Albert Le Floch and Guy Ropars

Left-right asymmetry of the Maxwell spot centroids in adults without and with dyslexia

publication date
18 October 2017
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In human vision, the brain has to select one view of the world from our two eyes. However, the existence of a clear anatomical asymmetry providing an initial imbalance for normal neural development is still not understood. Using a so-called foveascope, we found that for a cohort of 30 normal adults, the two blue cone-free areas at the centre of the foveas are asymmetrical. The noise-stimulated afterimage dominant eye introduced here corresponds to the circular blue cone-free area, while the non-dominant eye corresponds to the diffuse and irregular elliptical outline. By contrast, we found that this asymmetry is absent or frustrated in a similar cohort of 30 adults with normal ocular status, but with dyslexia, i.e. with visual and phonological deficits. In this case, our results show that the two Maxwell centroid outlines are both circular but lead to an undetermined afterimage dominance with a coexistence of primary and mirror images. The interplay between the lack of asymmetry and the development in the neural maturation of the brain pathways suggests new implications in both fundamental and biomedical sciences.


The noise-induced negative afterimage method enables us to determine the ocular dominance for an observer, which is correlated with the asymmetry of the outlines of the Maxwell spot centroids. Beyond genetics, the asymmetry between the two blue cone-free areas appears to be a necessary and fundamental condition for brain connectivity for a normal development. By contrast, the lack of asymmetry might be the biological and anatomical basis of reading and spelling disabilities in people with a normal ocular status but with dyslexia, perturbing the connectivity of different regions in the brain and inducing the observed common visual and phonological difficulties [54–56]. Our results suggest early anatomical diagnosis of dyslexia in young children and possible compensation for their potential lack of asymmetry, especially during the critical period. For adults, opportunities such as exploiting the small delays in the brain connections by using pulse-width modulation light-emitting diodes may provide novel strategies to overcome reading and other neurological difficulties.

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