Phonetic Categories Research Articles

Individual differences in the perception of phonetic category structure predict speech-in-noise performance.

Speech sounds exist in a complex acoustic-phonetic space, and listeners vary in the extent to which they are sensitive to variability within the speech sound category ("gradience") and the degree to which they show stable, consistent responses to phonetic stimuli. Here, we investigate the hypothesis that individual differences in the perception of the sound categories of one's language may aid speech-in-noise performance across the adult lifespan. Declines in speech-in-noise performance are well documented in healthy aging, and are, unsurprisingly, associated with differences in hearing ability. Nonetheless, hearing status and age are incomplete predictors of speech-in-noise performance, and long-standing research suggests that this ability draws on more complex cognitive and perceptual factors. In this study, a group of adults ranging in age from 18 to 67 years performed online assessments designed to measure phonetic category sensitivity, questionnaires querying recent noise exposure history and demographic factors, and crucially, a test of speech-in-noise perception. Results show that individual differences in the perception of two consonant contrasts significantly predict speech-in-noise performance, even after accounting for age and recent noise exposure history. This finding supports the hypothesis that individual differences in sensitivity to phonetic categories mediates speech perception in challenging listening situations.

Functional benefits of continuous vs. categorical listening strategies on the neural encoding and perception of noise-degraded speech

Acoustic information in speech changes continuously, yet listeners form discrete perceptual categories to ease the demands of perception. Being a more continuous/gradient as opposed to a more discrete/categorical listener may be further advantageous for understanding speech in noise by increasing perceptual flexibility and resolving ambiguity. The degree to which a listener’s responses to a continuum of speech sounds are categorical versus continuous can be quantified using visual analog scaling (VAS) during speech labeling tasks. Here, we recorded event-related brain potentials (ERPs) to vowels along an acoustic–phonetic continuum (/u/ to /a/) while listeners categorized phonemes in both clean and noise conditions. Behavior was assessed using standard two alternative forced choice (2AFC) and VAS paradigms to evaluate categorization under task structures that promote discrete vs. continuous hearing, respectively. Behaviorally, identification curves were steeper under 2AFC vs. VAS categorization but were relatively immune to noise, suggesting robust access to abstract, phonetic categories even under signal degradation. Behavioral slopes were correlated with listeners’ QuickSIN scores; shallower slopes corresponded with better speech in noise performance, suggesting a perceptual advantage to noise degraded speech comprehension conferred by a more gradient listening strategy. At the neural level, P2 amplitudes and latencies of the ERPs were modulated by task and noise; VAS responses were larger and showed greater noise-related latency delays than 2AFC responses. More gradient responders had smaller shifts in ERP latency with noise, suggesting their neural encoding of speech was more resilient to noise degradation. Interestingly, source-resolved ERPs showed that more gradient listening was also correlated with stronger neural responses in left superior temporal gyrus. Our results demonstrate that listening strategy modulates the categorical organization of speech and behavioral success, with more continuous/gradient listening being advantageous to sentential speech in noise perception.

Auditory Processing of Speech and Nonspeech in People Who Stutter.

We investigated speech and nonspeech auditory processing of temporal and spectral cues in people who do and do not stutter. We also asked whether self-reported stuttering severity was predicted by performance on the auditory processing measures. People who stutter (n = 23) and people who do not stutter (n = 28) completed a series of four auditory processing tasks online. These tasks consisted of speech and nonspeech stimuli differing in spectral or temporal cues. We then used independent-samples t-tests to assess differences in phonetic categorization slopes between groups and linear mixed-effects models to test differences in nonspeech auditory processing between stuttering and nonstuttering groups, and stuttering severity as a function of performance on all auditory processing tasks. We found statistically significant differences between people who do and do not stutter in phonetic categorization of a continuum differing in a temporal cue and in discrimination of nonspeech stimuli differing in a spectral cue. A significant proportion of variance in self-reported stuttering severity was predicted by performance on the auditory processing measures. Taken together, these results suggest that people who stutter process both speech and nonspeech auditory information differently than people who do not stutter and may point to subtle differences in auditory processing that could contribute to stuttering. We also note that these patterns could be the consequence of listening to one's own speech, rather than the cause of production differences.

