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Syntactic Structure Guides Prosody in Temporarily Ambiguous Sentences Catherine Anderson and Katy Carlson Language and Speech 2010 53: 472 originally published online 1 September 2010 DOI: 10.1177/0023830910372497 The online version of this article can be found at: http://las.sagepub.com/content/53/4/472

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Article

Syntactic Structure Guides Prosody in Temporarily Ambiguous Sentences

Language and Speech Language and Speech 53(4) 472–493 © The Author(s) 2010 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0023830910372497 las.sagepub.com

Catherine Anderson

McMaster University, Ontario, Canada

Katy Carlson

Morehead State University, Kentucky, USA

Abstract A pair of speaking and listening studies investigated the prosody of sentences with temporary Object/Clause and Late/Early Closure ambiguities. Speakers reliably produced prosodic cues that allowed listeners to disambiguate Late/Early Closure sentences, but only infrequently produced prosody that disambiguated Object/Clause sentences, as shown by the results of listening studies. The two continuations for Object/Clause sentences were not pronounced with identical prosody, but the differences in their productions were not helpful to listeners. Speakers’ different performance on the two sentence types is traced to their different syntactic structures. These results illustrate the importance of the syntax–prosody mapping in production and test the prosodic predictions of syntax–prosody models like that of Watson and Gibson (2004). Keywords ambiguity, prosody, sentence comprehension, sentence production

Introduction It is clear that prosody can influence parsing decisions (e.g., Carlson, Clifton, & Frazier, 2001; Cutler, Dahan, & van Donselaar, 1997; Lehiste, 1973; Nespor & Vogel, 1986; Price, Ostendorf, Shattuck-Hufnagel, & Fong, 1991; Speer, Kjelgaard, & Dobroth, 1996, and many others). Furthermore, a growing literature provides evidence as to the factors that predict speakers’ use of prosody in their productions of various kinds of sentences (Gahl & Garnsey, 2004; Kraljic & Brennan, 2005; Schafer, Speer, & Warren, 2005; Schafer, Speer, Warren, & White, Corresponding author: Catherine Anderson, McMaster University, Department of Linguistics and Languages, 1280 Main Street West, Hamilton, Ontario L8S 4M2, Canada Email: [email protected]

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2000; Snedeker & Trueswell, 2003; Watson, Breen, & Gibson, 2006; Watson & Gibson, 2004). In this paper, we argue that differences in the production and processing of Object/Clause and Late/Early Closure temporary ambiguities result from differences in the sentences’ syntactic structures. Our findings also allow us to test the predictions of models of the mapping between syntax and prosody proposed by Watson and Gibson (2004) and Cooper and Paccia-Cooper (1980). The Object/Clause temporary ambiguity is exemplified in (1), where the noun phrase (NP) following the verb believed could be either the direct object of that verb (1a) or the subject of an embedded clause complement of that verb (1b). (1)

The jury believed the defendant … a. without any hesitation. b. had committed the crime.

(Object) (Clause)

Late/Early Closure sentences, illustrated in (2), are superficially similar in that the ambiguous NP following the first verb, baked, could be either the direct object (2a) or the subject of the following clause (2b). (2)

As Janet baked the bread… a. the brownies cooled on a rack. b. cooled on a rack.

(late) (early)

The two kinds of local ambiguity are quite different syntactically, however. At the point of the critical NP in (1), both the verb phrase (VP) and the main clause are still open in both structures. At the point of the critical NP in (2), the preposed subordinate clause and its VP are closed in the Early Closure version and open on the Late Closure version. We assume the prosodic system of Pierrehumbert (1980; Beckman & Pierrehumbert, 1986; Pierrehumbert & Hirschberg, 1990) which also underlies the Tone and Boundary Indices (ToBI) analysis system (Beckman, Hirschberg, & Shattuck-Hufnagel, 2004). This system allows two levels of structure above the word: an intermediate phrase (ip) contains at least one pitch accent and ends with a phrase accent, and an Intonational Phrase (IPh) can include one or more ip’s and ends with a boundary tone. Prosodic theories which agree to this extent nevertheless vary in the predictions they make for the prosodic constituency of these sentence types. Watson and Gibson’s (2004) Left-hand side/Right-hand side Boundary (LRB) weight hypothesis is similar to the proposals of Cooper and Paccia-Cooper (1980), whose system predicts prosodic boundary placement on the basis of syntactic phrase bracketing. The LRB hypothesis claims that it is not only the number and type of syntactic brackets but also the size (in phonological phrases) of preceding and upcoming syntactic phrases that determines the likelihood of a prosodic boundary at any given location. The hypothesis also incorporates Selkirk’s (1984) notion that an IPh must be a “sense unit”, defined as a head along with its modifiers and arguments. Selkirk’s theory is explicitly non-syntactic, arguing that prosody is constrained by semantic relations, and predicts that it should be grammatical for (1) to be uttered with prosodic boundaries after the verb and/or the following noun phrase under either the Object or the Clause structure. Selkirk (2000) considers the phrasing possibilities of a prepositional dative sentence (e.g., She loaned her rollerblades to Robin), and concludes that phrasing the PP to Robin with the

