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The Journal of Neuroscience, April 1, 2002, 22(7):2936-2944

Cortical Activation during Spoken-Word Segmentation in Nonreading-Impaired and Dyslexic Adults

Päivi Helenius¹, Riitta Salmelin¹, Elisabet Service², John F. Connolly³, Seija Leinonen⁴, and Heikki Lyytinen⁴

¹ Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, FIN-02015 HUT, Espoo, Finland, ² Department of Psychology, University of Helsinki, FIN-00014 Helsinki, Finland, ³ Department of Psychology, Life Sciences Center, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 4J1, and ⁴ Department of Psychology, University of Jyväskylä, FIN-40351 Jyväskylä, Finland

We used magnetoencephalography to elucidate the cortical activation associated with the segmentation of spoken words in nonreading-impaired and dyslexic adults. The subjects listened to binaurally presented sentences where the sentence-ending words were either semantically appropriate or inappropriate to the preceding sentence context. Half of the inappropriate final words shared two or three initial phonemes with the highly expected semantically appropriate words. Two temporally and functionally distinct response patterns were detected in the superior temporal lobe. The first response peaked at ~100 msec in the supratemporal plane and showed no sensitivity to the semantic appropriateness of the final word. This presemantic N100m response was abnormally strong in the left hemisphere of dyslexic individuals. After the N100m response, the semantically inappropriate sentence-ending words evoked stronger activation than the expected endings in the superior temporal cortex in the vicinity of the auditory cortex. This N400m response was delayed for words starting with the same two or three first few phonemes as the expected words but only until the first evidence of acoustic-phonetic dissimilarity emerged. This subtle delay supports the notion of initial lexical access being based on phonemes or acoustic features. In dyslexic participants, this qualitative aspect of word processing appeared to be normal. However, for all words alike, the ascending slope of the semantic activation in the left hemisphere was delayed by ~50 msec as compared with control subjects. The delay in the auditory N400m response in dyslexic subjects is likely to result from presemantic-phonological deficits possibly reflected in the abnormal N100m response.

Key words: speech processing; lexical access; N400m; temporal cortex; reading impairment; magnetoencephalography

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Copyright © 2002 Society for Neuroscience  0270-6474/02/2272936-09$05.00/0

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