Phonetic categorization in phonological lexical neighborhoods: Facilitatory and inhibitory effects

Phonetic processing, whereby the bottom-up speech signal is translated into higher-level phonological representations such as phonemes, has been demonstrated to be influenced by phonological lexical neighborhoods. Previous studies show facilitatory effects of lexicality and phonological neighborhood density on phonetic categorization. However, given the evidence for lexical competition in spoken word recognition, we hypothesize that there are concurrent facilitatory and inhibitory effects of phonological lexical neighborhoods on phonetic processing. In Experiments 1 and 2, participants categorized the onset phoneme in word-nonword and nonword-word acoustic continua. The results show that the target word of the continuum exhibits facilitatory lexical influences whereas rhyme neighbors inhibit phonetic categorization. The results support the hypothesis that sublexical phonetic processing is affected by multiple facilitatory and inhibitory lexical forces in the processing stream.

Neural correlates of phonetic categorization under auditory (phoneme) and visual (grapheme) modalities.

We tested whether the neural mechanisms of phonetic categorization are specific to speech sounds or generalize to graphemes (i.e., visual letters) of the same phonetic label. Given that linguistic experience shapes categorical processing, and letter-speech sound matching plays a crucial role during early reading acquisition, we hypothesized sound phoneme and visual grapheme tokens representing the same linguistic identity might recruit common neural substrates, despite originating from different sensory modalities. Behavioral and neuroelectric brain responses (ERPs) were acquired as participants categorized stimuli from sound (phoneme) and homologous letter (grapheme) continua each spanning a /da/ - /ga/ gradient. Behaviorally, listeners were faster and showed stronger categorization of phoneme compared to graphemes. At the neural level, multidimensional scaling of the EEG revealed responses self-organized in a categorial fashion such that tokens clustered within their respective modality beginning ∼150-250 ms after stimulus onset. Source-resolved ERPs further revealed modality-specific and overlapping brain regions supporting phonetic categorization. Left inferior frontal gyrus and auditory cortex showed stronger responses for sound category members compared to phonetically ambiguous tokens, whereas early visual cortices paralleled this categorical organization for graphemes. Auditory and visual categorization also recruited common visual association areas in extrastriate cortex but in opposite hemispheres (auditory = left; visual=right). Our findings reveal both auditory and visual sensory cortex supports categorical organization for phonetic labels within their respective modalities. However, a partial overlap in phoneme and grapheme processing among occipital brain areas implies the presence of an isomorphic, domain-general mapping for phonetic categories in dorsal visual system.

Compactness of Native Vowel Categories in Monolingual, Bilingual, and Multilingual Speakers: Is Category Compactness Affected by the Number of Languages Spoken?

Phonetic category compactness pertains to the degree of variation or dispersion within a specific category. Previous research has shown that more compact native (L1) categories in production have been related to the discrimination of non-native sounds in perception and production. The understanding of the factors influencing L1 category compactness remains limited. Some proposals suggest that compactness may be influenced by individual differences in cognitive processes. Alternatively, category compactness could be linked to linguistic factors, such as the number of languages spoken or the density of the phonological system. This study investigates the latter hypothesis. This study examined category compactness in perception for three L1 Spanish vowels /i/, /e/, and /a/ across four participant groups: 12 monolinguals, 31 functional monolinguals, 24 bilinguals, and 19 multilinguals. To measure compactness in perception, the study employed a perceptual categorization task consisting of synthesized variants of /i/, /e/, and /a/. Participants were asked to label these variants as either acceptable or unacceptable members of their L1 /i/, /e/, and /a/ categories. The findings revealed significant differences in category compactness between monolingual and bi/multilingual speakers. More specifically, bilingual and multilingual speakers had larger/less compact L1 vowel categories than monolinguals. The substantial variability in compactness across all groups suggests that compactness may be influenced by a range of other individual differences, besides the number of languages spoken.

