Abstract
Grapheme–color synesthesia is a condition in which a visual grapheme induces a specific color sensation. A defining characteristic of synesthesia is the temporal (test-retest) consistency of synesthetic associations across the lifespan. However, Simner et al. (2017) compared synesthetes from two age groups (below and above 35 years of age) and demonstrated that the consistency of synesthetic associations is lower in older synesthetes. Therefore, further investigations are warranted to determine the longitudinal trends and factors contributing to age-related changes in these associations. We examined longitudinal synesthetic color consistency over five to ten years for 300 graphemes (alphanumeric letters and Japanese characters) in eight grapheme-color synesthetes in their late 20s at the beginning of the study. The results indicated that the synesthetic color for almost none of the graphemes disappeared, although the long-term consistency was lower than the short-term consistency over approximately three weeks, as measured at the beginning and the end of the study period. However, the long-term consistency of all synesthetes met the consistency criterion of synesthesia (Rothen et al., 2013). The results also showed that graphemes with unstable synesthetic color responses over short periods were also unstable over long periods. Graphemes with both low short- and long-term consistencies tended to have low familiarity and high visual complexity. It has been suggested that synesthetic grapheme-color associations are shaped by grapheme learning (Asano et al., 2019), which might facilitate stronger grapheme-color associations with increased frequency of the graphemes occurring. The correlation between high visual complexity and color inconsistency might be related to the finding that multiple colors are likely to be associated with characters and words consisting of multiple elements (Mankin et al., 2016; Uno et al., 2019). In such cases, synesthetic color reports would become less stable if synesthetes are asked to select just one color for each grapheme.