An evaluation of the cerebellar deficit hypothesis of developmental dyslexia

Abstract

The phonological processing deficit (PPD) hypothesis is the most influential theory proposed to explain the underlying cause of developmental dyslexia. However, alternative hypotheses have proposed that phonological deficits are just one manifestation of a more general learning disorder affecting other sensory, motor and cognitive functions. The cerebellar deficit hypothesis, proposes that, due to mild cerebellar dysfunction, dyslexic individuals demonstrate a general automatisation deficit on both motor and cognitive tasks. Individuals with cerebellar damage demonstrate deficits in temporal estimation ability, postural stability, muscle tone and coordination. It is claimed that similar deficits, comparable in magnitude with reading and spelling problems, have been found in dyslexic individuals.
Following a review of the literature, performance on a series of phonological, balance, auditory discrimination and cerebellar finger movement tests are measured in a group of boys, age 7-14, with a previous diagnosis of dyslexia. Results are compared to the performance of chronological-age and reading-age matched controls. Differences in performance relating to dyslexia severity, teacher ratings of inattention or ADHD characteristics and ability levels are also considered. Whilst there is strong support for the phonological processing deficit hypothesis (and double deficit hypothesis), there is no conclusive support for the cerebellar deficit hypothesis. Results fail to confirm significant deficits on measures of balance, temporal estimation ability or cerebellar finger movement tests. Test performance can equally be explained in terms of dyslexia or teacher ratings of inattention and/or ADHD characteristics. Increasing age and ability significantly moderate test performance. A fourth study investigating attention switching ability in students does, however, provide tentative support for the cerebellar deficit hypothesis, revealing a lack of autornatisation of shape recognition in dyslexic rather than attention deficit individuals.
The thesis concludes with an overall discussion of the implications of the results in terms of causal theories of dyslexia and its interaction with other learning disorders. Methodological issues relating to study design, definitions of learning disorders,
selection, inclusion and motivation of participants are considered. A theoretical basis for additional intervention programs within the education system is also discussed and the practical implications of implementing such strategies are examined.