Imprints of Dyslexia: Implicit Learning and the Cerebellum

Abstract

Dyslexia, a learning disability affecting reading and spelling, occurs in 3-10% of the general population. For many people diagnosed early in life the symptoms of dyslexia persist into adulthood causing them difficulties with functioning in the modern society so much reliant on the written word. This dissertation assesses the manifestations of developmental dyslexia in behavior, cognition and neuroanatomy as predicted by the procedural deficit hypothesis. According to the theory, a dysfunction in the cortico-cerebellar network responsible for procedural learning in both the motor and the language domains causes impoverished implicit motor and language learning abilities and eventually difficulties with language and motor skills in people with dyslexia. The study described in this dissertation was conducted in a group of adult participants with dyslexia and a group of IQ- and age-matched controls. The general design of the investigation included three parts, the neuropsychological, the cognitive and the neuroanatomical. In the neuropsychological part the language and the motor symptoms of dyslexia were examined. Along with the reading-related problems, difficulties with abilities associated with such functions of the cerebellum as fine motor skills and time estimation abilities in people with dyslexia were discovered. The cognitive part addressed the explicit and implicit learning abilities studied with the help of the artificial language learning and the serial reaction time paradigms. While the explicit learning ability, thought to be dissociated from the implicit learning one, was found to be intact in people with developmental dyslexia, indications of deficient implicit learning abilities in both the motor and the language domains were observed. Both groups of participants showed learning on the implicit motor learning task, with the group of participants with dyslexia being slightly delayed. Learning on the implicit language learning task was observed only in the control group, the group with dyslexia exhibiting chance performance. In the neuroanatomical part the participants’ cerebellar volume and the asymmetry between the cerebellar hemispheres was examined by means of structural MRI. No differences in the volume of the two hemispheres or in the asymmetry between the two hemispheres in the two groups were detected. Importantly, this study examined the three levels, behavior, cognition and neuroanatomy, in the same group of people with dyslexia. While differences between people with and without dyslexia were found in the neuropsychological and the cognitive investigations, they were not detected on the neurological level. The neuroanatomical manifestations of dyslexia might be more obvious in children or in the function of the cerebellum rather than its structure. Further investigations in these directions might appear fruitful.