The amplitude of the mismatch negativity response for acoustic within-category deviations in speech stimuli was investigated by presenting participants with different exemplars of the vowel /i/ in an odd-ball paradigm. The deviants differed from the standard either in terms of fundamental frequency, the first formant, or the second formant. Changes in fundamental frequency are generally more salient than changes in the first formant, which in turn are more salient than changes in the second formant. The mismatch negativity response was expected to reflect this with greater amplitude for more salient deviations. The fundamental frequency deviants did indeed result in greater amplitude than both first formant deviants and second formant deviants, but no difference was found between the first formant deviants and the second formant deviants. It is concluded that greater difference between standard and within-category deviants across different acoustic dimensions results in greater mismatch negativity amplitude, suggesting that the processing of linguistically irrelevant changes in speech sounds may be processed similar to nonspeech sound changes.

The mismatch negativity (MMN) response is influenced by the magnitude of the acoustic difference between standard and deviant, and the response is typically larger to linguistically relevant changes than to linguistically irrelevant changes. Linguistically relevant changes between standard and deviant typically co-occur with differences between the two acoustic signals. It is therefore not straightforward to determine the contribution of each of those two factors to the MMN response. This study investigated whether spectrally rotated speech can be used to determine the impact of the acoustic difference on the MMN response to a combined linguistic and acoustic change between standard and deviant. Changes between rotated vowels elicited an MMN of comparable amplitude to the one elicited by a within-category vowel change, whereas the between-category vowel change resulted in an MMN amplitude of greater magnitude. A change between rotated vowels resulted in an MMN ampltude more similar to that of a within-vowel change than a complex tone change did. This suggests that the MMN amplitude reflecting the acoustic difference between two speech sounds can be well approximated by the MMN amplitude elicited in response to their rotated counterparts, in turn making it possible to estimate the part of the response specific to the linguistic difference.

During the first year of life, infants shift their focus in speech perception from acoustic to linguistic information. This perceptual reorganization is related to exposure, and a direct relation has been demonstrated previously between amount of daily language exposure and mismatch response (MMR) amplitude to a native consonant contrast at around one year of age. The present study investigates the same relation between amount of speech exposure and MMR amplitude to a native vowel contrast at four to eight months of age. Importantly, the present study uses spectrally rotated speech in an effort to take general neural maturation into account. The amplitude of the part of the MMR that is tied specifically to speech processing correlates with amount of daily speech exposure, as estimated using the LENA system.

The aim of the present study is to further the understanding of the relationship between perceptual categorization and exposure to different frequency distributions of sounds. Previous studies have shown that speech sound discrimination proficiency is in- fluenced by exposure to different distributions of speech sound continua varying along one or several acoustic dimensions, both in adults and in infants. In the current study, adults were presented with either a bimodal or a unimodal frequency distri- bution of spectrally rotated sounds along a continuum (a vowel continuum before rotation). Categorization of the sounds, quantified as amplitude of the event-related potential (ERP) component mismatch negativity (MMN) in response to two of the sounds, was measured before and after exposure. It was expected that the bimodal group would have a larger MMN amplitude after exposure whereas the unimodal group would have a smaller MMN amplitude after exposure. Contrary to expectations, the MMN amplitude was smaller overall after exposure, and no difference was found between groups. This suggests that either the previously reported sensitivity to frequency distributions of speech sounds is not present for non-speech sounds, or the MMN amplitude is not a sensitive enough measure of categorization to detect an influence from passive exposure, or both.