Transient acquisition process of grapheme-color synesthesia on novel graphemes

Synesthesia is a cognitive trait in which stimulation in one sensory modality elicits an additional experience in another modality. Although both genetic and environmental factors have been proposed to contribute to the development of synesthesia, longitudinal studies focusing on the acquisition phase remain scarce. In the present longitudinal eye-tracking study, we examined whether gaze entropy reflects learning-related changes during the acquisition of a novel grapheme system (Thai numerals). Self-reported grapheme-color synesthetes with behavioral consistency evidence completed three laboratory sessions combined with at-home training while selecting colors for Thai and Arabic numerals. To quantify the degree of competition among candidate colors, Shannon entropy was calculated from fixation distributions. A linear mixed-effects model revealed a significant interaction between session and stimulus language, indicating that entropy decreased across sessions for Thai numerals, whereas entropy for well-learned Arabic numerals remained stable. This reduction suggests that gaze progressively converged toward fewer color candidates during learning.