Letter similarity (i.e., perceptual distance) is a critical measure to better understand letter perception and literacy development. Despite its importance, however, measurements of letter similarity for non-alphabetic scripts are limited, and the shortage of letter similarity for non-alphabetic script interferes with the identification of the universality and the uniqueness of letter perception systems across different scripts. In the present study, we provide a comprehensive matrix of letter similarity for Japanese kana letters (hiragana and katakana). We obtained the discrimination reaction times for simultaneously presented letter pairs and calculated the perceptual distance of 4,278 letter pairs by inversing the time. The matrix showed significant correlations with previously obtained letter similarity for hiragana and katakana. An additional experiment showed that letter pairs for the same sounds (え–エ) produced significantly slower responses compared with those for different sounds (え–コ). However, the differences in reaction times between the same and different sound conditions were smaller than the sequentially presented conditions, suggesting that the matrix was partially attributable to knowledge-based factors (e.g., letter-sound knowledge). This first comprehensive matrix of letter similarity (i.e., perceptual distance) for Japanese kana letters (hiragana and katakana) will be useful for researchers interested in letter perception and literacy development.

字母相似度（即感知距离）是更好地理解字母感知和读写能力发展的关键指标。然而，尽管它很重要，但对非字母文字的字母相似度的测量是有限的，并且非字母文字的字母相似度的缺乏会干扰对不同文字中字母感知系统的普遍性和唯一性的识别。在本研究中，我们为日语假名字母（平假名和片假名）提供了一个综合的字母相似度矩阵。我们获得了同时呈现的字母对的辨别反应时间，并通过时间倒转计算了 4,278 个字母对的感知距离。该矩阵显示出与先前获得的平假名和片假名的字母相似性显着相关。另一项实验表明，与不同声音（え-コ）的字母对相比，相同声音（え-エ）的字母对产生的反应明显更慢。然而，相同和不同声音条件之间的反应时间差异小于顺序呈现的条件，这表明矩阵部分归因于基于知识的因素（例如，字母发音知识）。日文假名（平假名和片假名）字母相似度（即感知距离）的第一个综合矩阵将对对字母感知和读写能力发展感兴趣的研究人员有用。相同和不同声音条件之间的反应时间差异小于顺序呈现的条件，这表明矩阵部分归因于基于知识的因素（例如，字母发音知识）。日文假名（平假名和片假名）字母相似度（即感知距离）的第一个综合矩阵将对对字母感知和读写能力发展感兴趣的研究人员有用。相同和不同声音条件之间的反应时间差异小于顺序呈现的条件，这表明矩阵部分归因于基于知识的因素（例如，字母发音知识）。日文假名（平假名和片假名）字母相似度（即感知距离）的第一个综合矩阵将对对字母感知和读写能力发展感兴趣的研究人员有用。