Grapheme-color synesthesia is a phenomenon in which letters and digits evoke perceptions of color. There is an ongoing debate on the levels of information processing and color perception required to induce a synesthetic experience. This study aims to identify the levels of sensory-perceptual processing at which synesthesia occurs by manipulating the visual forms and phonological pronunciation of Japanese Kana letters.

The results demonstrate that phonologic information (i.e. consonants and vowels) affects synesthetic color perception more than morphological forms. Our second finding is that synesthetic color perception that was elicited by different factors showed systematically characteristic distributions in the CIE L^*a^*b^* color space, based on E. Hering’s opponent color theory. These results imply that grapheme-color synesthesia is more likely to be explained by the opponent-process color perception theory, than by categorical color perception associated with higher processing.

We conclude that grapheme-color synesthesia is a phenomenon that results from the multiple interactions between color processing and character processing that occurs at the stage of color opponency and pre-categorical color perception.