Implicit associative learning in synesthetes and nonsynesthetes.

Although cross-modal neural connections and genetic underpinnings are prominent in most current theories regarding the development of synesthesia, the potential role of associative learning in the formation of synesthetic associations has recently been revitalized. In this study, we investigated implicit associative learning in synesthetes and nonsynesthetes by recording reaction times to a target whose color was probabilistically correlated with its shape. A continuous measure of target detection at multiple time points during learning revealed that synesthetes and nonsynesthetes learn associations differently. Specifically, our results demonstrated a "fast-facilitation" learning effect for nonsynesthetes and a "fast-interference, slow-facilitation" learning effect for synesthetes. Additionally, synesthetes exhibited superior long-term memory for the learned associations in a surprise delayed retest. After this retest, participants implicitly learned new (shuffled) shape-color associations. We found that synesthetes experienced greater interference while learning these new shape-color associations. These results detail ways in which implicit associative learning and memory differ between synesthetes and nonsynesthetes.