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previous NP, to the exclusion of the verb, is not allowed in a neutral informational context. For Object/ Clause sentences, this line of argument would suggest that a boundary after the NP is likely on the Object analysis. Likewise, Cooper and Paccia-Cooper’s (1980) syntax-based theory predicts a slight difference between the Object and Clause sentence prosodies. On the other hand, Watson and Gibson (2004) predict that Object/Clause sentences will have similar prosodic structures. Table 1, which summarizes the predictions of Watson and Gibson (2004) and of Cooper and Paccia-Cooper (1980) for Object/Clause and Late/Early Closure sentences, shows that both systems predict prosodic differentiation for Closure sentences. In fact, they both predict much stronger prosodic differentiation for Closure sentences than for Object/Clause sentences because of their different syntactic structures. The literature on the processing of these sentences supports the theoretical prediction that Object/Clause ambiguities are harder to differentiate prosodically than Late/Early Closure ambiguities. Numerous researchers (Beach, 1991; Marslen-Wilson, Tyler, Warren, Grenier, & Lee, 1992; Nagel, Shapiro, Tuller, & Nawy, 1996; Schepman, 1997; Stirling & Wales, 1996; Warren, 1985; Warren, Grabe, & Nolan, 1995; Watt & Murray, 1996) have investigated the effects of prosody on listeners’ responses to sentences with Object/Clause ambiguities. Warren’s (1985) study of the production of five Object/Clause sentences found shorter nouns and following pauses in the Clause versions of the sentences, as well as pitch resetting between the peak F0s on the verb and the noun (suggesting a prosodic boundary following the verb). The Object versions had longer noun durations and a continuing F0 fall over the verb and noun. These cues, which are most consistent with Cooper and Paccia-Cooper’s (1980) predictions, led listeners to choose the matching completion over 80% of the time when presented with recordings of the ambiguous sentences truncated after the post-verbal noun. Nagel et al.’s (1996) production study of a single speaker also found longer verbs for the Clause conditions, and Beach (1991) found that listeners chose the Clause continuation more often for synthesized sentences that had acoustic indicators of a prosodic boundary after the verb. However, when Stirling and Wales (1996) replicated Beach’s experiment using human-produced materials, they found no such effect. Marslen-Wilson et al. (1992) found evidence that listeners could

Table 1. Prosodic options predicted by existing theories Sentence type

Predicted prosody First verb

Watson & Gibson (2004) Object (The jury) 5 Clause (The jury) 5 Late (As Janet) 4 Early (As Janet) 3 Cooper & Paccia-Cooper (1980) Object (The jury) 2 Clause (The jury) 2 Late (As Janet) 1.3 Early (As Janet) 1.6

Critical NP

(believed) (believed) (baked) (baked)

1 1 1 6

(the defendant) (the defendant) (the bread) (the bread)

3 3 7 4

(without any hesitation) (had committed the crime) (the brownies) 4 (cooled) 2 (cooled) 2 (on a rack)

(believed) (believed) (baked) (baked)

3 5 2 7.2

(the defendant) (the defendant) (the bread) (the bread)

1.5 1.5 10 2.4

(without any hesitation) (had committed the crime) (the brownies) 2.7 (cooled) 1 (cooled) 1.2 (on a rack)

Note. Parentheses indicate predicted ip boundaries. Numbers indicate the likelihood of IPh boundaries as predicted by the authors’ algorithms.


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use prosody to avoid a garden-path in Object/Clause sentences in a cross-modal naming task, but Watt and Murray (1996) found no effect of prosody on response latency. Schepman (1997) found no significant effect of a longer verb and F0 differences in a cross-modal naming study and an off-line forced-choice continuation study. Some of these experiments suggest that it might be possible for speakers and listeners to use prosodic information to disambiguate Object/Clause ambiguities, but the variability of results across different experiments suggests that speakers have many prosodic options. In fact, given that prosodic theories predict little difference in the productions of these sentences, the amount of differentiation found by some researchers is somewhat surprising. In contrast, there is greater consistency across the production and processing results for Closure sentences. Speer, Kjelgaard, and Dobroth (1996) showed that trained speakers could facilitate listeners’ processing of Late/Early Closure sentences with well-placed prosodic boundaries. Schafer et al. (2000) then showed that naïve speakers also prosodically disambiguated such sentences in a communicative task, and the productions they collected were very successful in allowing listeners to predict the correct continuation of the sentences. Warren et al. (1995) also found that prosodic differences in Closure sentences led to different response times in a cross-modal naming study. Our hypothesis, following Watson and Gibson (2004) and Cooper and Paccia-Cooper (1980), is that speakers will use prosody to disambiguate Late/Early Closure sentences more frequently than they do for Object/Clause sentences. Furthermore, we predict that listeners will be able to disambiguate speakers’ productions of Closure sentences more frequently than Object/Clause sentences. If Watson and Gibson’s predictions are entirely right, there should be no consistent prosodic differences in Object/Clause sentences, while Cooper and Paccia-Cooper predict minor differences. An alternative hypothesis arises from the concern, raised by Allbritton, McKoon, and Ratcliff (1996), that the task of reading a list of sentences in a laboratory sound booth is not representative of normal speaking conditions. This artificial task might make speakers hyper-aware of their productions, leading them to produce clearer speech and larger prosodic cues than in normal conversation. If so, then we should observe disambiguating prosody in both Object/Clause sentences and Late/ Early Closure sentences.

