Native language shapes automatic neural processing of speech

Lexical tone perception and learning in older adults: A review and future directions

While the literature is well represented in accounting for how aging influences segmental properties of speech, less is known about its influences on suprasegmental properties such as lexical tones. In addition, foreign language learning is increasingly endorsed as being a potential intervention to boost cognitive reserve and overall well-being in older adults. Empirical studies on young learners learning lexical tones are aplenty in comparison with older learners. Challenges in this domain for older learners might be different due to aging and other learner-internal factors. This review consolidates behavioural and neuroscientific research related to lexical tone, speech perception, factors characterising learner groups, and other variables that would influence lexical tone perception and learning in older adults. Factors commonly identified to influence tone learning in younger adult populations, such as musical experience, language background, and motivation in learning a new language, are discussed in relation to older learner groups and recommendations to boost lexical tone learning in older age are provided based on existing studies.

Chinese-English Speakers' Perception of Pitch in Their Non-Tonal Language: Reinterpreting English as a Tonal-Like Language

Changing the F0-contour of English words does not change their lexical meaning. However, it changes the meaning in tonal languages such as Mandarin. Given this important difference and knowing that words in the two languages of a bilingual lexicon interact, the question arises as to how Mandarin-English speakers process pitch in their bilingual lexicon. The few studies that addressed this question showed that Mandarin-English speakers did not perceive pitch in English words as native English speakers did. These studies, however, used English words as stimuli failing to examine nonwords and Mandarin words. Consequently, possible pre-lexical effects and L1 transfer were not ruled out. The present study fills this gap by examining pitch perception in Mandarin and English words and nonwords by Mandarin-English speakers and a group of native English controls. Results showed the tonal experience of Chinese-English speakers modulated their perception of pitch in their non-tonal language at both pre-lexical and lexical levels. In comparison to native English controls, tonal speakers were more sensitive to the acoustic salience of F0-contours in the pre-lexical processing due to top-down feedback. At the lexical level, Mandarin-English speakers organized words in their two languages according to similarity criteria based on both F0 and segmental information, whereas only the segmental information was relevant to the control group. These results in perception together with consistently reported production patterns in previous literature suggest that Mandarin-English speakers process pitch in English as if it was a one-tone language.

Changes in Speech-Related Brain Activity During Adaptation to Electro-Acoustic Hearing

Objectives: Hearing improves significantly with bimodal provision, i.e., a cochlear implant (CI) at one ear and a hearing aid (HA) at the other, but performance shows a high degree of variability resulting in substantial uncertainty about the performance that can be expected by the individual CI user. The objective of this study was to explore how auditory event-related potentials (AERPs) of bimodal listeners in response to spoken words approximate the electrophysiological response of normal hearing (NH) listeners. Study Design: Explorative prospective analysis during the first 6 months of bimodal listening using a within-subject repeated measures design. Setting: Academic tertiary care center. Participants: Twenty-seven adult participants with bilateral sensorineural hearing loss who received a HiRes 90K CI and continued use of a HA at the non-implanted ear. Age-matched NH listeners served as controls. Intervention: Cochlear implantation. Main Outcome Measures: Obligatory auditory evoked potentials N1 and P2, and the event-related N2 potential in response to monosyllabic words and their reversed sound traces before, as well as 3 and 6 months post-implantation. The task required word/non-word classification. Stimuli were presented within speech-modulated noise. Loudness of word/non-word signals was adjusted individually to achieve the same intelligibility across groups and assessments. Results: Intelligibility improved significantly with bimodal hearing, and the N1-P2 response approximated the morphology seen in NH with enhanced and earlier responses to the words compared to their reversals. For bimodal listeners, a prominent negative deflection was present between 370 and 570 ms post stimulus onset (N2), irrespective of stimulus type. This was absent for NH controls; hence, this response did not approximate the NH response during the study interval. N2 source localization evidenced extended activation of general cognitive areas in frontal and prefrontal brain areas in the CI group. Conclusions: Prolonged and spatially extended processing in bimodal CI users suggests employment of additional auditory-cognitive mechanisms during speech processing. This does not reduce within 6 months of bimodal experience and may be a correlate of the enhanced listening effort described by CI listeners.

