The role of variation in the perception of accented speech

Phonetic variation has been considered a barrier that listeners must overcome in speech perception, but has been proved beneficial in category learning. In this paper, I show that listeners use within-speaker variation to accommodate gross categorical variation. Within the perceptual learning paradigm, listeners are exposed to p-initial words in English produced by a native speaker of French. Critically, listeners are trained on these words with either invariant or highly-variable VOTs. While a gross boundary shift is made for participants exposed to the variable VOTs, no such shift is observed after exposure to the invariant stimuli. These data suggest that increasing variation improves the mapping of perceptually mismatched stimuli.     

Generalization in perceptual learning for speech

Lexical context strongly influences listeners’ identification of ambiguous sounds. For example, a sound midway between /f/ and /s/ is reported as /f/ in “sheri_’” but as /s/ in “Pari_.” Norris, McQueen, and Cutler (2003) have demonstrated that after hearing such lexically determined phonemes, listeners expand their phonemic categories to include more ambiguous tokens than before. We tested whether listeners adjust their phonemic categories for a specific speaker: Do listeners learn a particular speaker’s “accent”? Similarly, we examined whether perceptual learning is specific to the particular ambiguous phonemes that listeners hear, or whether the adjustments generalize to related sounds. Participants heard ambiguous /d/ or /t/ phonemes during a lexical decision task. They then categorized sounds on /d/-/t/ and /b/-/p/ continua, either in the same voice that they had heard for lexical decision, or in a different voice. Perceptual learning generalized across both speaker and test continua: Changes in perceptual representations are robust and broadly tuned.     

Limited visual control of the intelligibility of speech in face-to-face dialogue

Speakers are thought to articulate individual words in running speech less carefully whenever additional nonacoustic information can help listeners recognize what is said (Fowler & Housum, 1987; Lieberman, 1963). Comparing single words excerpted from spontaneous dialogues and control tokens of the same words read by the same speakers in lists, Experiment 1 yielded a significant but general effect of visual context: Tokens introducing 71 new entities in dialogues in which participants could see one another’s faces were more degraded (less intelligible to 54 naive listeners) than were tokens of the same words from dialogues with sight lines blocked. Loss of clarity was not keyed to momentto-moment visual behavior. Subjects with clear sight lines looked at each other too rarely to account for the observed effect. Experiment 2 revealed that tokens of 60 words uttered while subjects were looking at each other were significantly less degraded (in length and in intelligibility to 72 subjects) vis-à-vis controls than were spontaneous tokens of the same words produced when subjects were looking elsewhere. Intelligibility loss was mitigated only when listeners looked at speakers. Two separate visual effects are discussed, one of the global availability and the other of the local use of the interlocutor’s face.     

Infants are sensitive to within-category variation in speech perception

Previous research on speech perception in both adults and infants has supported the view that consonants are perceived categorically; that is, listeners are relatively insensitive to variation below the level of the phoneme. More recent work, on the other hand, has shown adults to be systematically sensitive to within category variation [McMurray, B., Tanenhaus, M., & Aslin, R. (2002). Gradient effects of within-category phonetic variation on lexical access, Cognition, 86 (2), B33-B42.]. Additionally, recent evidence suggests that infants are capable of using within-category variation to segment speech and to learn phonetic categories. Here we report two studies of 8-month-old infants, using the head-turn preference procedure, that examine more directly infants' sensitivity to within-category variation. Infants were exposed to 80 repetitions of words beginning with either /b/ or /p/. After exposure, listening times to tokens of the same category with small variations in VOT were significantly different than to both the originally exposed tokens and to the cross-category-boundary competitors. Thus infants, like adults, show systematic sensitivity to fine-grained, within-category detail in speech perception.     

