Are you talking to me? How the choice of speech register impacts listeners' hierarchical encoding of speech

Speakers accommodate their speech to meet the needs of their listeners, producing different speech registers. One such register is L2 Accommodation (L2A), which is the way native speakers address non-native listeners, typically characterized by features such as slow speech rate and phonetic exaggeration. Here, we investigated how register impacts the cortical encoding of speech at different levels of language integration. Specifically, we tested the hypothesis that enhanced comprehension of L2A compared with Native Directed Speech (NDS) involves more than just a slower speech rate, influencing speech processing from acoustic to semantic levels. Electroencephalography (EEG) signals were recorded from Spanish native listeners, who were learning English (L2 learners), and English native listeners (L1 listeners) as they were presented with audio-stories. Speech was presented in English in three different speech registers: L2A, NDS, and a control register (Slow-NDS) which is a slowed down version of NDS. We measured the cortical encoding of acoustic, phonological, and semantic information with a multivariate temporal response function analysis (TRF) on the EEG signals. We found that L2A promoted L2 learners' cortical encoding at all the levels of speech and language processing considered. First, L2A led to a more pronounced encoding of the speech envelope. Second, phonological encoding was more refined when listening to L2A, with phoneme perception getting closer to that of L1 listeners. Finally, L2A also enhanced the TRF-N400, a neural signature of semantic integration. Conversely, L2A impacted acoustic but not linguistic speech encoding in L1 listeners. In contrast, slow-NDS altered the cortical encoding of sound acoustics in L1 listeners but did not impact semantic or phonological encoding. Taken together, these results support our hypothesis that L2A accommodates speech processing in L2 listeners beyond what can be achieved by simply speaking slowly, impacting the cortical encoding of sound and language at different abstraction levels. In turn, this study provides objective metrics that are sensitive to the impact of register on the hierarchical encoding of speech, which could be extended to other registers and cohorts.