Studying visual motion illusions in dyslexics supports magnocellular deficit hypothesis

Visual magnocellular-dorsal (M-D) deficit hypothesis is gaining an increasing consent in the study of developmental dyslexia. However, several experimental data supporting the M-D deficit hypothesis can also be interpreted as a consequence of a perceptual noise exclusion deficit. In our experiments, we measured sensitivity for two visual motion illusions proved to involve specifically the M-D pathway. The results show that dyslexics need more luminance contrast to perceive the motion illusions, although contrast sensitivity for these specific stimuli (measured by simple stimulus detection) was equal in the two groups. The individual data also confirmed that these two motion illusions are very sensitive in distinguishing dyslexics from controls suggesting that our tasks could become an important tool for early identification of at risk children for dyslexia. Our result is the first to support the M-D deficit hypothesis in dyslexia by measuring sensitivity for visual motion illusions, without involving any signal from noise extraction mechanism.