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Abstract:
This study looks at mismatch fields (MMFs) elicited by
preattentive speech discrimination and categorization from ten
adult subjects in the passive oddball paradigm. A synthetic
continuum of nine /ba-wa/ syllables were used as stimuli for
behavioral identification and discrimination. In the neuromagnetic
experiments, an examplar pair of /ba/ and /wa/ syllables
constituted the standards and deviants for discrimination. This
pair was replaced with three sounds from each category for
categorization. Standards and deviants were presented at 85%:15%,
and were reversed at a subsequent experiment. Three MMF
calculations were performed using: (a) the standard immediately
before the deviant, (b) the standard immediately after the deviant,
and (c) all the standards excluding (b). N1m and P2m were also
derived for each data set. Behavioral data replicated previous
findings of categorical perception. Repeated-measures ANOVA
analysis on the neuromagnetic data showed that the phonetic
categories of /ba/ and /wa/ were reflected in the MMF as well as in
N1m and P2m. Moreover, larger MMFs were observed for preattentive
discrimination, and the three types of MMFs showed significant
differences only for categorization. These results suggest that the
mismatch field encodes not only the cross-category information but
also the within-category acoustic differences.
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