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Abstract:
Developmental dyslexia (DD) was repeatedly shown to be associated with reduced phonological skills—evident, for instance, from a reduction in the amplitude of the auditory Mismatch Negativity (MMN) in the event-related brain potential. Yet, it has been suggested that phonological deficits in DD are in fact secondary to an underlying attention deficit. Under this view, dyslexics cannot disengage their attention quickly enough from a stimulus to attend to a subsequent stimulus, causing a reduced MMN response as an epiphenomenon. We investigated here whether dyslexics’ reduced MMN responses are preceded by markers of aberrant attention in the electroencephalogram (EEG), analyzing the EEG from 28 dyslexic school children (mean age = 9.69 years; SD = 0.50 years) and 25 control school children (mean age = 9.85; SD = 0.56 years). Participants received an audio-visual oddball paradigm, involving visually presented mouth movements forming the syllable /pa/, while either hearing the congruently produced syllable /pa/ as a standard or the mismatching syllable /ga/ as a deviant stimulus; and vice versa in the second experimental block. In addition to the auditory MMN as a measure of phonological processing, we assessed pre-stimulus alpha-band inter-trial phase coherence (ITPC) as a measure of automatic attention; ITPC was averaged across both standard and deviant trials, hence any potential ITPC group difference could not have been a result of MMN group differences. In line with our hypothesis, dyslexic children showed a highly significant pre-stimulus ITPC increase relative to control children. ITPC was a strong predictor of MMN amplitude, such that aberrantly high ITPC predicted an aberrantly reduced MMN amplitude. Moreover, the ITPC × MMN interaction predicted reading abilities, such that poor readers showed both high ITPC and a reduced MMN, the reverse being true in good readers. Increased pre-stimulus alpha-band phase-locking may thus be an overlooked EEG marker of reduced auditory attention switching in DD, consistent with the hypothesis that phonological deficits in DD are in fact secondary to an underlying automatic attention shifting deficit.
