Daniel Tammet has memorized Pi to the 22,514th digit. He speaks ten different languages, including one of his own invention, and he can multiply enormous sums in his head within a matter of seconds. However, he is unable to hold down a standard 9-to-5 job, in part due to his obsessive adherence to ritual, down to the precise times he has his tea every day.

Daniel is a savant. He is also autistic. And he is a synesthete.

Daniel experiences numbers as having color, as well as shape and texture. This helps him perform amazing mathematical feats seemingly without effort, the answer simply materializing to him rather than having to calculate it out.

In an interview he gave with The Guardian, Daniel explained, “When I multiply numbers together, I see two shapes. The image starts to change and evolve, and a third shape emerges. That’s the answer. It’s mental imagery. It’s like maths without having to think.”

Clearly this man has an extraordinary brain. However, Daniel is perhaps not entirely unique, and it appears that the link between autism and synesthesia is more common than originally thought. This suggests that there is a potential common mechanism between these two conditions, which may even help to explain some of Daniel’s special savant abilities.

A new study published in the journal Molecular Autism from a team of researchers at the University of Cambridge now empirically shows that there is an almost three-fold higher occurrence of synesthesia in individuals with autism (18.9%), compared with that of the general population (7.2%). This increased prevalence implies that there is indeed a significant link between autism and synesthesia.

Synesthesia can be thought of as a crossing of the senses, where one perceptual experience is accompanied by another unrelated one. For example, individuals with sound-color synesthesia experience visions and sensations of color in harmony with hearing music. In a similar (and the most common) rendition, grapheme-color, letters and numbers are perceived as having their own unique shade.

While at first seemingly unrelated, if you look closer at what is happening in the brain in both autism and synesthesia this overlap is not so surprising. Each condition is thought to at least partly stem from abnormalities in the way the brain is wired. For example, white matter tracts connecting different parts of the brain – shooting signals across regions and hemispheres – have been shown to be increased in both conditions.

While some of these connections traverse the entire brain, traveling from the frontal cortex back to the occipital lobe, others work more locally, connecting adjacent regions and sending information quickly back and forth. In autism, there is an increased volume of these short-distance white matter connections, either due to a proliferation of synaptogenesis (the creation of new connections), or a failure in the pruning out of these synaptic connections during childhood. Individuals with synesthesia also show a greater density of white matter, particularly in sensory regions implicated in the condition. In individuals with grapheme-color synesthesia, for instance, there is greater connection between visual area four, the region responsible for color perception, and the eponymous visual word-form area.

It has also been suggested that this increased white matter integrity and synesthetic ability may be behind the extraordinary faculty of individuals like Daniel who show signs of autistic savantism. One theory that has been proposed is that synesthesia may help people to better recall memories as they can tap into an automatic mnemonic device via their multiple sensory experiences – i.e. being able to rely on both visual and numeric memory when trying to remember a string of numbers.

However, while there is already an established link between autism and savantism, no such studies have been conducted on superior memory in people with synesthesia. Also, it should be noted that many savants do not report any presence of synesthesia, and certainly there are many synesthetes without any hint of superior memory or savantism. In fact, even I have traces of synesthesia, experiencing sensations of color in combination with days of the week, and I’ve certainly yet to experience any hint of enhanced cognitive ability.

So it looks like Daniel Tammet’s crown as the European champion for reciting Pi is safe for now. And certainly his method is unique – the current world leader, Chinese student Lu Chao, recited 67,890 digits of the number relying solely on ancient Chinese rote memorization techniques.

Baron-Cohen S et al. Is synaesthesia more common in autism? Molecular Autism 4:40 (2013).

Baron-Cohen S et al. Savant memory in a man with colour form-number synaesthesia and asperger. Journal of Consciousness Studies 14, 237-251 (2007).

Belmonte MK et al. Autism and abnormal development of brain connectivity. The Journal of Neuroscience 24, 9228-9231 (2004).

Bor D et al. Savant memory for digits in a case of synaesthesia and asperger syndrome is related to hyperactivity in the lateral prefrontal cortex. Neurocase 13, 311-319 (2007).

Courchesne et al. Evidence of brain overgrowth in the first year of life in autism. The Journal of the American Medical Association 290, 337-344 (2003).

Cytowic RE & Eagleman DM. Wednesday is Indigo Blue. Cambridge, MA: MIT Press (2009).

Hermelin B. Bright Splinters of the Mind: A Personal Story of Research with Autistic Savants. London: Jessica Kingsley (2002).

Hubbard EM et al. Individual differences among grapheme-color synesthetes: brain-behavior correlations. Neuron 45, 975-985 (2005).

Nunn JA et al. Functional magnetic resonance imaging of synesthesia: Activation of V4/V8 by spoken words. Nature Neuroscience 5, 371-375 (2002).

Rouw R & Scholte HS. Increased structural connectivity in grapheme-color synesthesia. Nature Neuroscience 10, 792-292 (2007).