AUDITORY TEMPORAL PROCESSING IN DEVELOPMENTAL DYSLEXICS

Developmental dyslexia is usually considered to result from a high lev el cognitive/linguistic deficit. But it is possible that the phonologi cal difficulties that most display may in part be due to impaired audi tory perception. This study investigates the ability of adult dyslexic s, screened for normal hearing sensitivity, to perform a non-speech au ditory task. The test (FM rate difference limen) was designed to be an alogous to visual motion sensitivity at which dyslexics are worse than controls. Subjects were identified as dyslexic by educational psychol ogists using standard criteria. Their persistent reading difficulties were confirmed using single real word and non-word reading tests. The 12 dyslexics not only made more errors than 12 matched controls in bot h tests but also took longer to complete the lists. When we ranked all 24 subjects on their word and non-word reading performance there was a strong correlation between them (r = 0.88; p < 0.001). There were no significant differences between dyslexics and controls in hearing 1 K Hz pure tones; but the dyslexics were significantly worse at detecting changes in the rate of modulation of frequency modulated tones (their FM rate difference limen was larger; t = 3.04, p < 0.005); and their FM depth difference limen was also worse (t = 3.55, p < 0.005). The ra nk of all the subjects on these tests correlated with their reading pe rformance (r = 0.5, P < 0.05). Results support the hypothesis that dys lexics have impaired development of neuronal systems responsible for p rocessing the timing of auditory frequency changes. As in the visual s ystem this function may be mediated by magnocellular neurones.