Asymmetries In Vowel Perception Research Articles

Asymmetries in Infants' Vowel Perception: Changes in Vowel Discrimination in German Learning 6- and 9-Month-Old Infants.

Infants' speech perception is characterized by substantial changes during the first year of life that attune the processing mechanisms to the specific properties of the ambient language. This paper focuses on these developmental changes in vowel perception. More specifically, the emergence and potential cause of perceptual asymmetries in vowel perception are investigated by an experimental study on German 6- and 9-month-olds' discrimination of a vowel contrast that is not phonemic in German. Results show discrimination without any asymmetry in the 6-month-olds but an asymmetrical pattern with better performance when the vowel changes from the less focal to the more focal vowel than vice versa by the 9-month-olds. The results concerning the asymmetries are compatible with the Natural Referent Framework as well as with the Native Language Magnet model. Our results foster two main conclusions. First, bi-directional testing must be mandatory when testing vowel perception. Second, when testing non-native vowel perception, the relation of the stimuli to the native language vowel system has to be considered very carefully as this system impacts the perception of non-native vowels.

Asymmetries in vowel perception align with focalization patterns, not peripherality: Acoustic evidence from Canadian French /e/ and /Ø/

The question of whether asymmetries in vowel perception stem from peripherality (location in F1/F2 vowel space) or focalization patterns (formant convergence) was examined. Prior studies with Danish and Dutch listeners reported asymmetric perception of /e-Ø/ such that discriminating a change from /e/ to /Ø/ was easier compared to the reverse direction (/Ø/ to /e/). Acoustic analyses of test stimuli suggest that focalization patterns predict this asymmetry whereas a peripherality-based prediction fails. Although this interpretation is compelling, it is based on analyses of several vowel tokens produced by a single talker. To confirm the reliability of these patterns we obtained acoustic and electromagnetic articulography recordings of Canadian French /e/ and /Ø/ productions from a broader sample of talkers (n = 20): 400 vowels/10 productions per vowel. Acoustic analyses revealed a robust pattern for all talkers: F1 and F2 (also F1 and F3) were spectrally closer for /Ø/ compared to /e/, and /e/ was also more peripheral than /Ø/ in F1/F2 space. These data align with the earlier (single-talker) analyses and confirm that /Ø/ is a more focal vowel than /e/. We will also discuss articulatory analyses of these French vowel productions that focus on how vocal-tract constrictions relate to focalization and peripherality.

Predicting asymmetries in vowel perception: Formant convergence succeeds where peripherality fails

Vowel discrimination is often asymmetric such that discriminating the same vowel contrast is easier in one direction compared to the opposite direction. According to the Natural Referent Vowel (NRV) framework, these asymmetries reveal a perceptual bias favoring acoustic vowel signals produced with more extreme vocalic gestures, which act as natural referent vowels. The NR vowel within a contrast typically falls in a more peripheral location within articulatory/acoustic vowel space (defined by F1 and F2) and with F1 and F2 in closer proximity. However, these properties do not always align, as in the case of the /e/-/Ø/ contrast. We here compared findings across three studies where asymmetries were observed during discrimination of this contrast. Peripherality predicts an asymmetry such that perception would be better in the /Ø/□/e/ direction. However, all three studies showed better performance in the opposite direction, which also aligns with the prediction based on formant convergence (derived from reported acoustic measures for each stimulus set). These findings suggest that this perceptual bias is shaped by formant convergence, rather than peripherality per se, which is presumably tied to the extent of vocal-tract constriction. A follow-up study will obtain articulographic recordings of these vocalic gestures to quantify this articulatory-acoustic relation.

