Voice-sensitive brain networks encode talker-specific phonetic detail

The speech stream simultaneously carries information about talker identity and linguistic content, and the same acoustic property (e.g., voice-onset-time, or VOT) may be used for both purposes. Separable neural networks for processing talker identity and phonetic content have been identified, but it is unclear how a singular acoustic property is parsed by the neural system for talker identification versus phonetic processing. In the current study, listeners were exposed to two talkers with characteristically different VOTs. Subsequently, brain activation was measured using fMRI as listeners performed a phonetic categorization task on these stimuli. Right temporoparietal regions previously implicated in talker identification showed sensitivity to the match between VOT variant and talker, whereas left posterior temporal regions showed sensitivity to the typicality of phonetic exemplars, regardless of talker typicality. Taken together, these results suggest that neural systems for voice recognition capture talker-specific phonetic variation.

Earlier age of second language learning induces more robust speech encoding in the auditory brainstem in adults, independent of amount of language exposure during early childhood

Learning a second language (L2) at a young age is a driving factor of functional neuroplasticity in the auditory brainstem. To date, it remains unclear whether these effects remain stable until adulthood and to what degree the amount of exposure to the L2 in early childhood might affect their outcome. We compared three groups of adult English-French bilinguals in their ability to categorize English vowels in relation to their frequency following responses (FFR) evoked by the same vowels. At the time of testing, cognitive abilities as well as fluency in both languages were matched between the (1) simultaneous bilinguals (SIM, N = 18); (2) sequential bilinguals with L1-English (N = 14); and (3) sequential bilinguals with L1-French (N = 11). Our results show that the L1-English group show sharper category boundaries in identification of the vowels compared to the L1-French group. Furthermore, the same pattern was reflected in the FFRs (i.e., larger FFR responses in L1-English > SIM > L1-French), while again only the difference between the L1-English and the L1-French group was statistically significant; nonetheless, there was a trend towards larger FFR in SIM compared to L1-French. Our data extends previous literature showing that exposure to a language during the first years of life induces functional neuroplasticity in the auditory brainstem that remains stable until at least young adulthood. Furthermore, the findings suggest that amount of exposure (i.e., 100% vs. 50%) to that language does not differentially shape the robustness of the perceptual abilities or the auditory brainstem encoding of phonetic categories of the language. Statement of significance: Previous studies have indicated that early age of L2 acquisition induces functional neuroplasticity in the auditory brainstem during processing of the L2. This study compared three groups of adult bilinguals who differed in their age of L2 acquisition as well as the amount of exposure to the L2 during early childhood. We demonstrate for the first time that the neuroplastic effect in the brainstem remains stable until young adulthood and that the amount of L2 exposure does not influence behavioral or brainstem plasticity. Our study provides novel insights into low-level auditory plasticity as a function of varying bilingual experience.

Using TMS to evaluate a causal role for right posterior temporal cortex in talker-specific phonetic processing

Theories suggest that speech perception is informed by listeners' beliefs of what phonetic variation is typical of a talker. A previous fMRI study found right middle temporal gyrus (RMTG) sensitivity to whether a phonetic variant was typical of a talker, consistent with literature suggesting that the right hemisphere may play a key role in conditioning phonetic identity on talker information. The current work used transcranial magnetic stimulation (TMS) to test whether the RMTG plays a causal role in processing talker-specific phonetic variation. Listeners were exposed to talkers who differed in how they produced voiceless stop consonants while TMS was applied to RMTG, left MTG, or scalp vertex. Listeners subsequently showed near-ceiling performance in indicating which of two variants was typical of a trained talker, regardless of previous stimulation site. Thus, even though the RMTG is recruited for talker-specific phonetic processing, modulation of its function may have only modest consequences.

Cerebellar representation during phonetic processing in tonal and non-tonal language speakers: An ALE meta-analysis

The role of the cerebellum in phonetic processing has been discovered and widely discussed for decades. However, with the idea that the cerebral representation of phonetic processing is different in tonal language and non-tonal language speakers, whether the cerebellar representation of phonetic processing differs based on language background remains unknown. In the present study, we conducted an activation likelihood estimation (ALE) analysis among 33 functional neuroimaging studies involving 541 healthy adults (213 tonal language speakers and 328 non-tonal language speakers). The aim was to explore the cerebellar representation of phonetic perception and phonetic production in these two language backgrounds. Our results demonstrated the involvement of cerebellum left Crus I, right Crus II, lobules VI, and VIIb in phonetic perception among tonal language speakers, whereas only one focal cluster (right Crus I and Crus II) was demonstrated in non-tonal language speakers. Conjunction analysis revealed overlapping regions located in the right Crus II both in tonal and non-tonal language speakers during phonetic perception. During phonetic production, no significant cluster was detected among tonal language speakers, whereas one focal cluster (within right lobule VI) was detected in non-tonal language speakers. These results highlight the specific cerebellar representation of phonetic processing in tonal and non-tonal languages. Overall, this ALE analysis provides a profound view of the neural mechanism of phonetic processing.