'Special cases': neural mechanisms and individual differences in synaesthesia.

The healthy ‘special case’ in neuropsychology! Synaesthesia (Galton, 1883) is a condition in which a particular stimulus (e.g., seeing the letter R) evokes a particular additional sensation (e.g., a deep purple colour). Synaesthesia is automatic in the sense that the cross-sensations are fast and seemingly effortless, and highly consistent as the same associations persist from early childhood. Importantly, synaesthesia is found unrelated to psychological, psychiatric, or neurological ‘illness’, or to substance abuse (Baron-Cohen, Wyke, & Binnie, 1987; Rich, Bradshaw, & Mattingley, 2005). Prevalence of synaesthesia is estimated to be between 1 and 5 percent (Simner et al., 2006). Recent years have seen a sharp rise in the number of publications on synaesthesia, including publications in top journals as ‘Nature’ and ‘Nature Neuroscience’. There is a growing interest in synaesthesia not only within, but also outside of the scientific community. Examples are documentaries on television, interviews on the topic of synaesthesia in the media, and the popularity of books on synaesthesia (e.g. ‘Born on a Blue Day’ by Tammet, 2006). One reason for this increased interest is that research has been able to prove that synaesthesia is a real condition (Baron-Cohen et al., 1987; Dixon, Smilek, Cudahy, & Merikle, 2000; Mattingley, Rich, Yelland, & Bradshaw, 2001). It has clear behavioural and biological characteristics that sets it apart from ‘normal’ associations, including perceptual nature of the synaesthetic experience (Palmeri, Blake, Marois, Flanery, & Whetsell, 2002; Ramachandran & Hubbard, 2001a; Smilek, Dixon, Cuday, & Merikle, 2001) structural differences between the brains of synaesthetes and non-synaesthetes (Hänggi, Beeli, Oechslin, & Jäncke, 2008; Jäncke, Beeli, Eulig, & Hänggi, 2009; Rouw & Scholte, 2007; Rouw & Scholte, 2010) and evidence for a genetic predisposition to having synaesthesia (Asher et al., 2009; Tomson et al., 2011). Synaesthesia is important to study not only because it is an interesting topic in its own right. The unusual experiences of synaesthetes provide an extraordinary opportunity to address some of the most hotly debated questions in the field of neuroscience, including what are the mechanisms behind cross-modal sensory integration, how can a conscious sensory experiences arise in the absence of the appropriate external stimulus, and what is the neural basis of perceptual awareness. A very different factor that has pushed forward research on synaesthesia is the availability of exciting new technology. These exciting developments have provided us with new knowledge on synaesthesia. However, the strong increase of knowledge is resulting from different types of studies, from different