The Specificity of Online Variation in Speech Production

The Specificity of Online Variation in Speech Production Christo Kirov(kirov@cogsci.jhu.edu) Department of Cognitive Science, 3400 N. Charles Street Baltimore, MD 21218 USA Colin Wilson (colin@cogsci.jhu.edu) Department of Cognitive Science, 3400 N. Charles Street Baltimore, MD 21218 USA Abstract Interestingly, offline phonetic enhancement effects seem to be rather general: they appear to depend only on the (frequency-weighted) density of a word’s neighborhood, not on the precise phonological relationships between the word and its neighbors. For example, Scarborough (2004) found that words that were particularly confusable by their nasal consonant (i.e., had one or more lexical neighbors that dif- fered in the position of the nasal) did not show greater vowel nasalization than words that were not similarly con- fusable. Words like stem, with minimal pair neighbor step, showed similar levels of nasalization as words like plank, with no nasal-differing neighbors in the lexicon. Similarly, Goldinger & Summers (1989) found more VOT enhancement for voiceless-initial words from dense neighborhoods than those from sparse neighborhoods, even though both sets of words had exactly one minimal pair lexical neighbor that be- gan with a voiced sound. The generality of offline phonetic enhancement suggests that it is driven by competition among entire lexical items, not among sublexical units. Unlike offline effects, online effects on phonetic realiza- tion by definition depend on the context in which a word is uttered, such as the discourse topic, transitional probabilities conditioned on preceding material, and other contextually- salient words. A classic online effect is the Lombard Re- flex, a set of vocal changes that include increases in ampli- tude and pitch that occur when speakers attempt to talk over noise (Lau, 2008; Zhao & Jurafsky, 2009). More recently, a number of corpus-based studies have found that the contex- tual predictability of speech elements, including phonemes and syllables, is inversely related to their length. Less pre- dictable elements tend to be longer (e.g., Cohen-Priva & Ju- rafsky 2008, Aylett & Turk 2004, van Son & Pols 2003). This paper aims to expand our understanding of online phonetic enhancement effects, looking not just at predictabil- ity effects but at how a word’s phonological neighborhood in context — the sound structure of contextually salient com- petitors — affects phonetic realization along several dimen- sions of possible hyperarticulation. This will provide further insight into how competition between similar words plays out during speech production. In previous work, Baese-Berke & Goldrick (2009) found that VOT is lengthened when a voiceless-stop initial word is pronounced in the context of a voiced-initial neighbor (in comparison to the context of a phonologically unrelated filler word). For example, cot shows increased initial VOT in the context of got, but not in Phonetic variation is sensitive to lexical properties of words, such as frequency and neighborhood density, as well as con- textual properties, such as predictability. Previous studies of lexically-induced variation have observed that both vowels and consonants are phonetically enhanced in words from dense neighborhoods, and have suggested that this effect is mod- ulated only by the number and frequency of the neighbors. To determine whether contextual variation is driven by cogni- tive processes similar to those underlying lexical enhancement, three experiments examined the effect of contextually salient neighbors on the phonetic realization of vowels and initial consonant aspiration. Enhancement was found only for con- sonants, and only when the neighbor differed from the target word in a single feature. Unlike lexical effects, contextually- driven phonetic enhancement reflects a highly specific compe- tition among words, a finding that can be rationalized in terms of the utility of speaker effort within a Bayesian model of word communication. Keywords: Speech production; lexical competition; commu- nication; Bayesian modeling Introduction Competition among alternatives, and the need to resolve com- petition efficiently and correctly, are pervasive in speech per- ception and speech production (e.g., Luce & Pisoni 1998, Marslen-Wilson & Zwitserlood 1989, Dell & Gordon 2003). Listeners must determine the speaker’s intended message as rapidly as possible given an inherently ambiguous signal. In speech production, words and sublexical units that are par- tially consistent with the intended message compete for real- ization at multiple levels of processing. A number of studies have examined how such competitive processes are reflected in the fine-grained phonetic realization of speech sounds. The number and relative frequency of phonologically- similar words in the lexicon (lexical neighbors) are known to affect phonetic realization. We refer to such affects as of- fline because they appear to depend on relatively static lex- ical relationships among words rather than dynamic contex- tual factors. Researchers have found that “hard” words, those with low lexical frequency and many high frequency neigh- bors, tend to be phonetically enhanced relative to “easy” words with high frequency and few neighbors. Hard words beginning with aspirated consonants have longer aspiration (as measured by voice onset time, VOT) than easy words (Goldinger & Summers, 1989). They are pronounced with an expanded vowel space (Munson & Solomon, 2004; Wright, 2003), and also show increased vowel nasalization and vowel- to-vowel coarticulation (Scarborough, 2004).