Elsevier

Cortex

Volume 47, Issue 1, January 2011, Pages 117-127
Cortex

Discussion forum
Dyslexia, dysgraphia, procedural learning and the cerebellum

https://doi.org/10.1016/j.cortex.2009.08.016Get rights and content

Introduction

In this review we focus on the developmental disorders of dyslexia (a disorder of reading) and dysgraphia (a disorder of writing), considering their commonalities and differences with a view to reflecting on the theoretical implications. Interest in dysgraphia was stimulated by the distinction between phonological and surface dyslexia (Castles and Coltheart, 1993), which claimed that orthographic problems (spelling) were separable from phonological reading problems. While this distinction has received mixed support (Snowling et al., 1996, Stanovich et al., 1997) it led to a fruitful analysis not only of the underlying causes of orthographic difficulties, but also to the widespread recognition of developmental difficulties in handwriting control (Deuel, 1995, Manis et al., 1996, Sprenger-Charolles et al., 2000). The result of this theoretical and empirical progress is that there are two usages of the term dysgraphia. One takes dysgraphia to refer to errors of writing that are analogous to errors in reading (e.g., surface, phonological or deep dysgraphia corresponding to surface, phonological and deep dyslexia), the other relating to difficulties in handwriting control. Furthermore, despite these attempts at differentiation, there remains some controversy in the literature as to whether motor difficulties in handwriting should be subsumed under the label dyslexia.

This review attempts to tease out the different strands of theoretical research underlying these confusions by analysing explanations of dyslexia and dysgraphia at the cognitive level and the brain level, considering both cortical and sub-cortical systems. First we outline theoretical approaches to developmental dyslexia, introducing causal explanations at the cognitive level, followed by an outline of recent developments in research into motor difficulties in handwriting. We then note the prevalence of comorbidities between developmental disorders, suggesting that this presents both a challenge and a potential stimulus for the disciplines. We then investigate a brain level causal explanation for dyslexia in terms of cerebellar deficit, because it provides a potential explanation of the co-existence of motor skill deficits and phonological deficits in dyslexia. The framework has strengths, but was also strongly criticised by theorists who advocated cortical foci of deficit. A promising further framework that may integrate cortical and sub-cortical accounts and provides a natural explanation for heterogeneity and comorbidity is that of neural systems and procedural learning (Nicolson and Fawcett, 2007). We extend this framework by applying it to dysgraphia, and conclude by arguing that the neural systems level of explanation provides a fruitful unifying framework for the developmental disabilities.

Section snippets

Developmental dyslexia

There is still considerable debate over the diagnosis of developmental dyslexia, but the traditional definition is “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” (World Federation of Neurology, 1968). It is known to be one of the most common of the learning disabilities with a prevalence of at least 4%. Prevalence may rise to as high as 10% or more if

Cognitive level explanations of dyslexia

There is considerable heterogeneity in the skills of dyslexic children, as might be expected given the very large numbers involved. A key strength of the phonological deficit hypothesis is that almost all children with dyslexia do show a core phonological problem, with variable secondary problems (Stanovich, 1988a). Nonetheless, it is important to attempt to explain secondary problems also. Arguably the broadest cognitive level description of the general type of performance difficulty in

Developmental dysgraphia

As noted earlier, dysgraphia is characterised by difficulties in writing. The diagnostic and statistical manual of mental disorders (DSM-IV) definitions dissociated learning disabilities into three categories: reading disability, mathematical disability and ‘disorder of written expression’, together with a fourth category, ‘learning disability not otherwise specified’. Dysgraphia as the disorder of written expression is characterised by “writing skills (that) …are substantially below those

Motor skill and dyslexia

From the first studies of dyslexia, there has been continuing evidence that mild clumsiness is associated with dyslexia. In a review of Orton's writings, Geschwind (1982) noted “… He pointed out the frequency of clumsiness in dyslexics. Although others have commented on this, it still remains a mysterious and not adequately studied problem. It is all the more mysterious in view of the fact that many of these clumsy children go on to successes in areas in which high degrees of manual dexterity

Brain level explanations for dyslexia

Understanding of brain processes has been transformed by functional imaging, with a major outcome being the discovery (Ackermann et al., 1999, Desmond and Fiez, 1998, Leiner et al., 1989) that the cerebellum plays a key role in linguistic and cognitive skills in addition to its well-established role in motor skill and coordination. See Ito (2008) for a recent review of the evidence. The established link between cerebellum and automaticity, and the emerging link between cerebellum and language

Comorbidities between the developmental disabilities

There is a considerable overlap between different developmental disorders, with an apparent ‘comorbidity’ between most (Bishop, 2002, Bishop et al., 2004, Bishop and Snowling, 2004, Fletcher et al., 1999, Gilger and Kaplan, 2001, Gillberg, 2003, Hill, 2001, Jongmans et al., 2003). Long-standing evidence for comorbidity derives from the literature on ADHD and dyslexia. School and clinic-based comorbidity rates for ADHD with dyslexia range from 25% to 40% (Semrud-Clikeman et al., 1992, Shaywitz

The neural systems framework

In a recent analysis (Nicolson and Fawcett, 2007) we have proposed a possible integration between the varied and apparently incompatible frameworks for dyslexia and other learning difficulties by undertaking an analysis of the neural systems underlying the different types of learning.

Five perspectives come together in this analysis. We started from our own analysis of learning, automaticity and the cerebellum in dyslexia, and the difficulties of disentangling the effects of impaired cerebellum

Summary and conclusions

In summary, we have taken an extended tour through dyslexia and other learning disabilities, focusing on the commonalities and differences between dyslexia and dysgraphia. We have put forward the viewpoint that both of these reflect lack of automaticity at the cognitive level, attributable to impairment of procedural learning circuits involving the cerebellum at the neural circuits level, with ‘pure’ dysgraphia involving impairment of the cerebellar-motor circuit and ‘pure’ dyslexia involving

Acknowledgment

We wish to acknowledge the valuable suggestions made by Alan Beaton, the joint editor of this special issue, with regard to both the content and the style of this article.

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