The neurobiology of reading difficulties.

Despite the high incidence of reading difficulties, which may trouble up to 10% of all children, there is no agreement about their causes. This controversy is encapsulated in arguments about whether extreme reading difficulties should be noncommittally called `specific reading difficulty' or, using the neurological description `developmental dyslexia'. Specific reading difficulty is preferred by those who believe reading problems are purely linguistic, whereas developmental dyslexia implies commitment to the idea that reading difficulties result from impaired neurological development. In our laboratory we take the latter view. We study the neurophysiological processes which underlie reading; hence we espouse the neurological model. Therefore, in this chapter I will discuss how normal reading depends on the quality of its sensory input. It requires both a highly sensitive visual magnocellular system to acquire good orthographic skills, and a sensitive auditory transient system to parse the phonological structure of words. Then I will speculate about the genetic and immunological mechanisms that may be responsible for the wide variety of abnormalities that are seen in developmental dyslexics. My overall conclusion will be that reading difficulties are neither specific to reading nor exclusively linguistically based, but a consequence of mildly impaired development of a particular kind of neurone in the brain, magnocellular neurones, so that dyslexia has widespread manifestations which are not at all confined to reading. However they are best thought of as individual differences between people rather than a consequence of neurological `disease'.