Underspecification of Syntactic Ambiguities: Evidence from Self-Paced Reading

Memory & Cognition 2008, 36 (1), 201-216 doi: 10.3758/MC.36.1.201 Underspecification of syntactic ambiguities: Evidence from self-paced reading BENJAMIN SWETS SUNY Stony Brook, Stony Brook, New York TIMOTHY DESMET Ghent University, Ghent, Belgium CHARLES CLIFTON, JR. University of Massachusetts, Amherst, Massachusetts AND FERNANDA FERREIRA University of Edinburgh, Edinburgh, Scotland Syntactically ambiguous sentences are sometimes read faster than disambiguated strings. Models of parsing have explained this tendency by appealing either to a race in the construction of alternative structures or to reanalysis. However, it is also possible that readers of ambiguous sentences save time by strategically underspecifying interpretations of ambiguous attachments. In a self-paced reading study, participants viewed sentences with relative clauses that could attach to one of two sites. Type of question was also manipulated between participants in order to test whether goals can influence reading/parsing strategies. The experiment revealed an ambiguity advantage in reading times, but only when participants expected superficial comprehension questions. When participants expected queries about relative clause interpretation, disambiguating regions were inspected with more care, and the ambiguity advantage was attenuated. However, even when participants expected relative clause queries, question-answering times suggested underspecified representa- tions of ambiguous relative clause attachments. The results support the construal and “good-enough” models of parsing. Many models of language comprehension assume that However, it has been difficult to demonstrate processing globally ambiguous sentences are more difficult to process costs for syntactic ambiguity itself, rather than for diffi- than unambiguous sentences. For instance, some constraint- culties caused when early information favors one analysis based parsing models (MacDonald, Pearlmutter, & Seiden- and later information favors a different analysis (Clifton, berg, 1994; McRae, Spivey-Knowlton, & Tanenhaus, 1998; Staub, & Rayner, 2007; Gibson & Pearlmutter, 2000; Spivey & Tanenhaus, 1998; Tabor & Tanenhaus, 1999) posit Lewis, 2000). Furthermore, this ambiguity disadvantage that sentences with syntactic ambiguities generally take assumption has been challenged by a line of studies show- longer to read than their unambiguous counterparts (see ing that some syntactic ambiguities can actually lead to Spivey-Knowlton & Sedivy, 1995, for a particularly clear shorter processing times (Traxler, Pickering, & Clifton, statement of this claim). According to these models, mul- 1998; van Gompel, Pickering, Pearson, & Liversedge, tiple structures are incrementally activated during sentence 2005; van Gompel, Pickering, & Traxler, 2001). In each comprehension. These interpretations compete to become of these studies, participants read ambiguous sentences the final one. When the sentential and contextual informa- and disambiguated variants while their eye movements tion being considered fails to produce a clear winner, such were monitored. The readers consistently displayed am- as with ambiguous strings, the various interpretations en- biguity advantages in these studies. For example, Traxler gage in a disruptive competition in which constraints from et al. studied a form of relative clause ambiguity (see Sen- multiple levels of representation must be considered and tences 1–3 below) in which a relative clause modifies ei- weighted over a longer period of time before the system set- ther a head noun (maid/son) or an adjunct of the head noun tles on a chosen interpretation. In sum, ambiguous strings ( princess). Traxler et al. showed that it took less time for in this view typically slow down reading.1 people to read ambiguous sentences, such as Sentence 1, B. Swets, [email protected] 201 Copyright 2008 Psychonomic Society, Inc. 202 SWETS, DESMET, CLIFTON, AND FERREIRA than to read sentences with temporary ambiguities that Although the probabilistic race model provides a com- were later disambiguated (Sentences 2 and 3). pelling account of the ambiguity advantage, other explana- tions can also be entertained. One is that readers simply (1) The maid of the princess who scratched herself in do not commit to a particular meaning when they are not public was terribly humiliated. provided with clearly disambiguating information. We (2) The son of the princess who scratched himself in will refer to this as an underspecification account. Previ- public was terribly humiliated. ous studies have offered little information about the final (3) The son of the princess who scratched herself in interpretations that readers are left with once a sentence public was terribly humiliated. has