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

Behavioral studies examining vowel perception in infancy indicate that, for many vowel contrasts, the ease of discrimination changes depending on the order of stimulus presentation, regardless of the language from which the contrast is drawn and the ambient language that infants have experienced. By adulthood, linguistic experience has altered vowel perception; analogous asymmetries are observed for non−native contrasts but are mitigated for native contrasts. Although these directional effects are well documented behaviorally, the brain mechanisms underlying them are poorly understood. In the present study we begin to address this gap. We first review recent behavioral work which shows that vowel perception asymmetries derive from phonetic encoding strategies, rather than general auditory processes. Two existing theoretical models–the Natural Referent Vowel framework and the Native Language Magnet model–are invoked as a means of interpreting these findings. Then we present the results of a neurophysiological study which builds on this prior work. Using event-related brain potentials, we first measured and assessed the mismatch negativity response (MMN, a passive neurophysiological index of auditory change detection) in English and French native-speaking adults to synthetic vowels that either spanned two different phonetic categories (/y/vs./u/) or fell within the same category (/u/). Stimulus presentation was organized such that each vowel was presented as standard and as deviant in different blocks. The vowels were presented with a long (1,600-ms) inter-stimulus interval to restrict access to short-term memory traces and tap into a “phonetic mode” of processing. MMN analyses revealed weak asymmetry effects regardless of the (i) vowel contrast, (ii) language group, and (iii) MMN time window. Then, we conducted time-frequency analyses of the standard epochs for each vowel. In contrast to the MMN analysis, time-frequency analysis revealed significant differences in brain oscillations in the theta band (4–8 Hz), which have been linked to attention and processing efficiency. Collectively, these findings suggest that early-latency (pre-attentive) mismatch responses may not be a strong neurophysiological correlate of asymmetric behavioral vowel discrimination. Rather, asymmetries may reflect differences in neural processing efficiency for vowels with certain inherent acoustic-phonetic properties, as revealed by theta oscillatory activity.

Disentangling the roles of formant proximity and stimulus prototypicality in adult vowel perception.

The present investigation examined the extent to which asymmetries in vowel perception derive from a sensitivity to focalization (formant proximity), stimulus prototypicality, or both. English-speaking adults identified, rated, and discriminated a vowel series that spanned a less-focal/prototypic English /u/ and a more-focal/prototypic French /u/ exemplar. Discrimination pairs included one-step, two-step, and three-step intervals along the series. Asymmetries predicted by both focalization and prototype effects emerged when discrimination step-size was varied. The findings indicate that both generic/universal and language-specific biases shape vowel perception in adults; the latter are challenging to isolate without well-controlled stimuli and appropriately scaled discrimination tasks.

Asymmetric discrimination of nonspeech tonal analogues of vowels.

Directional asymmetries reveal a universal bias in vowel perception favoring extreme vocalic articulations, which lead to acoustic vowel signals with dynamic formant trajectories and well-defined spectral prominences because of the convergence of adjacent formants. The present experiments investigated whether this bias reflects speech-specific processes or general properties of spectral processing in the auditory system. Toward this end, we examined whether analogous asymmetries in perception arise with nonspeech tonal analogues that approximate some of the dynamic and static spectral characteristics of naturally produced /u/ vowels executed with more versus less extreme lip gestures. We found a qualitatively similar but weaker directional effect with 2-component tones varying in both the dynamic changes and proximity of their spectral energies. In subsequent experiments, we pinned down the phenomenon using tones that varied in 1 or both of these 2 acoustic characteristics. We found comparable asymmetries with tones that differed exclusively in their spectral dynamics, and no asymmetries with tones that differed exclusively in their spectral proximity or both spectral features. We interpret these findings as evidence that dynamic spectral changes are a critical cue for eliciting asymmetries in nonspeech tone perception, but that the potential contribution of general auditory processes to asymmetries in vowel perception is limited. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Directional Asymmetries in Vowel Perception of Adult Nonnative Listeners Do Not Change Over Time With Language Experience.

This study tested an assumption of the Natural Referent Vowel (Polka & Bohn, 2011) framework, namely, that directional asymmetries in adult vowel perception can be influenced by language experience. Data from participants reported in Escudero and Williams (2014) were analyzed. Spanish participants categorized the Dutch vowels /aː/ and /ɑ/ in 2 separate sessions: before and after vowel distributional training. Sessions were 12 months apart. Categorization was assessed using the XAB task, where on each trial participants heard 3 sounds sequentially (first X, then A, then B) and had to decide whether X was more similar to A or B. Before training, participants exhibited a directional asymmetry in line with the prediction of Natural Referent Vowel. Specifically, Spanish listeners performed worse when the vowel change from X to A was a change from peripheral to central vowel (/ɑ/ to /aː/). However, this asymmetry was maintained 12 months later, even though distributional training improved vowel categorization performance. Improvements in adult nonnative vowel categorization accuracy are not explained by attenuation of directional asymmetries. Directional asymmetries in vowel perception are altered during native language acquisition, but may possibly be impervious to nonnative language experiences in adulthood.

Commentary: Revisiting vocal perception in non-human animals: a review of vowel discrimination, speaker voice recognition, and speaker normalization.

