Sentence Parsing

Given that syntactic analysis seems to be part of language comprehension, the question of exactly how the reader or listener determines the syntactic structure of a sentence (sentence parsing) becomes a central concern.  Researchers have examined this question by presenting sentences that contain a syntactic ambiguity, that is, a situation in which more than one well-formed synatctic analysis can be assigned to a string of words.  Usually, subsequent words indicate the correct analysis.  For example:

    (1) The lawyer charged that the defendant was lying.

    (2)  The lawyer charged the defendant was lying.

In (1), there is little syntactic ambiguity. By contrast, in (2) the grammatical role of the noun phrase the defendant is temporarily ambiguous between an "object of the verb" role and a "subject of an upcoming clause" role.  The fact that the subject role is appropriate in this case becomes clear only after encountering the disambiguating auxiliary verb was.   Current theories predict that in such situations the reader will assign the object role to the defendant.  This decision will result in a processing problem when the auxiliary verb is encountered; under the object analysis, the auxiliary cannot be attached to the preceding sentence material.  Consequently, this theory predicts that readers should (at least momentarily) perceive the auxiliary verb to be syntactically anomalous.  The prediction, then, is that the auxiliary verb in (2) should elicit a P600 effect, relative to the same word in (1).  The figure plots ERPs to the final three words in each sentence (defendant was lying).  Arrows indicate the onset of each word.  Consistent with the prediction, beginning at about 500 ms after onset, auxiliiary verbs in sentences like (2) elicited a P600-like positive shift.   One interpretation of these results is that readers commit themselves to a single syntactic analysis when confronted with ambiguity, rather than building all possible structures in parallel or waiting until disambiguating information indicates which analysis is the correct one before assigning grammatical roels to words.  Just as importantly, these results show that the P600 effect is elicited not only by outright ungrammaticalities, but also by perceived ungrammaticalities that result solely due to how people parse sentences. 

References: 

Osterhout, L. (1997). On the brain response to syntactic anomalies: Manipulations of word position and word class reveal individual differences. Brain and Language, 59, 494-522.

Osterhout, L. & Holcomb, P. J. (1992). Event-related brain potentials elicited by syntactic anomaly. Journal of Memory and Language, 31,        785-806.

Osterhout, L., & Holcomb, P. J. (1993). Event-related potentials and syntactic anomaly: Evidence of anomaly detection during the perception of continuous speech. Language and Cognitive Processes, 8, 413-438.

Osterhout, L., Holcomb, P. J., & Swinney, D. A. (1994). Brain potentials elicited by garden-path sentences: Evidence of the application of verb information during parsing. Journal of Experiment Psychology: Learning, Memory, & Cognition, 20, 786-803.