Learning phonetic categories in infancy: The role of word-context information

Identifying the type of mechanisms at the core of phonetic categorization remains a central subject of research in infant language learning. Amongst different theories, one is that infants compute distributional information of phonemes based on their surrounding sounds (i.e., word context) such that phonemes that appear in different word contexts are more likely to be discriminated and categorized separately than phonemes that appear in similar word contexts. Following the procedure of Feldman et al. (2013a), we investigated the role of contextual information in the acquisition of phonetic categories in 8-month-old infants, using a non-native vowel contrast (English /ɒ/-/ʌ/). In Experiment 1, we established lack of discrimination of the non-native contrast without prior exposure to it. In Experiment 2, we manipulated the type of exposure prior to testing: half of the infants were exposed to minimal pair carriers (words that differ by one phoneme only; e.g., lituh and litoh), and the other half of the infants were exposed to non-minimal pair carriers (words formed by different phonemes; e.g., lituh and nutoh). All infants were tested for discrimination of the contrast (tuh vs. toh) presented as alternating (e.g., tuh-toh-tuh-toh) and non-alternating trials (e.g., tuh-tuh-tuh), as in Experiment 1. Infants in both conditions looked on average longer at alternating rather than non-alternating trials, suggesting that they discriminated the /ɒ/-/ʌ/ contrast after a brief exposure to the vowels embedded into words. Crucially, discrimination occurred regardless of whether words were minimal pair carriers or non-minimal pair carriers. A cross-experiment comparison revealed that infants showed different patterns of looking times based on whether they were exposed to the contrast before testing (Experiment 2) or not (Experiment 1). Our study shows that any type of word context helps infants to re-establish discrimination of non-native contrasts once sensitivity has been lost. These findings aid to better understand how the speech input modulates learning mechanisms during the establishment of phonetic categories in the first year of postnatal life.

Acoustic Analyses of L1 and L2 Vowel Interactions in Mandarin–Cantonese Late Bilinguals

While the focus of bilingual research is frequently on simultaneous or early bilingualism, the interactions between late bilinguals’ first language (L1) and second language (L2) have rarely been studied previously. To fill this research gap, the aim of the current study was to investigate the production of vowels in the L1 Mandarin and L2 Cantonese of Mandarin–Cantonese late bilinguals in Hong Kong. A production experiment was conducted with 22 Mandarin–Cantonese bilinguals, as well as with 20 native Mandarin speakers and 21 native Cantonese speakers. Acoustic analyses, including formants of and Euclidean distances between the vowels, were performed. Both vowel category assimilation and dissimilation were noted in the Mandarin–Cantonese bilinguals’ L1 and L2 vowel systems, suggesting interactions between the bilinguals’ L1 and L2 vowel categories. In general, the findings are in line with the hypotheses of the Speech Learning Model and its revised version, which state that L1–L2 phonetic interactions are inevitable, as there is a common phonetic space for storing the L1 and L2 phonetic categories, and that learners always have the ability to adapt their phonetic space. Future studies should refine the data elicitation method, increase the sample size and include more language pairs to better understand L1 and L2 phonetic interactions.

Acoustic Characteristics of Greek Vowels Produced by Adult Heritage Speakers of Albanian

Investigating heritage language (HL)-contact effects on the dominant language has received limited attention despite its importance in understanding the dynamic interplay between linguistic systems in situations of bilingualism. This study compares the acoustic characteristics of Greek vowels produced by heritage speakers (HSs) of Albanian and monolingual Greek speakers, aiming to identify potential differences and explain them. The participants were adult second-generation HSs of Albanian with Greek as their dominant language, born and raised in Greece. A control group of age-matched monolingual Greek speakers was included for comparison purposes. All participants engaged in a controlled speech production task, with the data segmented to extract acoustic values pertaining to the first three formants and the duration of Greek vowels. Bayesian regression models were employed for the subsequent statistical analysis. The results demonstrated differences in the first three formants of certain vowels and the duration of all vowels. These differences can be attributed to the crosslinguistic effect of HL on the dominant language, as well as the interplay between the dynamic and internalized language system of the speakers and the complex effect of the sociophonetic context. These outcomes contribute to the hypothesis positing the emergence of deflected phonetic categories among a distinctive group of bilinguals, namely HSs. Furthermore, this study underscores the significance of a comprehensive exploration of the sociophonetic context of HSs for a nuanced understanding of their phonetic patterns.

