Grapheme-color synesthesia is a condition in which a visual letter or character induces a specific color sensation. Although grapheme-color association in grapheme-color synesthesia is characterized as idiosyncratic, some regularities in synesthetic sensation have also been reported. For example, Asano and Yokosawa (2012) showed that sound and meaning simultaneously affect the synesthetic color choice for Kanji script (a logographic script used in the Japanese language) in Japanese grapheme-color synesthesia. Many such regularities are related to psycholinguistic properties of graphemes, such as phonology, meaning or concepts, positions in a grapheme sequence, in addition to frequency of occurrence (e.g., Asano & Yokosawa, 2013; Simner, 2007). However, relatively little is known about the influence of more perceptual properties of graphemes on synesthetic colors, properties such as visual complexity. This study explored this issue by examining synesthetic colors for Japanese Kanji characters with both high and low visual complexity in 10 Japanese synesthetes. Results revealed that Kanji characters with high visual complexity elicited synesthetic colors that were darker (lower luminance) than those with low visual complexity. This visual complexity effect was observed even when the grapheme frequency, which is known to influence the luminance of synesthetic colors (Beeli, Esslen, & J?¤ncke, 2007), was controlled. Ten Japanese non-synesthetic controls were also presented with the same set of Kanji characters as the synesthetes; participants selected a color judged to "go well with the character" with each character. Results showed no effects of visual complexity. These findings suggest that not only psycholinguistic properties, but also more perceptual properties of graphemes influence grapheme-color associations of synesthetes.
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