Perceptual reorganization of vowels : Separating the linguistic and acoustic parts of the mismatch response

Sammanfattning: During the first year of life, infants go from perceiving speech sounds primarily based on their acoustic characteristics, to perceiving speech sounds as belonging to speech sound categories relevant in their native language(s). The transition is apparent in that very young infants typically discriminate both native and non-native speech sound contrasts, whereas older infants show better discrimination for native contrasts and worse or no discrimi­na­tion for non-native contrasts. The rate of this perceptual reorganization depends, among other things, on the salience of the relevant speech sounds within the speech signal. As such, the perceptual reorganization of vowels and lexical tone typically precedes the perceptual reorganization of consonants.Perceptual reorganizatoin of speech sounds is often demonstrated by measuring in­fants’ discrimination of specific speech sound contrasts across development. One way of measuring discriminatory ability is to use the mismatch response (MMR). This is a brain response that can be measured using external electroencephalography re­cord­ings. Pre­senting an oddball (deviant) stimulus among a series of standard stimuli elicits a response that, in adults, correlates well with behavioral discrimination. When the two stimuli are speech sounds contrastive in the listeners’ language, the response arguably reflects both acoustic and linguistic processing. In infants, the response is less studied, but has nevertheless already proven useful for studies on the perceptual reorganization of speech sounds.The present thesis documents a series of studies with the end game of investigating how amount of speech exposure influences the perceptual reorganization, and whe­ther the learning mechanisms involved in speech sound cate­gory learning is specific to speech or domain-general. In order to be able to compare MMR results across diffe­rent age groups in infancy, a non-speech control condition needed to be devised however, to account for changes in the MMR across development that are attributable to general brain matura­tion rather than language development specifically.Findings of studies incorporated in the thesis show that spectrally rotated speech can be used to approximate the acoustic part of the MMR in adults. Subtracting the acoustic part of the MMR from the full MMR thus estimates the part of the MMR that is linked to linguistic, rather than acoustic, processing. The strength of this linguistic part of the MMR in four- and eight-month-old infants is directly related to the daily amount of speech that the infants are exposed to. No evidence of distributional learning of non-speech auditory categories was demonstrated in adults, but the results together with previous research generated hypo­theses for future study.In conclusion, the research performed within the scope of this thesis highlight the need of a non-speech control condition for use in developmental speech perception studies using the MMR, demonstrates the viability of one such non-speech control condition, and points toward relevant future research within the topic of speech sound category development.

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