Regulatory factors influence grapheme-color associations in synesthete and non-synesthetes: a review and model

Grapheme-color synesthetes experience linguistic symbols (e.g., letters of the alphabet) as having a consistent color (e.g., “The letter S is burgundy red”). Intriguingly, when non-synesthetes are forced to choose colors for letters, similar non-random distributions are observed. Why are certain letters likelier to be associated with certain colors? Researchers have long sought to explain these trends, and in the past few decades numerous studies have reported correlations between synesthetic colors and various properties of letters, such as ordinal position, frequency in the language, and even pronunciation. These influences, which we call “Regulatory Factors” (RFs), each explain some fraction of the variation in observed associations. In the present work, we provide an updated review of the literature, covering all known studies of RFs. We describe each RF and the operationalization that was used to measure it. For each RF, we also replicate the results in our own database of synesthetes and non-synesthetes, in some cases testing for the first time whether the RF influences the associations of non-synesthete controls. Finally, we introduce a new statistical model of synesthetic associations, that can evaluate the effect of all RFs in a single model.