Pragmatic interpretation of contrastive prosody: It looks like speech adaptation

Pragmatic interpretation of contrastive prosody: It looks like speech adaptation Chigusa Kurumada kurumada@stanford.edu Dept. of Linguistics Stanford University Meredith Brown Michael K. Tanenhaus mbrown@bcs.rochester. edu mtan@bcs.rochester. edu Dept. of Brain and Cognitive Sciences Dept. of Brain and Cognitive Sciences University of Rochester University of Rochester Abstract Drawing on insights from recent work on phonetic adaptation, we examined how listeners interpret prosodic cues to two op- posing pragmatic meanings of the phrase “It looks like an X” (e.g., “It looks like a zebra (and it is one)” and “It LOOKS like a zebra (but its actually not)”. After establishing that differ- ent prosodic contours map onto these meanings (Experiment 1), we demonstrated that prosodic interpretation is shifted by inclusion of another alternative (Experiment 2); the reliabil- ity a speaker’s use of prosody to signal pragmatic alternatives (Experiment 3); and most importantly by the distribution of cue values along a continua (Experiment 4). We conclude that listeners derive linguistically meaningful categories from highly variable prosodic cues through rational inference about assumptions that are shared in the conversational context and adaptation to distributional characteristics of prosodic cues. Keywords: Prosody, contrastive accent, Gricean pragmatics, speech adaptation, rational inference Figure 1: Waveforms (top) and pitch contours (bottom) of the utterance “It looks like a zebra”. The affirmative interpreta- tion It is a zebra is typically conveyed by the pattern on the left, while the negative interpretation It is not a zebra is con- veyed by the pattern on the right. 1998). The other is to adapt phonetic categories to the dis- tributional characteristics of the acoustic input. For example, Clayards, Tanenhaus, Aslin, and Jacobs (2008) provided evi- dence that listeners adapt their perceptual categories accord- ing to the mean and the variance of a bimodal distribution along a VOT continuum (e.g. /b/-/p/). Importantly, the way listeners integrate different kinds of information in speech perception is compatible with the hy- pothesis that they make rational inferences (Kleinschmidt & Jaeger, 2011). Listeners can weight different kinds of in- formation according to their reliability, adjust phonetic cat- egories based on more reliable information, and ignore de- viation from the expected patterns when there is an ad-hoc source for the unfamiliar pronunciations (Kraljic, Brennan, & Samuel, 2008). The current study draws on these insights to explore the hypothesis that listeners navigate prosodic heterogeneity by adapting their interpretations through rational inference. We focus on the construction “It looks like an X”, which can evoke different pragmatic meanings depending on its prosodic realization. A canonical accent placement (as illus- trated in Figure 1, left panel, henceforth noun-focus prosody) typically elicits an affirmative interpretation (e.g. It looks like a zebra and I think it is one). When the verb “looks” is lengthened and emphasized with a contrastive accent (L+H*) and the utterance ends with a L-H% boundary tone (Figure 1, right, verb-focus prosody), it can trigger a negative interpre- tation (e.g. It LOOKS like a zebra but its actually not one; see also Dennison & Schafer, 2010). We explored the adaptation hypothesis in four rating ex- periments. Experiment 1 established that listeners system- atically derive different pragmatic interpretations based on noun- and verb-focus prosodic contours. Experiments 2 and 3 demonstrated that pragmatic interpretations are systemati- Introduction In a famous scene in the movie Taxi Driver, Robert DeNiros character repeatedly utters, You talkin’ to me. As he changes pitch contours, the intended meaning of the utterance shifts from a question to a challenge. As the example illus- trates, prosody carries information about a speakers inten- tions. However, the acoustic features of prosodic alterna- tives, as well as the mappings between prosodic patterns and intended meanings vary considerably across speakers. For example, although rising boundary tones can distinguish be- tween questions and assertions in many languages, many questions in fact do not end with a rising boundary tone. Also speakers who use “up-talk” often end assertions with a rising boundary tone. Likewise, a pitch accent preceded by an ini- tial drop (fall-rise: often annotated as L+H* in the ToBI con- vention (Silverman et al., 1992)) can signal the presence of contrast. However, characterizing the acoustic properties that signal this contrast in natural speech is far from straightfor- ward. L+H* and a simple rise (H*) have overlapping inter- pretations that are highly dependent on utterance context (Ito & Speer, 2008; Watson, Tanenhaus, & Gunlogson, 2008). How, then, do listeners navigate the lack of invariance in prosodic cues to pragmatic meaning? We propose that lis- teners solve the variability problem for prosody in the same way as they solve the variability problem for phonetic fea- tures, namely by adaptation. Just as prosodic contours vary according to both random and systematic factors, phonetic features of speech contain massive variability, which presents a challenge to listeners who are to derive discrete phonemic categories. It has been suggested that the speech perception system deals with the lack of invariance in two ways. One is to store separate, speaker-, group-, and context-specific rep- resentations of tokens from the same categories (Goldinger,