been read. Participants in Traxler et al. (1998) and van Gompel et al. (2001; van Gompel et al., 2005) were ques- Van Gompel et al. (2001) used a similar paradigm, tioned about their final interpretations of the sentences except that the difference between ambiguous sentences only on occasional trials, and the questions did not assess (4) and unambiguous sentences (5) centered on whether whether or how the participants resolved the attachment the prepositional phrase (with the rifle) could either mod- of the relative clause. In addition, data about question- ify the sentential object (the poacher) or be interpreted as answering time and accuracy were not consistently col- an instrument of the verb (killed). lected. Crucially, race models, as well as some constraint- (4) The hunter killed the dangerous poacher with the based competition models, assume that globally ambiguous rifle not long after sunset. sentences are assigned a definitive interpretation, just as disambiguated sentences are. In contrast, underspecifica- (5) The hunter killed the dangerous leopard with the tion models assume that the final interpretation of a glob- rifle not long after sunset. ally ambiguous sentence (one disambiguated neither by This study (see also van Gompel et al., 2005) yielded the discourse context nor by syntactic cues) is agnostic be- same result as Traxler et al. (1998): Participants spent tween the meanings corresponding to the two attachments more time reading sentences disambiguated by discourse of the relative clause. In other words, using our same ex- context or a syntactic cue than they spent reading globally ample, the underspecification approach assumes that the ambiguous sentences. comprehender will understand the sentence to mean that One way to explain this advantage for ambiguous sen- someone scratched herself in public, but the comprehender tences is to assume a probabilistic race model of sentence will leave undecided whether that person was the maid or comprehension (van Gompel et al., 2001). Race models the princess. Because this underspecification account al- maintain that sentences with unresolved syntactic ambigu- lows the comprehension system to sometimes compute in- ities are read quickly because the analysis that is computed terpretations that are shallow and incomplete, we view it as initially is never overturned at a later reanalysis stage; the consistent with what has been termed the “good-enough” first choice can always be used, since no information dis- approach to language comprehension (Ferreira, Bailey, & confirms it. This is not the case, however, when the am- Ferraro, 2002; Sanford & Sturt, 2002). biguity is resolved at some later point in the sentence. Consider the relative clause ambiguity examples above Syntactic Underspecification (Sentences 1–3). Given the somewhat equibiased nature of Recent studies (Christianson, Hollingworth, Halliwell, this ambiguity (see Swets, Desmet, Hambrick, & Ferreira, & Ferreira, 2001; Ferreira, 2003) in which comprehenders 2007), we can assume that some readers will initially take were asked directly about their understanding of sentences the head noun (the maid/the son) as the attachment site with a single specified meaning have demonstrated that for the relative clause, and about as many other readers often interpretations are based either on incomplete re- will select the adjunct of the head noun (the princess) as analysis (Christianson et al., 2001) or on shallow process- the attachment site. When a reader arrives at the reflexive ing (Ferreira, 2003). In a self-paced reading study (Chris- pronoun himself in Sentence 2 or herself in Sentence 3, tianson et al., 2001), after participants read sentences such the gender marking of the reflexive pronoun makes clear as While Anna dressed the baby played in the crib, they that the relative clause string beginning with who scratched were asked two kinds of questions. The first kind tested modifies the son (N1, or high attachment) and the princess whether they understood that the baby played in the crib. (N2, or low attachment), respectively. This means that high People answered this kind of question correctly. The sec- (N1) attachers will be forced to reanalyze in Sentence 3, ond kind of question tested whether the incorrect inter- and low (N2) attachers will be forced to reanalyze in Sen- pretation based on the garden path (that Anna dressed the tence 2. Averaged across all readers, the result is an overall baby) remained, even after this interpretation had been increase in reading times in these two conditions. However, disallowed by thematic-role-assigning principles. For this in the ambiguous condition (Sentence 1), the reflexive pro- type of question, participants were surprisingly inaccu- noun herself is consistent

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