Automaticity in sequence-space synaesthesia: a critical appraisal of the evidence

For many people, thinking about certain types of common sequence--for example calendar units or numerals--elicits a vivid experience that the sequence members occupy spatial locations which are in turn part of a larger spatial pattern of sequence members. Recent research on these visuospatial experiences has usually considered them to be a variety of synaesthesia, and many studies have argued that this sequence-space synaesthesia is an automatic process, consistent with a traditional view that automaticity is a key property of synaesthesia. In this review we present a critical discussion of data from the three main paradigms that have been used to argue for automaticity in sequence-space synaesthesia, namely SNARC-like effects (Spatial-Numerical-Association-of-Response-Codes), spatial cueing, and perceptual incongruity effects. We suggest that previous studies have been too imprecise in specifying which type of automaticity is implicated. Moreover, mirroring previous challenges to automaticity in other types of synaesthesia, we conclude that existing data are at best ambiguous regarding the automaticity of sequence-space synaesthesia, and may even be more consistent with the effects of controlled (i.e., non-automatic) processes. This lack of strong evidence for automaticity reduces the temptation to seek explanations of sequence-space synaesthesia in terms of processes mediated by qualitatively abnormal brain organization or mechanisms. Instead, more parsimonious explanations in terms of extensively rehearsed associations, established for example via normal processes of visuospatial imagery, are convergent with arguments that synaesthetic phenomena are on a continuum with normal cognition.