Temporal envelope processing in developmental dyslexics

Abstract

This study evaluates temporal-envelope processing in six dyslexic children (mean age: 10.10 years), six normal control children (11.6 years), and six normal control adults (24.8 years) by measuring: (1) temporal modulation transfer functions (TMTFs), i.e., the detection thresholds of sinusoidal amplitude-modulation applied to a white noise carrier, as a function of modulation frequency (4, 16, 64, 256, or 1024 Hz), (2) recognition performance for vowel-consonant-vowel stimuli over five sessions; speech stimuli were either unprocessed or processed to degrade the spectral information. Modulation thresholds were similar in normal children and adults; for both groups, TMTFs were low pass in shape. TMTFs measured in dyslexics were bandpass in four children, low pass and flat in the two others. Overall, modulation thresholds were higher in dyslexics than in normal children at 4 and 1024 Hz. Unprocessed-speech recognition performance was perfect in normal children and adults, and impaired in dyslexics. Processed-speech recognition performance was poorer in normal and dyslexic children compared to normal adults. Performance improved across sessions in normal children and adults, but remained constant in dyslexics. These results support the hypothesis that some dyslexics show abnormal auditory temporal-envelope processing. This may, in turn, explain their difficulties with speech perception. [Work supported by the Cognitique program.]