Magnocellular-dorsal pathway and sub-lexical route in developmental dyslexia

Developmental dyslexia (DD) is frequently associated to a phonological deficit, although the underlying neurobiological causes remain undetermined. One prominent hypothesis suggests a specific deficit in magnocellular-dorsal (MD) pathway. In the present study we investigated the visual MD and parvocellular-pathway in seventeen dyslexics and in twenty-four chronological age- and IQ-matched normally reading children by measuring dynamic (ie, spatial frequency doubling illusion) and static stimuli sensibility, respectively. The results revealed a specific deficit of the MD-pathway in dyslexics. More importantly, the MD-deficit was selectively presented in poor non-word readers, suggesting the specific role of MD-pathway on phonological decoding ability. Accordingly, we further showed that in dyslexic children, individual differences in the MD-sensibility accounted for 29% of unique variance in non-word reading fluency after controlling for age and IQ. Finally, MD-deficit appears to be frequent because the 75% of poor non-word readers were at least 1 standard deviation below the mean of the controls. These results suggest that a MD-deficit--probably linked to middle-temporal areas dysfunction--might impair the sub-lexical mechanisms that are critical for reading development.