The Interplay Of Syntactic Parsing Strategies And Prosodic Phrase Lengths In Processing Turkish Sentences

Many experiments have shown that the prosody (rhythm and melody) with which a sentence is uttered can provide a listener with cues to its syntactic structure (Lehiste, 1973, and since). A few studies have observed in addition that an inappropriate prosodic contour can mislead the syntactic parsing routines, resulting in a prosody-induced garden-path. These include, among others, Speer et al. (1996) and Kjelgaard and Speer (1999) for English. The studies by Speer et al. and Kjelgaard and Speer (SKS) showed that misplaced prosodic cues caused more processing difficulty in sentences with early closure of a clause (EC syntax) than in ones with late closure of a clause (LC syntax). One possible explanation for these results is that when prosody is misleading about the syntactic structure, the parser may ignore it and resort to a syntactic Late Closure strategy, as it does in reading where there is no overt prosodic boundary to inform the parser about the syntactic structure of the sentence. Augurzky\u27s (2006) observation of an LC syntax advantage for prosody-syntax mismatch conditions in her investigation of German relative clause attachment ambiguities provides support for this explanation. An alternative explanation considers the possibility that constituent lengths could have influenced the perceived informativeness of overt prosodic cues in these studies, as proposed in the Rational Speaker Hypothesis of Clifton et al. (2002, 2006). The Rational Speaker Hypothesis (RSH) maintains that prosodic breaks flanking shorter constituents are taken more seriously as indicators of syntactic structure than prosodic breaks flanking longer constituents, because the former cannot be justified as motivated by optimal length considerations. To test these two alternative hypotheses, four listening experiments were conducted. There was an additional reading experiment preceding the listening experiments to explore potential effects of the Late Closure strategy and constituent lengths in reading where there is no overt prosody. In all cases the target materials were temporarily ambiguous Turkish sentences which could be morphologically resolved as either LC or EC syntactic constructions. Constituent lengths were systematically manipulated in all target materials, such that the length-optimal prosodic phrasing was associated with LC syntax in one condition, and with EC syntax in the other. Experiment 1 employed a missing morpheme task developed for this study. In the missing morpheme task, underscores (length-averaged) replaced the disambiguating morphemes and participants had to insert them as they read the sentences aloud. Results revealed significant effects of phrase lengths in readers\u27 syntactic interpretations as indicated by the morphemes they inserted and the prosodic breaks they produced. Experiments 2A and 2B employed an end-of-sentence `got it\u27 task (Frazier et al., 1983), in which participants listened to spoken sentences and indicated after each one whether they understood or did not understand it. Sentences in Experiment 2A had phrase length distribution similar to the SKS English materials. Experiment 2B manipulated lengths in reverse. The stimuli had cooperating, conflicting or neutral prosody. Response time data supported an interplay of both syntactic Late Closure and RSH. Thus it was concluded that constituent lengths can indeed have a significant effect on listeners\u27 parsing decisions, in addition to the familiar syntactic parsing biases and prosodic influences. Experiments 3A and 3B used a lexical probe version of the phoneme restoration paradigm employed by Stoyneshka et al. (2010). In the phoneme restoration paradigm, the disambiguating phonemes (in the verb, in these materials) are replaced with noise (in this study, pink noise). In the lexical probe version of this paradigm (developed for this study) participants listened to the sentences with LC, EC or neutral prosody, and at the end of the sentence they were presented with a visual probe (one of the two possible disambiguating verbs, complete with all phonemes) that was congruent or incongruent or compatible with the prosody of the sentence they had heard. Their task was to respond to the visual probe either `yes\u27 (i.e., `I heard this word in the sentence I have just listened to\u27) or `no\u27 (i.e., `I didn\u27t hear this word\u27). Response time to the probe word indirectly taps which of the disambiguating morphemes on the verb the listener mentally supplies when it has been replaced by noise. The materials for Experiments 3A and 3B were identical to those used in Experiments 2A and 2B respectively except that the disambiguating phonemes were noise-replaced. Results of Experiments 3A and 3B showed that listeners were highly sensitive to the sentential prosody as revealed by their phoneme restoration responses and response time data, confirming Stoyneshka et al.\u27s findings establishing the reliability of the phoneme restoration paradigm in investigating effects of prosody in ambiguity resolution. Response time data showed a pattern similar to what SKS observed for English (except for one condition in Experiment 3A, with incongruent probes): despite the phrase length reversal in Experiment 3B, there was no influence of phrase length distribution on ambiguity resolution. This has a natural explanation in light of the difference between the `got it\u27 task with disambiguating morphology within the sentence stimulus, and the phoneme restoration task in which the listener can project onto the verb whatever morphology is compatible with the heard prosody. LC and EC were processed equally well for congruent probes, and there was an LC advantage in the incongruent and compatible probe conditions. Overall results support the hypothesis that syntactic Late Closure becomes evident in listening when prosody is absent or misleading, and also that phrase lengths can play a significant role