- Emeritus Professor of Neuropsychiatry, Institute of Psychiatry, London, UK
- Correspondence to: W A Lishman 9 Elwill Way, Beckenham, Kent BR3 3AB, UK;
The term developmental dyslexia ("specific reading retardation") refers to an unexpected difficulty in reading in children and adults who otherwise possess the intelligence, motivation, and schooling considered necessary for accurate and fluent reading.1 The concept has had a chequered career, with debates about its definition, origin, and causes, and indeed about its validity as a distinct entity. Increasingly, however, psychological and epidemiological investigations have lent it respectability and there are strong indications that genetic influences are at work.2 It is thought to affect between 4 and 7% of children, usually equivalent to a retardation of some 18–24 months in reading relative to expectation.3
A growing consensus highlights problems with phonological processing as a core deficit in the disorder, hence the focus on phonology in the majority of remedial programmes. The phoneme is the smallest identifiable unit of spoken or heard language, individual phonemes being assembled together in the construction of words. An important task in learning to read appears to consist in appreciating the correspondence between such phonemes and their equivalent representations ("graphemes") in written language.
A search is now afoot for possible physiological underpinnings to the condition, and in this, brain imaging has come to make a major contribution, as outlined below.
For obvious reasons, brain imaging was first explored in adults with a history of reading difficulties in childhood, even though by adulthood the problem may have ameliorated. In such subjects, a variety of imaging protocols have shown problems with activation of the language cortex during the performance of reading tasks. Thus, in a ground-breaking positron emission tomography (PET) study, Paulesu et al examined five young men who, although ultimately succeeding academically, had had marked dyslexia in childhood.4 In this sense they were "compensated" dyslexics. Appropriate normal reading controls were used for comparison.