Japanese readers show a crowding reduction even in vertically oriented strings of letters

Visual crowding affects reading performances. Related to this, the visual crowding effect is weaker in letter reading than in recognizing symbols in a string. A previous study reported that crowding reduction was observed exclusively in horizontal strings by participants with extensive reading experience in horizontal text strings. In the present study, we examined letter identification performance across various string orientations among Japanese readers with extensive experience reading both vertically and horizontally oriented texts. Through three experiments, we observed crowding reduction among Japanese readers in vertical test strings. Additionally, the observed crowding reduction was not robust when reading strings along unfamiliar orientations, such as diagonal orientations. These findings suggest that reading experience with specific text orientations adaptively shapes the spatial properties of letter-specific detection units. However, our results also indicate that reading experience has limited influence. For example, crowding reduction was not as robust for Japanese letters as it was for alphabet letters, showing that the influence of reading experience on crowding reduction depends on letter type, whereas letter type in previous reading does not matter. Furthermore, when the strings aligned with the zone where stronger crowding occurs, irrespective of letter type, crowding reduction was not robust, even for alphabet. These results imply that the reading experience could affect only a specific part of letter identification, which is likely important for alphabet identification in higher-level processing.