting in an intense focus on minute details, while with synesthesia, certain neurons share a link that they wouldn't otherwise.

The study's team leader — professor Simon Baron-Cohen of University of Cambridge, who also the director of the university's Autism Research Centre — suggests that the occurrence of both autism and synesthesia could be related to "apoptosis."

Apoptosis is "the natural pruning that occurs in early development, where we are programmed to lose many of our infant neural connections," Baron-Cohen explains to Psych Central. "In both autism and synesthesia, apoptosis may not occur at the same rate, so that these connections are retained beyond infancy." In this way, the neuronal connections persist, and their continued existence can result in over-connectivity or an atypical crossing of neurons.

Serotonin levels could also be a factor in the correlations between autism and synesthesia. Researchers have found that within the autistic brain, there tends to be discrepancies in the levels of serotonin produced between the left and right hemispheres. Berit Brogaard of the University of Miami says that within the autistic brain, there can be "decreased extracellular levels of serotonin in one brain hemisphere and compensatory increased levels in the other hemisphere," according to Psych Central.

Are high serotonin levels in children the cause of multi-sensory experiences? Brogaard suggests that the relationship between high serotonin levels and multi-sensory experience could "point to the possibility that increased extracellular levels of serotonin in the autistic brain may be a causal influence on the genesis of synesthesia." The result? A person within the autistic spectrum has a higher possibility of susceptibility to synesthesia.

'An exciting new lead'

boy sorts cars by color, denotes autism The research may help create more autism-friendly learning concepts. (Photo: PhotoUG/Shutterstock)

Since both autism and synesthesia are strongly influenced by genetics, researchers want to find out if there are similar genetic factors behind the conditions.

"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," team member Simon Fisher of the Max Planck Institute in Germany told Psych Central. This research offers insight into the inner workings of autism and synesthesia, but could also offer clues about genetics and brain development.

The correlation between autism and synesthesia could also open doors for innovations in education. Master’s student Donielle Johnson, who carried out the study for her program at Cambridge, told Psych Central that these findings have "major implications for educators and clinicians designing autism-friendly learning environments."

We might want to “[think] of synesthesia as the automatic retrieval of highly specific mnemonic associations, in which perceptual contents are brought to mind in a manner akin to mental imagery” says Nathan Witthoft and Jonathan Winawer of Stanford University in an article referenced by Baron-Cohen. In this way, synesthesia can be seen as a unique form of memory recall — one that allows for deeper associations with mnemonic devices than a person outside of the autistic or synesthetic spectrum might have.