Experiment 1 Method Participants. Twenty-four Northwestern University students participated in the experiment and received course credit. The recordings from eight participants were excluded from the analysis: six who were not native speakers of English, one who had already participated in a related experiment, and one whose production included many errors, corrections, and disfluencies. Of the remaining 16 participants, four were men. All participants reported normal speech and hearing. Stimuli. The stimuli consisted of 16 sets of Object/Clause sentences and 20 pairs of Late/Early Closure sentences. The set of Object/Clause sentences included a manipulation of the length of the subject NP: the long-subject version was at least three syllables longer than the short-subject version. Examples are given in (3)–(6); the entire list of stimulus sentences is provided in the Appendix.

476 (3) (4) (5) (6)

Language and Speech 53(4) [ Tom / Thomas Morgencrantz ] noticed his roommate during the long lecture. [ Tom / Thomas Morgencrantz ] noticed his roommate seemed kind of depressed. While the violinist practiced the concerto the symphony blared from the neighbor’s radio. While the violinist practiced the concerto blared from the neighbor’s radio.

(Object) (Clause) (Late Closure) (Early Closure)

The 36 sentences were arranged in scripts that included 40 filler sentences with a variety of structures. Four versions of the script were prepared using a Latin Square design so that no participant saw more than one version of each sentence. Each script was arranged in pseudorandom order, with no two sentences with the same type of continuation appearing adjacent to each other. Each sentence in the script was followed by a comprehension question that was designed to ensure that participants had parsed the sentence correctly, since they were presented without punctuation (except for an initial capital letter and a final period). For example, the comprehension question following sentence (6) is shown in (7): (7)

What do you know about the concerto? a. It blared from the neighbor’s radio. b. The violinist wrote it.

In the sentences with temporary ambiguities, only one of the two answers to the question was consistent with the grammatical interpretation of the sentence. In the filler sentences, one of the answers was obviously correct and one obviously incorrect. Procedure. The recordings were made in a sound-attenuated booth, using a Shure SM 81 microphone connected through an Ariel Proport soundboard into an Apple computer. Each participant was given a script and asked to read each sentence silently, answer the comprehension question by circling the correct answer on the page, and then read the sentence aloud. The recordings were sampled at a rate of 22,050 Hz and stored digitally on computer disk. The entire experiment took about 45 minutes to complete.

Results For each of the 16 speakers, there were 36 experimental items for a total of 576 recorded sentences. Nineteen tokens were excluded from the analysis, either because the speaker had read the sentence incorrectly or because of a major disfluency. The experimenters performed acoustic and phonological analysis on the remaining 557 tokens using Praat (Boersma, 2001; Boersma & Weenink, 2000). Duration measurements. The durations of the verb and the ambiguous NP were measured for each sentence from the beginning of each region to the beginning of the following region, with any following pauses included in the measurement. Figures 1 and 2 show the mean durations of these


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regions calculated from the analysis by participants. In these and all figures, error bars represent the 95% confidence interval on the mean. The duration data were entered into an ANOVA with fixed factors of subject length and sentence continuation, and random factors of participants (F1) or items (F2). The results of this analysis are given in Table 2. Analysis of variance was performed on the LogNormal transformation of the verb durations for Closure sentences and the NP durations in both sentence types since the durations themselves were not normally distributed. The results reveal some differences in speakers’ productions in the two kinds of ambiguities. In Object/Clause sentences, speakers produced a slightly longer verb in Clause sentences than in Object sentences, but this difference was significant only in the analysis by items. The ambiguous NP was significantly longer in Object sentences than in Clause sentences. There was no evidence that the length of the sentence’s subject had any effect on the durations of the verb or of the ambiguous NP. In Closure sentences, the pattern was much clearer: the duration of the verb was significantly longer in Early Closure than in Late Closure sentences, while the ambiguous NP was significantly longer in Late Closure than in Early Closure sentences. We also calculated the durations of the verb and the ambiguous NP as a proportion of the total sentence duration, to take account of variation in speakers’ rate of speech. The overall pattern of results and of the statistical tests was the same for these relative duration measurements as for the raw durations.

Figure 1. Mean duration of verb

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Figure 2. Mean duration of noun phrase

Table 2. Analysis of variance results for verb and noun phrase durations Phrase region

Source of variance

Object/Clause sentences Verb Length (L) Continuation (C) L*C Noun phrase Length (L) Continuation (C) L*C Closure sentences Verb Continuation Noun phrase Continuation

By participants

By items

df

df

1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15

F1 0.00 1.67 0.02 0.36 45.73** 0.31 115.2** 60.5**

1,15 1,15 1,15 1,15 1,15 1,15 1,19 1,19

Min F′ F2 0.06 8.43* 0.25 2.72 43.60** 0.59 392.7** 99.6**

df

min F′

1,17 1,21 1,17 1,19 1,30 1,27

0.00 1.39 0.02 0.32 22.32** 0.20

1,29 1,34

89.1** 37.6**

*

p < .05, **p < .01.

