Date Published: November 4, 2011
Publisher: Public Library of Science
Author(s): Julie A. Kirkby, Hazel I. Blythe, Denis Drieghe, Simon P. Liversedge, Anne Castles. http://doi.org/10.1371/journal.pone.0027105
Children with developmental dyslexia show reading impairment compared to their peers, despite being matched on IQ, socio-economic background, and educational opportunities. The neurological and cognitive basis of dyslexia remains a highly debated topic. Proponents of the magnocellular theory, which postulates abnormalities in the M-stream of the visual pathway cause developmental dyslexia, claim that children with dyslexia have deficient binocular coordination, and this is the underlying cause of developmental dyslexia. We measured binocular coordination during reading and a non-linguistic scanning task in three participant groups: adults, typically developing children, and children with dyslexia. A significant increase in fixation disparity was observed for dyslexic children solely when reading. Our study casts serious doubts on the claims of the magnocellular theory. The exclusivity of increased fixation disparity in dyslexics during reading might be a result of the allocation of inadequate attentional and/or cognitive resources to the reading process, or suboptimal linguistic processing per se.
Despite normal intelligence and educational opportunities, some children show a persistent difficulty in learning to read; these children are often diagnosed with developmental dyslexia . The most widely accepted theory of dyslexia is the phonological deficit theory , , , in which it is argued that children with dyslexia have a cognitive-level deficit in representing phonological information. The theory stipulates that underspecified phonological representations and difficulties in associating printed letters with relevant speech sounds cause the reading difficulties experienced by these individuals.
For both experiments, data were analyzed by means of linear mixed effects modelling specifying participants as random factors. For the reading experiment the sentence being read was also entered as a random factor. For the dot scanning experiment the number of dots in the dot string was entered as a random factor. Standard procedures were employed in the construction of the initial models in that all factors potentially influencing binocular disparity as suggested by prior research were entered into the model: participant group, the amplitude of the incoming saccade and fixation position relative to the centre of the screen; (see Table 1 for the basic characteristics of eye movements during reading and dot scanning, obtained in the current experiments). Comparisons between the initial and reduced models were carried out to obtain the most parsimonious model that was not statistically inferior in terms of fit of the data relative to the initial model.
In the present research, we aimed to investigate whether poor binocular coordination might be causally related to the reading difficulties experienced by children with dyslexia. The results from the study showed: (i) children with dyslexia have an increased magnitude of fixation disparity when they are reading compared to dot scanning – within the same group of children, binocular coordination was affected by the task; (ii) in comparison with other participant groups, when reading, the magnitude of fixation disparity was significantly greater in the dyslexic children; and (iii) adults’ and typically developing children’s binocular coordination was equivalent, and this was the case during both reading and dot scanning. This pattern of results formally demonstrated that when children with dyslexia were required to read sentences, the magnitude of fixation disparity was greater than that found when they were scanning simple dot stimuli.