Ang Cao ^* Kazuhiro Ueda ^*

• The University of Tokyo, Bunkyo, Japan

This study investigates the mechanisms underlying pitch class-color synesthesia, a cognitive trait in which musical pitches evoke color perceptions. Synesthesia in music particularly involves the association of pitch classes (e.g., do, re, and mi) with specific colors. Itoh et al. (2017) introduced the two-step hypothesis, which suggests that pitch class identification precedes color association, and proposed a rainbow-like theory based on color gradients selected by synesthetes. The primary objective is to retest these theories to evaluate their generalizability in explaining pitch class-color synesthesia. We employed a dual-task paradigm to assess the robustness of the two-step hypothesis and conducted qualitative interviews to explore the nature of synesthetic experiences. The results indicated that the two-step hypothesis may not be always applicable, because it effectively accounts for only a subset of synesthetes. The presence of one-step synesthetes, who experience direct pitch-to-color associations without intermediate steps, implied a more varied synesthetic mechanism. Moreover, rainbow-like theory predominantly characterized two-step synesthetes, while one-step synesthetes exhibited distinct color perceptions. Furthermore, we found that the differentiation between two-and one-step synesthesia may be associated with the methods through which participants develop synesthetic associations. The findings highlighted the diversity of synesthetic experiences in pitch class-color synesthesia, which challenges the generalizability of the current theories and poses the need for a further nuanced understanding of this phenomenon.