Fundamental frequency measurements. Speakers’ F0 (fundamental frequency) heights were measured in three critical sentence regions: the subject NP, the verb, and the following ambiguous NP. Within each region, one F0 measurement (labeled “center”) was taken on the main stressed syllable of the phrase head. The peak F0 was measured for syllables with a non-low pitch target, and the minimum F0 was measured for concave and flat low syllables. The other F0 measurement (labeled

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“end”) was taken at the end of each region. The peak F0 was measured for final syllables higher than the stressed syllable, and the minimum F0 for lower final syllables. Final syllable values differing by less than 10 Hz (for women) or 6 Hz (for men) from the stressed syllable measurement were not counted, being within the range of segmental perturbations of F0. In the long-subject Object/Clause items, the F0 measurements were taken on the head nouns of the definite descriptions (e.g., the tired and overworked jury) and the last names in lengthened proper names in order to examine a similar position in the phrase (e.g., Alexandra Douglas). The averaged F0s in each sentence position for the Object/Clause sentences are shown in Figures 3 and 4, for female speakers. The pattern for the four male speakers was very similar, but averaging over both genders led to distracting variations in F0 depending on how many male speakers produced any particular pattern. Creaky voice prevented analysis of 11 final lows within verbs, and 27 final lows within ambiguous NPs. The F0 data were entered into an ANOVA with fixed factors of subject length and sentence continuation, and random factors of participants (F1) or items (F2). The results of this analysis are given in Table 3. At the beginning of the sentences, a significant main effect of subject length reflected that the stressed and final syllables of the long subjects were lower than those syllables in the short-subject conditions. The stressed syllable effect is primarily due to pitch declination: the long-subject NPs started high, but the head nouns or last names which were measured occurred late in each phrase, where the F0s were lower. Also, longer NPs were likely to be prosodic phrases on their own, so the ends of long-subject NPs were low because of boundary tones.

330 Object Sentences Clause Sentences

310 290

F0 (Hz)

270 250 230 210 190 170 150 Subject center

Subject end

Verb center

Verb end

Noun Phrase center

Noun Phrase end

Figure 3. Mean fundamental frequency for critical regions in Object/Clause sentences with short subjects, women speakers

480


240

Object Sentences Clause Sentences

230

F0 (Hz)

220

210

200

190

180

170 Subject center

Subject end

Verb center

Verb end

Noun Phrase center

Noun Phrase end

Figure 4. Mean fundamental frequency for critical regions in Object/Clause sentences with long subjects, women speakers

Table 3. Analysis of variance results for F0s for Object/Clause sentences, women speakers Phrase-region

Subject center Subject end Verb center Verb end NP center NP end

Source of variance

Length Continuation L*C Length Continuation L*C Length Continuation L*C Length Continuation L*C Length Continuation L*C Length Continuation L*C

p < .10, *p < .05, **p < .01.

By participants

By items

Min F′

df

F1

df

F2

df

min F′

1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11 1,11

33.06** 0.47 1.47 57.84** 0.23 0.02 3.96+ 7.29* 0.003 2.87 12.67** 0.37 0.68 4.55+ 0.09 4.26+ 13.95** 1.86

1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15 1,15

55.73** 1.04 1.41 545.32** 0.004 0.15 4.32+ 5.81* 0.28 0.02 14.44** 2.62 0.44 1.71 0.39 5.57* 10.70** 4.17+

1,22 1,20 1,25 1,13 1,15 1,13 1,25 1,25 1,11 1,15 1,24 1,14 1,25 1,23 1,16 1,23 1,25 1,20

20.75** 0.32 0.72 52.29** 0 0.02 2.07 3.23+ 0 0.02 6.75* 0.32 0.27 1.24 0.07 2.41 6.06* 1.29

481


330 320

Late Closure

310

Early Closure

300 290 280 270 F0 (Hz)

260 250 240 230 220 210 200 190 180 170 160 Subject center

Subject end

Verb center

Verb end

Noun Phrase center

Noun Phrase end

Figure 5. Mean fundamental frequency for critical regions in Closure sentences, women speakers

On the verb, there was a significant main effect of continuation on the stressed syllable and the final syllable, such that sentences with an Object continuation had higher verbs than sentences with a Clause continuation. Finally, at the end of the ambiguous NP there was a significant main effect of continuation, since sentences with an Object continuation often had a lower F0 at the end of that phrase. These patterns suggest that the Object continuation was more likely to have a boundary after the NP than after the verb. The F0 effects on the verb and following ambiguous NP were quite small, though, even when they were significant. The average F0s (over female participants) for the Closure sentences are shown in Figure 5. The male patterns were very similar. The results of an analysis of variance on the fixed factor of Closure, with participants (F1) and items (F2) as random factors, are shown in Table 4. There were significant effects of Closure condition on the verb and the ambiguous NP. In particular, Early Closure sentences had lower F0s on the stressed and final syllables of the verb than Late Closure sentences, and they had higher F0s on the stressed and final syllables of the head noun of the following ambiguous NP. These patterns are consistent with an earlier prosodic boundary (after the verb) in the Early Closure sentences. ToBI analysis. In addition to the acoustic measurements, we analyzed the sentences using the criteria of ToBI transcription (Beckman & Ayers, 1997; Beckman et al., 2004), as a way of summarizing the patterns. The two authors independently analyzed each token to determine whether a prosodic phrase ended after the verb and/or after the ambiguous NP. The two locations were

482


Table 4. Analysis of variance results for F0s for Closure sentences, women speakers Phrase region

Subject center Subject end Verb center Verb end NP center NP end

Source of variance

closure closure closure closure closure closure

By participants

By items

df

F1

df

1,11 1,11 1,11 1,11 1,11 1,11

3.17 15.28** 8.46* 82.58** 8.33* 32.56**

1,19 1,19 1,19 1,19 1,19 1,19

Min F′ F2 1.32 1.33 24.13** 179.67** 7.41* 29.97**

df

min F′

1,29 1,22 1,18 1,20 1,28 1,28

.93 1.22 6.26* 56.58** 3.92+ 15.61**

+

p < .10, *p < .05, **p < .01.

