Comparing implicit and synaesthetic number-space associations: visuospatial and verbal SNARC effects

In Dehaene’s (1992) theory of the mental number line, number and space are implicitly associated, giving rise to the spatial-numerical association of response codes (SNARC) effect, in which smaller numbers are more readily associated with the left side of space, larger numbers with the right, during a parity judgement task. Others, however, have argued that the SNARC effect is flexible (e.g. Bächtold, Baumüller, & Brugger, 1998) and better explained by verbal rather than spatial associations (Proctor & Cho, 2006). A few single-case studies on the SNARC effect have tested number-space synaesthetes, who make explicit associations between number and space. Here, we present data from experiments conducted on groups of synaesthetes and non-synaesthetes on the classic SNARC parity judgement task with lateralised response keys and a modified version in which they responded to labels appearing on-screen (Gevers et al., 2010). Synaesthetes’ behaviour was expected to differ from non-synaesthetes’ due to the explicit, fixed nature of their number-space associations, but both experiments show the two groups behaving in the same way, indicating that parity judgement tasks may not be tapping the same representation of number that gives rise to synaesthetic number-space experiences.