DYSLEXIA: DEVELOPMENTAL

Delayed acquisition of reading skills and/or the adoption of reading processes which deviate markedly from those that are generally observed in children.

 The first stage of learning to read, based purely on sight-reading, does not appear to be critical. In the second, children use parts of words to form matches (often inaccurate) with known written forms. There is evidence that some dyslexics may not achieve this kind of analytic processing. At the third stage, phonics begins to play a part: the child establishes a set of grapheme-phoneme correspondence rules which enable it to deconstruct written words into sounds. Many dyslexics appear to experience problems at this point, which result in delay or deviance in their reading. An important indication is whether, at the age of about 8, the child is able to transcribe non-words. Reading difficulties of this type are characterised as phonological dyslexia.

The final stage, especially important for orthographies like English which are not transparent, involves the ability to achieve whole-word matches, and possibly the associated ability to trace analogies between word rimes (LEAD (n.) with HEAD; LEAD (v.) with BEAD). Here, dyslexics may experience problems with spellings that permit of two interpretations (e.g. PINT/MINT) and with homophones such as SAIL/SALE. This type is sometimes termed surface dyslexia.

 Developmental dyslexia varies enormously between individuals. Some show strong signs of a phonological impairment, some of a surface impairment; but most cases represent a combination of both. Clear cases of the semantic errors which characterise acquired deep dyslexia are not common.

 One research approach is to compare the performance of dyslexic children with that of children of a similar reading age. Dyslexics tend to perform less well on phonological tasks, naming tasks and tasks involving working memory.

Another method is to treat reading as a set of sub-skills and to analyse a child’s performance in terms of which sub-skills form part of its reading repertoire and which are absent. There have also been attempts to trace connections between dyslexia and other cognitive processes related to language. Links are sought with poor higher-level comprehension and short phonological and visual memory. Some phonological dyslexics may have problems with the rapid processing of speech sounds; it has been suggested that this may lead to difficulty in sequencing the sounds they hear.

There may be a genetic factor in developmental dyslexia. The children of parents with reading problems are more likely to experience problems themselves; and there are recorded cases of similar types of dyslexia in twins but not in other family members. Word-level problems (SAIL/SALE) do not appear to be inherited, whereas difficulties with phonological processing (e.g. the ability to read non-words) may be.

Recent neurological evidence supports a view that developmental dyslexia may be partly attributable to differences in brain configuration. In non-dyslexics, an area of the brain known as the planum temporale tends to be larger in the left (language-associated) hemisphere than the right. However, in many dyslexics the two appear to be the same size. There have also been suggestions that some dyslexics manifest a larger right-hemisphere, indicating a bias towards higher level language processing rather than decoding. Dyslexia appears to be more common in left-handers, who sometimes manifest an unusual right brain lateralisation for language.

See also: Disorder, Dysgraphia: developmental, Dyslexia: acquired

 Further reading: Ellis (1993); Harris and Coltheart (1986); Miles (1993)

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