Back to the future: synaesthesia could be due to associative learning

INTRODUCTION Over the last century, researchers and laypeople alike have been captivated by synaesthesia—a developmental condition present in approximately 5% of the adult population, where the presentation of an “inducer” stimulus gives rise to additional “concurrent” sensation absent from the veridical sensory world (Ward, 2013). For example, the most common variant is grapheme-color synaesthesia, where letters and digits induce concurrent sensations of color (Day, 2005). Early theories proposed that synaesthesia resulted from associative learning between the inducer and concurrent sensory representations (Calkins, 1893; Claparede, 1903). Specifically, the inducer and concurrent stimuli have been present in a correlated, or “contingent,” fashion in the synaesthete’s environment; hence presentation of the inducer alone now activates the representation of the concurrent. However, these theories have been widely rejected. A range of structural and functional mechanisms are proposed by contemporary theorists, including hyperconnectivity between brain regions (Hubbard and Ramachandran, 2005) and feedback disinhibition (Grossenbacher and Lovelace, 2001), but all are united in the assertion that “associative learning in and of itself cannot explain synaesthesia” (Marks and Odgaard, 2005). Given recent evidence consistent with learning explanations of synaesthesia, we consider here whether the associative account may have been dismissed prematurely. We suggest that common objections to this account may be rooted in misunderstandings about what it would predict. We consider these objections in turn, highlighting how they are easily countered by contemporary learning theory. The objections are that synaesthetic experiences (1) do not reflect environmental statistics, (2) remain consistent over time, (3) cannot be trained into neurotypicals, and (4) are domainspecific. We close by describing the types of experiment that would provide the associative account of synaesthesia with the rigorous empirical assessment it deserves.