Synesthesia strengthens sound-symbolic cross-modal correspondences

Synesthesia is a phenomenon in which an experience in one domain is accompanied by an involuntary secondary experience in another, unrelated domain; in classical synesthesia, these associations are arbitrary and idiosyncratic. Cross-modal correspondences refer to universal associations between seemingly unrelated sensory features, e.g., auditory pitch and visual size. Some argue that these phenomena form a continuum, with classical synesthesia being an exaggeration of universal cross-modal correspondences, whereas others contend that the two are quite different, since cross-modal correspondences are non-arbitrary, non-idiosyncratic, and do not involve secondary experiences. Here, we used the implicit association test to compare synesthetes' and non-synesthetes' sensitivity to cross-modal correspondences. We tested the associations between auditory pitch and visual elevation, auditory pitch and visual size, and sound-symbolic correspondences between auditory pseudowords and visual shapes. Synesthetes were more sensitive than non-synesthetes to cross-modal correspondences involving sound-symbolic, but not low-level sensory, associations. We conclude that synesthesia heightens universally experienced cross-modal correspondences, but only when these involve sound symbolism. This is only partly consistent with the idea of a continuum between synesthesia and cross-modal correspondences, but accords with the idea that synesthesia is a high-level, post-perceptual phenomenon, with spillover of the abilities of synesthetes into domains outside their synesthesias. To our knowledge, this is the first demonstration that synesthetes, relative to non-synesthetes, experience stronger cross-modal correspondences outside their synesthetic domains.