Cerebral dominance in cases of reading disability.

In an extensive review of the literature on reading disability Hallgren (1950) finds a consensus of opinion among most authors that the incidence of severe cases of dyslexia among otherwise normal school-children is about 1% ; and this agrees well with the most recent findingin this country, reported in the Ministry of Education (1950) pamphlet " Reading Ability ", that 1-4% of children leaving school were illiterate. Hinshelwood (1917) described individual cases of this disability under the designation "congenital word blindness ", but it was Orton (1937) who attracted attention to the disturbances of lateral dominance in the patients he studied. (I am inclined to subscribe to Orton's and Hallgren's criticism of this term, and to agree with the latter that the diagnosis " specific dyslexia " is suitably obscure, adequately descriptive, and carries no theoretical implications.) Orton observed that hand or eye preferences were often not consistently unilateral; and set up the hypothesis that "the existence of demonstrable mixtures between right and left motor preferences ... implies that comparable intergrading may exist between the critical areas for the various fractions of the language faculty in the two hemispheres of the brain, thus giving rise to a series of developmental disorders in language" (1937). These disorders arise because "learning to read ... entails the elision of one of the two antitropic records or engrams (in opposite hemispheres), and faulty or incomplete elision would result in uncertainty in mnemonic recall of orientation and progression" (1943). Subsequently, a number of investigations has been reported of the lateral preferences of dyslexic children, the evidence in some cases supporting, and in others opposing, Orton's interpretation. Although not confirmed by Hallgren's investigation, perhaps the most consistent observation has been that a high proportion of these patients show a left-eye preference (Macmeeken, 1939) or cross dominance, right-handed but left-eyed (Harris, 1947). Some authors (e.g. Wile, 1942; Blau, 1946) contend that right-eye dominance is more conducive to the left-