Developmental Dyslexia

The possible causes and correlates of such difficulty have been the focus of a great deal of theorizing since before the turn of the twentieth century, when W. Pringle Morgan (1896) described a 14-year-old boy named Percy who suffered from pronounced difficulty learning to read and spell, despite normal achievement in other academic areas. Because he could find no evidence of definitive brain injury that might have caused the boy’s reading and spelling problems, Morgan theorized that these problems were caused by a congenital defect that resulted in difficulty in storing visual impressions of words. James Hinshelwood (1900, 1917) held a similar view and provided the field with the first extensive description of reading impairment in otherwise normal children, which he characterized as “congenital word blindness.” The writings of Morgan and Hinshelwood called initial attention to the possibility that reading difficulties in some children may represent a neurodevelopmental disorder affecting cognitive abilities underlying the ability to learn to read, rather than frank brain injury or environmental causes such as a limited home background or inadequate instruction. Serious consideration was given to this possibility in Samuel T. Orton’s seminal monograph in 1925 describing reading difficulties and correlated symptom patterns in poor readers judged to be afflicted by what he called strephosymbolia (“twisted symbols”). Orton believed strephosymbolia to be a perceptual disorder manifested in an aberrant tendency to perceive visual symbols as reversed images (“seeing” b as d or was as saw) and suggested that such difficulties are caused by a developmental delay in the establishment of hemispheric dominance. Delay in establishing hemispheric dominance was said to disrupt development of the child’s ability to suppress mirror image counterparts of letters and words and this was presumed to cause optical reversibility in visual perception along with letter orientation and letter sequencing errors in oral reading and writing (Vellutino, 1979). 19