Savant Memory for Digits in a Case of Synaesthesia and Asperger Syndrome is Related to Hyperactivity in the Lateral Prefrontal Cortex

Abstract

Single case: DT is a savant with exceptional abilities in numerical memory and mathematical calculations. DT also has an elaborate form of synaesthesia for visually presented digits. Further more, DT also has Asperger syndrome (AS). We carried out two preliminary investigations to establish whether these conditions may contribute to his savant abilities. Neuroimaging: In an fMRI digit span study, DT showed hyperactivity in lateral prefrontal cortex when encoding digits, compared with controls. In addition, while controls showed raised lateral prefrontal activation in response to structured (compared to unstructured) sequences of digits, DT's neural activity did not differ between these two conditions. In addition, controls showed a significant performance advantage for structured, compared with unstructured sequences whereas no such pattern was found for DT. We suggest that this performance pattern reflects that DT focuses less on external mathematical structure, since for him all digit sequences have internal structure linked to his synaesthesia. Finally, DT did not activate extra-striate regions normally associated with synaesthesia, suggesting that he has an unusual and more abstract and conceptual form of synaesthesia. This appears to generate structured, highly-chunked content that enhances encoding of digits and aids both recall and calculation. Neuropsychology: People with AS preferentially attend to local features of stimuli. To test this in DT, we administered the Navon task. Relative to controls, DT was faster at finding a target at the local level, and was less distracted by interference from the global level. Discussion: The propensity to focus on local detail, in concert with a form of synaesthesia that provides structure to all digits, may account for DT's exceptional numerical memory and calculation ability. This neural and cognitive pattern needs to be tested in a series of similar cases, and with more constrained control groups, to confirm the significance of this association.

Keywords:

• Prodigy
• fMRI
• Digit span
• Chunking
• Asperger syndrome
• Synaesthesia

Acknowledgments

We thank DT and the other volunteers for their participation in the study, the radiographers at the Wolfson Brain Imaging Centre for their assistance, Dr Matthew Brett and Dr Rik Henson for technical assistance, and MaryAnn Noonan for help with behavioural analyses. Julian Asher kindly confirmed the diagnosis of DT's synaesthesia using the Test of Genuineness-Revised. We are grateful to Martin Weitz and the production team of ‘Brain Man’ (Focus Productions) for introducing us to DT as part of their science documentary. This work was supported by the MRC, UK. This study is dedicated to the memory of the late Professor Ati Hermelin, who inspired us to test if DT had AS, given her research into savantism in autism.