Phonological Symmetry Does Not Predict Generalization of Perceptual Adaptation to Vowels

Speech perception is inherently adaptive, with context-dependent rather than fixed perceptual phoneme boundaries. Exposure to novel vowel variants in lexically biasing contexts induces lasting boundary shifts toward the novel variant. Studies have found mixed results as to whether perceptual adaptation generalizes to phonologically related vowels and suggest that phonological symmetry might predict generalization. We test this hypothesis in Turkish, which has a fully symmetrical 8-vowel inventory. Listeners were exposed to words with either lowered /i/s or raised /ɛ/s and identified vowels on /i-e/ and /u-o/ continua to assess perceptual adaptation and generalization. We found perceptual adaptation to only lowered /i/s in /i-e/ identification and no generalization of perceptual adaptation in /u-o/ identification. We argue that phonological symmetry might not be sufficient for generalization, and that vowel inventory size and organization might also play a role.