Spiky figures correspond to Japanese names written in square characters.

Several studies have characterized the bouba-kiki effect as a cross-modal correspondence between spoken sounds and shapes. Although the effect is consistent across languages, some studies have indicated that it may be influenced by the visual resemblance between alphabetic letters and figures. Since the bouba-kiki effect is a cross-linguistic phenomenon, the orthographical bouba-kiki effect should be investigated in nonalphabetic letters. Here, the term "orthographical" refers to the visual shapes of characters and letters. However, no study on the orthographical bouba-kiki effect has focused on the difference between the two Japanese phonograms with the same sound but different orthographical appearances: hiragana and katakana. Hiragana is orthographically rounded, while katakana is square. Therefore, this study investigated whether hiragana and katakana are associated with rounded and spiky figures, respectively. In the experiment, native Japanese participants rated the appropriateness of nonsense words as the names of abstract figures on a scale from 0 to 100. The results showed that katakana and spiky figures were likely to be associated. Our findings suggest that orthographical similarity between specific character types (i.e., square characters) and spiky figures strengthens the word-shape association, revealing an asymmetrical orthographical effect.