Lateral biases in the representation of the mental number line among healthy Japanese young adults

Studies have presented evidence for mental number representation in Western cultures, where smaller numbers are assumed on the left and larger numbers on the right.Writing systems to represent numbers vary according to culture.Although, in Japan, Arabic numerals are primarily used,traditional Kanji and Kana characters are also employed.However, studies investigating mental number line representation involving Kanji and Kana characters are lacking. This study examined the characteristics of the mental number line in Japanese culture using a numerical bisection task.A total of 36right-handed Japanese university students participated in this study. Each participant was asked to judge which flanker number in a triplet was farther from the middle number (e.g., 19-42-55). Similar trials were conducted for each digit triplet of Arabic, Kanji, and Kana numerals.Despite changing the spatial configuration of the stimuli, the participants consistently overestimated the numerical length to the left for each digit triplet of Arabic numerals. Although the sequence effect was a leftward bias for descending sequences than for ascending sequences, no numerical distance effect was observed, in which the leftward bias would be stronger for a higher numerical distance between the stimuli. Although a similar leftward bias was observed for Kanji numerals, no unidirectional bias was found for Kana numerals.The reason a leftward bias was not observed for Kana numerals is that, unlike Arabic and Kanji numerals, they were considered in relation to the nature of number notation in Japanese.