The neurological basis of developmental dyslexia
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.