Comparative research provides a unique window into our understanding of human vocal perception. We commend Kriengwatana, Escudero, and ten Cate (KEtC) for providing a much-needed review of this diverse literature. Their appraisal of three research areas highlights conceptual and empirical gaps, while also pointing to fruitful directions for future research. This commentary addresses the literature on asymmetries in vowel perception. In their review of this topic KEtC focus on vowel contrasts that have revealed directional asymmetries in infants and non-human animals. We offer some clarification with respect to these stimulus issues and highlight another aspect of this research landscape—the role of task demands—that must also guide future comparative investigations.

Asymmetries in vowel perception: Effects of formant convergence and category “goodness”

The mechanisms underlying directional asymmetries in vowel perception have been the subject of considerable debate. One account—the Natural Referent Vowel (NRV) framework—suggests that asymmetries reflect a language-universal perceptual bias, such that listeners are predisposed to attend to vowels with greater formant convergence (Polka & Bohn, 2011). A second (but not mutually exclusive) account—the Native Language Magnet (NLM) theory—suggests that asymmetries reflect an experience-dependent language-specific bias favoring “good” exemplars of native language vowel categories (Kuhl, 1993). We tested the above hypotheses by investigating whether listeners, from different language backgrounds, display asymmetries influenced by formant proximity and/or language experience. Specifically, we examined monolingual English and French listeners’ performance in a within-category AX vowel discrimination task, using variants of /u/ that systematically differed in both their degree of formant proximity (between F1 and F2) and category “goodness” judgments. Results revealed asymmetries that pattern as predicted by NRV when pairs of /u/ tokens exhibited a relatively larger difference in their F1-F2 convergence patterns, and as predicted by NLM when the pairs of /u/ tokens exhibited a relatively smaller difference in their F1-F2 convergence patterns. These findings suggest that language-universal perceptual biases and specific language experience interact to shape vowel perception.

Revisiting vocal perception in non-human animals: a review of vowel discrimination, speaker voice recognition, and speaker normalization.

The extent to which human speech perception evolved by taking advantage of predispositions and pre-existing features of vertebrate auditory and cognitive systems remains a central question in the evolution of speech. This paper reviews asymmetries in vowel perception, speaker voice recognition, and speaker normalization in non-human animals – topics that have not been thoroughly discussed in relation to the abilities of non-human animals, but are nonetheless important aspects of vocal perception. Throughout this paper we demonstrate that addressing these issues in non-human animals is relevant and worthwhile because many non-human animals must deal with similar issues in their natural environment. That is, they must also discriminate between similar-sounding vocalizations, determine signaler identity from vocalizations, and resolve signaler-dependent variation in vocalizations from conspecifics. Overall, we find that, although plausible, the current evidence is insufficiently strong to conclude that directional asymmetries in vowel perception are specific to humans, or that non-human animals can use voice characteristics to recognize human individuals. However, we do find some indication that non-human animals can normalize speaker differences. Accordingly, we identify avenues for future research that would greatly improve and advance our understanding of these topics.

Asymmetries in vowel perception: Do they arise from focalization, perceptual magnets, or both?

While directional asymmetries are ubiquitous in cross-language studies of vowel perception, their underlying mechanisms have not been established. One hypothesis is that listeners display a universal perceptual bias favoring vowels with greater formant frequency convergence, or focalization (Polka and Bohn, 2011). A second, but not mutually exclusive, hypothesis is that listeners are biased toward prototypical vowel exemplars in their native language (Kuhl, 1993). In a test of these hypotheses, English listeners discriminated synthesized English /u/ and French /u/ vowels presented in pairs. While the French /u/ tokens exhibit greater formant convergence (between F1 and F2), English listeners have previously been shown to rate the English /u/ tokens as “better” instances of the category (Molnar, 2010). Preliminary results demonstrate that the degree of focalization affects vowel discrimination. When discriminating vowel changes presented in the direction going from the more focal (French) to less focal (English) /u/ vowels, English listeners' reaction times were slower, relative to the same changes presented in the reverse direction. These results suggest that listeners treat the more focal vowels as perceptual reference points. Additional data collection with French listeners is ongoing. The implications of these findings for theories of vowel perception will be discussed.

