Speech versus tone processing in compensated dyslexia: discrimination and lateralization with a dichotic mismatch...

This study investigated the possibility of a pervasive auditory-processing deficit in 10 adult dyslexics who had compensated for their reading disability, compared to 10 matched controls. Unlike previous studies [Baldeweg, T., Richardson, A., Watkins, S., Foale, C. & Gruzelier, J. (1999). Impaired auditory frequency discrimination in dyslexia detected with mismatch evoked potentials. Annals of Neurology, 45(4): 495-503], the current EEG study used a dichotic presentation of stimuli in order to probe the relationship between ear advantage and left- and right-hemisphere dominance for processing speech and non-speech stimuli respectively. A dichotic presentation is thought to maximise lateralization effects, as well being a more ecologically valid paradigm. The Mismatch Negativity (MMN) was measured for both speech [consonant-vowel pairs: /ta/ vs. /ka/ and ba/ vs. /da/] and non-speech stimuli (pure tones: 1 kHz vs. 1.2 kHz). Smaller MMNs to tone stimuli were obtained for dyslexics versus controls, but no differences were found for speech stimuli. Controls differentiated between speech and tones, with larger MMNs to tone stimuli. Dyslexics showed significantly greater MMNs to one stop consonant discrimination (/ta/ vs /ka/) than the other (/ba/ vs /da/), but did not differentiate speech from tones, and no or minimal lateralization was found for either group or stimulus type, in line with recent studies [Kershner and Micallef, 1992; Bellis, T.J., Nicol, T., & Kraus, N. (2000). Aging affects hemispheric asymmetry in the neural representation of speech sounds. Journal of Neuroscience, 20, 791-797]. However, analysis of left and right dipole source activity suggested right hemispheric preference for tones in controls, and the lack of such a preference in dyslexics. Lateralization of the auditory system in general may be less specialized in compensated dyslexia, although no specific differences in speech lateralization were seen. The present study also extends previous findings to show that the frequency range over which dyslexics are shown to display impaired frequency discrimination can be extended to 20% (1 kHz vs. 1.2 kHz) if using a dichotic presentation.