Why we are not all synesthetes (not even weakly so).

A little over a decade ago, Martino and Marks (Current Directions in Psychological Science 10:61-65, 2001) put forward the influential claim that cases of intuitive matchings between stimuli in different sensory modalities should be considered as a weak form of synesthesia. Over the intervening years, many other researchers have agreed-at the very least, implicitly-with this position (e.g., Bien, ten Oever, Goebel, & Sack NeuroImage 59:663-672, 2012; Eagleman Cortex 45:1266-1277, 2009; Esterman, Verstynen, Ivry, & Robertson Journal of Cognitive Neuroscience 18:1570-1576, 2006; Ludwig, Adachi, & Matzuzawa Proceedings of the National Academy of Sciences of the United States of America 108:20661-20665, 2011; Mulvenna & Walsh Trends in Cognitive Sciences 10:350-352, 2006; Sagiv & Ward 2006; Zellner, McGarry, Mattern-McClory, & Abreu Chemical Senses 33:211-222:2008). Here, though, we defend the separatist view, arguing that these cases are likely to form distinct kinds of phenomena despite their superficial similarities. We believe that crossmodal correspondences should be studied in their own right and not assimilated, either in terms of the name used or in terms of the explanation given, to synesthesia. To conflate these two phenomena is both inappropriate and potentially misleading. Below, we critically evaluate the evidence concerning the descriptive and constitutive features of crossmodal correspondences and synesthesia and highlight how they differ. Ultimately, we wish to provide a general definition of crossmodal correspondences as acquired, malleable, relative, and transitive pairings between sensory dimensions and to provide a framework in which to integrate the nonsystematic cataloguing of new cases of crossmodal correspondences, a tendency that has increased in recent years.