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Abstract
Key Points Dyslexia is a neurological disorder with a genetic origin, but the underlying biological and cognitive causes are still being investigated.This study compares the brain activation pattern while reading in Spanish, a semitransparent language, in three groups of children: typically developing readers, dyslexic readers and readers with functional monocular vision.Based on our results Dyslexia would be a neurological disorder not related to vision impairments and would require a multidisciplinary treatment based on improving phonological awareness and language development.Developmental dyslexia is a neurological disorder the underlying biological and cognitive causes of which are still being investigated, a key point, because the findings will determine the best therapeutic approach to use. Using functional magnetic resonance imaging, we studied the brain activation pattern while reading in the language-related cortical areas from the two reading routes, phonological and orthographic, and the strength of their association with reading scores in 66 Spanish-speaking children aged 9–12 years divided into three groups: typically developing readers (controls), dyslexic readers and readers with monocular vision due to ocular motility disorders but with normal reading development, to assess whether (or not) the neuronal network for reading in children with dyslexia has similarities with that in children with impaired binocular vision due to ocular motility disorders. We found that Spanish-speaking children with dyslexia have a brain circuit for reading that differs from that in children with monocular vision. Individuals with dyslexia tend to hypoactivate some of the language-related areas in the left hemisphere engaged by the phonological route, especially the visual word form area and left Wernicke's area, and try to compensate this deficit by activating language-related areas related to the orthographic route, such as the anterior part of the visual word form area and the posterior part of both middle temporal gyri. That is, they seem to compensate for impairment in the phonological route through orthographic routes of both hemispheres. Our results suggest that ocular motility disturbances do not play a causal role in dyslexia. Dyslexia seems to be a neurological disorder that is unrelated to vision impairments and requires early recognition and multidisciplinary treatment, based on improving phonological awareness and language development, to achieve the best possible outcome.
Highlights
Language, unlike reading, is predefined in our genome
In the left fusiform gyrus, three regions of interest (ROI) were identified and studied due to the activations found in previous group-level analysis; thereby, we found activation in an anterior region we call visual word form area (VWFA) 3 (BA 20), it being more intense in dyslexics (p = 0.026, K-W test) than the MVRs (p = 0.016, M-W test) or the typical reader (TDR) (p = 0.025, M-W test)
In our research, we have conducted a comprehensive functional MRI (fMRI) study including three different cognitive tasks, two tasks of lexical decision and lexical/orthographic matching, read preferentially through the sublexical phonological route based on grapheme– phoneme correspondences, and one of semantic categorization, that enhances the orthographic route based on lexical units, to explore the two pathways of language in an attempt to reproduce the neural network involved in reading (Pugh et al, 1996; Saur et al, 2008; Ramus and Ahissar, 2012)
Summary
Unlike reading, is predefined in our genome. Language acquisition is natural and inherent to the human species. Writing and reading, viewed from an evolutionary perspective, are very recent inventions (Artigas-Pallarés, 2011). Dehaene proposed the theory of “neuronal recycling” to describe such adaptations in the function of an organ. This is the case of the fusiform gyrus, a region used in primates and other species to display visual forms (e.g., predators, prey or potential mates), that has been adapted in humans to visualize the shapes of the letters of the alphabet (Paulesu et al, 2001; Dehaene and Cohen, 2007; Dehaene et al, 2010). Recent studies suggest that nearly 20% of the population has some degree of learning disability, in many cases attributable to reading difficulties, reading being a complex cognitive process that is required for complicated and sophisticated learning (National Center for Learning Disabilities, 2010)
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