Revisiting the Relationship between Visual Functions and Reading Difficulty: A Large Sample Perspective

Among the various etiologies of developmental dyslexia, the hypothesis of the defected specific subcortical structure, known as the magnocellular (M) pathway, is still contentious. Its supporting evidence is limited in that some task comparisons, which normally show group differences between dyslexic and normal readers, fail to consider the prevalence rate of dyslexia. Such negligence may have missed some poor visual task performers who have normal reading, and the normal vs. dyslexic-at-risk poor visual performers may be equal ratio or larger in the former, making magnocellular hypothesis much less persuasive. So far, this assertion remains untested. In the current study, large sample were collected from the second and third Taiwan elementary school graders, on reading comprehension (as well as their academic performance on mathematics and language for corroboration) and the two often cited M-related tasks: dot coherence (DC, N = 381) and contrast sensitivity (CS, N = 165). The results disagreed with the previously suggested 1 ~ 2:1 poor-DC/CS with normal or with poor reading ratio, suggesting that the real ratios could be larger (4 ~ 8:1). In addition, significant DC-reading and CS-reading, but insignificant DC-CS, correlations were found, indicative of shared yet distinct reading-related mechanisms that are less likely from a single origin. Among other possible but unfavorable explanations (such as test position variability, light conditions during testing, and the students’ visual acuities), these results concur the relations between visual functions and reading, but unsupportive of the magnocellular hypothesis of dyslexia. Lastly, the current study exemplifies an efficient group testing/screening protocol applicable to developmental dyslexia and other research domains.