Synaesthesia and the SNARC effect

In number-form synaesthesia, numbers become explicitly mapped onto portions of space in the mind’s eye or around the body. However, non-synaesthetes are also known to map number onto space, though in an implicit way. For example, those who are literate in a language that is written in a left-to-right direction are likely to assign small numbers to the left side of space and large numbers to the right side of space (e.g., Dehaene et al., 1993). In non-synaesthetes, this mapping is flexible (e.g., numbers map onto a circular form if the participant is primed to do so by the appearance of a clock-face), which has been interpreted as a response to task demands (e.g., Bachtold et al., 1998) or as evidence of a linguistically-mediated, rather than a direct, link between number and space (e.g., Proctor and Cho, 2006). We investigated whether synaesthetes’ number forms show the same flexibility during an odd-or-even judgement task that tapped linguistic associations between number and space (following Gevers et al., 2010). Synaesthetes and non-synaesthetes alike mapped small numbers to the verbal label ‘left’ and large numbers to the verbal label ‘right’. This surprising result may indicate that synaesthetes’ number forms are also the result of a linguistic link between number and space, instead of a direct link between the two, or that performance on tasks such as these is not mediated by the number form.