Daniel Tammet was born with congenital childhood epilepsy. Experiencing numbers as colors or sensations is a well-documented form of synesthesia, but the detail and specificity of Tammet's mental imagery of numbers is unique.
Tammet has been "studied repeatedly" by researchers in Britain and the United States, and has been the subject of several peer-reviewed scientific papers. Professor Allan Snyder at the Australian National University has said of Tammet: "Savants can't usually tell us how they do what they do. It just comes to them. Daniel can describe what he sees in his head. That's why he's exciting. He could be the 'Rosetta Stone'."
In his mind, he says, each positive integer up to 10,000 has its own unique shape, colour, texture and feel. He has described his visual image of 289 as particularly ugly, 333 as particularly attractive, and pi as beautiful. The number 6 apparently has no distinct image yet what he describes as an almost small nothingness, opposite to the number 9 which he calls large, towering, and quite intimidating. In his memoir, Tammet states experiencing a synaesthetic and emotional response for numbers and words.
Tammet holds the European record for reciting pi from memory to 22,514 digits in five hours and nine minutes on 14 March 2004.
Tammet has reportedly learned ten languages, including Romanian, Gaelic, Welsh, and Icelandic which he learned in a week for a TV documentary.
After the World Memory Championships, Tammet participated in a group study, later published in the New Year 2003 edition of Nature Neuroscience. The researchers investigated the reasons for the memory champions' superior performance. They reported that they used "strategies for encoding information with the sole purpose of making it more memorable", and concluded that superior memory was not driven by exceptional intellectual ability or differences in brain structure.
In another study, Simon Baron-Cohen and others at the Autism Research Centre at the University of Cambridge tested Tammet's abilities in around 2005. He was found to have synaesthesia according to the "Test of Genuineness-Revised" which tests the subjects' consistency in reporting descriptions of their synaesthesia. He performed well on tests of short term memory (with a digit-span of 11.5, where 6.5 is typical). Conversely, test results showed his memory for faces scored at the level expected of a 6–8 year old child in this task. The authors of the study speculated that Tammet's savant memory could be a result of synaesthesia combined with Asperger syndrome, or it could be the result of mnemonic strategies.
Baron-Cohen, Bor and Billington investigated whether his synaesthesia and Asperger syndrome explained his savant memory abilities in a further study published in Neurocase in 2008. They concluded that his abilities might be explained by hyperactivity in one brain region (the left prefrontal cortex), which results from his Asperger syndrome and synaesthesia. On the Navon task, relative to non-autistic controls, Tammet was found to be faster at finding a target at the local level and to be less distracted by interference from the global level. In an fMRI scan, "Tammet did not activate extra-striate regions of the brain normally associated with synaesthesia, suggesting that he has an unusual and more abstract and conceptual form of synaesthesia". Published in Cerebral Cortex (2011), an fMRI study lead by Professor Jean-Michel Hupé at the University of Toulouse (France) observed no activation of colour areas in ten synaesthetes. Hupé suggests that synaesthetic colour experience lies not in the brain's colour system, but instead results from "a complex construction of meaning in the brain, involving not only perception, but language, memory and emotion".
In his book Moonwalking with Einstein (2011), science journalist and former US Memory Champion Joshua Foer speculates that study of conventional mnemonic approaches has played a role in Tammet's feats of memory. While accepting that Tammet meets the standard definition of a prodigious savant, Foer suggests that his abilities may simply reflect intensive training using standard memory techniques, rather than any abnormal psychology or neurology per se. In a review of his book for the New York Times, psychologist Alexandra Horowitz described Foer's speculation as among the book's few "missteps", because the issue of whether or not savants such as Tammet make use of such strategies is irrelevant.