labeled as to the status of the prosodic boundary: either no boundary, an intermediate phrase (ip) boundary, or an intonational phrase (IPh) boundary. The initial judgments of the two raters had a Cohen’s Kappa (unweighted) coefficient of 0.68 (where the maximum is 0.80) (Cohen, 1960). The raters achieved agreement about the remaining judgments after discussion. We follow the lead of Schafer et al. (2000) in categorizing the sentences according to the location of the biggest prosodic boundary, using ToBI criteria, either after the verb or after the following NP. The distribution of prosodic boundaries for the two sentence types is shown in Figures 6–8. The most common contour in Object/Clause sentences had a bigger boundary following the ambiguous NP; this was also by far the preferred contour for Object sentences. Speakers produced equal boundaries at both locations most often in Clause sentences, but they also did so occasionally in Object sentences. In both the short-subject and long-subject conditions, there was a significant difference between speakers’ use of these two prosodic patterns in Object vs. Clause sentences (short-subject: Pearson’s c2 = 78, p < 0.001; long-subject: c2 = 32, p < 0.001). The pattern that speakers used least frequently (which was excluded from the chi-squared analysis because of the small number of observations) had a bigger boundary following the verb. When speakers did produce this contour, it was nearly always in Clause sentences. Speakers most often produced this contour (eight tokens) on sentences containing the verb observed, which Trueswell, Tanenhaus, and Kello (1993) report has a 0.86 bias towards the Object continuation. Speakers’ productions on the observe sentences, then, were consistent with Gahl and Garnsey’s (2004) finding that Clause prosody was more common after Object-biased verbs. But since we did not have bias information for all of our verbs, we could not thoroughly test Gahl and Garnsey’s claims. In Closure sentences, speakers were very consistent in differentiating their prosody. Speakers nearly always placed a bigger boundary after the NP in Late Closure sentences, and almost always placed a bigger boundary after the verb in Early Closure sentences. Contours with equal boundaries at these two locations were rarely produced. In addition, the Closure sentences as a group were produced with 192 IPh boundaries at some location, while the Object/Clause sentences contained only 43 IPh boundaries. This fact suggests a greater magnitude of prosodic differentiation for the Closure sentences.


Figure 6. Distribution of prosodic boundaries in Object/Clause sentences with short subjects

Figure 7. Distribution of prosodic boundaries in Object/Clause sentences with long subjects

483

484


Figure 8. Distribution of prosodic boundaries in Closure sentences

Discussion The results of the production experiment were remarkably consistent for Closure sentences. The duration, F0, and ToBI analyses converged to show that speakers frequently produced clear prosodic boundaries following the verb in Early Closure sentences, and clear prosodic boundaries following the ambiguous NP in Late Closure sentences. These results are consistent with previous production research, such as Schafer et al. (2000). In the Object/Clause sentences, patterns were much less clear. F0 measurements showed that the long-subject NPs differed prosodically from the short NPs, illustrating that the speakers were sensitive to the phonological requirements of prosodic phrases. Duration measurements showed longer post-verbal NPs in the Object sentences. Small F0 effects were also correlated with the sentence continuation, with higher verbs and lower final syllables of ambiguous NPs in the Object sentences. The ToBI analysis of the Object/Clause sentences found that speakers produced a variety of contours for each continuation, with a tendency to produce post-NP boundaries in the Object conditions (consistent with the F0 trends) and equal boundaries in the Clause conditions. The contour with the largest boundary following the verb was used rarely, but when it was used it was most often for Clause continuations. The question is whether the prosodic variation in these sentences is useful for listeners.


485

Experiment 2 Experiment 2 investigated the effect of these prosodic differences on listeners’ ability to identify the continuation of temporarily ambiguous sentences.

Method Stimuli. One quarter of the recorded sentences from Experiment 1 were used as stimuli for the listening experiment. The items were selected pseudo-randomly, such that each sentence appeared once in each of the original conditions. The ambiguous target region was excised from each recorded sentence. The root-mean-square (RMS) amplitudes of the extracted target phrases were equated. The stimuli were arranged in a pseudo-random order, such that no two utterances of the same sentence by the same speaker, or with the same subject length and continuation, occurred adjacent to each other. The stimuli were arranged into sound files with a 12-second inter-trial interval and recorded onto an audio CD. Participants. The participants were 16 Northwestern University students who received course credit for their participation, and 16 Morehead State University students who each received $5 for their participation. The participants ranged in age from 18 to 45 years. All participants reported normal speech and hearing. Procedure. The participants were told that they would hear a sentence fragment that was temporarily ambiguous. Their job was to decide which continuation the original sentence had, based on what the sentence fragment sounded like. Each participant had a printed answer sheet. For each test item, the ambiguous target phrase (i.e., the phrase that the listener heard) appeared on the first line, followed by the two possible continuations on the second line, as shown in (8): (8)

Tom noticed his roommate ... a. seemed kind of depressed. b. during the long lecture.

There were two versions of the answer sheet, with the order of presentation of the two possible continuations for each trial counterbalanced across the two answer sheets. Listeners were seated in a quiet room. The recorded sentences were played on a sound system with speakers in several locations throughout the room. Listeners first completed two practice trials with novel sentences produced by a naïve speaker whose voice did not appear in the experimental items. The experimental items were separated by 12 seconds. There was a 90-second break after every 26 items. The entire listening task (104 items) took about 30 minutes to complete.

