Impaired auditory frequency discrimination in dyslexia detected with mismatch evoked potentials

Deficits in phonological skills appear to be at the heart of reading disabi lity; however, the nature of this impairment is not yet known. The hypothes is that dyslexic subjects are impaired in auditory frequency discrimination was tested by using an attention-independent auditory brain potential, ter med mismatch negativity (MMN) while subjects performed a visual distracter task. In separate blocks, MMN responses to graded changes in tone frequency or tone duration were recorded in 10 dyslexic and matched control subjects . MMN potentials to changes in tone frequency but not to changes in tone du ration were abnormal in dyslexic subjects. This physiological deficit was c orroborated by a similarly specific impairment in discriminating tone frequ ency, but not tone duration, which was assessed separately. Furthermore, th e pitch discrimination and MMN deficit was correlated with the degree of im pairment in phonological skills, as reflected in reading errors of regular words and nonwords. It is possible that in dyslexia a persistent sensory de ficit in monitoring the frequency of incoming sound may impair the feedback control necessary for the normal development of phonological skills.