A mechanistic investigation of processing costs associated with cross-talker acoustic-phonetic variability

The acoustic realization of speech sounds may differ substantially across talkers, such that (for example) a sound interpreted as “s” in one voice might be considered a “sh” in another. In this talk, I will review select neurobiological and computational data illustrating how listeners cope with this variability. I will also review behavioral data showing how talker variability can hinder processing, with slower and/or less accurate processing when the listening environment contains multiple voices compared to one voice. I argue that at least three mechanisms underlie these multi-talker processing costs. First, multi-talker processing costs may arise because talker changes disrupt a listener’s ability to attend to the target talker. Second, I make the novel claim that talker changes might impose processing penalties by inducing uncertainty about whether the extant acoustic-to-phonetic mapping is appropriate, motivated by recent data that when multi-talker processing costs are attenuated by a preceding carrier phrase, this manifests as slowed responses to single-talker speech rather than improved processing of mixed-talker speech. Finally, talker variability costs may result from making on-the-fly adjustments to the mapping between acoustics and phonetic categories. Overall, this talk highlights the ways in which listeners contend with and are hindered by talker variability.

Age and category structure in phonetic category learning

When learning new speech sound categories in a second language, listeners must decide which sounds belong to the same category and which belong to different categories. This learning process is contingent on perceptual systems that change across the lifespan, which may, in turn, make certain categories easier to learn depending on which systems are employed by learners. In the present study, learners across a variety of ages categorize speech sounds taken from German that vary in their category structure (i.e., the complexity of the categories being learned within the stimulus space). Participants aged 7 to 70 are recruited and run at community sites such as museums and libraries. The preliminary dataset indicates that the simpler category structure saw strong age effects (with poorer performance on the younger and older ends of the age range and better performance at intermediate ages), while differences across ages were much smaller for the more complex category structure. These findings suggest that learning simpler category structures may rely on age-dependent learning systems, while learning more complex categories may rely on systems that are less age-dependent.

An auditory perspective on phonological development in infancy.

The auditory system encodes the phonetic features of languages by processing spectro-temporal modulations in speech, which can be described at two time scales: relatively slow amplitude variations over time (AM, further distinguished into the slowest <8-16 Hz and faster components 16-500 Hz), and frequency modulations (FM, oscillating at higher rates about 600-10 kHz). While adults require only the slowest AM cues to identify and discriminate speech sounds, infants have been shown to also require faster AM cues (>8-16 Hz) for similar tasks. Using an observer-based psychophysical method, this study measured the ability of typical-hearing 6-month-olds, 10-month-olds, and adults to detect a change in the vowel or consonant features of consonant-vowel syllables when temporal modulations are selectively degraded. Two acoustically degraded conditions were designed, replacing FM cues with pure tones in 32 frequency bands, and then extracting AM cues in each frequency band with two different low-pass cut- off frequencies: (1) half the bandwidth (Fast AM condition), (2) <8 Hz (Slow AM condition). In the Fast AM condition, results show that with reduced FM cues, 85% of 6-month-olds, 72.5% of 10-month-olds, and 100% of adults successfully categorize phonemes. Among participants who passed the Fast AM condition, 67% of 6-month-olds, 75% of 10-month-olds, and 95% of adults passed the Slow AM condition. Furthermore, across the three age groups, the proportion of participants able to detect phonetic category change did not differ between the vowel and consonant conditions. However, age-related differences were observed for vowel categorization: while the 6- and 10-month-old groups did not differ from one another, they both independently differed from adults. Moreover, for consonant categorization, 10-month-olds were more impacted by acoustic temporal degradation compared to 6-month-olds, and showed a greater decline in detection success rates between the Fast AM and Slow AM conditions. The degradation of FM and faster AM cues (>8 Hz) appears to strongly affect consonant processing at 10 months of age. These findings suggest that between 6 and 10 months, infants show different developmental trajectories in the perceptual weight of speech temporal acoustic cues for vowel and consonant processing, possibly linked to phonological attunement.

