The causal link between magnocellular-dorsal pathway functioning and dyslexia

Impaired auditory-phonological processing is widely assumed to characterize dyslexic individuals. However, the magnocellular-dorsal (MD) pathway deficit theory, while controversial, has long been argued to play an important role in developmental dyslexia. The debate has centered on the critique that the visual MD deficit found in individuals with developmental dyslexia could just be a consequence of an impoverished reading experience. Here, we employ a comprehensive approach that incorporates all the accepted methods required to test the relationship between the MD pathway and developmental dyslexia: (i) a comparison with reading level controls, that are younger controls who read at the same level as the dyslexics (Experiment 1); (ii) a prospective-longitudinal approach, in which MD functioning was measured in pre-readers and a correlation then established with the future reading development (Experiment 2); and (iii) two remediation studies, in which the MD pathway is specifically trained and reading improvement is established (Experiment 3 and 4). The results of all the four experiments point strongly in the direction of a causal relationship between MD deficit and developmental dyslexia. Since an MD dysfunction can be diagnosed much earlier than a reading and language disorders, our findings pave the way for low resource-intensive, early prevention programs that could drastically reduce the incidence of reading disorders. Meeting abstract presented at VSS 2015