A neural instantiation of the motor theory of speech perception Comment from Richard B. Ivry and Timothy C. Justus to Nicolson et al.

The dissociation of articulatory rehearsal and phonological representation also makes clear that there could be multiple neural bases for dyslexia. Poorly developed phonological representations could result from (1) an impairment in articulation, (2) the consolidation of such representations, or (3) the access of these representations from systems involved in the conversion of orthography to phonology. The striking degree of clumsiness in dyslexia 12xPerformance of dyslexic children on cerebellar and cognitive tests. Fawcett, A.J and Nicolson, R.I. J. Mot. Behav. 1999; 31: 68–78Crossref | PubMedSee all References12, in addition to the impairments on temporal processing tasks associated with the cerebellum 13xTime-estimation deficits in developmental dyslexia: evidence of cerebellar involvement. Nicolson, R.I et al. Proc. Roy. Soc. London Series B. 1995; 259: 43–47Crossref | PubMed | Scopus (133)See all References13 are consistent with the idea that an articulatory problem is at the head of the causal chain.However, it might also be that these children exhibit a host of subclinical neural abnormalities, only one of which involves the cerebellum. Unlike acquired language disorders such as alexia without agraphia in which, by definition, the deficit is restricted, people with developmental dyslexia typically exhibit below normal performance on a host of linguistic and non-linguistic tasks. Even if future imaging studies were to identify structural abnormalities in the cerebellum, it is probable that abnormalities will also be found in other brain areas. Anatomical studies using in vivo MRI have identified cerebellar hypoplasia in several psychiatric disorders including autism 14xBrainstem, cerebellar and limbic neuroanatomical abnormalities in autism. Courchesne, E. Curr. Opin. Neurobiol. 1997; 7: 269–278Crossref | PubMed | Scopus (259)See all References14, schizophrenia 15xAn MRI study of cerebellar vermis morphology in patients with schizophrenia: evidence in support of the cognitive dysmetria concept. Nopoulos, P.C et al. Biol. Psychiatry. 1999; 46: 703–711Abstract | Full Text | Full Text PDF | PubMed | Scopus (136)See all References15, and attention deficit and hyperactivity disorder 16xCerebellum in attention-deficit hyperactivity disorder: a morphometric MRI study. Berquin, P.C et al. Neurology. 1998; 50: 1087–1093Crossref | PubMedSee all References16. Correspondingly, causal accounts based on the cerebellar abnormalities have been proposed for autism and schizophrenia. Nonetheless, the pathology does not appear to be restricted to the cerebellum: structural differences have also been found in cortical regions.What then are we to make of this revisionist literature in which the cerebellum is suddenly being elevated from the low-level slave of the motor system to the key link of disorders as varied as schizophrenia and dyslexia? A sceptical position would be to argue that the cerebellar impairment is a correlate of the disorders, but not causal. For unknown reasons, the cerebellum is especially sensitive to problems that occur during neurodevelopment, but abnormalities in other, cortical regions are pre-eminent. Interestingly, the hypoplasia in autism, schizophrenia, and attention-deficit hyperactive disorder (ADHD) is associated with different cerebellar lobules. Knowing the time course of cerebellar neurogenesis and maturation might provide clues to windows during which the development of the CNS goes awry 17xEmbryonic development of the rat cerebellum. III. Regional differences in the time of origin, migration, and settling of Purkinje cells. Altman, J and Bayer, S. J. Comp. Neurol. 1985; 231: 42–65Crossref | PubMedSee all References17.Alternatively, it is an important enterprise to consider causal accounts that include the cerebellum. The target article offers a fine example of how these can be developed and tested. Nicolson et al.'s working hypothesis for dyslexia builds upon a well-specified psychological model and, by emphasizing articulation as a skilled motor process, connects with more-traditional views of the cerebellum. As such, the ideas resonate with the central premise of the motor theory of speech perception and this theory's more recent progeny (e.g. mirror neurons, see Ref. 18xLanguage within our grasp. Rizzolatti, G and Arbib, M.A. Trends Neurosci. 1998; 21: 188–194Abstract | Full Text | Full Text PDF | PubMed | Scopus (1566)See all ReferencesRef. 18). Perception and action are intimately linked, interactive processes. Our knowledge, be it linguistic, perceptual, or conceptual is constrained by the actions we can produce, for it is their production, as well as our ability to understand the actions of others, that renders this knowledge adaptive.