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Implicit and explicit categorization of speech sounds--dissociating behavioural and neurophysiological data.
Heidrun Bien1, Lothar Lagemann, Christian Dobel
1Psychological Institute II, University of Muenster, 48149 Münster, Germany. heidrun.bien@uni-muenster.de
The European Journal of Neuroscience
|July 21, 2009
Summary
Speech perception systems immediately categorize sounds correctly, even with assimilation. Early brain responses (N100m) reflect underlying phonemic categories, ignoring surface form changes during speech processing.
Area of Science:
- Psycholinguistics
- Auditory Neuroscience
- Phonetics
Background:
- Speech perception involves mapping sounds to phonological categories.
- Connected speech processes like assimilation can challenge accurate categorization.
- Investigating whether speech perception corrects assimilation errors immediately or automatically is crucial.
Purpose of the Study:
- To determine if speech perception systems correct assimilation errors in real-time or if they are processed automatically.
- To compare behavioral categorization with early neural responses to assimilated speech sounds.
- To investigate the neural processing of phonemic contrasts and acoustic variations.
Main Methods:
- Behavioral two-alternative forced-choice tasks with isolated German nasal sounds (/m/, /n/, assimilated /n'/).
- Magnetoencephalography (MEG) to record the N100m brain response to the same stimuli.
- Distributed source modeling to analyze neural processing of phonemic differences and pitch variation.
Main Results:
- Behavioral tasks showed participants correctly categorized /n/ and /m/, but perceived assimilated nasals as /m/.
- Magnetoencephalography (MEG) revealed that the early N100m response automatically ignored assimilation, reflecting the underlying phonemic category.
- Phonemic differences were processed left-laterally, while pitch variation was processed bilaterally in temporal regions.
Conclusions:
- Explicit categorization focuses on surface acoustic form and perceived changes.
- Early, automatic neural responses (N100m) accurately reflect underlying phonemic categories, overriding surface assimilation.
- The brain's automatic categorization mechanism is sensitive to subtle phonetic cues, even when they deviate from the surface form.
