In “Dyslexia, Dysgraphia, Procedural Learning and the Cerebellum,” Roderick Nicolson and Angela Fawcett present a fascinating and, to me, strong argument for unifying theoretical views of dyslexia and dysgraphia. To be sure, their analysis is preliminary and basic, but my first read left me feeling as if they’d hit lots of good points. They’ve emphasized impairment of automatic procedural learning in the cerebellum at the level of neural circuits, but in dysgraphia the problems are with motor circuits and in dyslexia they are in the language circuits. In developing their case, they integrate a broad range of neurological and psychological research.
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] and [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] and Sprenger-Charolles et al., 2000 L. Sprenger-Charolles, P. Cole, P. Lacert and W. Serniclaes, On subtypes of developmental dyslexia: Evidence from processing time and accuracy scores, Canadian Journal of Experimental Psychology-Revue Canadienne De Psychologie Experimentale 54 (2000), pp. 87–104. Abstract | Full Text via CrossRef[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.
For those of us who find theory helpful in advancing thinking about Learning Disabilities, this is a worthwhile read.
Nicolson, R. I., & Fawcett, A. J. (2011). Dyslexia, dysgraphia, procedural learning and the cerebellum. Cortex, 47, 117-127. http://dx.doi.org/10.1016/j.cortex.2009.08.016