The possible relationship between visual deficits and dyslexia: examination of a critical assumption.

Numerous studies have found that visual deficits are associated with dyslexia. The prevailing theory regarding this association is that dyslexia is the result of a deficit in the magnocellular system (earlier called the transient system) in the visual pathway. An essential assumption of this theory is that the parvocellular system (formerly called the sustained system) is suppressed by the magnocellular system at the time of saccadic eye movements. This assumption is examined on the basis of published studies of saccadic suppression. The evidence from six studies indicates quite unequivocally that the magnocellular system, not the parvocellular system, is suppressed during saccadic eye movements. It seems, therefore, that an essential premise of the magnocellular deficit theory of dyslexia is incorrect.