Effects of formant proximity and stimulus prototypicality on the neural discrimination of vowels: Evidence from the auditory frequency-following response

Cross-language speech perception experiments indicate that for many vowel contrasts, discrimination is easier when the same pair of vowels is presented in one direction compared to the reverse direction. According to one account, these directional asymmetries reflect a universal bias favoring "focal" vowels (i.e., vowels with prominent spectral peaks formed by the convergence of adjacent formants). An alternative account is that such effects reflect an experience-dependent bias favoring prototypical exemplars of native-language vowel categories. Here, we tested the predictions of these accounts by recording the auditory frequency-following response in English-speaking listeners to two synthetic variants of the vowel /u/ that differed in the proximity of their first and second formants and prototypicality, with stimuli arranged in oddball and reversed-oddball blocks. Participants showed evidence of neural discrimination when the more-focal/less-prototypic /u/ served as the deviant stimulus, but not when the less-focal/more-prototypic /u/ served as the deviant, consistent with the focalization account.

Tone language experience-dependent advantage in pitch representation in brainstem and auditory cortex is maintained under reverberation

Long-term language and music experience enhances neural representation of temporal attributes of pitch in the brainstem and auditory cortex in favorable listening conditions. Herein we examine whether brainstem and cortical pitch mechanisms-shaped by long-term language experience-maintain this advantage in the presence of reverberation-induced degradation in pitch representation. Brainstem frequency following responses (FFR) and cortical pitch responses (CPR) were recorded concurrently from Chinese and English-speaking natives, using a Mandarin word exhibiting a high rising pitch (/yi²/). Stimuli were presented diotically in quiet (Dry), and in the presence of Slight, Mild, and Moderate reverberation conditions. Regardless of language group, the amplitude of both brainstem FFR (F0) and cortical CPR (NaPb) responses decreased with increases in reverberation. Response amplitude for Chinese, however, was larger than English in all reverberant conditions. The Chinese group also exhibited a robust rightward asymmetry at temporal electrode sites (T8 > T7) across stimulus conditions. Regardless of language group, direct comparison of brainstem and cortical responses revealed similar magnitude of change in response amplitude with increasing reverberation. These findings suggest that experience-dependent brainstem and cortical pitch mechanisms provide an enhanced and stable neural representation of pitch-relevant information that is maintained even in the presence of reverberation. Relatively greater degradative effects of reverberation on brainstem (FFR) compared to cortical (NaPb) responses suggest relatively stronger top-down influences on CPRs.

Linguistic effect on speech perception observed at the brainstem

Linguistic experience affects speech perception from early infancy, as previously evidenced by behavioral and brain measures. Current research focuses on whether linguistic effects on speech perception can be observed at an earlier stage in the neural processing of speech (i.e., auditory brainstem). Brainstem responses reflect rapid, automatic, and preattentive encoding of sounds. Positive experiential effects have been reported by examining the frequency-following response (FFR) component of the complex auditory brainstem response (cABR) in response to sustained high-energy periodic portions of speech sounds (vowels and lexical tones). The current study expands the existing literature by examining the cABR onset component in response to transient and low-energy portions of speech (consonants), employing simultaneous magnetoencephalography (MEG) in addition to electroencephalography (EEG), which provide complementary source information on cABR. Utilizing a cross-cultural design, we behaviorally measured perceptual responses to consonants in native Spanish- and English-speaking adults, in addition to cABR. Brain and behavioral relations were examined. Results replicated previous behavioral differences between language groups and further showed that individual consonant perception is strongly associated with EEG-cABR onset peak latency. MEG-cABR source analysis of the onset peaks complimented the EEG-cABR results by demonstrating subcortical sources for both peaks, with no group differences in peak locations. Current results demonstrate a brainstem-perception relation and show that the effects of linguistic experience on speech perception can be observed at the brainstem level.

