Syntactic presupposition in sentence comprehension - Science Direct

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Syntactic presupposition

in sentence comprehension*

J. LANGFORD

Department of Melbourne,

and V. M. HOLMES Psychology,

Parkville,

Australia

University

of

**

Abstract Two experiments investigated the role of syntactic presupposition in sentence comprehension. In Experiment I subjects verified cleft, pseudocleft and factive complement sentences with respect to preceding context paragraphs, which contradicted either the assertion or the presupposition of the target sentence. Subjects took significantly longer to verify sentences with false presuppositions than sentences with false assertions. In Experiment II subjects verified cleft and pseudocelft sentences with respect to subsequently) presented pictures. Once again, verification times for sentences with false presuppositions were significantly longer than verification times for sentences with false assertions. It was argued that these findings are more adequately> explained by a “‘structural” hypothesis, than in terms of strategies designed to locate given and new information.

Introduction Although a large body of psycholinguistic research has been devoted to the study of sentences in isolation, it is now widely recognized that any approach which ignores the role of context is severely limited. One way of formulating the relationship between sentences and their contexts is in terms of their presuppositional content. For this reason the phenomenon of presupposition has received considerable attention from psychologists concerned with sentence comprehension (e.g., Haviland and Clark, 1974; Hornby, 1974), sentence memory (e.g., Offir, 1973; Singer, 1976; Hupet and Le Bouedec, 1977) and sentence production (e.g., Osgood, 1971; Bock, 1977). Similarly, by studying presupposition, the present research aimed to further elucidate the mechanisms by which sentences are understood in context. Our particular interest was in the type of presupposition which is created by the sentence’s surface structure. *This research was partly supported by an Australian Research Grants Committee award to V. M. Holmes. **Requests for reprints should be addressed to: J. Langford, Department of Psychology, University of Melbourne, Parkville, Victoria 3052, Australia.

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J. Langford and V. M. Holmes

A syntactic presupposition may be identified as that part of a sentence’s meaning which is not affected by negation of the sentence. It may be distinguished from the focus, the part of the sentence falling within the scope of negation, and from the assertion, the message produced by the focus in combination with the presupposition. In both sentences (1) and (2), the presupposition is John embroidered something and the focus is tlzc napkin. The assertions of the two sentences concern whether the napkin was or was not embroidered by John. The relationship of a sentence to a given context may be specified in terms of the nature of the information contained within the presupposition and focus. A contextually appropriate sentence is one which presupposes established or given information and which focusses new or contrasting information. For example, in the context of the question What did John embroider? ( 1) and (2) would be appropriate replies, (though (2) is unhelpful), while (3) and (4) would be inappropriate. Because of its obvious association with contextual antecedents, presupposition is often referred to as given information, while focus and assertion are referred to as new information. (1) (2) (3) (4)

It was a napkin that John embroidered. It was not a napkin that John embroidered. It was John who embroidered a napkin. The one who embroidered a napkin was John.

The most fully developed account of the role of presupposition in the comprehension process is the Given-New strategy of Haviland and Clark (1974). This strategy, which Clark and Haviland (1977) have characterized as “a three step procedure for relating the current sentence to.. . [a] knowledge base”, involves the following stages: “At Step 1, the listener isolates the given and the new information in the current sentence. At Step 2, he searches memory for a direct antecedent, a structure containing propositions that match the given information precisely. Finally, at Step 3 the listener integrates the new information into the memory structure by attaching it to the antecedent found in Step 2”. (Clark and Haviland, 1977, p. 5.) The Given-New strategy was originally based on a series of experiments which investigated the processing of sentences containing lexical presuppositions (i.e., presuppositions produced by individual word meanings rather than by syntactic structure). Haviland and Clark (1974) found that these sentences were understood more rapidly when preceded by a context sentence which established a direct, as opposed to indirect, antecedent for the presupposition. Thus, the test sentence (7) was understood faster when it was preceded by the context sentence (5) than (6).

Syntactic presupposition in sentence comprehension

(5) (6) (7)

365

Ed was given an alligator for his birthday. Ed wanted an alligator for his birthday. The alligator was his favorite present.

These results may be taken as evidence that people find it more difficult to integrate a sentence with its context when the sentence’s presuppositions are not established directly by the context, and that some additional inferential processing is necessary to understand such sentences. However, because they only considered the processing of unfulfilled presuppositions, Haviland and Clark have not directly established that asserted and presupposed information are processed differently. It would seem likely that if the sentence’s assertion did not follow directly from the context, then a similar increase in comprehension time would be observed. For example, in the context of (8), sentence (9) would follow directly but (10) would not, even though in both cases the presupposition is fulfilled. (8) Ed wanted an alligator for his birthday (9) The alligator was his favourite present. ( 10) The alligator was his worst present.

and was given one.

