Cross-speaker generalisation in two phoneme-level perceptual adaptation processes

Speech perception is shaped by listeners' prior experience with speakers. Listeners retune their phonetic category boundaries after encountering ambiguous sounds in order to deal with variations between speakers. Repeated exposure to an unambiguous sound, on the other hand, leads to a decrease in sensitivity to the features of that particular sound. This study investigated whether these changes in the listeners' perceptual systems can generalise to the perception of speech from a novel speaker. Specifically, the experiments looked at whether visual information about the identity of the speaker could prevent generalisation from occurring. In Experiment 1, listeners retuned auditory category boundaries using audiovisual speech input. This shift in the category boundaries affected perception of speech from both the exposure speaker and a novel speaker. In Experiment 2, listeners were repeatedly exposed to unambiguous speech either auditorily or audiovisually, leading to a decrease in sensitivity to the features of the exposure sound. Here, too, the changes affected the perception of both the exposure speaker and the novel speaker. Together, these results indicate that changes in the perceptual system can affect the perception of speech from a novel speaker and that visual speaker identity information did not prevent this generalisation

Lexically-guided perceptual learning in speech processing

During listening to spoken language, the perceptual system needs to adapt frequently to changes in talkers, and thus to considerable interindividual variability in the articulation of a given speech sound. This thesis investigated a learning process which allows listeners to use stored lexical representations to modify the interpretation of a speech sound when a talker's articulation of that sound is consistently unclear or ambiguous. The questions that were addressed in this research concerned the robustness of such perceptual learning, a potential role for sleep, and whether learning is specific to the speech of one talker or, alternatively, generalises to other talkers. A further study aimed to identify the underlying functional neuroanatomy by using magnetic resonance imaging methods. The picture that emerged for lexically-guided perceptual learning is that learning occurs very rapidly, is highly specific, and remains remarkably robust both over time and under exposure to speech from other talkers

Perceptual learning of context-sensitive phonetic detail

[Abstract abbreviated due to inability of DSpace@Cambridge to display phonetic symbols. Please see the full abstract in the attached pdf file.] Although familiarity with a talker or accent is known to facilitate perception, it is not clear what underlies this phenomenon. Previous research has focused primarily on whether listeners can learn to associate novel phonetic characteristics with low-level units such as features or phonemes. However, this neglects the potential role of phonetic information at many other levels of representation. To address this shortcoming, this thesis investigated perceptual learning of systematic phonetic detail relating to higher levels of linguistic structure, including prosodic, grammatical and morphological contexts. Furthermore, in contrast to many previous studies, this research used relatively natural stimuli and tasks, thus maximising its relevance to perceptual learning in ordinary listening situations. This research shows that listeners can update their phonetic representations in response to incoming information and its relation to linguistic-structural context. In addition, certain patterns of systematic phonetic detail were more learnable than others. These findings are used to inform an account of how new information is integrated with prior experience in speech processing, within a framework that emphasises the importance of phonetic detail at multiple levels of representation.This work was funded by an AHRC grant

The impact of regional accent variation on monolingual and bilingual infants’ lexical processing

Phonetic variation is inherent in natural speech. It can be lexically relevant, differentiating words, as well as lexically irrelevant indexical variation, which gives information about the talker or context, such as the gender, mood, regional or foreign accent. Efficient communication requires perceivers to discern how lexical versus indexical sources of variation affect the phonetic form of spoken words. While ample evidence is available on how children acquiring a single language handle variability in speech, less is known about how children simultaneously acquiring two languages deal with phonetic variation. This thesis investigates how the bilingual language environment affects children’s ability to accommodate accented speech. We consider three hypotheses. One is that bilingual infants may have an advantage relative to monolinguals due to their greater experience with phonetic variability across their two phonological systems. This is because the lexical representations in bilingual children, who have more experience with accent variation than monolingual children, might be more open to phonetic variation than monolinguals. Representations that are more open to variation might lead to higher flexibility in the word recognition of children with multi-accent input (bilinguals), resulting in accommodation benefits when processing an unfamiliar accent. An alternative hypothesis, however, is that bilingual children may have less stable lexical representations than monolinguals because their vocabulary size in each language is smaller. This could lead to processing costs in accent adaptation, resulting in accommodation disadvantages for bilinguals. The third and final hypothesis is that there would be no difference between bilinguals and their monolingual peers. This is because the effects of greater accent experience but less stable lexical representations in bilinguals may essentially neutralise each other, resulting in equivalent accent accommodation by bilinguals and monolinguals. To evaluate these hypotheses, three experiments were conducted with 17- and 25-month-old bilingual and monolingual children. Their ability to accommodate unfamiliar accented speech was analysed based on their language experience, pre-exposure to the unfamiliar accent, the type of phonetic variation (easy versus difficult phonetic change), and the cognitive demands of the experimental procedure. Taken together, the findings of Experiments 1-3 suggest that bilingual language input neither benefits nor hampers accent adaptation in bilingual children relative to monolingual children. The results carry implications for our current understanding of bilingualism and phonological development

Production and perception of speaker-specific phonetic detail at word boundaries

Experiments show that learning about familiar voices affects speech processing in many tasks. However, most studies focus on isolated phonemes or words and do not explore which phonetic properties are learned about or retained in memory. This work investigated inter-speaker phonetic variation involving word boundaries, and its perceptual consequences. A production experiment found significant variation in the extent to which speakers used a number of acoustic properties to distinguish junctural minimal pairs e.g. 'So he diced them'—'So he'd iced them'. A perception experiment then tested intelligibility in noise of the junctural minimal pairs before and after familiarisation with a particular voice. Subjects who heard the same voice during testing as during the familiarisation period showed significantly more improvement in identification of words and syllable constituents around word boundaries than those who heard different voices. These data support the view that perceptual learning about the particular pronunciations associated with individual speakers helps listeners to identify syllabic structure and the location of word boundaries

