Université de Poitiers
Centre National de la Recherche Scientifique
Centre de Recherche Sur la Cognition et l’Apprentissage
Rapport technique : 2009/02/M.GIM
The effect of noun phrase type on working memory saturation during sentence comprehension Manuel Gimenes; François Rigalleau; Daniel Gaonac'h, University of Poitiers
A paraître dans / To appear in : Gimenes, M., Rigalleau, F., & Gaonac’h, D.. The effect of noun phrase type on working memory saturation during sentence comprehension. European Journal of Cognitive Psychology.
Address for correspondence to the first author at: Center for Research on Learning and Cognition 99 avenue du Recteur Pineau F-86000 Poitiers France
[email protected]
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KEYWORDS: Syntax; Working memory; Complexity; Recursion.
ABSTRACT Double centre-embedded structures such as “the rat the cat the boy chased ate was brown” seem ungrammatical to many human subjects. Using an offline complexity judgement task, Gibson and Thomas (1999) demonstrated that people found such sentences no more difficult to understand when the second verb phrase (VP) was omitted, relative to a condition where all the required VPs were present. According to the Syntactic Prediction Locality Theory (SPLT; Gibson, 1998), this syntactic illusion is determined by the high working memory cost associated with the integration of the second VP. This cost could be reduced by replacing the third noun phrase (the boy) by a pronoun, making the reader more sensitive to the omission of the second VP. This hypothesis was tested in two experiments using French sentences. Both experiments confirmed the syntactic illusion when the second VP was not a pronoun. The second experiment measured the reading times of the VPs and showed that the pronoun induced a longer reading time of the final VP when the second VP was omitted. The overall results indicate a condition under which human subjects could process the most complex part ofa sentence with more than one embedded relative clause. The overall results are consistent with most of the hypotheses derived from the SPLT although offline complexity judgements could not be the most sensitive measure to test some of these hypotheses.
INTRODUCTION A fundamental issue in psycholinguistic research concerns the human capacity to process complex linguistic structures where nonlocal dependencies occur between elements. Some recent studies were conducted about this question in the domain of artificial grammar learning (AGL). For instance, Fitch and Hauser (2004) tested the ability of human subjects and cotton-top tamarins to process syllable strings where the dependent elements where made distant by inserting other dependent elements between them. The structure of a string could be A3A2A1B1B2B3 where the index values indicate the dependencies between Ai and Bi elements. This structure contains a double-centre embedding, because a dependency (A1-B1) is centre-embedded within another dependency (A2-B2) which is also centreembedded within another dependency (A3-B3). Their results suggested that only human participants were able to process such structures. However, several authors (de Vries, Monaghan, Knecht, & Zwitserlood, 2008; Perruchet & Rey, 2005) conducted similar experiments, and they showed that participants could perform correctly these tasks without exploiting the centreembedded structure. Instead, participants seemed to perform the task by applying strategies such as counting or repetition detection. Therefore, de Vries et al. (2008) concluded that the learnability of double-centre embeddings in AGL tasks remains to be demonstrated. Moreover, these authors acknowledged that there is a difference between AGL and natural language, and it will be crucial to determine the computations in natural language processing of centreembedded structures. At the sentence level, such complex structures can be illustrated by double centre-embedded relative sentences like Sentence 1, which has the same meaning as the right-branching construction in Sentence 2. 1. The patient who the nurse who the clinic had hired admitted met Jack. 2. The clinic had hired the nurse who admitted the patient who met Jack.
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Sentence 1 illustrates a structure A3A2A1B1B2B3 where A elements correspond to noun phrases (NP), and B elements to verbal phrases (VP). The index values indicate the dependencies between Ai and Bi elements, for instance A3 (i.e., “the patient”) is the grammatical subject of B3 (i.e., “met Jack”). Sentences like Sentence 1 seem ungrammatical to most human subjects (Blumenthal, 1966; Marks, 1968). Most of the authors considered that human beings are not able to process such structures, in particular because of a working memory overload. Our main goal in the current paper is to verify, in natural language processing, whether human beings can process the most complex part of double centreembedded sentences when the working memory cost is reduced.