Rapid acquisition of phonological alternations by infants

We explore whether infants can learn novel phonological alternations on the basis of distributional information. In Experiment 1, two groups of 12-month-old infants were familiarized with artificial languages whose distributional properties exhibited either stop or fricative voicing alternations. At test, infants in the two exposure groups had different preferences for novel sequences involving voiced and voiceless stops and fricatives, suggesting that each group had internalized a different familiarization alternation. In Experiment 2, 8.5-month-olds exhibited the same patterns of preference. In Experiments 3 and 4, we investigated whether infants' preferences were driven solely by preferences for sequences of high transitional probability. Although 8.5-month-olds in Experiment 3 were sensitive to the relative probabilities of sequences in the familiarization stimuli, only 12-month-olds in Experiment 4 showed evidence of having grouped alternating segments into a single functional category. Taken together, these results suggest a developmental trajectory for the acquisition of phonological alternations using distributional cues in the input.     

Comparing lexically guided perceptual learning in younger and older listeners

Numerous studies have shown that younger adults engage in lexically guided perceptual learning in speech perception. Here, we investigated whether older listeners are also able to retune their phonetic category boundaries. More specifically, in this research we tried to answer two questions. First, do older adults show perceptual-learning effects of similar size to those of younger adults? Second, do differences in lexical behavior predict the strength of the perceptual-learning effect? An age group comparison revealed that older listeners do engage in lexically guided perceptual learning, but there were two age-related differences: Younger listeners had a stronger learning effect right after exposure than did older listeners, but the effect was more stable for older than for younger listeners. Moreover, a clear link was shown to exist between individuals’ lexical-decision performance during exposure and the magnitude of their perceptual-learning effects. A subsequent analysis on the results of the older participants revealed that, even within the older participant group, with increasing age the perceptual retuning effect became smaller but also more stable, mirroring the age group comparison results. These results could not be explained by differences in hearing loss. The age effect may be accounted for by decreased flexibility in the adjustment of phoneme categories or by age-related changes in the dynamics of spoken-word recognition, with older adults being more affected by competition from similar-sounding lexical competitors, resulting in less lexical guidance for perceptual retuning. In conclusion, our results clearly show that the speech perception system remains flexible over the life span.     

Feature parsing: feature cue mapping in spoken word recognition

For listeners to recognize words, they must map temporally distributed phonetic feature cues onto higher order phonological representations. Three experiments are reported that were performed to examine what information listeners extract from assimilated segments (e.g., place-assimilated tokens of cone that resemble comb) and how they interpret it. Experiment 1 employed form priming to demonstrate that listeners activate the underlying form of CONE, but not of its neighbor (COMB). Experiment 2 employed phoneme monitoring to show that the same assimilated tokens facilitate the perception of postassimilation context. Together, the results of these two experiments suggest that listeners recover both the underlying place of the modified item and information about the subsequent item from the same modified segment. Experiment 3 replicated Experiment 1, using different postassimilation contexts to demonstrate that context effects do not reflect familiarity with a given assimilation process. The results are discussed in the context of general auditory grouping mechanisms.     

Perceptual learning for speech: Is there a return to normal?

Recent work on perceptual learning shows that listeners' phonemic representations dynamically adjust to reflect the speech they hear (Norris, McQueen, & Cutler, 2003). We investigate how the perceptual system makes such adjustments, and what (if anything) causes the representations to return to their pre-perceptual learning settings. Listeners are exposed to a speaker whose pronunciation of a particular sound (either /s/ or /integral/) is ambiguous (e.g., halfway between /s/ and /integral/). After exposure, participants are tested for perceptual learning on two continua that range from /s/ to /integral/, one in the Same voice they heard during exposure, and one in a Different voice. To assess how representations revert to their prior settings, half of Experiment 1's participants were tested immediately after exposure; the other half performed a 25-min silent intervening task. The perceptual learning effect was actually larger after such a delay, indicating that simply allowing time to pass does not cause learning to fade. The remaining experiments investigate different ways that the system might unlearn a person's pronunciations: listeners hear the Same or a Different speaker for 25 min with either: no relevant (i.e., 'good') /s/ or /integral/ input (Experiment 2), one of the relevant inputs (Experiment 3), or both relevant inputs (Experiment 4). The results support a view of phonemic representations as dynamic and flexible, and suggest that they interact with both higher- (e.g., lexical) and lower-level (e.g., acoustic) information in important ways.     