Auditory perceptual learning and changes in the conceptualization of auditory cortex

Perceptual learning, improvement in discriminative ability as a consequence of training, is one of the forms of sensory system plasticity that has driven profound changes in our conceptualization of sensory cortical function. Psychophysical and neurophysiological studies of auditory perceptual learning have indicated that the characteristics of the learning, and by implication the nature of the underlying neural changes, are highly task specific. Some studies in animals have indicated that recruitment of neurons to the population responding to the training stimuli, and hence an increase in the so-called cortical "area of representation" of those stimuli, is the substrate of improved performance, but such changes have not been observed in other studies. A possible reconciliation of these conflicting results is provided by evidence that changes in area of representation constitute a transient stage in the processes underlying perceptual learning. This expansion - renormalization hypothesis is supported by evidence from studies of the learning of motor skills, another form of procedural learning, but leaves open the nature of the permanent neural substrate of improved performance. Other studies have suggested that the substrate might be reduced response variability - a decrease in internal noise. Neuroimaging studies in humans have also provided compelling evidence that training results in long-term changes in auditory cortical function and in the auditory brainstem frequency-following response. Musical training provides a valuable model, but the evidence it provides is qualified by the fact that most such training is multimodal and sensorimotor, and that few of the studies are experimental and allow control over confounding variables. More generally, the overwhelming majority of experimental studies of the various forms of auditory perceptual learning have established the co-occurrence of neural and perceptual changes, but have not established that the former are causally related to the latter. Important forms of perceptual learning in humans are those involved in language acquisition and in the improvement in speech perception performance of post-lingually deaf cochlear implantees over the months following implantation. The development of a range of auditory training programs has focused interest on the factors determining the extent to which perceptual learning is specific or generalises to tasks other than those used in training. The context specificity demonstrated in a number of studies of perceptual learning suggests a multiplexing model, in which learning relating to a particular stimulus attribute depends on a subset of the diverse inputs to a given cortical neuron being strengthened, and different subsets being gated by top-down influences. This hypothesis avoids the difficulty of balancing system stability with plasticity, which is a problem for recruitment hypotheses. The characteristics of auditory perceptual learning reflect the fact that auditory cortex forms part of distributed networks that integrate the representation of auditory stimuli with attention, decision, and reward processes.

Music training enhances the automatic neural processing of foreign speech sounds

Growing evidence shows that music and language experience affect the neural processing of speech sounds throughout the auditory system. Recent work mainly focused on the benefits induced by musical practice on the processing of native language or tonal foreign language, which rely on pitch processing. The aim of the present study was to take this research a step further by investigating the effect of music training on processing English sounds by foreign listeners. We recorded subcortical electrophysiological responses to an English syllable in three groups of participants: native speakers, non-native nonmusicians, and non-native musicians. Native speakers had enhanced neural processing of the formant frequencies of speech, compared to non-native nonmusicians, suggesting that automatic encoding of these relevant speech cues are sensitive to language experience. Most strikingly, in non-native musicians, neural responses to the formant frequencies did not differ from those of native speakers, suggesting that musical training may compensate for the lack of language experience by strengthening the neural encoding of important acoustic information. Language and music experience seem to induce a selective sensory gain along acoustic dimensions that are functionally-relevant-here, formant frequencies that are crucial for phoneme discrimination.

Phonological experience modulates voice discrimination: Evidence from functional brain networks analysis

Numerous behavioral studies have found a modulation effect of phonological experience on voice discrimination. However, the neural substrates underpinning this phenomenon are poorly understood. Here we manipulated language familiarity to test the hypothesis that phonological experience affects voice discrimination via mediating the engagement of multiple perceptual and cognitive resources. The results showed that during voice discrimination, the activation of several prefrontal regions was modulated by language familiarity. More importantly, the same effect was observed concerning the functional connectivity from the fronto-parietal network to the voice-identity network (VIN), and from the default mode network to the VIN. Our findings indicate that phonological experience could bias the recruitment of cognitive control and information retrieval/comparison processes during voice discrimination. Therefore, the study unravels the neural substrates subserving the modulation effect of phonological experience on voice discrimination, and provides new insights into studying voice discrimination from the perspective of network interactions.