Asymmetrical Equivalence Classification – Cluster Affrication vs. Lenis Stops in the Speech of Polish Learners of English

According to the Speech Learning Model (Flege 1995), successful L2 phonological acquisition is facilitated by the formation of new phonetic categories in the L2. However, category formation may be hindered by equivalence classification, wherein speakers perceptually merge L1 and L2 sounds. This study examines L1 Polish learners of English, including a phonetic parameter that has received minimal attention: affrication of /tr-dr/ clusters in English. Two groups of speakers, comprising B1 level and C2 level learners, produced word lists containing both initial /tr-dr/ clusters, as well as singleton voiced stops /b, d, g/. The results revealed an asymmetry: both groups failed to suppress pre-voicing in /b, d, g/, but were successful in producing affricated clusters. A new category has therefore been formed for the clusters, but not for the singleton stops. Phonological implications of this finding are discussed.

Variability in cross-language and cross-dialect perception. How Irish and Chinese migrants process Australian English vowels.

We investigate how three adult groups- experienced L2 English listeners; experienced D2 (second dialect) listeners; and native L1/D1 listeners- categorise Australian English (AusE) lax front vowels /ɪ e æ/ in /hVt/, /hVl/ and /mVl/ environments in a forced-choice categorisation task of synthesised continua. In study 1, AusE listeners show predictable categorisations, with an effect of coarticulation raising the vowel in perception for nasal onset stimuli, and a following lateral lowering the vowel in perception. In study 2, Irish (D2) and Chinese listeners (L2) have different categorisations than AusE listeners, likely guided by their D1/L1. Coarticulation influences the D1/D2 groups in similar ways, but results in more difficulty and less agreement for the Chinese. We also investigate the role of extralinguistic factors. For the Chinese listeners, higher proficiency in English does not correlate with more Australian-like categorisation behaviour. However, having fewer Chinese in their social network results in more Australian-like categorisation for some stimuli. These findings lend partial support to the role of experience and exposure in L2/D2 contexts, whereby categorisation is likely still driven by native categories, with increased exposure leading to better mapping, but not to a restructuring of underlying phonetic categories.

Multisensory and lexical information in speech perception.

Both multisensory and lexical information are known to influence the perception of speech. However, an open question remains: is either source more fundamental to perceiving speech? In this perspective, we review the literature and argue that multisensory information plays a more fundamental role in speech perception than lexical information. Three sets of findings support this conclusion: first, reaction times and electroencephalographic signal latencies indicate that the effects of multisensory information on speech processing seem to occur earlier than the effects of lexical information. Second, non-auditory sensory input influences the perception of features that differentiate phonetic categories; thus, multisensory information determines what lexical information is ultimately processed. Finally, there is evidence that multisensory information helps form some lexical information as part of a phenomenon known as sound symbolism. These findings support a framework of speech perception that, while acknowledging the influential roles of both multisensory and lexical information, holds that multisensory information is more fundamental to the process.

Emergence of the cortical encoding of phonetic features in the first year of life

Even prior to producing their first words, infants are developing a sophisticated speech processing system, with robust word recognition present by 4–6 months of age. These emergent linguistic skills, observed with behavioural investigations, are likely to rely on increasingly sophisticated neural underpinnings. The infant brain is known to robustly track the speech envelope, however previous cortical tracking studies were unable to demonstrate the presence of phonetic feature encoding. Here we utilise temporal response functions computed from electrophysiological responses to nursery rhymes to investigate the cortical encoding of phonetic features in a longitudinal cohort of infants when aged 4, 7 and 11 months, as well as adults. The analyses reveal an increasingly detailed and acoustically invariant phonetic encoding emerging over the first year of life, providing neurophysiological evidence that the pre-verbal human cortex learns phonetic categories. By contrast, we found no credible evidence for age-related increases in cortical tracking of the acoustic spectrogram.

