A Morphological Analysis of the Cerebellar Deficit Hypothesis of Dyslexia Developmental dyslexia has been defined as a disorder in children who fail to attain reading and spelling skills commensurate with their intellectual abilities despite conventional classroom experience1. Along with deficits in reading, children with dyslexia tend to display a variety of cognitive and visual-motor deficits in one or more of the following areas: phonological awareness, rapid phonological retrieval, information processing speed, automaticity, motor skills, and balance2. Some also present with reduced verbal short-term/working memory3, 4 and linguistic functioning5. Most causal theories of dyslexia make a satisfactory attempt at explaining the primary behavioral symptom – poor word identification; however, deficits frequently go beyond reading and may vary across individuals. Thus, the crucial test of these theories needs to address common neuropsychological deficits in addition to poor reading ability2.

Currently, there are several theories of dyslexia. Three widely researched theories include the phonological deficit hypothesis, the double deficit hypothesis, and the cerebellar deficit hypothesis. The phonological deficit hypothesis suggests that reading deficits can be attributed to a core deficit in phonological awareness, or the ability to determine the constituent sounds which comprise spoken words. This deficit in phonological awareness leads to difficulty learning grapheme-phoneme correspondence early on and to later difficulty learning decoding skills6-9. According to the double deficit hypothesis, some children with reading problems have poor phonological awareness skills while others have difficulty rapidly retrieving phonemes and words from long-term memory10 (rapid naming). Poor phonological awareness leads to reduced decoding accuracy; whereas poor rapid naming leads to slow reading rate. A double deficit occurs when children have both types of problems; these children tend to have the most severe reading deficits according to the theory and are most likely to develop dyslexia10. The cerebellar deficit hypothesis states that children with dyslexia have an automatization deficit and reduced speed of processing, along with motor and oral-motor deficits, because of a cerebellum that does not function comparably to typically developing children. These deficits lead to subsequent problems in cognitive processing and reading2,11. According to Nicolson and Fawcett, a deficit in cerebellar performance provides a complete explanation for the range of problems demonstrated by children with dyslexia, and provides a better explanation of their deficits than the phonological deficit and double deficit theories2,11,12. This manuscript seeks to examine the cerebellar deficit hypothesis through studying the morphology of the cerebellum and its relationship to cognition.

Nicolson and Fawcett have proposed two mechanisms by which the cerebellum may play a role in dyslexia2,11. One route is related to the motor theory of speech perception13, 14, which suggests recognition of the phonological units of words is based upon inferring the corresponding articulatory gestures. According to Nicolson and colleagues2, cerebellar dysfunction leads to mild motor problems in the infant, which lead to articulation difficulties. Poor quality articulatory representations lead to impaired sensitivity to the phonemic structure of language and to reduced phonological awareness2,15. In addition, decreased articulation speed can reduce verbal short-term/working memory functioning, as subvocal rehearsal is important in keeping memory traces in the store2,16. Reduced verbal working memory functioning may cause difficulties with language acquisition17, 18. The other route is related to processing speed. Cerebellar dysfunction may lead to reduced processing speed, which would affect cognitive functioning on a more global scale than merely producing deficits in phonological processing11. Based upon these two routes, the cerebellar deficit hypothesis attempts to explain the phonological deficit hypothesis and the double deficit hypothesis2. The oral-motor difficulties lead to deficits in phonological awareness whereas the processing speed deficits lead to difficulties with rapid naming. The cerebellum in particular may be involved with rapid naming given its role in speech, inner speech, and speeded processing.

Several studies have focused upon the cerebellum recently in their investigation of individuals with dyslexia. Rae and colleagues19 found the cerebellar hemispheres to be symmetric in adult males with dyslexia but asymmetric in controls. Controls had larger right cerebellar hemispheres than left when analyzing gray matter. For those with dyslexia, the more symmetric the cerebellar hemispheres were, the greater the deficits in phonological decoding. Leonard and colleagues20 found adults with phonological dyslexia to differ from controls and adults with other types of dyslexia in four key brain regions, one of which was marked leftward asymmetry of the anterior lobe of the cerebellum. When the four brain measures were normalized and summed into a single variable, it predicted phonological memory. In a follow-up study focused upon children with dyslexia, Eckert and colleagues21 found children with dyslexia to have smaller right anterior lobes of the cerebellum, pars triangularis bilaterally, and cerebral volume. Measurements of the right anterior lobe of the cerebellum and bilateral pars triangularis classified 72% of children with dyslexia and 88% of controls correctly. These measurements also were correlated with reading, spelling and language ability.

