Neural substrates of speech and nonspeech processing in compensated dyslexica

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 similar previous studies (e.g. Baldeweg et al, 1999), the current auditory event related potential (ERP) 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. This is thought to maximise lateralisation effects, as well being a more ecologically valid paradigm. The Mismatch Negativity (MMN) was measured for both speech (consonant-vowel pairs) and non-speech stimuli (pure tones). 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. In contrast 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. No lateralization was found for either group or stimulus type, in line with recent studies (Kershner and Micallef, 1992; Bellis et al, 2000). However, analysis of left and right dipole activation suggested a left hemisphere preference for speech in the controls, but no such difference between the hemispheres in the dyslexics. This might reflect a compensatory mechanism for speech processing. Results are discussed in terms of implications for the auditory processing deficit theory of dyslexia (e.g. Miller and Tallal, 1995).