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Left hemisphere's dominance in speech determined by long-term memory traces not by physical sound features: an MEG study
SHTYROV, Y. & PULVERMULLER, F.
Society for Psychophysiological Research, Abstracts of the 44rd Annual Meeting, Psychophysiology 41: s1, Santa Fe, USA, 2004
Year of publication:
According to an influential view on brain mechanisms of speech perception, the left-hemisphereís dominance in speech is explained by its putative advantage in the processing of any spectrally and temporally rich sounds. We scrutinised this view by recording magnetic brain responses to the same rapid-onset stop consonant incorporated in four contexts: those of noun, verb, acoustically matched pseudoword and non-speech complex sound. We found that reliable left hemispheric dominance could only be elicited by the sounds forming real meaningful words. The same identical acoustic contrasts failed to produce significant hemispheric asymmetry not only in the case of non-speech sound but also when incorporated in phonologically and phonotactically legal but meaningless pseudoword. We conclude that acoustic stimulus properties are not sufficient for eliciting left-dominant activation. Instead, it appears that such lateralised processing requires the existence of long-term memory traces. Laterality may therefore be tied to the process of memory-trace formation but not to physical properties per se. We also show enhanced mismatch negativity responses elicited by words as opposed to pseudowords thus indexing the presence of long-term memory traces for meaningful linguistic items in the brain. The described phenomena were found in non-attend paradigm already at ~150 ms after the relevant acoustic information became available; this suggests that the language-specific processing of auditory information commences rather early in time and is largely independent of the focussed attention.