The Resolution Of Local Syntactic Ambiguity By The Human Sentence Processing Mechanism

The resolution of local syntactic ambiguity by the Human Sentence Processing Mechanism is a topic which has provoked considerable interest in recent years. At issue is whether such ambiguities are resolved on the basis of syntactic information alone (cf. Minimal Attachment Frazier, 1979), or whether they are resolved on some other basis. Crain & Steedman (1982) suggest that the resolution process is governed not by Minimal Attachment but instead by whether or not a referring expression provides sufficient information with which to identify a unique referent. Such an approach relies on the provision of adequate contextual information, something which has been lacking in experiments which have been claimed to support Minimal Attachment. In this paper I shall consider a number of such experiments, and the different patterns of results which emerge once contextual information is provided. Although the importance of contextual information will be stressed, I shall briefly consider reasons why parsing preferences arise in the absence of any explicit prior context. The conclusion is that computational models of syntactic ambiguity resolution which are based on evidence which has ignored contextual considerations are models of something other than natural language processing. There has been much controversy recently surrounding the processes responsible for the "garden path" effect Ln the following kind of example: The oil tycoon sold the off-shore oil tracts for a lot of money wanted to kill J.R. The garden path effect arises here because the Human Sentence Processing Mechanism ("HSPM") encounters, during the processing of this sentence, a local syntactic ambiguity. The word "sold" is ambiguous: it can be interpreted either as a simple active, or it can be interpreted as a past participle, in a reduced passive. The only way to make the whole string into a sentence is to interpret it as a reduced form of the passive. However, what seems to happen in this (and similar) examples is that people tend to interpret the word "sold" as the main verb. This tendency leads them down a syntactic garden path. So the HSPM exhibits a preference for one analysis over another when faced with a local ambiguity. But why? A number of suggestions have been made concerning this. One suggestion, originally proposed by Kimball (1973) and followed up more recently by Frazier (1979) and Rayner, Carlson, & Frazier (1983), is that the HSPM takes into account the syntactic structure of these sentences. There are two possible structures which could be assigned to the ambiguous sentence fragment The oil tycoon sold the off-shore oil tracts . . . The reduced passive interpretation requires an extra NP node as compared to the main verb interpretation. Kimball (1975) and Frazier suggest that when more than one interpretation is possible, one pursues that interpretation which creates the structure with fewest nodes. This is what Frazier calls the Principle of Minimal Attachment. This structural hypothesis proposes, then, that an initial decision is made on grounds of syntactic structure alone. If it subsequently turns out to be the wrong decision (on grounds of "implausibility"), the alternative analysis (which is identified on the basis of "thematic selection" Rayner et al., 1983) is then, and only then, attempted. In support of this claim, Rayner et al. collected reading times and eye movement data for sentences which, syntactically speaking, allow two attachment sites for a prepositional phrase: one attachment, to an NP, requires an extra NP node as compared to the other attachment, which is to a VP.