Language experience-dependent advantage in pitch representation in the auditory cortex is limited to favorable signal-to-noise ratios

Long-term experience enhances neural representation of temporal attributes of pitch in the brainstem and auditory cortex in favorable listening conditions. Herein we examine whether cortical pitch mechanisms shaped by language experience are more resilient to degradation in background noise, and exhibit greater binaural release from masking (BRM). Cortical pitch responses (CPR) were recorded from Mandarin- and English-speaking natives using a Mandarin word exhibiting a high rising pitch (/yi²/). Stimuli were presented diotically in Quiet, and in noise at +5, and 0 dB SNR. CPRs were also recorded in binaural conditions, SONO (where signal and noise were in phase at both ears); or S0Nπ (where signal was in phase and noise 180° out of phase at each ear), using 0 dB SNR. At Fz, both groups showed increase in CPR peak latency and decrease in amplitude with increasing noise level. A language-dependent enhancement of Na-Pb amplitude (Chinese > English) was restricted to Quiet and +5 dB SNR conditions. At T7/T8 electrode sites, Chinese natives exhibited a rightward asymmetry for both CPR components. A language-dependent effect (Chinese > English) was restricted to T8. Regarding BRM, both CPR components showed greater response amplitude for the S0Nπ condition compared to S0N0 across groups. Rightward asymmetry for BRM in the Chinese group indicates experience-dependent recruitment of right auditory cortex. Restriction of the advantage in pitch representation to the quiet and +5 SNR conditions, and the absence of group differences in the binaural release from masking, suggest that language experience affords limited advantage in the neural representation of pitch-relevant information in the auditory cortex under adverse listening conditions.

Human Brainstem Exhibits higher Sensitivity and Specificity than Auditory-Related Cortex to Short-Term Phonetic Discrimination Learning

Phonetic discrimination learning is an active perceptual process that operates under the influence of cognitive control mechanisms by increasing the sensitivity of the auditory system to the trained stimulus attributes. It is assumed that the auditory cortex and the brainstem interact in order to refine how sounds are transcribed into neural codes. Here, we evaluated whether these two computational entities are prone to short-term functional changes, whether there is a chronological difference in malleability, and whether short-term training suffices to alter reciprocal interactions. We performed repeated cortical (i.e., mismatch negativity responses, MMN) and subcortical (i.e., frequency-following response, FFR) EEG measurements in two groups of participants who underwent one hour of phonetic discrimination training or were passively exposed to the same stimulus material. The training group showed a distinctive brainstem energy reduction in the trained frequency-range (i.e., first formant), whereas the passive group did not show any response modulation. Notably, brainstem signal change correlated with the behavioral improvement during training, this result indicating a close relationship between behavior and underlying brainstem physiology. Since we did not reveal group differences in MMN responses, results point to specific short-term brainstem changes that precede functional alterations in the auditory cortex.

Changes in pitch height elicit both language-universal and language-dependent changes in neural representation of pitch in the brainstem and auditory cortex

Language experience shapes encoding of pitch-relevant information at both brainstem and cortical levels of processing. Pitch height is a salient dimension that orders pitch from low to high. Herein we investigate the effects of language experience (Chinese, English) in the brainstem and cortex on (i) neural responses to variations in pitch height, (ii) presence of asymmetry in cortical pitch representation, and (iii) patterns of relative changes in magnitude of pitch height between these two levels of brain structure. Stimuli were three nonspeech homologs of Mandarin Tone 2 varying in pitch height only. The frequency-following response (FFR) and the cortical pitch-specific response (CPR) were recorded concurrently. At the Fz-linked T7/T8 site, peak latency of Na, Pb, and Nb decreased with increasing pitch height for both groups. Peak-to-peak amplitude of Na-Pb and Pb-Nb increased with increasing pitch height across groups. A language-dependent effect was restricted to Na-Pb; the Chinese had larger amplitude than the English group. At temporal sites (T7/T8), the Chinese group had larger amplitude, as compared to English, across stimuli, but also limited to the Na-Pb component and right temporal site. In the brainstem, F0 magnitude decreased with increasing pitch height; Chinese had larger magnitude across stimuli. A comparison of CPR and FFR responses revealed distinct patterns of relative changes in magnitude common to both groups. CPR amplitude increased and FFR amplitude decreased with increasing pitch height. Experience-dependent effects on CPR components vary as a function of neural sensitivity to pitch height within a particular temporal window (Na-Pb). Differences between the auditory brainstem and cortex imply distinct neural mechanisms for pitch extraction at both levels of brain structure.