Native phonological and phonetic influences in perceptual assimilation of monosyllabic Thai lexical tones by Mandarin and Vietnamese listeners

•Mandarin listeners Categorised Thai level tone with high percentage but medium ratings.•The two Vietnamese dialects differ in their assimilation of Thai tones.•Categorisation responses were faster in Categorised than Uncategorised assimilation. A cross tone-language perceptual assimilation study investigated native categorisations and goodness ratings of non-native Thai tones by Thai-naive listeners differing in their native tone systems: Mandarin, Northern Vietnamese and Southern Vietnamese. We derived hypotheses from the Perceptual Assimilation Model (PAM: Best, 1995), which considers both native phonological and phonetic influences on perceptual assimilation of non-native speech contrasts. Mandarin listeners reliably categorised the Thai mid level tone to their single native level tone category, reflecting a native phonological effect, but they also showed high residual phonetic sensitivity to differences between the non-native tone and the native tone it was assimilated to, indicated by low category-goodness ratings. Native phonological and phonetic differences in tones of the two Vietnamese dialects also affected perceptual assimilation of the Thai high level and rising tones. In addition, categorisation responses were faster overall for Categorised than Uncategorised assimilations, revealing the processing cost of perceptual uncertainty due to phonological competition among and/or phonetic discrepancies from multiple native categories. This indicates, furthermore, a more focused and thus stronger native phonological contribution for Categorised than Uncategorised assimilations. PAM principles thus extend to non-native tone assimilations and indicate the importance of both native phonological and phonetic contributions to non-native speech perception.