The magnocellular deficit theory of dyslexia: the evidence from contrast sensitivity.

A number of authors have made the claim that dyslexia is the result of a deficit in the magnocellular part of the visual system. Most of the evidence cited in support of this claim is from contrast sensitivity studies. The present review surveys this evidence. The result of this survey shows that the support for the magnocellular deficit theory is equivocal. In the case of spatial contrast sensitivity there clearly are results that are consistent with the magnocellular deficit theory; however, these results are outnumbered both by studies that have found no loss of sensitivity and by studies that have found contrast sensitivity reductions that are inconsistent with a magnocellular deficit. Many of the studies of temporal contrast sensitivity are also difficult to reconcile with a magnocellular deficit. The evidence from studies of contrast sensitivity is therefore highly conflicting with regard to the magnocellular system deficit theory of dyslexia.