Factors influencing vowel categorisation flexibility

Phonological category boundaries are flexible, and listeners are highly capable of interpreting variation in speech. This is particularly evident in vowel categorisation. Some factors that may facilitate or impede this flexibility are spectral distance from the listener’s native vowel category, listener dialect, vowel frontness, and lexical bias. The current project explored the relative impact these factors may have on flexibility of category boundaries, to understand more about tolerance of variation. Continua of VC and CVC-structured sequences were used in a forced choice categorisation task. The stimuli varied in their lexicality; some continua contained only words or nonwords, and some contained both. The continua also varied in vowel frontness. Listeners from two different dialect groups (Standard Southern British English and New Zealand English) took part in the categorisation task. Results showed that lexical bias influences vowel categorisation, but only across an ambiguous boundary and categorising away from an underspecified vowel. This offers support for a range of speech perception theories such as Ideal Adapter and Featural Underspecification.