Visual magnocellular impairment in adult developmental dyslexics
Talcott JB., Hansen PC., Willis-Owen C., McKinnell IW., Richardson AJ., Stein JF.
Previous research has suggested that visual magnocellular impairment may be characteristic of up to 75% of developmental dyslexics. In this study we compared 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 and nonword reading, a sensitive measure of phonological awareness. Our results showed that dyslexics were significantly less sensitive than controls for detection of coherent motion in random-dot kinematograms (RDKs) and also the highest frequency at which temporal modulation at full contrast was detectable, 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 perception measures were able to discriminate 72.2% of the dyslexics from controls, so this type of visual deficit may be an important feature of dyslexia. Our results support the hypothesis that dyslexics' reading problems are not entirely caused by a specific deficit in language processing. These visual deficits are also found in younger subjects; hence visual temporal perception measures may be used to identify children at risk for dyslexia prior to actual reading failure.