Linking visual span and reading speed in dyslexia.

This study examines visual processing in dyslexia using a psychophysical measure linked to reading speed, the visual span, defined as the number of letters reliably identified within a single fixation. We asked whether reduced visual span is associated with slower reading in dyslexia. In a laboratory study, adults with dyslexia and typical readers completed standardized assessments, reading speed, and visual span measurements. Individuals with dyslexia showed significantly slower reading speeds and narrower visual span profiles than typical readers. Visual span size was positively correlated with reading speed across participants. The paradigm was then adapted for online testing. In a validation study with typical readers, online visual span and reading speed test showed patterns consistent with laboratory patterns, including a strong positive relationship between visual span size and reading speed. Applying this paradigm to a larger online sample of individuals with self-reported dyslexia revealed markedly slower reading speeds, reduced visual span sizes, and a robust correlation between visual span and reading speed despite substantial individual variability. Together, these findings provide the first direct evidence that reduced visual span is a reliable characteristic of dyslexia and is closely linked to reading performance. Moreover, they demonstrate that a brief, psychophysically grounded visual span assessment can be implemented reliably online, enabling scalable investigation of visual factors in dyslexia across diverse populations.