Dyslexia could be caused by light receptors in the eyes that confuse the brain by producing "mirror" images, scientists have said.
Two French scientists concluded that receptor cells were arranged in matching patterns in both eyes in people with reading difficulties.
This, they suggest, could prevent them from distinguishing between letters such as "b" and "d".
In non-dyslexic people, the cells are arranged asymmetrically, allowing signals from the one eye to override 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," said Guy Ropars, the study co-author, from the University of Rennes.
It offers a "relatively simple" method of diagnosis, he said, by simply examining a subject's eyes.