Grapheme-color synesthesia is a phenomenon that can only be felt by synesthetes, in which characters act as an inducer and excite the sensation of a specific color, and the high temporal consistency of the combination of characters and colors is the basis of synesthesia. However, recent studies have reported that this temporal consistency may change over time. Therefore, we conducted a longitudinal study of Japanese synesthesia and investigated the long-term consistency of grapheme-color synesthesia （Uno, Asano, & Yokosawa, 2021）. To compare short-term and long-term temporal consistency, we examined temporal consistency over relatively short periods （up to several months） and long periods （5-8 years）. In particular, it was predicted that the correspondence with the synesthetic color of characters with high familiarity may be excited more frequently in daily life, leading to high temporal consistency in both short and long term.
As a result, characters with low short-term temporal consistency had low long-term temporal consistency. What is important is that the temporal consistency of synesthesia depends on the familiarity of the character. The synesthetic correspondence between characters and colors might be not fully integrated in childhood, suggesting that it also has a fluid aspect in adult synesthesia.
It is also consistent with our recent finding that learning new attributes （sounds or meanings） reduces the temporal consistency of characters and colors. In addition, the long-term （5-8 years） temporal consistency of the letter-color association of each character can be predicted from the short-term temporal consistency. The synesthetic color of characters with the low familiarity is likely to change over time, which is considered to reflect the weak integration of such letters with the synesthesia color.
