Numbers, Synesthesia, and Directionality

Numbers are fundamental to our understanding of, and survival in, the environment. Not surprisingly, numbers represent an important psychological dimension in triggering synaesthetic experiences, such as in digit-colour synaesthesia (where digits trigger colour experiences), or number-space synaesthesia (where digits are seen in particular spatial arrays). Another important consideration is directionality in synaesthesia, in that we might ask whether the stimulus and response in any given synaesthetic variant can also work on the opposite way (e.g., if numbers trigger colours, do colours also give a sensation of numerosity?). Most studies have documented the typical direction of the synaesthetic experience from the inducer (e.g., digit) to the concurrent (e.g., colour). However, it seems that some synaesthetes do show bi-directionality, that is, their concurrent (e.g., colour) might also trigger the inducer (e.g., digits) either at an implicit level, or even at the explicit level that reaches perceptual awareness. We discuss these two issues in this book chapter. We will first explore synesthesia and automaticity where numbers are concerned, followed by a discussion on synaesthesia and directionality