Multilevel analysis of individual differences in regularities of grapheme-color associations in synesthesia

- Individual differences in regularities of grapheme-color associations have been shown.
- A necessity of a multilevel analysis to investigate regularities of grapheme-color associations were confirmed.
- We tested whether individual differences in regularities were explained by a type of synesthetic experience.
- We found evidence for impacts of the type of synesthetic experience on some regularities.
- We discuss the implications of our findings for developmental process of synesthesia.

Grapheme-color synesthesia is a neurological phenomenon where visual perception of letters and numbers stimulates perception of a specific color. Grapheme-color correspondences have been shown to be systematically associated with grapheme properties, including visual shape difference, ordinality, and frequency. However, the contributions of grapheme factors differ across individuals. In this study, we applied multilevel analysis to test whether individual differences in regularities of grapheme-color associations could be explained by individual styles of processing grapheme properties. These processing styles are reflected by the type of synesthetic experience. Specifically, we hypothesized that processing focusing on shape differences would be associated with projector synesthetes, while processing focusing on ordinality or familiarity would be associated with associator synesthetes. The analysis revealed that ordinality and familiarity factors were expressed more strongly among associators than among projectors. This finding suggests that grapheme-color associations are partly determined by the type of synesthetic experience.