The Psychological Reality of Local Coherences in Sentence Processing

The Psychological Reality of Local Coherences in Sentence Processing Lars Konieczny (lars@cognition.uni-freiburg.de) Center for Cognitive Science, University of Freiburg, Friedrichstr. 50 D-79098 Freiburg i. Br. , Germany Abstract Dynamical systems of language processing predict that sentence processing complexity is not only a function of the globally coherent structure ranging from the beginning of the sentence to its current point of processing, but also a function of locally coherent sub-parses. This paper presents an experiment that tests whether locally coherent, yet globally false continuations affect on-line anomaly detection times. The results indicate that they do interfere with processing, but only if the global analysis is not too demanding. This result can be seen as an indicator of the psychological reality of local coherence processing, and hence supports the dynamic system view on language processing. Keywords: local coherence; sentence processing; parsing complexity; relative clauses; anomaly detection; dynamical systems; simple recurrent networks. Introduction There has been considerable debate as to why some sentences are more difficult to process than others, and why some people are more affected by this difference than others. Sentences can differ on a variety of dimensions affecting working memory demands, such as the number and place (left, centre, right) of embeddings, the number and range of open dependencies within a sentence, word order regularity, and the number and locality of integrations to be made at each word. The language-related, working memory-oriented literature has emphasized the notion of one or several memory resources, whose capacity or capacities may vary between individuals (Just and Carpenter, 1992, Caplan and Waters, 1996). Exceeding this capacity leads to an impaired comprehension quality, as intermediate processing results may be forgotten. More complex sentences draw more on these resources, leading to reduced reading speed. For instance, relative clauses (RCs) are harder to process when the relative pronoun represents the object of the RC (so called object-extracted relative clauses, ORCs, as in 1.) than when it is the subject (SRCs, as in 2., King and Just, 1991; Gibson, 1998; Traxler, Morris, and Seely, 2002). The reporter who i the senator attacked t i admitted the error (ORC) The reporter who i t i attacked the senator admitted the error (SRC) In English, whether the relative pronoun (who) is the subject or the object of the RC is determined by the word order within the RC: in ORCs, where who is followed by an NP and then the verb, the pronoun represents the object of the sentence, while the NP is the subject. When who is immediately followed by the verb, it must be the clause- subject (SRC). There have been numerous proposals to explain the RC-type effect. Sheldon (1974) pointed out that the same element (reporter) bears the same function (subject) in an SRC as it does in the matrix clause, whereas it carries different functions when it is the object of the ORC. Processing and memorizing parallel functions for the same entity should be less demanding than doing so with different functions. Wanner and Maratsos (1978) attribute the ORC difficulty to the fact that the filler (i.e. the host of the relative pronoun, here reporter) must be carried unattached longer in the ORCs than in SRCs, where the verb immediately follows the pronoun. Clifton and Frazier (1989) proposed the active filler strategy as a heuristics employed by the human parser. According to that strategy, which assumes that a trace co- indexed with the filler is proposed as soon as it is grammatically permissible during incremental parsing, memory load is held minimal. Of course, the earliest permissible filler position is right after the relative-pronoun who, as in SRCs. Gibson (1998) provided a metric of integration and memory costs associated with the number of new discourse elements intervening dependents. Both cost components are higher for ORCs: the prediction of a verb and a trace must be kept in memory across the RC subject (where memory cost is highest), and integration of the clause final verb with its complements requires crossing more discourse entities (the subject and the verb). Gordon, Hendrick, and Johnson (2001) propose similarity-based interference as the primary source of difficulty when the complements must be retrieved from memory. MacWhinney and Pleh (1988) pointed out that comprehenders want to take the perspective of the subject of the sentence, which requires them to shift perspective when an ORC is entered, and shift back when processing continues with the matrix clause. In SRCs on the other hand, they can keep the same perspective throughout the entire sentence, which is favourable. Interestingly, readers with a low reading span show more difficulties with ORCs than others (King and Just, 1991). The reading span test used here (Daneman and Carpenter, 1980) was designed to involve both processing and memory skills. Just and Carpenter (1992) proposed a model, named CC- Reader, which has at its core a parallel production system, whose rules spread activation to memory elements when they fire. Once a memory element receives enough activation to pass a threshold, it can be retrieved from memory. Crucially, the amount of activation available is limited (capacity constrained, hence CC), so that an activation increase of one