[The reader brain: natural and cultural story]

Abstract

The report of cases of pure alexia suggests that some regions of the neural system are dedicated to reading-specific visual processing abilities. Pure alexia results from the disruption or the disconnection of the visual word form area, a region reproducibly located within the left occipito-temporal sulcus and encoding the abstract identity of strings of visual letters. The functional specialization of this area suggests that it is initially plastic and becomes attuned to the orthographic regularities that constrain letter combination during the acquisition of literacy. The visual word form area further belongs to the network of areas that are consistently implicated in studies of people with developmental dyslexia. Developmental dyslexia is typically interpreted as resulting from a core phonological disorder and most neuroimaging studies showed reduced activity in the left perisylvian regions which have a role in phonological processes. Although low level visual and/or visual attentional disorders have been consistently reported suggesting a visual basis of developmental dyslexia, these disorders typically co-occurred with phonological problems so that the phonological deficit was viewed as the most plausible cause of the poor reading outcome of dyslexic children. In the last years however, dissociations have been reported in developmental dyslexia between phonological processing deficits and a particular kind of visual disorder, a visual attention span deficit characterised by a reduction in the number of distinct orthographic units which can be processed simultaneously in a single fixation. Large sample studies revealed that a non trivial number of dyslexic children exhibit a visual attention span disorder and that this disorder typically dissociates from phonological impairments in the dyslexic population. Neuroimaging studies suggest involvement of the parietal lobes - in particular the superior parietal lobules - in visual attention span and these brain regions are less active in people with developmental dyslexia. A visual attention span disorder thus appears as a second core disorder related to a parietal dysfunction in developmental dyslexia. Further studies are required to determine whether the phonological disorder or the visual attention span disorder independently contribute to the development of the visual word form area during literacy acquisition. These neurobiological dysfunctions are further modulated by environmental factors such as language characteristics, remedial interventions or socio-economic status. Future studies would help better understanding the interactions between neurobiological and environmental factors and the potential influence of the later on the development of the visual word form area.