Auditory frequency discrimination learning is affected by stimulus variability

We explored the effects of training set variability on learning and generalization of pure-tone frequency discrimination (FD) in three groups of untrained, normally hearing adult listeners. Group 1 trained using a fixed standard tone at 1 kHz (fixed), Group 2 on slightly varying (roving) tones around 1 kHz, and Group 3 on widely varying standard frequencies (wide-roving). Initially, two thirds of all listeners had low FD thresholds (good listeners) and one third had intermediate to high thresholds (poor listeners). For good listeners, slight variations in the training set slowed learning but wide variations did not. Transfer to untrained frequencies (up to 4 kHz) and to the fixed condition was equivalent regardless of training condition, but Group 1 listeners did not fully transfer learning to the roving condition. For poor listeners, any variation in the training condition slowed learning and impaired transfer to other frequencies but did not affect transfer to untrained conditions. Thus, the effects of training set on progress and outcome depend on set variability and individual FD ability.     

The link between statistical segmentation and word learning in adults

Many studies have shown that listeners can segment words from running speech based on conditional probabilities of syllable transitions, suggesting that this statistical learning could be a foundational component of language learning. However, few studies have shown a direct link between statistical segmentation and word learning. We examined this possible link in adults by following a statistical segmentation exposure phase with an artificial lexicon learning phase. Participants were able to learn all novel object-label pairings, but pairings were learned faster when labels contained high probability (word-like) or non-occurring syllable transitions from the statistical segmentation phase than when they contained low probability (boundary-straddling) syllable transitions. This suggests that, for adults, labels inconsistent with expectations based on statistical learning are harder to learn than consistent or neutral labels. In contrast, a previous study found that infants learn consistent labels, but not inconsistent or neutral labels.     

Learnability of embedded syntactic structures depends on prosodic cues

The ability to process center-embedded structures has been claimed to represent a core function of the language faculty. Recently, several studies have investigated the learning of center-embedded dependencies in artificial grammar settings. Yet some of the results seem to question the learnability of these structures in artificial grammar tasks. Here, we tested under which exposure conditions learning of center-embedded structures in an artificial grammar is possible. We used naturally spoken syllable sequences and varied the presence of prosodic cues. The results suggest that mere distributional information does not suffice for successful learning. Prosodic cues marking the boundaries of the major relevant units, however, can lead to learning success. Thus, our data are consistent with the hypothesis that center-embedded syntactic structures can be learned in artificial grammar tasks if language-like acoustic cues are provided.     

Neurological evidence in support of a specialized phonetic processing module

Event-related potentials (ERPs) were utilized to study brain activity while subjects listened to speech and nonspeech stimuli. The effect of duplex perception was exploited, in which listeners perceive formant transitions that are isolated as nonspeech "chirps," but perceive formant transitions that are embedded in synthetic syllables as unique linguistic events with no chirp-like sounds heard at all (Mattingly et al., 1971). Brain ERPs were recorded while subjects listened to and silently identified plain speech-only tokens, duplex tokens, and tone glides (perceived as "chirps" by listeners). A highly controlled set of stimuli was developed that represented equivalent speech and nonspeech stimulus tokens such that the differences were limited to a single acoustic parameter: amplitude. The acoustic elements were matched in terms of number and frequency of components. Results indicated that the neural activity in response to the stimuli was different for different stimulus types. Duplex tokens had significantly longer latencies than the pure speech tokens. The data are consistent with the contention of separate modules for phonetic and auditory stimuli.     

