Real-time expectations based on context speech rate can cause words to appear or disappear

Real-time expectations based on context speech rate can cause words to appear or disappear Meredith Brown (mbrown@bcs.rochester.edu) Department of Brain & Cognitive Sciences, University of Rochester Meliora Hall, Box 270268, Rochester, NY 14627-0268 Laura C. Dilley (ldilley@msu.edu) Department of Communicative Sciences & Disorders, Michigan State University 116 Oyer, East Lansing, MI 48824 Michael K. Tanenhaus (mtan@bcs.rochester.edu) Department of Brain & Cognitive Sciences, University of Rochester Meliora Hall, Box 270268, Rochester, NY 14627-0268 Abstract To test predictions of a forward modeling framework for spo- ken language processing, we characterized effects of context speech rate on the real-time interpretation of indefinite noun phrases using the visual world paradigm. The speech rate of sentence material distal to the onset of the noun phrase was ma- nipulated such that the segments surrounding the determiner a in singular noun phrases had a faster speech rate than the sur- rounding context and the segments surrounding the onset of plural noun phrases had a relatively slow rate. These manipu- lations caused listeners to fail to perceive acoustically present determiners and to falsely perceive determiners not present in the signal. Crucially, fixations to singular and plural target pic- tures revealed effects of distal speech rate during the real-time processing of target expressions, strongly suggesting a locus in perceptual expectations. These results set the stage for quanti- tative tests of forward models of spoken language processing. Keywords: Speech rate; prosody; expectations; speech processing; visual world paradigm Introduction Expectation-based approaches in which perceptual input is evaluated with respect to internally generated forward mod- els provide compelling and increasingly influential explana- tions of phenomena in the perception and motor control liter- atures (e.g. Jordan & Rumelhart, 1992; Kawato, 1999). For example, DIVA, an influential model of speech production, incorporates a forward model component that predicts the auditory signal likely to result from a particular configura- tion of articulators within the vocal tract (Guenther & Micci Barreca, 1997). The forward model component accounts for the speed and efficiency with which the system can control speech movements, given the relatively slow mechanisms by which acoustic feedback influences speech production. Similar forward modeling approaches may also provide a promising explanatory framework for spoken language com- prehension. As in the domain of motor control, forward mod- eling of the perceptual attributes of upcoming speech pro- vides a compelling explanation for the remarkable speed and efficiency of speech perception and spoken word recognition in the face of considerable variability. This is a particularly attractive feature of expectation-based approaches to higher- level language comprehension as well, which propose a cen- tral role for expectations in processes such as syntactic com- prehension and lexical processing (e.g. Levy, 2008; Altmann & Kamide, 1999). We propose that comprehension also in- volves developing expectations about the acoustic realization of upcoming speech, conditioned on various contextual fac- tors such as prosodic phrasing, speech rate, discourse his- tory, and speaker-specific characteristics. Previous work sug- gests that these expectations are best characterized as prob- ability distributions, with listeners representing not only the expected form of a spoken word given the set of contextual conditioning factors, but also a measure of the variance or uncertainty of their estimate (Clayards et al., 2008; Levy et al., 2009). The degree of congruence between these percep- tual expectations and the incoming acoustic signal then con- tributes to the differential activation of competing lexical al- ternatives. Finally, when perceptual expectations are incon- gruent with the actual realization of a word, listeners should update their beliefs about the cues that condition their percep- tual expectations, resulting in adaptation that more closely aligns listeners’ expectations with the characteristics of the signal in context. Speech prosody generates acoustic regularities that are likely to foster expectations about the acoustic realization of upcoming speech sounds, including pitch and temporal char- acteristics that listeners perceive as patterning. This perceived patterning has been shown to influence real-time spoken lan- guage processing. For example, manipulations of pitch and duration early in an utterance influence the interpretation of cues to prosodic structure several syllables downstream (Dilley & McAuley, 2008; Dilley et al., 2010; Brown et al., 2011). The distal locus of these effects suggests that they are rooted in listeners’ expectations about the acoustic-phonetic realization of upcoming segments. Speech rate is particularly likely to systematically influ- ence listeners’ expectations about upcoming material. Speech sounds are interpreted relative to the global speech rate of an utterance, affecting perceived phoneme distinctions such as voicing contrasts (e.g. Miller, 1987; Reinisch et al., 2011). Therefore, listeners must evaluate the incoming signal with respect to a speaker’s estimated rate. Dilley and Pitt (2010) demonstrated that effects of context speech rate scale up to the perceived rate of articulation of larger constituents, in-