The acquisition of a novel phonetic category in a foreign language setting: Input versus phonological awareness

Input-related factors are fundamental for the acquisition of new second language (L2) phones (e.g., Flege et al., 1995). Nonetheless, evidence from instructional settings suggests that, in foreign language contexts, input alone may not be sufficient for the formation of new phonetic categories. In the present study, we investigated to what extent the acquisition of a novel L2 phone (/ð/) is associated with input-related variables and phonological awareness. First language (L1) Brazilian Portuguese speakers of English answered a language background, a phonological self-awareness questionnaire, and completed a paragraph reading task and a phonological awareness test. The recordings were submitted to acoustic analysis and accuracy assessment by Brazilian teachers of English. Linear mixed-effects models revealed that perceived accuracy was predicted by input quality and phonological self-awareness, suggesting that greater interaction with L1 speakers of the target language and heightened phonological self-awareness play an important role in the acquisition of the tested L2 phone.

Neural evidence of switch processes during semantic and phonetic foraging in human memory

Humans may retrieve words from memory by exploring and exploiting in "semantic space" similar to how nonhuman animals forage for resources in physical space. This has been studied using the verbal fluency test (VFT), in which participants generate words belonging to a semantic or phonetic category in a limited time. People produce bursts of related items during VFT, referred to as "clustering" and "switching." The strategic foraging model posits that cognitive search behavior is guided by a monitoring process which detects relevant declines in performance and then triggers the searcher to seek a new patch or cluster in memory after the current patch has been depleted. An alternative body of research proposes that this behavior can be explained by an undirected rather than strategic search process, such as random walks with or without random jumps to new parts of semantic space. This study contributes to this theoretical debate by testing for neural evidence of strategically timed switches during memory search. Thirty participants performed category and letter VFT during functional MRI. Responses were classified as cluster or switch events based on computational metrics of similarity and participant evaluations. Results showed greater hippocampal and posterior cerebellar activation during switching than clustering, even while controlling for interresponse times and linguistic distance. Furthermore, these regions exhibited ramping activity which increased during within-patch search leading up to switches. Findings support the strategic foraging model, clarifying how neural switch processes may guide memory search in a manner akin to foraging in patchy spatial environments.

Rapid neural changes during novel speech-sound learning: An fMRI and DTI study

While the functional and microstructural changes that occur when we learn new language skills are well documented, relatively little is known about the time course of these changes. Here a combined functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) study that tracks neural change over three days of learning Arabic phonetic categorization as a new language (L-training) is presented. Twenty adult native English-speaking (L-native) participants are scanned before and after training to perceive and produce L-training phonetic contrasts for one hour on three consecutive days. A third (Chinese) language is used as a control language (L-control). Behavioral results show significant performance improvement for L-training in both learnt tasks; the perception and production task. Imaging analysis reveals that, training-related hemodynamic fMRI signal and fractional anisotropy (FA) value increasing can be observed, in the left inferior frontal gyrus (LIFG) and positively correlated with behavioral improvement. Moreover, post training functional connectivity findings show a significant increasing between LIFG and left inferior parietal lobule for L-training. These results indicate that three hours of phonetic categorization learning causes functional and microstructural changes that are typically associated with much more long-term learning.

Effects of lexical frequency on the post-exposure magnitude of recalibration in lexically guided perceptual learning: An explorative analysis.

Listeners use lexical information to guide the mapping between acoustic signals and representations of speech sound. This process is known as perceptual learning and results in recalibration of phonetic categories. The current work examines the effect of lexical frequency of exposure words on the magnitude of recalibration. Results showed comparable levels of perceptual learning for listeners exposed to high-frequency vs low-frequency critical words, in line with empirical findings that suggest that if frequency affects recalibration, such effects may be difficult to detect. These findings warrant further empirical probing and theoretical characterization of the role of lexical frequency in perceptual learning.