Socially learned preferences for differentially rewarded tokens in the brown capuchin monkey (Cebus apella)

Social learning is assumed to underlie traditions, yet evidence indicating social learning in capuchin monkeys (Cebus apella), which exhibit traditions, is sparse. The authors tested capuchins for their ability to learn the value of novel tokens using a previously familiar token-exchange economy. Capuchins change their preferences in favor of a token worth a high-value food reward after watching a conspecific model exchange 2 differentially rewarded tokens, yet they fail to develop a similar preference after watching tokens paired with foods in the absence of a conspecific model. They also fail to learn that the value of familiar tokens has changed. Information about token value is available in all situations, but capuchins seem to pay more attention in a social situation involving novel tokens.     

Procedural learning: A developmental study of motor sequence learning and probabilistic classification learning in school-aged children

In this study, we investigated motor and cognitive procedural learning in typically developing children aged 8–12 years with a serial reaction time (SRT) task and a probabilistic classification learning (PCL) task. The aims were to replicate and extend the results of previous SRT studies, to investigate PCL in school-aged children, to explore the contribution of declarative knowledge to SRT and PCL performance, to explore the strategies used by children in the PCL task via a mathematical model, and to see whether performances obtained in motor and cognitive tasks correlated. The results showed similar learning effects in the three age groups in the SRT and in the first half of the PCL tasks. Participants did not develop explicit knowledge in the SRT task whereas declarative knowledge of the cue–outcome associations correlated with the performances in the second half of the PCL task, suggesting a participation of explicit knowledge after some time of exposure in PCL. An increasing proportion of the optimal strategy use with increasing age was observed in the PCL task. Finally, no correlation appeared between cognitive and motor performance. In conclusion, we extended the hypothesis of age invariance from motor to cognitive procedural learning, which had not been done previously. The ability to adopt more efficient learning strategies with age may rely on the maturation of the fronto-striatal loops. The lack of correlation between performance in the SRT task and the first part of the PCL task suggests dissociable developmental trajectories within the procedural memory system.     

Are we "experienced listeners"? A review of the musical capacities that do not depend on formal musical training

The present paper reviews a set of studies designed to investigate different aspects of the capacity for processing Western music. This includes perceiving the relationships between a theme and its variations, perceiving musical tensions and relaxations, generating musical expectancies, integrating local structures in large-scale structures, learning new compositional systems and responding to music in an emotional (affective) way. The main focus of these studies was to evaluate the influence of intensive musical training on these capacities. The overall set of data highlights that some musical capacities are acquired through exposure to music without the help of explicit training. These capacities reach such a degree of sophistication that they enable untrained listeners to respond to music as "musically experienced listeners" do.     

The impact of Parkinson's disease on vocal-prosodic communication from the perspective of listeners

An expressive disturbance of speech prosody has long been associated with idiopathic Parkinson's disease (PD), but little is known about the impact of dysprosody on vocal-prosodic communication from the perspective of listeners. Recordings of healthy adults (n=12) and adults with mild to moderate PD (n=21) were elicited in four speech contexts in which prosody serves a primary function in linguistic or emotive communication (phonemic stress, contrastive stress, sentence mode, and emotional prosody). Twenty independent listeners naive to the disease status of individual speakers then judged the intended meanings conveyed by prosody for tokens recorded in each condition. Findings indicated that PD speakers were less successful at communicating stress distinctions, especially words produced with contrastive stress, which were identifiable to listeners. Listeners were also significantly less able to detect intended emotional qualities of Parkinsonian speech, especially for anger and disgust. Emotional expressions that were correctly recognized by listeners were consistently rated as less intense for the PD group. Utterances produced by PD speakers were frequently characterized as sounding sad or devoid of emotion entirely (neutral). Results argue that motor limitations on the vocal apparatus in PD produce serious and early negative repercussions on communication through prosody, which diminish the social-linguistic competence of Parkinsonian adults as judged by listeners.     

