Cerebellar theory of dyslexia

The cerebellar theory of dyslexia asserts that a mildly dysfunctional cerebellum can cause dyslexia. This theory was initially proposed by Harold Levinson and Jan Frank in 1973 and further developed by Levinson and other researchers. Angela Fawcett and Rod Nicolson later proposed that the cerebellum contributes to motor control during the articulation of speech, and that articulation problems can contribute to the phonological processing deficits that can cause dyslexia. They also reasoned that the cerebellum contributes to the automatisation of learned behaviors, which may include learning the grapheme-phoneme relationships when reading text.^[1][2]

Overview

The cerebellar theory is considered to be one of the three main theories for what causes dyslexia, based largely on the association of dyslexia with a lack of balance, coordination, and time estimation as well as the cerebellum's suggested role in modulating sensory, memory, concentration, cognitive, communication and emotional skills. Additional research has shown that those with dyslexia show impairments in both the right and left sides of their cerebellum, including a lower than normal blood-flow to these regions while performing tasks the cerebellum is responsible for. Those with dyslexia can also exhibit earlier delays in motor milestones associated with the cerebellum, such as crawling, walking, talking, and coordination. However, research into the association between the cerebellum and dyslexia has led to no consensus on the specific role of the cerebellum in dyslexia.^[3]

Levinson cerebellar-vestibular theory

The origin of the cerebellar theory of dyslexia starts with medical physicians Harold Levinson and Jan Frank. In 1973 they published an article in the Journal of Child Psychiatry, first hypothesizing that dyslexia was caused by a faulty connection between the cerebellum and the other parts of the brain. Prior clinicians and researchers had instead believed that the root of dyslexia would be a primary defect within the cerebrum, however Levinson and Frank posited new evidence that a primary impairment within the cerebellum was the root and secondarily disrupts normal cerebral and related brain processing.

Levinson believes that a cerebellum impairment causes dyslexia in conjunction with impairment of the vestibular system—basically the inner-ear. This inner-ear and cerebellum dysfunction was first established in a study with 115 dyslexic children, a sample in which 97% showed clear signs of cerebellar-vestibular impairment. In 1988 further studies showed that out of 4000 dyslexic children the average demonstrating cerebellar impairment remained the same, with a total of 96.3% of the children showing cerebellar neurological dysfunction. Levinson also invented the optokinetic apparatus, which measures visual tracking ability, in trying to diagnose a cerebellar-vestibular cause of dyslexia. He used this device to measure the cerebellar-vestibular performance of dyslexic children versus control groups, which further established the possibility of the theory's efficacy. Levinson also showed that cerebellar theory may be able to account for aspects of ADHD and anxiety disorders. However, his practice focused mostly on the clinical aspects of neurophysiological diagnosis and medical treatment of dyslexic individuals with anti-motion sickness medications rather than academic research studies.^[4]

Fawcett and Nicolson

In the 1990s researchers Angela Fawcett and Rod Nicolson also supported the idea that dyslexia and other mental disabilities were caused by problems in the cerebellum. Their research focused on the role of the cerebellum in motor control and its effects on speech articulation, which could in turn lead to poor phonological representations. The second thing that they focused on was the role of the cerebellum in the automatisation of learned tasks, which could affect the ability of an individual to form sound-letter correlations as easily as others, making it more difficult to read. Later research showed their first hypothesis to be unlikely, but did not disprove their second hypothesis. It also showed that while difficulty with balance associated with an impaired cerebellum could be found in individuals with dyslexia, it was also found within individuals having ADHD or intelligence impairment. Therefore, a direct correlation between dyslexia and balance was not clear.^[5]

Other research

Henrietta and Alan Leiner, together with Robert Dow's cerebellar research in 1986 and 1993, as well as prior neurophysiological data reported by Ray Snider and Averill Stowell,^[6] suggested that the cerebellum processes much more than motor functioning. These neurophysiological researchers showed that the cerebellum, together with the cerebrum, is capable of processing the entire sensory input as well as contributes to mental, memory, concentration, emotional and communication skills.^[7][8]

In 1998 a research team used MRI evidence to determine the differences in cerebellar and tempero-parietal cortex brain activity using a group of 14 dyslexic men and 15 additional men on a control group. They found biochemical differences in these parts of the brains of the dyslexic men from a normally functioning brain. Specifically when it comes to the balance between chlorine-containing compounds and N-acetylaspartate in the brains of individuals who have dyslexia. They hypothesized that this imbalance may result in abnormal cell formation in the brain, thus potentially leading to dyslexia. The study also mentioned the aid that anti-motion sickness medication can give to helping dyslexic people cope with their disorder, referring to them as cerebellar-vestibular stabilizing.^[9]

References