The presence of a magnocellular defect depends on the type of dyslexia

Previous studies have identified a magnocellular pathway defect in approximately 75 % of dyslexics. Since these experiments have not classified dyslexia into subtypes, the purpose of this experiment was to determine if adult dyseidetic dyslexics or dysphoneidetic dyslexics suffer from a defect in the magnocellular pathway. Nine dyseidetic dyslexics, eight dysphoneidetic dyslexics, and nine normal readers participated in the experiment. Contrast sensitivity functions (CSF) were determined with vertically oriented sine wave gratings (0.5, 1.0, 2.0, 4.0, 8.0, 12.0 c/deg drifting at 1 and 10 Hz) by employing a two-alternative, forced-choice technique. The results of the experiment indicated that dysphoneidetic dyslexics had reduced sensitivity to low spatial frequencies at 10 Hz, whereas dyseidetic dyslexics did not have reduced sensitivity at either 1 or 10 I-It. These results suggest that the type of dyslexia influences whether losses in perception are found which are consistent with a magnocellular deficit.