Categorical perception: effects of the extent and rate of spectral change

ABSTRACT

Two experiments evaluated a potential explanation of categorical perception (CP) for place of articulation – namely, that listeners derive limited information from rapid spectral changes. Experiment 1 examined vowel context effects for /b/–/d/ continua that included consonant–vowel tokens with F2 onset frequencies that varied systematically from the F2 frequencies of their corresponding steady-states. Phoneme categorisation sharply shifted with F2 direction at locations along the continuum where discrimination performance peaked, indicating CP. Experiment 2 compared findings for a replicated condition against conditions with vowels reduced to match consonant duration or consonants extended to match vowels. CP was similarly obtained for replicated and vowel-reduced conditions. However, listeners frequently perceived diphthongs centrally on the consonant-extended continuum. Some listeners demonstrated CP, although aggregate performance appeared more continuous. These experiments indicate a model based upon the perceived direction of frequency transitions.     

Perception of tones by infants learning a non-tone language

This article examines the perception of tones by non-tone-language-learning (non-tone-learning) infants between 5 and 18 months in a study that reveals infants’ initial sensitivity to tonal contrasts, deterioration yet plasticity of tonal sensitivity at the end of the first year, and a perceptual rebound in the second year. Dutch infants in five age groups were tested on their ability to discriminate a tonal contrast of Mandarin Chinese as well as a contracted tonal contrast. Infants are able to discriminate tonal contrasts at 5–6 months, and their tonal sensitivity deteriorates at around 9 months. However, the sensitivity rebound sat 17–18 months. Non-tone-learning infants’ tonal perception is elastic, as is shown by the influence of acoustic salience and distributional learning: (1) a salient contrast may remain discriminable throughout infancy whereas a less salient one does not; (2) a bimodal distribution in tonal exposure increases non-tone-learning infants’ discrimination ability during the trough in sensitivity to tonal contrasts at 11–12 months. These novel findings reveal non-tone-learning infants’ U-shaped pattern in tone perception, and display their perceptual flexibility.     

Infant-directed speech supports phonetic category learning in English and Japanese

Across the first year of life, infants show decreased sensitivity to phonetic differences not used in the native language [Werker, J. F., & Tees, R. C. (1984). Cross-language speech perception: evidence for perceptual reorganization during the first year of life. Infant Behaviour and Development, 7, 49-63]. In an artificial language learning manipulation, Maye, Werker, and Gerken [Maye, J., Werker, J. F., & Gerken, L. (2002). Infant sensitivity to distributional information can affect phonetic discrimination. Cognition, 82(3), B101-B111] found that infants change their speech sound categories as a function of the distributional properties of the input. For such a distributional learning mechanism to be functional, however, it is essential that the input speech contain distributional cues to support such perceptual learning. To test this, we recorded Japanese and English mothers teaching words to their infants. Acoustic analyses revealed language-specific differences in the distributions of the cues used by mothers (or cues present in the input) to distinguish the vowels. The robust availability of these cues in maternal speech adds support to the hypothesis that distributional learning is an important mechanism whereby infants establish native language phonetic categories.     

Cue integration with categories: Weighting acoustic cues in speech using unsupervised learning and distributional statistics

During speech perception, listeners make judgments about the phonological category of sounds by taking advantage of multiple acoustic cues for each phonological contrast. Perceptual experiments have shown that listeners weight these cues differently. How do listeners weight and combine acoustic cues to arrive at an overall estimate of the category for a speech sound? Here, we present several simulations using a mixture of Gaussians models that learn cue weights and combine cues on the basis of their distributional statistics. We show that a cue‐weighting metric in which cues receive weight as a function of their reliability at distinguishing phonological categories provides a good fit to the perceptual data obtained from human listeners, but only when these weights emerge through the dynamics of learning. These results suggest that cue weights can be readily extracted from the speech signal through unsupervised learning processes.