Results We calculated for each item the percentage of correct responses and the ratio of Object to Clause or of Late Closure to Early Closure responses. Listeners chose the correct response on 61% of trials for Object/Clause sentences and on 89% of trials for Late/Early Closure sentences, a significant

486


difference, t1(15) = 11.4, p < 0.0001; t2(34) = 9.7, p < 0.0001; t3(31) = 14.3, p < 0.0001. For both types of sentence, the rate of correct identification differed significantly from chance (Object/ Clause: t1(15) = 4.9, t2(15) = 6.2, t3(31) = 9.4, all p’s < 0.001; Late/Early Closure: t1(15) = 34, t2(19) = 20, t3(31) = 18, all p’s < 0.001). There was no evidence of a difference in responses between the Northwestern students and the Morehead State students, so we collapsed the remaining analyses across both groups. There was no speaker whose productions received significantly more correct responses than others. Likewise, there was no sentence that received significantly more correct responses than the others. We also categorized listeners’ responses according to the location of the prosodic boundary in the ambiguous sentence fragment. Table 5 summarizes these results. In Object/Clause sentences, listeners were more successful at identifying the correct continuation for a sentence when speakers had placed a bigger boundary following the verb than when the bigger boundary followed the NP or the boundaries at the two locations were of equal size. Pairwise comparisons revealed that this difference was significant in the analysis by listeners (after verb vs. equal boundaries: t(31) = 2.8, p < 0.01; after verb vs. after NP: t(31) = 4.5, p < 0.001). Because speakers produced so few sentences with a larger boundary following the verb, it was not possible to perform analyses by speakers or by items. It is noteworthy that all three prosodic options allowed listeners to select both continuations a sizable proportion of the time. None of the prosodies was a clear disambiguator towards either continuation for an ambiguous Object/Clause sentence fragment. In Late/Early Closure sentences, the pattern is very different from that observed in Object/Clause sentences. Listeners were very successful at identifying the original continuation of the ambiguous fragment both when the biggest boundary followed the NP and when it followed the verb. But on the few occasions when the speaker had produced equal-sized boundaries at both locations, listeners’ correct identifications were at chance. The difference in correct identifications between items with the bigger boundary following the NP and those with the bigger boundary following the verb was not significant. There were not enough items with equal-sized boundaries to include this category in the statistical analysis.

Discussion Listeners nearly always correctly identified the continuations of ambiguous Late Closure/ Early Closure fragments, which is not surprising since speakers’ productions of these sentences

Table 5. Listeners’ categorizations of ambiguous sentence fragments Prosodic option

Object/Clause sentences

Closure sentences

Equal boundaries Biggest boundary after verb Biggest boundary after noun phrase

61% 74% 60%

48% 91% 89%

Equal boundaries Biggest boundary after verb Biggest boundary after noun phrase

Object

Clause

Late Closure

Early Closure

39% 26% 63%

61% 74% 37%

27% 9% 89%

73% 91% 11%

487


were so consistent and prosodically distinct. But for Object/Clause fragments, listeners were only slightly (though significantly) better than chance at identifying the continuations. The prosodic option that allowed listeners the greatest success at identifying a Clause continuation was a larger boundary following the verb than the NP, which led to 74% correct identifications. However, this was the prosodic option that speakers employed the least often. The results of the listening task add to the findings of the acoustic and phonological measures, showing that speakers’ use of prosody in Late/Early Closure sentences reliably disambiguates the sentence’s continuation, while the prosody used in Object/Clause sentences was much less helpful to listeners.

General discussion This pair of experiments tested the prediction, supported by several models of prosody, that the underlying syntactic differences between Object/Clause sentences and Late/Early Closure sentences would lead to differences in how speakers and listeners use prosody to produce them. Experiment 1 showed that speakers frequently produced strong disambiguating prosody for Late/Early Closure sentences, but much smaller differences in Object/Clause sentences. Correspondingly, Experiment 2 showed that listeners were able to accurately identify the completion of a syntactically ambiguous fragment using the prosodic cues that speakers had produced for Late/Early Closure sentences, but were much less accurate for speakers’ renditions of Object/Clause sentences. Contrary to Allbritton et al.’s (1996) concern, our speakers did not show signs of using unnaturally clear prosody, since the most frequent contour that they produced for Object/Clause sentences included no prosodic boundaries in the ambiguous region. A crucial syntactic difference between these two types of sentences is that an Object/Clause sentence is ambiguous as to whether a clause-beginning syntactic boundary exists after the first verb, while a Closure sentence is ambiguous as to whether a clause-ending syntactic boundary is located after the first verb. (9)

(10)

The jury believed the defendant … a. without any hesitation. b. had committed the crime.

(Object) (Clause)

As Janet baked the bread … a. the brownies cooled on a rack. b. cooled on a rack.

(Late Closure) (Early Closure)

In the Clause version of sentence (1), repeated above as (9), the clause the defendant had committed the crime is embedded inside the verb phrase headed by believed. In the Early Closure version of (10), on the other hand, as Janet baked is a preposed adverbial clause that is not part of the argument structure of the brownies cooled on a rack, and it is completed before the beginning of the main clause. Sentences like (10), we suggest, can be disambiguated with prosody because their syntactic structure is similar to prosodic structure. Selkirk (1984) and Shattuck-Hufnagel and Turk (1996) describe prosodic structure as strictly layered, which means that nested structures are not possible: at the point where a prosodic phrase ends, another prosodic phrase of the same type begins. Likewise, for a phrase to begin, the preceding phrase of the same type must end. The idea that prosodic structure is strictly layered is not universally accepted (see, for example, Ladd, 1986).

