The rational speaker hypothesis (Clifton, Carlson, & Frazier, 2002) claims that speakers are self-consistent, employing intonation in a manner consistent with their intended message. Preceding a constituent by a prosodic boundary that is not required by the grammar often signals that this constituent is not part of the immediately preceding phrase. However, speakers tend to place prosodic boundaries before and after long constituents. The question is whether prosodic boundaries will have a larger influence on listeners' choice of an analysis when they flank short constituents than when they flank long ones. The results of two listening experiments indicate that they do, suggesting that listeners attend not just to properties of the input signal, but also to the reasons why speakers produce those properties.
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Speakers apparently do not tailor their utterances to make them maximally easy for listeners to comprehend. Instead, speakers seem to choose structures on the basis of their own needs, delaying the articulation of phrases that take a long time to plan (Arnold, Wasow, Losongco, & Ginstrom, 2000; Wasow, 1997) and uttering alreadyplanned phrases as soon as possible, without regard for structural ambiguities (Ferreira, 1996; Ferreira & Dell, 2000). But although speakers may not try to accommodate the needs of listeners, listeners may have to pay close attention to the behavior of speakers in order to determine which aspects of an utterance are informative with respect to the speaker's intentions.
In the present experiments, we investigate whether one aspect of a spoken sentence, the presence of an intonational phrase (IPh) boundary, becomes less informative to the listener under conditions in which more than one reason for the boundary exists.1 The way adjacent words are spoken depends on many factors, including their phonological length and the syntactic structure they occur in. Whether there is a prosodie boundary separating the words and, if so, whether it is an intermediate phrase (ip) or an IPh boundary in the ToBI analysis system (Beckman & Ayers, 1993; Pierrehumbert, 1980), will depend, in part, on whether the two words occur in the same syntactic phrase (Nespor & Vogel, 1986; Selkirk, 1984; Truckenbrodt, 1995), as well as on the length of the constituents (Gee & Grosjean, 1983; Watson & Gibson, 2001, 2004). Listeners' interpretations of sentences with ambiguous syntactic constituency are influenced by the presence and placement of prosodic boundaries (see, among others, Carlson, Clifton, & Frazier, 2001; Price, Ostendorf, Shattuck-Humagel, & Fong, 1991). Furthermore, listeners'judgments of the prosodic appropriateness of prosodie boundaries are affected by the lengths of the phrases they precede (Frazier, Clifton, & Carlson, 2004), and speakers are more likely to place a prosodic boundary before a long phrase than before a short one (Fodor, 1998; Watson & Gibson, 2004).
We examined how the listener deals with the alternative possible reasons for an IPh boundary in the context of the rational speaker hypothesis (Clifton, Carlson, & Frazier, 2002). According to this hypothesis, listeners interpret intonation by assuming that speakers do not make prosodic choices without some reason (and are, therefore, rational). In particular, if a speaker intends a larger syntactic boundary before X than before Y in the sequence X . . . Y, the speaker cannot then place a larger prosodic boundary before Y than before X for no reason. If constituent Y is long, though, the speaker might place a large boundary before Y in order to produce the sentence fluently. In this case, a rational speaker might intend a larger syntactic boundary at X but place the larger prosodic boundary at Y. The present question is whether listeners adjust. Do they discount prosodie boundaries flanking long constituents because they can be justified by the length of the constituents? …