The role of certain genes, particularly FOXP2, in the development of language is mentioned in Chap. 22 and summarized by Konopka and Roberts. Here it is pointed out that there is an isolated developmental verbal dyspraxia that is caused by a point mutation in this gene but that other disorders, such as dyslexia, have not yielded clearly to genetic analysis as discussed in the following text. However, Vernes and colleagues have found that FOXP2 downregulates a gene (CNTNAP2) that encodes neurexin in the developing cortex. Polymorphisms of the gene are found in children with a number of specific but seemingly unrelated language deficits. They propose that this is a mechanistic link between different developmental language syndromes.
This condition, first described by Hinshelwood in 1896, becomes manifest in an older child who is found to lack the aptitude for one or more of the specific skills necessary to derive meaning from the printed word. Also defined as a significant discrepancy between “measured intelligence” and “reading achievement” (Hynd et al), it has been found in 3 to 6 percent of all schoolchildren. There have been several excellent writings on the subject over the past century, to which the interested reader is referred for a detailed account (Orton; Critchley and Critchley; Rutter and Martin; Shaywitz; Rosenberger).
The main problem is an inability to read, spell, and to write words despite the ability to see and recognize letters. There is no loss of the ability to recognize the meaning of objects, pictures, and diagrams. According to Shaywitz, these children lack an awareness that words can be broken down into individual units of sound and that each segment of sound is represented by a letter or letters. This has been summarized as a problem in “phonologic processing,” referring to the smallest unit of spoken language, the phoneme, and as a parallel inability of dyslexic individuals to appreciate a correspondence between phonemes and their written representation (graphemes). A defect in the decoding of acoustic signals is one postulated mechanism. In addition to the essential visuoperceptual defect, some individuals also manifest a failure of sequencing ability and altered cognitive processing of language. De Renzi and Luchelli have found a deficit of verbal and visual memory in some affected children as noted below.
Much of what has been learned about dyslexia applies to native speakers of English more than to those who speak Romance languages. English is more complex phonologically than most other languages; for example, it uses 1,120 graphemes to represent 40 phonemes, in contrast to Italian, which uses 33 graphemes to represent 22 phonemes (see Paulesu et al). Children with native orthographic languages, such as Chinese and Japanese, apparently have a far lower incidence of dyslexia.
Often, before the child enters school, reading failure can be anticipated by a delay in attending to spoken words, difficulty with rhyming games, and speech characterized by frequent mispronunciations, hesitations, and dysfluency; or there may be a delay in learning to speak or in attaining clear articulation. In the early school years there are difficulties in copying, color naming, and formation of number concepts as well as the persistent reversal of letters. The child’s writing reflects faulty perception of form and a kind of constructional and directional apraxia. Often, there is an associated vagueness about the serial order of letters in the alphabet and months in the year, as well as difficulty with numbers (acalculia) and an inability to spell and to read music. The complex of dyslexia, dyscalculia, finger agnosia, and right-left confusion is found in a small number of these children and is interpreted as a developmental form of the Gerstmann syndrome described in Chap. 21.
Lesser degrees of dyslexia are found in a large segment of the school population and are more common than the severe ones. Approximately 10 percent of schoolchildren in some surveys have some degree of this disability. The disorder is stable and persistent; however, as a result of effective methods of training, only a few children are unable to read at all after many years in school.