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Nevertheless, it is generally agreed that prosodic phrases do not contain constituents of a higher order (e.g., an Intonational Phrase inside an intermediate phrase), while this sort of recursive nesting is manifestly possible in syntactic phrases (e.g., a clause inside a verb phrase). An Early Closure sentence is easy to map onto a prosodic structure because its syntax is also layered. The subordinate clause ends before the first word of the next clause (the bread cooled on a rack), so each clause can be mapped onto an intonational phrase. The Late Closure sentence can likewise be straightforwardly disambiguated, with the clause ending after bread in one prosodic phrase and the main clause in a second prosodic phrase. In contrast, speakers rarely placed a prosodic boundary after the verb in the Clause version of Object/Clause sentences because of the recursive structure of these sentences. In order to begin an intonational phrase when the embedded clause (the defendant had committed the crime) begins, a speaker must end an intonational phrase after the jury believed. But the main clause and its verb phrase are still open at this point. The speakers’ mapping from syntax to prosody does not provide any encouragement to end an intonational phrase at this point. Watson and Gibson (2004) specifically prohibit an intonational phrase boundary between a head and its complement. We could make a similar argument on thematic rather than syntactic grounds. Both an object NP (his roommate) and an embedded clause (his roommate seemed depressed) are arguments of the verb noticed, while the clause the soup grew cold is not an argument of the preceding verb ate. Since the present experiments were not designed to distinguish between an account based on syntactic structure and one based on argumenthood, future research will be necessary to determine whether syntactic or thematic structure is more likely to guide prosodic production. Cooper and Paccia-Cooper (1980), unlike Watson and Gibson (2004), predicted that speakers would be slightly more likely to place a prosodic boundary after the main clause verb in Clause sentences than in Object sentences. Although this was uncommon, on the few occasions that our speakers produced a contour with a larger boundary after the verb than after the ambiguous NP, this contour led to the highest accuracy in listeners’ identifications of the sentence continuation. This mismatch between what speakers do and what listeners find helpful is actually a direct result of the mapping between syntax and prosody. Speakers, who are mapping from syntax to prosody, rarely place a boundary after the verb for the reasons summarized above. But listeners are mapping in the opposite direction, from the prosodic structure of the spoken sentence to a syntactic representation in the mind. For a listener, the end of an IPh after the verb indicates the beginning of a new IPh (because of the strictly layered nature of prosodic structure) and, consequently, is easy to map onto the beginning of a new syntactic clause. Manipulating the length of the subject in the Object/Clause sentences allowed us to rule out the possibility that our speakers’ infrequent use of this helpful prosodic contour was simply a side effect of phonological phrasing. It is well-known that speakers tend to create prosodic phrases within a sentence that are approximately equal in length (Cooper & Paccia-Cooper, 1980; Gee & Grosjean, 1983), so speakers might have been reluctant to place a prosodic boundary after the verb if the combination of the subject plus verb was too short in relation to the remainder of the sentence. However, in Clause sentences with long-subject phrases, speakers placed a boundary after the verb even less frequently than they did in short-subject Clause sentences. Instead, in those sentences they often divided the sentence by placing a prosodic boundary after the long-subject NP and grouping the verb in a prosodic phrase with the following clause. As Watson and Gibson’s (2004) theory predicts, phonological weight influences prosodic phrasing independently of syntactic structure.


489

Watson and Gibson’s (2004) LRB hypothesis differed from Cooper and Paccia-Cooper’s in its prediction of no prosodic differences between Object and Clause sentences. Even though our speakers did not often produce the cue that was most helpful to listeners, they did not produce these two sentence types with the same prosody. Rather, they produced a boundary after the ambiguous NP in 89% of Object sentences but in only 26% of Clause sentences. The syntax of Object sentences makes it natural to produce a prosodic boundary at this location, since the boundary would coincide with the end of the V′ constituent containing the verb plus its complement. Furthermore, this prosody groups the verb with its obligatory complement and places the optional adjunct PP in its own prosodic phrase, consistent with the discussion in Watson et al. (2006) and Selkirk (2000). It seems probable that some constraint in the spirit of Selkirk’s (1984) “sense unit” is at play here, which leads speakers to favor grouping a verb with its direct object but avoid separating an SVO string within an embedded clause. A future experiment could test this proposal while controlling for the phonological weight of the two kinds of verb complements. The idea that speakers’ production of prosody is guided by grammatical principles that map between syntax and prosody could account for some of the variability in the earlier literature. Instances where speakers successfully disambiguate are those where syntactic structure coincides fairly closely with a layered prosodic structure. The kinds of sentences that have been most consistently shown to be disambiguated are prepositional phrase-attachment structures (Kraljic & Brennan, 2005; Schafer et al., 2005; Snedeker & Trueswell, 2003) and closure ambiguities (Kjelgaard & Speer, 1999; Schafer et al., 2000). Adverb attachment ambiguities may be resolved in some cases but not in others (Allbritton et al., 1996), depending on whether the adverb is attached inside another phrase or on its own. We have shown that sentences with Object/Clause ambiguities are difficult for speakers to disambiguate because they involve a recursive structure that cannot be mapped directly onto a prosodic structure. Further research may add to this catalogue of structures which are easy or difficult to disambiguate prosodically. Manuscript received: 2/25/08 Manuscript accepted: 5/4/09 Acknowledgments Portions of this research were supported by SSHRC (Canada) Doctoral Fellowship #752–2001–0076 to Catherine Anderson at Northwestern University, an Andrew W. Mellon Postdoctoral Fellowship at Northwestern University and Kentucky NSF EPSCoR REG grant EPS-0132295 to Katy Carlson, and by the Department of Linguistics at Northwestern University, which support the authors hereby acknowledge with gratitude. We are indebted to our colleagues at Northwestern, especially Ann Bradlow and Janet Pierrehumbert, for extensive feedback on an earlier version of the manuscript. Any weaknesses in the paper are of course entirely our own responsibility.

