Nonlinear dynamics in auditory cortical activity reveal the neural basis of perceptual warping in speech categorization

Surrounding context influences speech listening, resulting in dynamic shifts to category percepts. To examine its neural basis, event-related potentials (ERPs) were recorded during vowel identification with continua presented in random, forward, and backward orders to induce perceptual nonlinearities. Behaviorally, sequential order shifted listeners' categorical boundary vs. random delivery revealing perceptual warping (biasing) of the heard phonetic category dependent on recent stimulus history. ERPs revealed later (~300 ms) activity localized to superior temporal and middle/inferior frontal gyri that predicted listeners' hysteresis magnitudes. Findings demonstrate that top-down, stimulus history effects on speech categorization are governed by interactions between frontotemporal brain regions.

Neural indicators of articulator-specific sensorimotor influences on infant speech perception

While there is increasing acceptance that even young infants detect correspondences between heard and seen speech, the common view is that oral-motor movements related to speech production cannot influence speech perception until infants begin to babble or speak. We investigated the extent of multimodal speech influences on auditory speech perception in prebabbling infants who have limited speech-like oral-motor repertoires. We used event-related potentials (ERPs) to examine how sensorimotor influences to the infant’s own articulatory movements impact auditory speech perception in 3-mo-old infants. In experiment 1, there were ERP discriminative responses to phonetic category changes across two phonetic contrasts (bilabial–dental /ba/-/ɗa/; dental–retroflex /ɗa/-/ɖa/) in a mismatch paradigm, indicating that infants auditorily discriminated both contrasts. In experiment 2, inhibiting infants’ own tongue-tip movements had a disruptive influence on the early ERP discriminative response to the /ɗa/-/ɖa/ contrast only. The same articulatory inhibition had contrasting effects on the perception of the /ba/-/ɗa/ contrast, which requires different articulators (the lips vs. the tongue) during production, and the /ɗa/-/ɖa/ contrast, whereby both phones require tongue-tip movement as a place of articulation. This articulatory distinction between the two contrasts plausibly accounts for the distinct influence of tongue-tip suppression on the neural responses to phonetic category change perception in definitively prebabbling, 3-mo-old, infants. The results showing a specificity in the relation between oral-motor inhibition and phonetic speech discrimination suggest a surprisingly early mapping between auditory and motor speech representation already in prebabbling infants.

Phonetic Category Formation is Perceptually Driven During the Early Stages of Adult L2 Development

Research on the acquisition of L2 phonology in sequential language learners has stressed the importance of language use and input as a means to accurate production and perception; however, the two constructs are difficult to evaluate and control. This study focuses on the role of language use during the initial stages of development of phonetic categories related to stop voicing and analyzes the relationship between production and perception. Native English-speaking late learners of Spanish provided production/perception data on a weekly basis throughout the course of a seven-week immersion program in which L1 use was prohibited. The production/perception data were analyzed using generalized linear mixed effects models. Generalized additive mixed models were used to analyze and compare the learning trajectories of each modality. The analyses revealed phonetic learning in both production and perception over the course of the program. Perception gains paralleled those of native bilinguals by the conclusion of the program and preceded production gains. This study is novel in that it provides production/perception data in a semi-longitudinal design. Moreover, the beginning adult learners are examined in a learning context in which L1 use was minimal and L2 input was maximized. Taken together, the experiments suggest that L2 phonetic category formation can occur abruptly, at an early stage of development, is perceptually driven, and appears to be particularly fragile during the initial stages of learning.

The categorical neural organization of speech aids its perception in noise

ABSTRACTWe investigated whether the categorical perception (CP) of speech might also provide a mechanism that aids its perception in noise. We varied signal-to-noise ratio (SNR) [clear, 0 dB, -5 dB] while listeners classified an acoustic-phonetic continuum (/u/ to /a/). Noise-related changes in behavioral categorization were only observed at the lowest SNR. Event-related brain potentials (ERPs) differentiated phonetic vs. non-phonetic (category ambiguous) speech by the P2 wave (∼180-320 ms). Paralleling behavior, neural responses to speech with clear phonetic status (i.e., continuum endpoints) were largely invariant to noise, whereas responses to ambiguous tokens declined with decreasing SNR. Results demonstrate that phonetic speech representations are more resistant to degradation than corresponding acoustic representations. Findings suggest the mere process of binning speech sounds into categories provides a robust mechanism to aid perception at the “cocktail party” by fortifying abstract categories from the acoustic signal and making the speech code more resistant to external interferences.

Phonetic category formation is perceptually driven during the early stages of adult L2 development

Research on the acquisition of L2 phonology in sequential language learners has stressed the importance of language use and input as a means to accurate production and perception; however, the two constructs are difficult to evaluate and control. This study focuses on the role of language use during the initial stages of development of phonetic categories related to stop voicing and analyzes the relationship between production and perception. Native English speaking late learners of Spanish provided production/perception data on a weekly basis throughout the course of a 7-week immersion program in which L1 use was prohibited. The production/perception data were analyzed using generalized linear mixed effects models. Generalized additive mixed models were used to analyze and compare the learning trajectories of each modality. The analyses revealed phonetic learning in both production and perception over the course of the program. Perception gains paralleled those of native bilinguals by the conclusion of the program and preceded production gains. This study is novel in that it provides production/perception data in a semi-longitudinal design. Moreover, the beginning adult learners are examined in a learning context in which L1 use was minimal and L2 input was maximized. Taken together, the experiments suggest that L2 phonetic category formation can occur abruptly, at an early stage of development, is perceptually driven, and appears to be particularly fragile during the initial stages of learning.

Understanding individual variation in levels of second language attainment through the lens of critical period mechanisms

Mayberry and Kluender (2017) present an important and compelling argument that in order to understand critical periods (CPs) in language acquisition, it is essential to disentangle studies of late first language (L1) acquisition from those of second language (L2) acquisition. Their primary thesis is that timely exposure to an L1 is crucial for establishing language circuitry, thus providing a foundation on which an L2 can build. They note that while there is considerable evidence of interference from the L1 on acquisition of the L2 – especially in late L2 learners (as in our work on cascading influences on phonetic category learning and visual language discrimination, e.g., Werker & Hensch, 2015 and Weikum, Vouloumanos, Navarra, Soto-Faraco, Sebastián-Gallés & Werker, 2013, respectively) – there are other examples of ways in which the L1 can scaffold L2 acquisition. Mayberry and Kluender take this evidence of L1 scaffolding L2 as undermining the value of considering CPs as useful in understanding L2 acquisition.