French study links vision abnormalities and dyslexia

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Irregularities in tiny light receptors in the human eye may explain why some people have dyslexia, French scientists say.

Guy Ropars and Albert le Floch from the University of Rennes, discovered a significant difference between the arrangement of tiny receptor cones in the eyes of dyslexic and non-dyslexic people.

According to their study, published in the journal Proceedings of the Royal Society B, these tiny cells are arranged in matching patterns in both eyes in people with dyslexia, whereas in non-dyslexic people they are arranged asymmetrically.

They believe that these matching patterns may be confusing the brain by producing ‘mirror’ images, causing familiar errors such as letter reversal — for example, a ’b’ being read as a ’d’. The ‘normal’ asymmetrical arrangement allows signals from the one eye to be over-ridden by the other to create a single image in the brain.

Our observations lead us to believe that we indeed found a potential cause of dyslexia.

Guy Ropars told Agence France Presse (AFP).

The study involved examining the eyes of 30 individuals with dyslexia and 30 without the condition. The researchers used a novel method to establish the perceived brightness of the after-image in each subject’s left and right eye.

In the 30 subjects without dyslexia, 19 saw a brighter after-image with their right eye, and 11 with their left eye — i.e. one eye was dominant. In the case of the group with dyslexia 27 had no eye dominance — i.e. the after-image in each eye was equally bright.

The researchers then linked this perceptual difference to the arrangement of the cells concentrated in the fovea, a small dimple-like spot at the centre of the retina. In subjects with a dominant eye these cells were circular in the dominant eye but elliptical in the weaker eye. In subjects with dyslexia the cells were circular in both eyes.

The findings suggest that a relatively simple test could be devised to identify those with the condition. It may also lead to it being treatable. Because of a delay of about 10 thousandths of a second between the primary image and the mirror image Ropars and le Floch found they could use a flashing LED lamp to erase the mirror image.

Professor John Stein, dyslexia expert and emeritus professor in neuroscience at the University of Oxford, told the BBC that while the study was ‘really interesting’ it was unlikely to explain everyone’s dyslexia. ‘No one problem is necessary to get dyslexia and no one problem is behind it,’ he said.

Research carried out at the universities of Bristol and Newcastle, UK, which analysed the eye test results of 5,822 children found that the majority of the dyslexic children had perfect vision.

Dr Cathy Williams, a paediatric ophthalmologist and the study’s lead author, said:

These population-based results give the “bigger picture” and show us that vision problems are rare in dyslexic children. The few vision impairments we did see in the dyslexic children also occurred in their non-dyslexic classmates.

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