Synaesthesia in its protean guises.

Synaesthesia is a condition characterized by unusual perceptual or cognitive pairings – for example, words might trigger tastes (Ward & Simner, 2003), letters may trigger the sensation of texture (Eagleman & Goodale, 2009), or music may induce the sensation of shapes and colour (Ward, Huckstep, & Tsakanikos, 2006). To date, there are estimated to be up to 150 reported forms of synaesthesia (Cytowic & Eagleman, 2009). Perhaps because of its varied incarnations, synaesthesia has proven difficult to capture under a single definition that rigorously marks the boundaries of inclusion and exclusion criteria. Adding to the bewilderment is the problem that almost every paper on synaesthesia examines a single sub-type, and the temptation is great in young fields to extrapolate the findings from single studies to the field more generally. In an article in this issue, Julia Simner aims to build scaffolding for a ‘single shared understanding of the definition of synaesthesia’. Taking a nuanced approach, she identifies the key qualities that would compose a working definition. First, Simner points out that synaesthesia should not be over-simplistically defined as a ‘merging of the senses’. That is, the criteria should not be limited only to sensory categories, but instead be broad enough to encompass conceptual categories. For example, colours can be triggered by the concept of a letter (rather than simply the physical form, Dixon, Smilek, Cudahy, & Merikle, 2000); in another sub-type, a number can trigger the conceptual experience of gender or personality type (Simner & Hubbard, 2006). Thus, Simner argues, joined sensation (the root words of syn + aesthesia) might represent a misnomer. Second, Simner asks important questions about the method by which researchers test for synaesthesia: consistency testing. This involves asking participants to pick, for example, the colour that best matches their synaesthetic perception for a particular letter. Participants are tested multiple times with all letters, and their colour choices are