Discussion forumDyslexia, dysgraphia, procedural learning and the cerebellum
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.
References (133)
- et al.
Cerebellar contributions to the perception of temporal cues within the speech and nonspeech domain
Brain and Language
(1999) - et al.
ADHD and dysgraphia: Underlying mechanisms
Cortex
(2007) Dyslexia and the cerebellar deficit hypothesis
Cortex
(2002)- et al.
Inflammation in areas of remote changes following focal brain lesion
Progress in Neurobiology
(2005) - et al.
The role of the basal ganglia and cerebellum in language processing
Brain Research
(2007) - et al.
Prisms throw light on developmental disorders
Neuropsychologia
(2007) - et al.
Developmental coordination disorder: Exploration of a cerebellar hypothesis
Human Movement Science
(2007) - et al.
Varieties of developmental dyslexia
Cognition
(1993) - et al.
Developmental dyslexia
Lancet
(2004) - et al.
Rapid ‘automatized’ naming (R.A.N.). Dyslexia differentiated from other learning disabilities
Neuropsychologia
(1976)
Neuroimaging studies of the cerebellum: Language, learning and memory
Trends in Cognitive Sciences
Distinct contribution of the cortico-striatal and cortico- cerebellar systems to motor skill learning
Neuropsychologia
The visual deficit theory of developmental dyslexia
NeuroImage
Comorbidity of learning and attention disorders – separate but equal
Pediatric Clinics of North America
The effect of damage to the cerebellum on sensorimotor and cognitive function in children and adolescents
Neuroscience and Biobehavioural Reviews
Anatomical risk factors that distinguish dyslexia from SLI predict reading skill in normal children
Journal of Communication Disorders
On the bases of two subtypes of development dyslexia
Cognition
The lateralized linguistic cerebellum: A review and a new hypothesis
Brain and Language
Automaticity: a new framework for dyslexia research?
Cognition
Procedural learning difficulties: Re-uniting the developmental disorders?
Trends in Neurosciences
Association of abnormal cerebellar activation with motor learning difficulties in dyslexic adults
Lancet
Developmental dyslexia: the cerebellar deficit hypothesis
Trends in Neurosciences
From single to multiple deficit models of developmental disorders
Cognition
On the automaticity/cerebellar deficit hypothesis of dyslexia: Balancing and continuous rapid naming in dyslexic and adhd children
Neuropsychologia
Cerebellar morphology in developmental dyslexia
Neuropsychologia
The basal ganglia: A vertebrate solution to the selection problem?
Neuroscience
Comorbidity between ADHD and learning disability. A review and report in a clinically referred sample
Journal of the American Association of Child and Adolescent Psychiatry
Reading-disabled students with and without comorbid arithmetic disability
Developmental Neuropsychology
Diagnostic and Statistical Manual of Mental Disorders
Guidelines for teachers, parents and learners
Can the wppsi be of aid in identifying young children at risk for reading disability
Journal of Learning Disabilities
Reading disability in an epidemiological context: Incidence and environmental correlates
Journal of Learning Disabilities
Cerebellar lesions and prism adaptation in macaque monkeys
Journal of Neurophysiology
Dyslexia, Reading and the Brain: a Sourcebook of Psychological and Biological Research
Poor penmanship in children correlates with abnormal rhythmic tapping: A broad functional temporal impairment
Journal of Child Neurology
Some conclusions about dyslexia
Motor immaturity and specific speech and language impairment: Evidence for a common genetic basis
American Journal of Medical Genetics
Using nonword repetition to distinguish genetic and environmental influences on early literacy development: A study of 6-year-old twins
American Journal of Medical Genetics Part B-Neuropsychiatric Genetics
Developmental dyslexia and specific language impairment: Same or different?
Psychological Bulletin
M-stream deficits and reading-related visual processes in developmental dyslexia
Psychological Bulletin
Categorising sounds and learning to read: a causal connection
Nature
Motor coordination in children with dyslexia: Theoretical and clinical implications
Motor proficiency in dyslexic children with and without attentional disorders
Archives of Neurology
Developmental dysgraphia and motor-skills disorders
Journal of Child Neurology
Reorganization and plasticity in the adult brain during learning of motor skills
Current Opinion in Neurobiology
Functional anatomy of motor skill learning
Anatomical correlates of dyslexia: Frontal and cerebellar findings
Brain
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