TRENDS in Neurosciences Vol.24 No.9 September 2001 http://tins.trends.com 0166-2236/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: 508 Opinion Opinion http://tins.trends.com 0166-2236/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0166-2236(00)01896-8 Opinion Opinion Developmental dyslexia is traditionally defined 1 as ‘a disorder in children who, despite conventional classroom experience, fail to attain the language skills of reading, writing and spelling commensurate with their intellectual abilities’. Dyslexia researchers have focused on two alternative hypotheses: the phonological deficit account 2–4 , which holds that the reading difficulties derive initially from problems in breaking spoken words down into their constituent sounds (syllables or phonemes), and the magnocellular deficit account, which holds that the reading problems derive from impaired sensory processing, caused by abnormal auditory 5 and/or visual 6,7 magnocellular pathways. Unfortunately, in spite of extensive research, these approaches have failed to account for the full range of difficulties established for dyslexic children. It is therefore timely to present the case for our alternative hypothesis that the full range of deficits might be accounted for in terms of cerebellar deficit. There is no space here to survey the rich evidence relating to alternative hypotheses. We fully expect the commentators to present cogent alternative data and views. In preview, we claim that: (1) The behavioural symptoms of dyslexia can be characterized as difficulties in skill automatisation 8,9 (the process by which, after long practice, skills become so fluent that they no longer need conscious control). (2) The pattern of difficulties in cognitive, information processing and motor skills is predicted by the cerebellar deficit hypothesis 9–12 . (3) Dyslexic adults showing the above behavioural manifestations of cerebellar impairment also show direct neurobiological evidence of cerebellar impairment 13 . This is consistent with other evidence of cerebellar abnormalities in dyslexia 14 . A TINS debate – Hindbrain versus the forebrain:a case for cerebellar deficit in developmental dyslexia Progressive improvement in reading and writing skills, through school and beyond, is something that many of us take for granted. However, for people suffering from dyslexia, these skills are not acquired in the usual manner. For many years it has been thought that brain differences in the cortical areas related to language are the likely source of the problems. However, it has recently been established that the problems associated with this syndrome go beyond reading-related problems: balance, motor skills and sensory processes can also be affected. An explanation for this multitude of seemingly disparate problems has proved elusive. Roderick Nicolson, Angela Fawcett and Paul Dean believe that a deficit in cerebellar performance might provide a complete explanation, and it is this argument, presented in the first article below, that forms the focus of this debate. But can cerebellar deficit explain all deficits experienced by dyslexics? Are those that experience cerebellar damage later in life similarly affected? Is the cerebellum the sole contributor to dyslexia? Two pairs of experts in this field, Thomas Zeffiro and Guinevere Eden, and Richard B. Ivry and Timothy C. Justus, discuss these and other questions. It becomes clear during the debate that the acquisition of reading-related skills requires the co-ordination of many areas of the brain involved in visual, motor and cognitive activities, and that an increased understanding of dyslexia could provide insights far beyond the disorder itself. The conclusion to this debate is provided by Roderick Nicolson and his colleagues. Developmental dyslexia:the cerebellar deficit hypothesis Roderick I. Nicolson, Angela J. Fawcett and Paul Dean Surprisingly, the problems faced by many dyslexic children are by no means confined to reading and spelling. There appears to be a general impairment in the ability to perform skills automatically, an ability thought to be dependent upon the cerebellum. Specific behavioural and neuroimaging tests reviewed here indicate that dyslexia is indeed associated with cerebellar impairment in about 80% of cases. We propose that disorders of cerebellar development can in fact cause the impairments in reading and writing characteristic of dyslexia, a view consistent w ith the recently appreciated role of the cerebellum in language-related skills. This proposal has implications for early remedial treatment.