Absolute pitch is not necessary for pitch class-color synesthesia

Sounds evoke color sensations in sound-color synesthesia. Recently, we showed that pitch classes (do, re, mi, etc.) have rainbow hues and that colors are linked to the names of pitch classes rather than to their sounds in 15 subjects who had "pitch class-color synesthesia." However, all synesthetes in our previous study had high levels of absolute pitch (AP); therefore the effects of AP on the condition remained unclear. The present study investigated 18 additional pitch class-color synesthetes who had no or lower levels of AP, and confirmed the generality of the above findings. Furthermore, behavioral experiments indicated a two-step process underlying color sensations: pitches are first associated with their pitch class names, and then the pitch class names evoke color sensations. Two separable brain functions underlie pitch-to-color conversion in pitch class-color synesthesia: a musical function of pitch class identification, and the synesthetic association between pitch class and color.