Neurophysiological Correlates of Asymmetries in Vowel Perception: An English-French Cross-Linguistic Event-Related Potential Study

German infants' discrimination of the English /æ/-/ɛ/ contrast: Evidence from a cross-sectional and a longitudinal study

Young infants can discriminate many non-native sounds, but the discrimination ability is thought to decrease within the first year of life due to perceptual attunement. However, most studies tested infants' perception cross-sectionally, without examining within-group change. To this end, the current study tested German infants' discrimination of the English /æ/-/ɛ/ contrast both cross-sectionally and longitudinally using the visual habituation technique. In Experiment 1, 96 German-learning infants were tested cross-sectionally at 5-6, 8-9 and 12-13 months. Linear mixed-effects models revealed that while the 5-6-month-olds did not discriminate the contrast, the 8-9- and 12-13-month-olds showed signs of discrimination only when they were habituated with /ɛ/, in line with previous findings suggesting that changes from central to peripheral vowels in the F1/F2 vowel space are more noticeable than in the reverse direction. Moreover, the 8-9-month-olds showed a novelty preference, while the 12-13-month-olds showed a familiarity preference. In Experiment 2, the infants tested at 5-6 months in Experiment 1 were tested again at 8-9 and 12-13 months. Fifteen infants completed the three experiments. Here, only the 12-13-month-olds discriminated the contrast by showing a novelty preference but only when habituated with /æ/. Overall, both experiments showed gradual development of discrimination ability across the first year, which challenges the assumptions of perceptual attunement. We propose that the perceptual sensitivity for a non-native vocalic contrast can improve during development. The change in perceptual asymmetry tells us that the direction of asymmetry is not universal and can be altered by linguistic experience. The change from novelty to familiarity preference may be due to the emerging preference for the more native-like vowel as well as the effect of repeating experiments with the same infants. In sum, our cross-sectional and longitudinal results overlap broadly, but the potential effect of repeating experiments must be considered when interpreting longitudinal studies.

Can the Mismatch Negativity Really Be Elicited by Abstract Linguistic Contrasts?

The mismatch negativity (MMN) is an event-related potential component that reflects pre-attentive change detection in the brain. As an electrophysiological index of processing that responds to differences in incoming consecutive stimuli, the MMN can be elicited through, for example, the presentation of two different categories of sounds in an oddball paradigm where sounds from the "standard" category occur frequently and sounds from the "deviant" category occur rarely. The specificity of what can elicit the MMN is yet to be fully defined. Here we test whether the MMN can be generated by an abstract linguistic contrast with no reliable acoustic cue. Previous studies have shown that the way in which an acoustic cue is used to elicit MMN is influenced by linguistic knowledge, but have not shown that a nonacoustic, abstract linguistic contrast can itself elicit MMN. In this study, we test the strongest interpretation of the claim that the MMN can be generated through a purely linguistic contrast by contrasting tenses in ablauting irregular English verbs (where there is no reliable acoustic cue for tense). We find that this contrast elicits a negativity, as do other linguistic contrasts previously shown to elicit MMN (a contrast between phonologically voiced and phonologically voiceless segments and a purely acoustic contrast between aspirated and unaspirated segments). The findings provide evidence that the MMN is indeed sensitive to purely abstract linguistic categories.

Reaction time for correct identification of vowels in consonant-vowel syllables and of vowel segments

Reaction times for correct vowel identification were measured to determine the effects of intertrial intervals, vowel, and cue type. Thirteen adults with normal hearing, aged 20-38 years old, participated. Stimuli included three naturally produced syllables (/ba/ /bi/ /bu/) presented whole or segmented to isolate the formant transition or static formant center. Participants identified the vowel presented via loudspeaker by mouse click. Results showed a significant effect of intertrial intervals, no significant effect of cue type, and a significant vowel effect-suggesting that feedback occurs, vowel identification may depend on cue duration, and vowel bias may stem from focal structure.

Unified Coding of Spectral and Temporal Phonetic Cues: Electrophysiological Evidence for Abstract Phonological Features

Spoken word recognition models and phonological theory propose that abstract features play a central role in speech processing. It remains unknown, however, whether auditory cortex encodes linguistic features in a manner beyond the phonetic properties of the speech sounds themselves. We took advantage of the fact that English phonology functionally codes stops and fricatives as voiced or voiceless with two distinct phonetic cues: Fricatives use a spectral cue, whereas stops use a temporal cue. Evidence that these cues can be grouped together would indicate the disjunctive coding of distinct phonetic cues into a functionally defined abstract phonological feature. In English, the voicing feature, which distinguishes the consonants [s] and [t] from [z] and [d], respectively, is hypothesized to be specified only for voiceless consonants (e.g., [s t]). Here, participants listened to syllables in a many-to-one oddball design, while their EEG was recorded. In one block, both voiceless stops and fricatives were the standards. In the other block, both voiced stops and fricatives were the standards. A critical design element was the presence of intercategory variation within the standards. Therefore, a many-to-one relationship, which is necessary to elicit an MMN, existed only if the stop and fricative standards were grouped together. In addition to the ERPs, event-related spectral power was also analyzed. Results showed an MMN effect in the voiceless standards block—an asymmetric MMN—in a time window consistent with processing in auditory cortex, as well as increased prestimulus beta-band oscillatory power to voiceless standards. These findings suggest that (i) there is an auditory memory trace of the standards based on the shared (voiceless) feature, which is only functionally defined; (ii) voiced consonants are underspecified; and (iii) features can serve as a basis for predictive processing. Taken together, these results point toward auditory cortex's ability to functionally code distinct phonetic cues together and suggest that abstract features can be used to parse the continuous acoustic signal.

Newborns' neural processing of native vowels reveals directional asymmetries

Prenatal learning of speech rhythm and melody is well documented. Much less is known about the earliest acquisition of segmental speech categories. We tested whether newborn infants perceive native vowels, but not nonspeech sounds, through some existing (proto-)categories, and whether they do so more robustly for some vowels than for others. Sensory event-related potentials (ERP), and mismatch responses (MMR), were obtained from 104 neonates acquiring Czech. The ERPs elicited by vowels were larger than the ERPs to nonspeech sounds, and reflected the differences between the individual vowel categories. The MMRs to changes in vowels but not in nonspeech sounds revealed left-lateralized asymmetrical processing patterns: a change from a focal [a] to a nonfocal [ɛ], and the change from short [ɛ] to long [ɛ:] elicited more negative MMR responses than reverse changes. Contrary to predictions, we did not find evidence of a developmental advantage for vowel length contrasts (supposedly most readily available in utero) over vowel quality contrasts (supposedly less salient in utero). An explanation for these asymmetries in terms of differential degree of prior phonetic warping of speech sounds is proposed. Future studies with newborns with different language backgrounds should test whether the prenatal learning scenario proposed here is plausible.

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Newborns’ processing of native vowels and comparable nonspeech sounds differ.

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Durational and spectral differences in stimuli were more clearly reflected by the ERPs to vowels, compared to tone complexes.

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Directional asymmetries were detected in the mismatch responses to vowel deviants.

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In the left hemisphere, a change in vowels from focal to nonfocal and from short to long resulted in a more negative MMR.

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The findings may be explained by phonetic learning prior to the 3rd day after birth.