Phonetic learning can transfer between bilingual speakers' languages, despite cross-language phonetic mismatch

Listeners exposed to an unfamiliar accent can rapidly adjust their sound category representations in response, a phenomenon known as phonetic recalibration. When exposed to words where a sound category has been altered to be phonetically ambiguous (e.g., super→ [ʃ/s]uper), listeners can expand the category to accommodate this new pronunciation. We investigated the mechanisms underlying such learning in Basque-English bilinguals (N = 38). While both Basque and English contrast alveo-palatal and alveolar fricatives (/ʃ/ versus /s/), English and Basque /s/ are articulatorily and acoustically distinct. To test how phonetic similarity constrains the transfer of learning within and across languages, we exposed listeners to Basque words produced with an unfamiliar accent where either /s/ (session 1) or /ʃ/ (session 2) were replaced with ambiguous [ʃ/s]. We then assessed learning in both Basque and English via a standard categorization task that involved phonetic continua between critical fricatives. Results show listeners recalibrated their Basque /ʃ/-/s/ boundary but failed to generalize learning to other Basque fricative contrasts. However, there was a generalization of learning to the English /ʃ/-/s/ contrast, despite cross-language phonetic mismatch. This suggests that similar cross-language sound categories may be linked, despite production differences between languages. Crucially, however, such learning effects were only consistently present within the most ambiguous portion of each phonetic continuum.