THE MISSING -VP2 EFFECT We started from a study conducted by Gibson and Thomas (1999) who showed that human subjects did not seem sensitive to the omission of the second VP in a sentence like Sentence 1. They asked subjects to read sentences like Sentence 1 and to assign an intuitive complexity score to the sentence. They compared the “theoretically” grammatical version (with the three VPs) with three versions where each of the VPs could be deleted. Although the versions with deleted VP1 or VP3 were judged less easy to understand than the version with all-three VPs, the omission of VP2 did not induce a higher score than the version where all the VPs were present. This last result sheds doubts about the ability of human subjects to process all the grammatical dependencies in double-centre embedded relative sentences, and so confirmed the claims of Perruchet et al. (2005) and de Vries et al. (2008). The study of Gibson and Thomas showed that participants computed the syntactic relations between the first VP and his arguments, and between the last VP and his arguments, but failed to compute the syntactic relations about the VP2. To explain the missing VP2 effect, Gibson and Thomas (1999) used a theoretical hypothesis about the role of working memory in sentence processing. The Syntactic Prediction Locality Theory (Gibson, 1998) assumes that the human parser formulates predictions about the phrases required to complete the phrase structure of a sentence. A prediction is maintained until the parser encounters a word satisfying the prediction. For instance, after reading “the patient who the nurse” in Sentence 1, there are three syntactic predictions: a verb for the matrix clause, a verb for the embedded clause, and an empty category noun phrase (NP) for the whpronoun “who”. Importantly, the intuitive complexity judgement of a sentence is assumed to reflect the maximal memory cost during sentence processing. In addition to the Syntactic Prediction Locality Theory (Gibson, 1998), the authors proposed a memory cost pruning assumption: at points of high memory cost, the system should forget the syntactic prediction(s) associated with the most expensive memory cost. For Sentence 1, this point is when the noun phrase “the clinic” is encountered. The two predictions associated with the higher memory load both concern the second VP. Because these predictions are forgotten, the missing-VP2 condition should be judged easier to understand than the all-three VPs condition. Forgetting the two most expensive predictions in Sentence 1 results in a difficulty to integrate VP2, which becomes unexpected; on the other hand, the omission of VP2 caused no specific problem because VP1 and VP3 are still predicted. For the two other missing-VP conditions (i.e., missing-VP1 and missing-VP3), the omitted VPs are associated with less expensive memory loads: The omission was thus noticed. The SPLT emphasises that the most complex part of a double-centre relative clause is at the second VP where forgotten predictions could engender an integration difficulty for this VP. The lack of difference between the all-three VPs condition and the missing VP2 condition was not predicted by Gibson and Thomas (1999); they predicted that the missing VP2 condition would be judged easier to understand than the all-three VPs condition). Gibson and Thomas speculated that the offline nature of the task allowed some participants to look back to see whether the initial NPs were licensed by the verbs that occurred. It should be noticed that Gimenes, Rigalleau, and Gaonac’h (in press) found the predicted difference in an experiment preventing rereading and using more participants and more sentences. In this paper, we wanted to establish whether human subjects could become sensitive to the omission of VP2 if the syntactic dependencies involving this VP were made easier to compute. Such a result, in addition to the results of Gibson and Thomas (1999), would demonstrate the capacity of human beings to compute the syntactic dependencies in double centreembedded structures. In order to reduce the memory cost in processing double centre-embedded 3
structures, we manipulated the nature of the most embedded NP (the clinic in Sentence 1). Warren and Gibson (2002) reported an experiment where they asked participants to judge the intuitive complexity of sentences with double centre-embedded relative clauses. They did not consider versions where a VP was missing. The most embedded NP was either a new discourse referent or an indexical pronoun (e.g., “I”), as illustrated in Sentence 3. 3. The patient who the nurse who (the clinic)/(I) had hired admitted met Jack. Because an indexical pronoun is not a new discourse referent, at the position of the most embedded NP, the SPLT assigns a lower maximal memory cost in the indexical pronoun condition than in the full NP condition (cf. Gibson, 1998, p. 35). So the reduced complexity of the indexical pronoun version is consistent with SPLT. Following the logic proposed by Gibson and Thomas to explain the missing VP2 effect, if a pronoun occurs at the position of the most embedded subject, the memory cost reduction should allow the parser to not drop out the prediction of VP2. This is precisely what will be tested in the current paper. An important prediction is that the parser should be more sensitive to an omission of VP2 when the most embedded subject is an indexical pronoun than when it is a definite description.
OVERVIEW Two experiments are reported. In Experiment 1, participants had to read double centre-embedded structures presented in a paper-and-pencil task. The all-three VPs were present, or the VP1 or the VP2 was missing. There was a second factor manipulated: The most embedded NP was a definite description or a pronoun. The results were only partly consistent with SPLT. In Experiment 2, the same factors were manipulated, but a self-paced reading paradigm allowed the measurement of reading times (RT) of the last VP. The offline complexity scores reproduced those obtained in Experiment 1. However, the RT results were consistent with SPLT.
EXPERIMENT 1 The aim of this experiment was to check whether the nature of the most embedded NP (i.e., NP1) could have offline influences on the missing VP2 effect. In particular, we hypothesised that the sensitivity to the missing VP2 should be greater when the nature of the NP1 reduces the maximal memory cost during sentence processing. The NP1 was of two types: a definite description, as in Sentence 4, or the first-person pronoun “I”, as in Sentence 5. 4. Le plat mexicain que le critique culinaire que le magazine a engagé a goûté dans le nouveau restaurant avait une odeur étrange. [The Mexican meal that the gastronomic critic that the journal hired tasted in the new restaurant had a strange smell.] 5. Le plat mexicain que le critique culinaire que j’ai engagé a goûté dans le nouveau restaurant avait une odeur étrange. [The Mexican meal that the gastronomic critic that I hired tasted in the new restaurant had a strange smell.] The authors who reported a missing VP2 effect noticed that a version where the VP2 was missing was judged easier to understand than a version where either VP1 or VP3 was omitted (Gibson & Thomas, 1999). A missing VP1 condition was also tested as a control condition for the comparisons involving the offline complexity score. It should be noticed that the SPLT also predicts a lower complexity score when VP2 is missing than when the three VPs are present. According to Gibson and Thomas, this prediction was not checked by their own study because of a methodological problem. Gimenes et al. (in press) reported the expected complexity difference in a study involving more participants and more items with a self-paced reading paradigm that did not allow rereading the sentence. Another prediction concerned the beneficial effect of the indexical pronoun. In the all-three VPs condition, we expected to replicate the result reported by Warren and Gibson (2002): a better complexity judgement when the most embedded subject was a pronoun than when it was a definite description.
