There is a new article posted in Nature Neuroscience (sorry, not available free online yet) about Dyslexia and visual perception. The article below heralds "Dyslexia Redefined", but that's hype. The research provides more evidence that dyslexia involves perceptual abnormalities in vision...so not sound only. More specifically, Seidenberg and colleagues at Georgetown found that dyslexics had problems perceiving visual stimuli in the presence of visual 'noise'. They hypothesize that these problems could be part of a general problem in noise exclusion (sound discrimination problems), or speculate that it might account for why dyslexics have trouble with visual crowding (seeing all the letters at once).
Visual and Perceptual Difficulties in Dyslexia News