Cerebral Cortex Advance Access originally published online on November 24, 2004
Cerebral Cortex 2005 15(7):1054-1063; doi:10.1093/cercor/bhh206
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Cerebral Cortex V 15 N 7 © Oxford University Press 2004; all rights reserved
Cortical differentiation of speech and nonspeech sounds at 100 ms: implications for dyslexia
Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, Espoo, Finland
Address correspondence to Tiina Parviainen, Brain Research Unit, Low Temperature Laboratory, PO Box 2200, FIN-02015 HUT, Finland. Email: tiina{at}neuro.hut.fi.
Neurophysiological measures indicate cortical sensitivity to speech sounds by 150 ms after stimulus onset. In this time window dyslexic subjects start to show abnormal cortical processing. We investigated whether phonetic analysis is reflected in the robust auditory cortical activation at 
100 ms (N100m), and whether dyslexic subjects show abnormal N100m responses to speech or nonspeech sounds. We used magnetoencephalography to record auditory responses of 10 normally reading and 10 dyslexic adults. The speech stimuli were synthetic Finnish speech sounds (/a/, /u/, /pa/, /ka/). The nonspeech stimuli were complex nonspeech sounds and simple sine wave tones, composed of the F1+F2+F3 and F2 formant frequencies of the speech sounds, respectively. All sounds evoked a prominent N100m response in the bilateral auditory cortices. The N100m activation was stronger to speech than nonspeech sounds in the left but not in the right auditory cortex, in both subject groups. The leftward shift of hemispheric balance for speech sounds is likely to reflect analysis at the phonetic level. In dyslexic subjects the overall interhemispheric amplitude balance and timing were altered for all sound types alike. Dyslexic individuals thus seem to have an unusual cortical organization of general auditory processing in the time window of speech-sensitive analysis.
Key Words: dyslexia • MEG • N100 • speech perception
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