Merav Ahissar and colleagues now report that a subpopulation of dyslexics, those with learning disabilities, cannot use sound repetition to their advantage in auditory tasks, a deficit that seems to account for many prior reports of poor psychoacoustic performance. Dyslexic subjects performed as well as control subjects on two auditory discrimination tasks when the sound stimuli were drawn from a large set. When the stimulus sets were small, the performance of the control subjects improved, presumably because they could use the regularities to help them perform the task. For instance, when asked to judge which of two tones was higher in pitch, control subjects performed much better when one of the tones was always at the same frequency, perhaps because they formed an internal representation of the standard tone that they could compare to the other tone when making the judgment. Dyslexic subjects, in contrast, failed to benefit from small stimulus sets involving sound repetitions, performing much worse than control subjects under those conditions. The authors suggest that this might be because dyslexics are unable to form a memory trace of the repeated stimuli, suggesting that a problem with sensory memory could underlie many previous reports of abnormal performance by dyslexics on basic sensory tasks. Whether this impairment generalizes to dyslexics who are not learning disabled, and whether it is causally related to reading difficulties, remains to be seen.
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