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Finally, we hypothesised that when the NP1 is a definite description, the all-three VPs condition will result in a higher intuitive complexity score than the missing VP2 condition. On the contrary, when the NP1 is a pronoun, the all-three VPs condition will result in a lower intuitive complexity score than the missing VP2 condition. Finally, the missing VP1 condition should result in higher scores than the missing VP2 condition, whatever the nature of the NP1.
METHOD PARTICIPANTS. Thirty native French speakers served as participants in the experiment. MATERIALS. Forty-eight double centre-embedded experimental sentences were constructed in French. Sentences 4 and 5 are examples; the complete set of sentences is given in the Appendix. Each verb had strong semantic/ pragmatic selectional restrictions, so that only one of the preceding NPs was plausible as its subject. The variation concerning the type of NP1 (definite description versus pronoun “I”) required an additional constraint. In the allthree VPs condition, the most embedded clause had the form “NP2 that NP1 VP1 ”. In the missing VP1 condition, the most embedded clause had the form “NP2 that NP1 VP2”. The sequence NP1 VP2 could not be semantically integrated because NP1 was an implausible subject for VP2 (e.g., “the gastronomic critic that the journal tasted”). However, when NP1 was an indexical pronoun, “I VP2” became acceptable. To maintain an equivalency between the two NP1 conditions, we selected NP2s that were implausible objects for VP2, so that “NP2 that I VP2” was not plausible (e.g., “the gastronomic critic that I tasted”).
PROCEDURE. Each participant saw the three conditions of the structure factor and the two conditions of the NP1 type factor. The experimental items were counterbalanced across the lists so that each list contained exactly one version of every item. At the end of a sentence, the participants had to rate its complexity with a pencil, using a scale from “1” (“easy to understand”) to 5 (“hard to understand”).There were five practice items.
RESULTS The mean ratings for each condition are presented in Figure 1. The main effect of NP1 was significant, F1(1, 29) = 10.09, MSE= 0.223, p < .004; F2(1, 47) = 6.35, MSE= 0.567, p < .02: The pronoun condition was rated lower than the definite description condition.
Figure 1. Mean complexity ratings (max =5) in Experiment 1, for each structure condition and for each NP1 type (error bars are the 95% confidence intervals). 5
The main effect of structure was significant, F1(2, 58) = 14.7, MSE = 0.345, p < .0001; F2(2, 94) = 13.44, MSE= 0.604, p < .0001: The missing VP1 condition was rated higher than the combined missing VP2 and all-three VPs conditions, F1(1, 29) = 15.94, MSE= 0.41, p < .0004; F2(1, 47) = 18.61, MSE= 0.56, p < .0001, and the missing VP2 condition was rated lower than the all-three VPs condition, F1(1, 29) = 12.92, MSE= 0.28, p < .001; F2(1, 47) = 9.03, MSE= 0.65, p < .004. However, this main effect of structure was influenced by the nature of NP1, F1(2, 58) = 11.43, MSE= 0.226, p < .0001; F2(2, 94) = 12.41, MSE= 0.333, p < .0001. First, we compared the missing VP1 condition to the combined missing VP2 and all-three VPs conditions, for each type of NP1: The contrast interaction was significant, F1(1, 29) = 25.98, MSE= 0.16, p < .0001; F2(1, 47) = 16.33, MSE= 0.4, p < .0002. When the NP1 was a pronoun, the missing VP1 condition was rated higher than the combined missing VP2 and all-three VPs conditions, F1(1, 29) = 31.38, MSE = 0.33, p < .0001; F2(1, 47) = 28.59, MSE= 0.59, p < .0001. However, when the NP1 was a definite description, this difference disappeared, F1(1, 29) = 0.57, MSE = 0.23, p < .46; F2(1, 47) = 0.56, MSE = 0.37, p < .46. More importantly, we tested whether the difference between the missing VP2 condition and the all-three VPs condition was influenced by the nature of the NP1: The contrast interaction was significant by items and almost by subjects, F1(1, 29) = 3.59, MSE= 0.294, p < .07; F2(1, 47) = 6.41, MSE = 0.263, p < .01. When the NP1 was a definite description, the missing VP2 condition was rated lower than the all-three VPs condition, F1(1, 29) = 11.64, MSE= 0.372, p < .002; F2(1, 47) = 17.4, MSE = 0.398, p < .0001. However, when the NP1 was the first-person pronoun, there was no significant difference between the all-three VPs condition and the missing VP2 condition, F1(1, 29)=1.92, MSE=0.206, p
