Experts have been trying to elucidate the biological basis of dyslexia since the condition was first identified in 1881 by Oswald Berkhan and described as dyslexia in 1887 by Rudolf Berlin.
Several theories about dyslexia have been suggested over the years. It was in 1896 that W. Pringle Morgan described the condition in a report that was published in the British Medical Journal. The paper was entitled “Congenital Word Blindness” and was about a boy who had not learned to read by the age of 14, despite having a normal level of intelligence and being generally competent in all other areas.
A series of articles on dyslexia were published in several medical journals in the 1890s and early 1900s by an ophthalmologist called James Hinshelwood. Hinshelwood released a book, also called “Congenital Word Blindness” in 1917, in which he suggested that the main problem in dyslexia was a poor visual memory for words and letters. He described how children with the condition tended to write letters the wrong way around or had difficulty reading and spelling words.
Brain injury was another suggested cause of dyslexia but in 1925, Samuel T. Orton wrote that dyslexia was independent of brain injury or damage. He described the condition as “strephosymbolia,” which referred to the twisting of words and how dyslexics find it difficult to match the visual word information with spoken word forms. Along with psychologist and educator Anna Gillingham, Orton developed the educational interventions that formed the basis of the multisensory teaching that is still being used to teach dyslexic children today.
A 1951 thesis by G. Mahec explored the role sight dynamics play in dyslexia. Using a sequence of letters 5 mm in height and spaced 5 mm apart, children with and without dyslexia read the letters form left to right and from right to left. The results showed that children without dyslexia read from left to right more easily, while a large proportion of children who had dyslexia read the letters at equal speed, regardless of the direction. Furthermore, 10% of those with dyslexia even found it easier to read form right to left.
It was only in the 1970s that phonological awareness was identified as playing a major role in dyslexia. In 1979, Galaburda and Kemper reported their findings after observing the post-autopsy brains of dyslexic people. They noted that the language centres in these individuals were different to those of people without dyslexia. This work and further studies by Cohen et al in 1989 suggested that cortical development was impaired during the first sixth months of fetal brain growth in individuals with dyslexia.
In the 1990s, development of neuroimaging techniques such as positron emission tomography and magnetic resonance imaging provided further clues about phonological processing as well as enabling the neural signature of normal reading to be identified.
Reviewed by Sally Robertson, BSc