Neural correlates of phonetic adaptation as induced by lexical and audiovisual context

When speech perception is difficult, one way listeners adjust is by reconfiguring phoneme category boundaries, drawing on contextual information. Both lexical knowledge and lipreading cues are used in this way, but it remains unknown whether these two differing forms of perceptual learning are similar at a neural level. This study compared phoneme boundary adjustments driven by lexical or audiovisual cues, using ultra-high-field 7-T fMRI. During imaging, participants heard exposure stimuli and test stimuli. Exposure stimuli for lexical retuning were audio recordings of words, and those for audiovisual recalibration were audio–video recordings of lip movements during utterances of pseudowords. Test stimuli were ambiguous phonetic strings presented without context, and listeners reported what phoneme they heard. Reports reflected phoneme biases in preceding exposure blocks (e.g., more reported /p/ after /p/-biased exposure). Analysis of corresponding brain responses indicated that both forms of cue use were associated with a network of activity across the temporal cortex, plus parietal, insula, and motor areas. Audiovisual recalibration also elicited significant occipital cortex activity despite the lack of visual stimuli. Activity levels in several ROIs also covaried with strength of audiovisual recalibration, with greater activity accompanying larger recalibration shifts. Similar activation patterns appeared for lexical retuning, but here, no significant ROIs were identified. Audiovisual and lexical forms of perceptual learning thus induce largely similar brain response patterns. However, audiovisual recalibration involves additional visual cortex contributions, suggesting that previously acquired visual information (on lip movements) is retrieved and deployed to disambiguate auditory perception

Phonetic vowel training for child second language learners: the role of input variability and training task

Acquiring a second language speech contrast that does not exist in the native language is often difficult. High variability phonetic training (HVPT) is a wellestablished method used to train learners on specific non-native phoneme contrasts: it critically uses high variability (HV) input after earlier attempts using low variability (LV) input had proved unsuccessful. HVPT has since been successfully applied in many different adult studies. However, there is no consensus on the effect of input variation on children’s learning of non-native phoneme contrasts. This thesis aims to further investigate the effect of input variability on phonetic training for children, and examining whether they show the same HV benefit which has been argued to hold for adults. In the first set of studies, native English speaking adults and children were taught Dutch vowels in a single computerised training session, during which they received either HV or LV input. Additionally, the traditional HVPT paradigm was adapted to see if mapping vowels to orthography-like symbols representing phoneme categories was more or less effective than a vocabulary training method without such representations. Learning was stronger with training most akin to vocabulary learning, particularly for children, suggesting a benefit for a more meaningful learning context. Crucially, there was no evidence of a HV benefit for either children or adults. The second study was a two-week training study in which Dutch children of two age groups were trained on Standard Southern British English vowel contrasts. Since picture-based training had proved beneficial, this study combined both orthography and pictures in training. Potential effects of HV or LV input in training were investigated using a pre/post-test design. Older children outperformed younger children throughout, and again no evidence for a variability benefit was found. This indicates children might not benefit from high input variability

Perceptual learning, talker specificity, and sound change

Perceptual learning is when listeners hear novel speech input and shift their subsequent perceptual behavior. In this paper we consider the relationship between sound change and perceptual learning. We spell out the connections we see between perceptual learning and different approaches to sound change and explain how a deeper empirical understanding of the properties of perceptual learning might benefit sound change models. We propose that questions about when listeners generalize their perceptual learning to new talkers might be of of particular interest to theories of sound change. We review the relevant literature, noting that studies of perceptual learning generalization across talkers of the same gender are lacking. Finally, we present new experimental data aimed at filling that gap by comparing cross-talker generalization of fricative boundary perceptual learning in same-gender and different-gender pairs. We find that listeners are much more likely to generalize what they have learned across same-gender pairs, even when the different-gender pairs have more similar fricatives. We discuss implications for sound change

Robust Lexically Mediated Compensation for Coarticulation: Christmash Time Is Here Again

First published: 20 April 2021A long-standing question in cognitive science is how high-level knowledge is integrated with sensory input. For example, listeners can leverage lexical knowledge to interpret an ambiguous speech sound, but do such effects reflect direct top-down influences on perception or merely postperceptual biases? A critical test case in the domain of spoken word recognition is lexically mediated compensation for coarticulation (LCfC). Previous LCfC studies have shown that a lexically restored context phoneme (e.g., /s/ in Christma#) can alter the perceived place of articulation of a subsequent target phoneme (e.g., the initial phoneme of a stimulus from a tapes-capes continuum), consistent with the influence of an unambiguous context phoneme in the same position. Because this phoneme-to-phoneme compensation for coarticulation is considered sublexical, scientists agree that evidence for LCfC would constitute strong support for top–down interaction. However, results from previous LCfC studies have been inconsistent, and positive effects have often been small. Here, we conducted extensive piloting of stimuli prior to testing for LCfC. Specifically, we ensured that context items elicited robust phoneme restoration (e.g., that the final phoneme of Christma# was reliably identified as /s/) and that unambiguous context-final segments (e.g., a clear /s/ at the end of Christmas) drove reliable compensation for coarticulation for a subsequent target phoneme.We observed robust LCfC in a well-powered, preregistered experiment with these pretested items (N = 40) as well as in a direct replication study (N = 40). These results provide strong evidence in favor of computational models of spoken word recognition that include top–down feedback