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Appendix: Experiment 1 stimuli Object/Clause sentences 1. 2. 3. 4. 5.

The [tired and overworked] jury believed the defendant without any hesitation. The [tired and overworked] jury believed the defendant had committed the crime. The [eminent] botanist observed the cells through the microscope. The [eminent] botanist observed the cells had replicated themselves overnight. Louise [Mortensen] noticed the neighbors through the front window. Louise [Mortensen] noticed the neighbors had painted their house. Sarah [Johanssen] learned her lines in a matter of hours. Sarah [Johanssen] learned her lines would be cut from the play. [Patty | Patricia Steeves] recognized the novel by its peculiar title. [Patty | Patricia Steeves] recognized the novel still had many flaws.

492 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Language and Speech 53(4) The [talented and famous] soloist learned the song with the help of a recording. The [talented and famous] soloist learned the song had been composed by her voice teacher. [Sandy | Alexandra Douglas] remembered the music with feelings of nostalgia. [Sandy | Alexandra Douglas] remembered the music was lying on the piano. [Pam | Pamela Vandenburg] knew the teacher through the campus volunteer network. [Pam | Pamela Vandenburg] knew the teacher would grade on a bell curve. [Sue | Susan Robinson] found the marinara after a search through the freezer. [Sue | Susan Robinson] found the marinara needed more garlic. The [anxious and frightened] prisoner revealed the names by tapping them out in Morse code. The [anxious and frightened] prisoner revealed the names were part of the code. The [experienced] scientist observed the mice through a transparent partition. The [experienced] scientist observed the mice had learned the route through the maze. The [worried young] patient understood the results with the help of the doctor. The [worried young] patient understood the results would take several days to analyze. [Alice | Alison Kennedy] suspected the guest despite his clever alibi. [Alice | Alison Kennedy] suspected the guest had eaten the entire cake. [Tony | Antonio Covello] forgot the noodles in spite of his mother’s reminder. [Tony | Antonio Covello] forgot the noodles needed cooking first. Elaine [Cunningham] discovered her neighbor in a compromising position. Elaine [Cunningham] discovered her neighbor had flown planes in the war. [Tom | Thomas Morgencrantz] noticed his roommate during the long lecture. [Tom | Thomas Morgencrantz] noticed his roommate seemed kind of depressed.

Late/Early Closure sentences 17. While the skipper sailed the ship the dinghy began to leak. While the skipper sailed the ship began to leak. 18. While the clown juggled the knives the apples sat on the table. While the clown juggled the knives sat on the table. 19. While Thomas walked the poodle the collie barked at the neighbors. While Thomas walked the poodle barked at the neighbors. 20. While Pamela sketched the girl the dog bothered the cat. While Pamela sketched the girl bothered the cat. 21. As the artist painted the model the landscape baked in the sun. As the artist painted the model baked in the sun. 22. As Angela cleaned the floor the kitchen began to sparkle. As Angela cleaned the floor began to sparkle. 23. As Michael smoked the cigar the pipe glowed in the dimness. As Michael smoked the cigar glowed in the dimness. 24. As Janet baked the bread the brownies cooled on a rack. As Janet baked the bread cooled on a rack. 25. As the principal lectured the seniors the parents planned a party. As the principal lectured the seniors planned a party. 26. While the violinist practiced the concerto the symphony blared from the neighbor’s radio. While the violinist practiced the concerto blared from the neighbor’s radio. 27. As the kid drank the juice the milk dripped on the counter. As the kid drank the juice dripped on the counter.

Anderson and Carlson 28. While Rupert ate the soup the main dish grew cold. While Rupert ate the soup grew cold. 29. After Kendra parked the van the SUV rolled backwards. After Kendra parked the van rolled backwards. 30. While Eric played the piano the harp snapped a string. While Eric played the piano snapped a string. 31. As the author typed the novel the story got more complicated. As the author typed the novel got more complicated. 32. When Burt answered the telemarketer his family was quite surprised. When Burt answered the telemarketer was quite surprised. 33. After Betty woke up her son she coughed loudly for a while. After Betty woke up her son coughed loudly for a while. 34. While the boy washed the dog its puppies barked intermittently. While the boy washed the dog barked intermittently. 35. While the thief hid the elegant jewelry a stray diamond sparkled in the light. While the thief hid the elegant jewelry sparkled in the light. 36. As Louisa undressed the baby her grandmother started to cry. As Louisa undressed the baby started to cry.

493