Reading fluency and word segmentation agreement modulate the benefits of word boundary cues for older readers in traditional Chinese.

Despite having extensive reading experience, older readers suffer from declines in visual acuity and processing speed, which may undermine their ability to segment words in unspaced scripts like Chinese. While using text colors to highlight word boundaries can aid reading and eye movement control for readers of simplified Chinese, it remains unclear whether the benefits extend to older readers, especially those who read the visually more complex traditional Chinese script. This study investigated this question in three conditions: a baseline monocolor condition, a word segmentation condition where words were marked by alternating text colors, and a nonword segmentation condition. By tracking the eye movements of 76 older readers, we found a robust interference effect from nonword segmentation across all reading and oculomotor measures. In contrast, benefits of word segmentation cues were strikingly specific, emerging only for readers with lower vocabulary knowledge and for words with clear, unambiguous boundaries. This reveals that the utility of explicit word boundary cues depends on a dynamic interplay between visual processing and vocabulary knowledge. These findings have important implications. Theoretically, they underscore that models of reading should account for word boundary ambiguity and readers' experience. Practically, the development of assistive reading technologies needs to be tailored to the needs of less proficient readers, who benefit most from external support.