In general, there is now a considerable body of evidence that supports the role of the cerebellum in cognition from neuroimaging studies, most of which are on normal adults22-25, and studies of cerebellar patients26, 27. Much of this research has demonstrated greater right than left cerebellar hemisphere involvement in linguistic functioning, but several studies have found bilateral involvement in cognition and even vermal involvement. Fawcett and colleagues11 hypothesized that the cerebellar regions involved with dyslexia include the lateral posterior lobes of the cerebellum, as lesions here are associated with dysmetria and hypotonia. This region, along with the vermis, showed atypical activation in adults with dyslexia on tasks measuring motor skills2.

While some studies have demonstrated cerebellar involvement in dyslexia, others point to alternative explanations. For example, Ramus and colleagues28 found little support for the cerebellar deficit hypothesis in their behavioral study of adults with dyslexia, but their results did support the phonological deficit hypothesis. Zeffiro and Eden29 stated a cerebellar deficit is not likely the cause of dyslexia, as individuals with developmental dyslexia often do not display significant cerebellar signs such as ataxia and hyptonia in their research. In addition, those with acquired cerebellar damage often do not display reading problems. They suggested that the main source of dysfunction in dyslexia may lie outside of the cerebellum (such as in the perisylvian region), exerting its influence on the cerebellum through the cerebro-cerebellar connections. Thus, according to Zeffiro and Eden, while the cerebellum may appear to be mildly affected in dyslexia, the true deficit may lie outside of the cerebellum causing poor quality input to the cerebellum. The cerebellum is unable to function properly because of this poor input.

Given the controversy surrounding whether or not the cerebellum is involved with dyslexia, this study sought to test whether those with dyslexia differed from those without it in cerebellar anatomy and its relationship to cognition. It was hypothesized that children without dyslexia would present with greater rightward asymmetry compared to those with dyslexia based upon the work of Rae and colleagues19. In addition, it was hypothesized that right cerebellar hemisphere volume would predict performance on measures of phonological awareness, rapid naming, and verbal/phonological short-term memory based upon the possible routes whereby the cerebellum may be involved with dyslexia2 and the neuroimaging and lesion literature implicating greater right than left cerebellar hemisphere involvement in these functions. In general, the hypotheses on the cerebellar contribution to cognition are exploratory given the small sample size (n = 20 per cell).

This study includes children with dyslexia and ADHD. Approximately 15−40% of children with dyslexia meet criteria for ADHD30, 31, and around 10−30% of children with ADHD also have dyslexia30,31. Given this high comorbidity, findings are more generalizable to the population of children with dyslexia at large if those children with comorbid dyslexia and ADHD are included in the sample. In addition, the cerebellum has been suggested to play a role in ADHD. For example, structural MRI studies have found children with ADHD to have a smaller posterior-inferior vermis than controls32-34, with Castellanos and colleagues33 finding a significant correlation between posterior-inferior vermis morphology and several ratings of ADHD severity. Durston and colleagues35 found boys with ADHD to have reduced right cerebellar volume but not their siblings. They concluded that reduced cerebellum volume may be directly related to the pathophysiology of the disorder. In order to control for the specific effects that ADHD may have on cerebellar morphology, presence of ADHD was distributed evenly in numbers and severity across groups: those with and without dyslexia. In addition, it was hypothesized that those with and without ADHD would differ on poster-inferior vermis volume and right cerebellar hemisphere volume. Asymmetry was not expected to differ between groups, in contrast to those with and without dyslexia. Furthermore, it was hypothesized that size of the posterior-inferior vermis and right cerebellar hemisphere would correlate with measures of ADHD severity. This hypothesis also is exploratory given the small sample size.