Motion detection, letter position encoding, and single word reading

Abstract

Recent research has shown that many people with dyslexia find it unusually difficult to detect flickering or moving visual stimuli, consistent with impaired processing in the magnocellular visual stream. Nonetheless, it remains controversial to suggest that reduced visual sensitivity of this kind might affect reading. We first show that the accuracy of letter position encoding may depend on input from the magnocellular pathway. We then suggest that when children read, impaired magnocellular function may degrade information about where letters are positioned with respect to each other, leading to reading errors which contain sounds not represented in the printed word. We call these orthographically inconsistent nonsense errors letter errors. In an unselected sample of primary school children, we show that the probability of children making “letter” errors in a single word reading task was best explained by independent contributions from motion detection (magnocellular function) and phonological awareness (assessed by a spoonerism task). This result held even when controlling for chronological age, reading ability, and IQ. Together, these findings suggest that impaired magnocellular visual function, as well as phonological deficits, may affect reading.