Sentence-Internal Prosody Does not Help Parsing the Way Punctuation Does

Sentence-Internal Prosody Does not Help Parsing the Way Punctuation Does Michelle L Gregory Brown University mgregory@cog.brown.edu Mark Johnson Brown University Mark Johnson@Brown.edu Eugene Charniak Brown University ec@cs.brown.edu Abstract This paper investigates the usefulness of sentence-internal prosodic cues in syntac- tic parsing of transcribed speech. Intu- itively, prosodic cues would seem to pro- vide much the same information in speech as punctuation does in text, so we tried to incorporate them into our parser in much the same way as punctuation is. We com- pared the accuracy of a statistical parser on the LDC Switchboard treebank corpus of transcribed sentence-segmented speech using various combinations of punctua- tion and sentence-internal prosodic infor- mation (duration, pausing, and f0 cues). With no prosodic or punctuation informa- tion the parser’s accuracy (as measured by F-score) is 86.9%, and adding punctuation increases its F-score to 88.2%. However, all of the ways we have tried of adding prosodic information decrease the parser’s F-score to between 84.8% to 86.8%, de- pending on exactly which prosodic infor- mation is added. This suggests that for sentence-internal prosodic information to improve speech transcript parsing, either different prosodic cues will have to used or they will have be exploited in the parser in a way different to that used currently. 1 Introduction Acoustic cues, generally duration, pausing, and f0, have been demonstrated to be useful for auto- S INTJ UH Oh , , NP PRP I VP VBD loved NP PRP it . . Figure 1: A treebank style tree in which punctuation is coded with terminal and preterminal nodes. matic segmentation of natural speech (Baron et al., 2002; Hirschberg and Nakatani, 1998; Neiman et al., 1998). In fact, it is generally accepted that prosodic information is a reliable tool in predict- ing topic shifts and sentence boundaries (Shriberg et al., 2000). Sentences are generally demarcated by a major fall (or rise) in f0, lengthening of the final syllable, and following pauses. However, the usefulness of prosodic information in sentence- internal parsing is less clear. While assumed not to be a one-to-one mapping, there is evidence that there is a strong correlation between prosodic boundaries and sentence-internal syntactic bound- aries (Altenberg, 1987; Croft, 1995). For exam- ple, Schepman and Rodway (2000) have shown that prosodic cues reliably predict ambiguous attach- ment of relative clauses within coordination con- structions. Jansen et al. (2001) have demonstrated that prosodic breaks and an increase in pitch range can distinguish direct quotes from indirect quotes in a corpus of natural speech. This paper evaluates the accuracy of a statistical parser whose input includes prosodic cues. The pur- pose of this study to determine if prosodic cues im- prove parsing accuracy in the same way that punc- tuation does. Punctuation is represented in the vari- ous Penn treebank corpora as independent word-like tokens, with corresponding terminal and pretermi- nal nodes, as shown in Figure 1 (Bies et al., 1995). Even though this seems linguistically highly un- natural (e.g., punctuation might indicate supraseg- mental prosodic properties), statistical parsers gen- …

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