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<Paper uid="H05-1104">
  <Title>Proceedings of Human Language Technology Conference and Conference on Empirical Methods in Natural Language Processing (HLT/EMNLP), pages 827-834, Vancouver, October 2005. c(c)2005 Association for Computational Linguistics Parallelism in Coordination as an Instance of Syntactic Priming: Evidence from Corpus-based Modeling</Title>
  <Section position="9" start_page="832" end_page="833" type="concl">
    <SectionTitle>
6 Conclusions
</SectionTitle>
    <Paragraph position="0"> In this paper, we have demonstrated a robust, pervasive effect of parallelism for noun phrases. We found the tendency for structural repetition in two different corpora of written English, and also in a dialog cor- null pus. The effect occurs in a wide range of contexts: within coordinate structures (Experiment 1), within sentences for NPs in an arbitrary structural configuration, between sentences, and within documents (Experiment 2). This strongly indicates that the parallelism effect is an instance of a general processing mechanism, such as syntactic priming (Bock, 1986), rather than specific to coordination, as suggested by (Frazier and Clifton, 2001). However, we also found that the parallelism effect is strongest in co-ordinate structures, which could explain why comprehension experiments so far failed to demonstrate the effect for other structural configurations (Frazier et al., 2000). We leave it to future work to explain why adaptation is much stronger in co-ordination: is co-ordination special because of extra constrains (i.e., some kind of expected contrast/comparison between co-ordinate sisters) or because of fewer constraints (i.e., both co-ordinate sisters have a similar grammatical role in the sentence)? Another result (Experiment 3) is that the parallelism effect occurs between speakers in dialog. This finding is compatible with Pickering and Garrod's (2004) interactive alignment model, and strengthens the argument for parallelism as an instance of a general priming mechanism.</Paragraph>
    <Paragraph position="1"> Previous experimental work has found parallelism effects, but only in comprehension data. The present work demonstrates that parallelism effects also occur in production data, which raises an interesting question of the relationship between the two data types. It has been hypothesized that the human language processing system is tuned to mirror the probability distributions in its environment, including the probabilities of syntactic structures (Mitchell et al., 1996). If this tuning hypothesis is correct, then the parallelism effect in comprehension data can be explained as an adaptation of the human parser to the prevalence of parallel structures in its environment (as approximated by corpus data) that we demonstrated in this paper.</Paragraph>
    <Paragraph position="2"> Note that the results in this paper not only have an impact on theoretical issues regarding human sentence processing, but also on engineering problems in natural language processing, e.g., in probabilistic parsing. To avoid sparse data problems, probabilistic parsing models make strong independence assumptions; in particular, they generally assume that sentences are independent of each other. This is partly due to the fact it is difficult to parameterize the many possible dependencies which may occur between adjacent sentences. However, in this paper, we show that structure re-use is one possible way in which the independence assumption is broken. A simple and principled approach to handling structure re-use would be to use adaptation probabilities for probabilistic grammar rules, analogous to cache probabilities used in caching language models (Kuhn and de Mori, 1990). We are currently conducting further experiments to investigate of the effect of syntactic priming on probabilistic parsing.</Paragraph>
  </Section>
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