Interactions among lexical and discourse characteristics in vowel production

Various factors are known to affect vowel production, including lexical frequency, neighborhood density, contextual predictability, mention in the discourse, and speaking style. This study explores interactions among all five of these factors on vowel duration and dispersion. Participants read paragraphs containing target words varying in frequency, density, and predictability. Each target word appeared twice in the paragraph. Participants read each paragraph twice: as if they were talking to a friend ("plain speech") and as if they were talking to a hearing-impaired or non-native interlocutor ("clear speech"). Measures of vowel duration and dispersion were obtained. Results revealed that high frequency words and words in plain speech were shorter and less dispersed in the vowel space than low frequency words and words in clear speech, and that second mentions were less dispersed in the vowel space than first mentions, as expected. A series of interactions among the lexical and discourse factors was also observed for both vowel duration and vowel dispersion, suggesting maximization of phonetic reduction in some contexts that impose limited processing costs on the listener and maximization of phonetic enhancement in other contexts that impose heavy processing costs on the listener.