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Mismatch negativity cortical event-related potential measures of cross-linguistic phoneme perception Tsui, Vicki Chi Ki

Abstract

It is a well-established fact that infants are born with the ability to discriminate a universal set of phonetic categories. However, the ability to consciously perceive phonetic contrasts which are not relevant in one's native language gradually decreases within the first year of life. The objective of this investigation is to determine whether non-native category changes are still recognized by adults at early levels of auditory processing even though they find it difficult, or impossible, to discriminate them behaviourally. Electrophysiological and behavioural responses to native (/ba/ vs. /da/) and non-native contrasts (/da/ vs. /Da/) were obtained from native English listeners. The component of the event-related potential (ERP) of particular interest is the mismatch negativity (MMN), generated in the auditory cortex. The MMN is a waveform difference which occurs, independent of attention, in response to a discriminable change in a repeated sound stimulus. Therefore, if the ability to perceive the non-native category change is preserved at the level of the primary auditory cortex, we would expect to find an MMN response to that change. For the passive ERP measures, stimuli were presented in an oddball paradigm, with conditions counterbalanced for order. The behavioural condition consisted of a forced-choice task in which participants were presented with various stimulus pairs and asked to decide whether the stimuli within the pair were "same" or "different". Behavioural results confirm that the majority of the English listeners could not discriminate the non-native category change. Performance in the native category was significantly better. Surprisingly, ERP results not only indicate that there is a distinct MMN in response to the non-native category change, but that this response actually tends to be larger in amplitude than the response to the native category change. The order of presentation of the stimuli also has a significant effect on the amplitude of the MMN. The MMN response appears to be stronger when /da/ is the standard stimuli and when /Da/ is the deviant. It is clear from these results that the auditory cortex is indeed able to recognize non-native phonetic category changes even when they are not consciously perceived by the listener. These findings support the theory that the change in categorical perceptual ability from infancy to adulthood involves a functional reorganization rather than a permanent loss of ability.

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