Encoding of temporal speech features (formant transients) during binaural and dichotic stimulus application: A whole-head magnetencephalography study

Spoken-word recognition depends upon the encoding of relevant 'information bearing elements' of the acoustic speech signal. For example, relatively ra pid shifts of spectral energy distribution (formant transients:) cue the pe rception of stop consonant-vowel (CV) syllables such as /ba/, /ga/, and /da /. A variety of data indicate left-hemisphere superiority with respect to t he processing of formant transients. To further delineate the underlying ne urophysiological mechanisms, evoked cortical fields in response to CV sylla bles (oddball design; frequent stimulus=binaural /ga/; four deviant constel lations: Binaural /ba/, binaural /da/, left /da/ (left ear deviant)-right / ga/, right /da/ (right ear deviant)-left /ga/) were recorded by means of wh ole-head magnetencephalography (MEG; 151 channels) under two different cond itions of attentional demands (visual distraction versus reaction to prespe cified stimuli). (a) During binaural stimulus presentation attention toward target events resulted in a significantly enhanced mismatch field (MMNm, m agnetic analogue to the mismatch negativity) over the left as compared to t he right hemisphere. In contrast, preattentive processing of the CV syllabl es failed MMNm lateralization effects. (b) Dichotic application of /da/ eli cited a larger contralateral MMNm amplitude in subjects with right ear adva ntage (REA) at behavioral testing. In addition, right ear deviants yielded a stronger ipsilateral response than the left ear cognates. Taken together, these data indicate bilateral preattentive processing and subsequent atten tion-related predominant left-hemisphere encoding of formant transients at the level of the supratemporal plane. Furthermore, REA during dichotic appl ication of CV syllables seems to be linked to functional dissociation of th e two hemispheres during auditory processing. (C) 2000 Elsevier Science B.V . All rights reserved.