Unfixed reference, monocular occlusion, and developmental dyslexia--a critique.

Stein and Fowler have proposed that poor binocular control of vergence eye movements is responsible for reading problems in a subset of dyslexic children, and that this subgroup is characterised by unstable performance on Dunlop's reference eye test. Four predictions from this hypothesis are evaluated in the light of published evidence. First, it is shown that a substantial minority of good readers have unfixed reference. Second, the evidence for a raised prevalence of unfixed reference in dyslexics is reviewed and contradictory findings are discussed. Third, it is argued that there is little support for the view that dyslexics with unfixed reference make different types of reading errors from those with fixed reference: indeed many dyslexics with unfixed reference have non-visual, phonological difficulties. Finally, it is argued that studies which claim that monocular occlusion is a successful treatment for 'visual dyslexia' are methodologically flawed and do not provide convincing evidence for this view.