The Interplay Between Input and Initial Biases: Asymmetries in Vowel Perception During the First Year of Life

Vowels with extreme articulatory-acoustic properties act as natural referents. Infant perceptual asymmetries point to an underlying bias favoring these referent vowels. However, as language experience is gathered, distributional frequency of speech sounds could modify this initial bias. The perception of the /i/-/e/ contrast was explored in 144 Catalan- and Spanish-learning infants (2 languages with a different distribution of vowel frequency of occurrence) at 4, 6, and 12 months. The results confirmed an acoustic bias at 4 and 6 months in all infants. However, at 12 months, discrimination was not affected by the acoustic bias but by the frequency of occurrence of the vowel.

The role of spectral and durational properties on hemispheric asymmetries in vowel perception

The aim of the current study is to investigate potential hemispheric asymmetries in the perception of vowels and the influence of different time scales on such asymmetries. Activation patterns for naturally produced vowels were examined at three durations encompassing a short (75ms), medium (150ms), and long (300ms) integration time window in a discrimination task. A set of 5 corresponding non-speech sine wave tones were created with frequencies matching the second formant of each vowel. Consistent with earlier hypotheses, there was a right hemisphere preference in the superior temporal gyrus for the processing of spectral information for both vowel and tone stimuli. However, observed laterality differences for vowels and tones were a function of heightened right hemisphere sensitivity to long integration windows, whereas the left hemisphere showed sensitivity to both long and short integration windows. Although there were a number of similarities in the processing of vowels and tones, differences also emerged suggesting that even fairly early in the processing stream at the level of the STG, different mechanisms are recruited for processing vowels and tones.

Training of English vowel perception by Finnish speakers to focus on spectral rather than durational cues

This study used the High-Variability Phonetic Training (HVPT) technique to train Finnish speakers to distinguish English vowels. It was found that Finnish speakers tend to use durational cues (which are phonemically relevant in their own language) to make a vowel category distinction rather than the relevant spectral cues. We used duration-modified stimuli with a HVPT program to 'force' the use of spectral cues. We focused on the /i/ (as in 'feet') vs /I/ (as in 'fit') vowel contrast and tested behavioural performance using a perceptual identification task. We also measured the Mismatch negativity (MMN) component of auditory event-related potential (ERP) before and after the training to look at changes in brain responses. The worst pre-test performance was for the 'modified duration' condition (i.e. where the learner had no choice but to rely on spectral cues). There were also asymmetries in vowel perception in both behavioural and MMN tasks, with the detection of /i/ being more difficult compared to the detection of /I/. Nevertheless, training did result in marked improvement of the most problematic contrasts for Finnish speakers.

Asymmetries in vowel perception, in the context of the Dispersion–Focalisation Theory

In a recent paper in this journal, Polka and Bohn [Polka, L., Bohn, O.-S., 2003. Asymmetries in vowel perception. Speech Communication 41, 221–231] display a robust asymmetry effect in vowel discrimination, present in infants as well as adults. They interpret this effect as a preference for peripheral vowels, providing an anchor for comparison. We discuss their data in the framework of the Dispersion–Focalisation Theory of vowel systems. We show that focalisation, that is the convergence between two consecutive formants in a vowel spectrum, is likely to provide the ground for anchor vowels, by increasing their perceptual salience. This enables to explain why [y] is an anchor vowel, as well as [i], [a] or [u]. Furthermore, we relate the asymmetry data to an old experiment we had done on the discrimination of focal vs. non-focal vowels. Altogether, it appears that focal vowels, more salient in perception, provide both a stable percept and a reference for comparison and categorisation.

Asymmetries in vowel perception

Asymmetries in vowel perception occur such that discrimination of a vowel change presented in one direction is easier compared to the same change presented in the reverse direction. Although such effects have been repeatedly reported in the literature there has been little effort to explain when or why they occur. We review studies that report asymmetries in vowel perception in infants and propose that these data indicate that babies are predisposed to respond differently to vowels that occupy different positions in the articulatory/acoustic vowel space (defined by F1–F2) such that the more peripheral vowel within a contrast serves as a reference or perceptual anchor. As such, these asymmetries reveal a language-universal perceptual bias that infants bring to the task of vowel discrimination. We present some new data that support our peripherality hypothesis and then compare the data on asymmetries in human infants with findings obtained with birds and cats. This comparison suggests that asymmetries evident in humans are unlikely to reflect general auditory mechanisms. Several important directions for further research are outlined and some potential implications of these asymmetries for understanding speech development are discussed.