The neurological basis of developmental dyslexia: An overview and working hypothesis

Abstract

Five to ten per cent of school-age children fail to learn to read in spite of normal intelligence, adequate environment and educational opportunities. Thus defined, developmental dyslexia (hereafter referred to as dyslexia) is usually considered of constitutional origin, but its actual mechanisms are still mysterious and currently remain the subject of intense research endeavour in various neuroscientific areas and along several theoretical frameworks. This article reviews evidence accumulated to date that favours a dysfunction of neural systems known to participate in the normal acquisition and achievement of reading and other related cognitive functions. Historically, the first arguments for a neurological basis of dyslexia came from neuropathological studies of brains from dyslexic individuals. These early studies, although open to criticism, for the first time drew attention towards a possible abnormality in specific stages of prenatal maturation of the cerebral cortex and suggested a role of atypical development of brain asymmetries. This has prompted a large amount of subsequent work using in vivo imaging methods in the same vein. These latter studies, however, have yielded less clear-cut results than expected, but have globally confirmed some subtle differences in brain anatomy whose exact significance is still under investigation. Neuropsychological studies have provided considerable evidence that the main mechanism leading to these children's learning difficulties is phonological in nature, namely a basic defect in segmenting and manipulating the phoneme constituents of speech. A case has also been made for impairment in brain visual mechanisms of reading as a possible contributing factor. This approach has led to an important conceptual advance with the suggestion of a specific involvement of one subsystem of vision pathways (the so-called magnosystem hypothesis). Both phonological and visual hypotheses have received valuable contribution from modern functional imaging techniques. Results of recent PET and functional MRI studies are reported here in some detail. Finally, one attractive interpretation of available evidence points to dyslexia as a multi-system deficit possibly based on a fundamental incapacity of the brain in performing tasks requiring processing of brief stimuli in rapid temporal succession. It is proposed that this so-called 'temporal processing impairment' theory of dyslexia could also account for at least some of the perceptual, motor and cognitive symptoms very often associated with the learning disorder, a coincidence that has remained unexplained so far.

Cognitive functions correlate with white matter architecture in a normal pediatric population: A diffusion tensor MRI study

“…The DTI results in the reading-impaired (adult) individuals thus reflect the disruption of a normal (anatomical and functional) connection between cooperating brain regions. Our results, however, reflect normal variations in healthy children that correlate with their individual, overall cognitive abilities as measured by a broad range of tasks in a test of "general" intelligence (which is normal in most dyslexic individuals (Habib 2000)). To further investigate we looked at the average FA from the ROI of correlated voxels in the slice at Z = +25 in left temporo-parietal white matter (detailed in Figure 1) and computed the partial correlation coefficients for the average FA versus Full-scale IQ (R = 0.58), Verbal IQ (R = 0.6), and Performance IQ (R=0.43), with subject age as the covariate, indicating that the majority of the FA differences in this area may be traced to verbal proficiency.…”

“…They found a positive correlation of FA in a left temporo-parietal area with higher reading skills, and the spatial location of that region is extremely close to the region exhibiting changes in FA with general cognitive ability (Figure 1, middle slice, middle row). Reading is a task that heavily relies on highly specialized brain areas in frontal, temporo-parietal, and occipito-temporal regions and their fast and efficient connection (Habib 2000;Pugh, et al 2000). In fact, it has been hypothesized that in reading-impaired individuals, "temporo-parietal difficulties disrupt this developmental trajectory" (Pugh, et al 2001), which is consistent with neuroimaging studies demonstrating pathological patterns of posterior activation in reading-impaired subjects, with a compensatory shift involving stronger activation in frontal areas (Pugh, et al 2001).…”

Processamento auditivo (central) em crianças com dislexia: avaliação comportamental e eletrofisiológica

“…Theories based on anatomical, physiological and behavioral disorders try to explain the etiology of dyslexia, and two of them are more broadly discussed (2)(3)(4) . The first and oldest one affirms that difficulties are phonological in origin and comprise a purely linguistic deficit, that is, difficulties concern specifically one aspect of the language processing (2,3) .…”

“…The first and oldest one affirms that difficulties are phonological in origin and comprise a purely linguistic deficit, that is, difficulties concern specifically one aspect of the language processing (2,3) . The second one, which will be studied in this research, concerns perceptual sensory disorders found in people with dyslexia.…”

Perfil motor de crianças e adolescentes com deficiência mental moderada

“…Tem sido reconhecido que alterações no processamento temporal podem ser atribuídas a sinais cognitivos e motor-perceptuais, comumente associados a desordens no aprendizado 25 . Além disso, crianças com essas desordens têm capacidade limitada de assimilar informações e precisam de mais tempo para realizá-las em um contexto de atividades motoras 10 .…”