(Caveat: here I am speaking of children who have normal vision, or normal corrected vision). I
Advocates for visual treatment of dyslexia (through special glasses, vision therapy, tinted lenses and overlays, and so on) make the case that there is a visual basis for reading difficulties. don't think there is a visual component to dyslexia or reading difficulty.
Here is an interesting and relatively new piece of research that suggests that vision isn't involved in dyslexia:
http://www.eric.ed.gov/ERICWebPortal/search/recordDetails.jsp?searchtype=keyword&pageSize=10&ERICExtSearch_SearchValue_0=visual+dyslexia&eric_displayStartCount=1&ERICExtSearch_SearchType_0=kw&_pageLabel=RecordDetails&objectId=0900019b804400c3&accno=EJ886840&_nfls=false
Ziegler, Johannes C.; Pech-Georgel, Catherine; Dufau, Stephane; Grainger, Jonathan Developmental Science, v13 n4 pF8-F14 Jul 2010
Rapid Processing of Letters, Digits and Symbols: What Purely Visual-Attentional Deficit in Developmental Dyslexia?
Visual-attentional theories of dyslexia predict deficits for dyslexic children not only for the perception of letter strings but also for non-alphanumeric symbol strings. This prediction was tested in a two-alternative forced-choice paradigm with letters, digits, and symbols. Children with dyslexia showed significant deficits for letter and digit strings but not for symbol strings. This finding is difficult to explain for visual-attentional theories of dyslexia which postulate identical deficits for letters, digits and symbols. Moreover, dyslexics showed normal W-shaped serial position functions for letter and digit strings, which suggests that their deficit is not due to an abnormally small attentional window. Finally, the size of the deficit was identical for letters and digits, which suggests that poor letter perception is not just a consequence of the lack of reading. Together then, our results show that symbols that map onto phonological codes are impaired (i.e. letters and digits), whereas symbols that do not map onto phonological codes are not impaired. This dissociation suggests that impaired symbol-sound mapping rather than impaired visual-attentional processing is the key to understanding dyslexia.