Visual magnocellular impairment in adult developmental dyslexics

Previous research has suggested that visual magnocellular impairment may be characteristic of up to 75% of developmental dyslexics. In this study we c ompared 18 adult dyslexics and 18 controls on two tasks of putative visual magnocellular function. We examined whether these tasks could discriminate dyslexics from controls and also the relationship between these measures an d nonword reading, a sensitive measure of phonological awareness. Our resul ts showed that dyslexics were significantly less sensitive than controls fo r detection of coherent motion in random-dot kinematograms (RDKs) and also the highest frequency at which temporal modulation at full contrast was det ectable, the critical flicker fusion frequency (CFF). Across the two groups and within each group examined separately, motion and flicker sensitivity correlated strongly with nonword reading ability. Together, the temporal pe rception measures were able to discriminate 72.7% of the dyslexics from con trols, so this type of visual deficit may be an important feature of dyslex ia. Our results support the hypothesis that dyslexics' reading problems are not entirely caused by a specific deficit in language processing. These vi sual deficits are also found in younger subjects; hence visual temporal per ception measures may be used to identify children at risk for dyslexia prio r to actual reading failure.