Presumably, (10) would take longer to integrate with the context because it would require an additional bridging inference, for example, that Ed changed his mind about alligators. Since there is no evidence in Haviland and Clark’s experiment that new information is treated any differently from given, their Given-New model remains unsubstantiated. Quite a different model has been proposed to describe how people process presupposition and focus when verifying sentences. Presumably, assigning truth to a statement might introduce different strategies from those used in comprehension without verification. Hornby (1974) and Clark and Clark (1977) have suggested that, because speakers generally assign presupposition and focus appropriately, listeners are likely to assume that the presupposition is true (since normally this contains information they already know) and to examine more critically the focus, where new information is normally located. Thus, while the Given-New strategy suggests that the listener first corroborates the presupposition and then proceeds to assimilate the assertion, Hornby’s account suggests that the listener critically examines the focus while taking for granted the truth of the presupposition. The evidence for this model, which might perhaps be designated the New-Given strategy, is not particularly convincing. Hornby’s experiment investigated the processing of syntactic, rather than lexical, presupposition in a sentence verification task. In this task each acoustically presented sentence was followed by the tachistoscopic exposure of a picture. Hornby found that more errors were

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J. Langford

and V. M. Holmes

made in recognizing a discrepancy between sentence and picture when the discrepancy involved a presupposed noun than when it involved a focussed noun. While this result appears to demonstrate a differential effect of presupposition and focus, it is open to alternative interpretations. Firstly, the extremely short presentation time ensured that not more than a single aspect of the picture could be attended to. In this situation all that the observed difference indicates is that subjects tended to examine focussed information first. This result does not, however, directly establish that subjects were “taking for granted” the truth of the presupposition, since there may have been no time left to examine the presupposed information. More importantly, in the experiment sentential focus coincided with the locus of heaviest stress. It is therefore quite possible that the superior recognition of discrepant focussed information was due to its dominant acoustic trace in short-term memory, rather than to a selective search for new information. The marking of focus by acoustic stress also leaves open the question of whether it is the syntax of the sentence that was used as a basis for distinguishing asserted and presupposed information. It seems, then, that the evidence does not unequivocally implicate syntax as a means by which people distinguish between presupposition and assertion in sentence comprehension. Nor do either of the hypothesized Given-New or New-Given strategies have very strong supporting evidence. The experiments reported below thus aimed to determine whether the structural distinction between presupposition and assertion really is utilized in the processing of sentences in context. They also aimed to evaluate the relevance of the givennew distinction to the comprehension process. Experiment

I

Most previous studies concerned with presupposition have examined the performance consequences of presupposition failure. Similarly, this experiment was designed to compare the processing of contradicted presuppositions with the processing of contradicted assertions. The comprehension task chosen was a paragraph-sentence verification task. With this task it was possible to set up the two experimental conditions by constructing for each test sentence two contexts -- one contradicting the sentence’s assertion and the other the sentence’s presupposition. By comparing the verification times for a given sentence in the two context conditions it was hoped to determine whether presupposition and assertion are in fact processed differently. In order to avoid any confounding between acoustic salience and structural marking of presupposition and assertion, all sentences were visually presented. To ensure that the results would not be limited to any one particular sentence structure,


367

two groups of sentences were used. One group comprised cleft and pseudocleft sentences and the other, factive complement sentences. Because the task in the present experiment necessitated relating a target sentence back to a preceding paragraph, the Given-New strategy would seem an appropriate model of the processing involved. Yet, since the task was one of verification, the New-Given strategy is also applicable. The two models predict rather different outcomes of the experiment. Subjects using a GivenNew strategy would first search memory for information corresponding to the presupposition in the target sentence and then would proceed to verify the assertion. These subjects would presumably detect information contradicting the presupposition before they would detect information contradicting the assertion. The Given-New Strategy, therefore, would predict longer verification times for items with false assertions. Subjects using a New-Given strategy, on the other hand, would tend to ignore the presupposed information, assuming it to be true, and would selectively search memory for information relevant to the assertion. These subjects would be expected to succeed in detecting false assertions but highly likely to overlook false presuppositions. The New-Given strategy, then, would predict more errors on items with false presuppositions than on items with false assertions. Materials and design

Materials consisted of 24 true and 24 contradicted, or false items. Each set contained 12 cleft-pseudocleft sentences and I2 factive complement sentences. To control for the order in which the target sentence mentioned assertion and presupposition, two target versions were constructed for each item. One version, either a cleft or an object complement sentence, mentioned the assertion first and the other version, a pseudocleft or a subject complement mentioned the presupposition first. Examples of target versions mentioning the assertion first are It was the coffee that ruined our carpet and The keeper was annoyed by their feeding the monkeys. Target versions which mention the presupposition first are What ruined our carpet was the coffee, and Their feeding

the monkeys

annoyed

the keeper.

The major experimental manipulation was achieved by constructing two contexts for each pair of target sentences. The two contexts were similar in length and content, but differed in that one contained information which contradicted the assertion in the target sentence while the other contained information which contradicted the target’s presupposition. There were thus four related versions of a given item. For the cleft-pseudocleft items, both inconsistencies involved the nouns in the target, while in the factive complements the inconsistencies involved the verbs in the target. To control for

368 J. Langford and V. M. Holmes

where in the context the discrepant information occurred, half of the items were constructed so that information relevant to the assertion was mentioned last and the other half were constructed so that information relevant to the target’s presupposition was mentioned last. The 24 true items were as similar as possible to the false items. Table 1 shows examples of the context and target conditions for two false items used in Experiment I. In sum, the item design consisted of one between-items factor, referring to whether the last information in the context related to the assertion or the presupposition (Context Order). There were also two within-item factors, one referring to the type of proposition, assertion or presupposition, contradicted by the context (Proposition Type), and one referring to whether the target sentence mentioned the assertion or the presupposition first (Target Order). To prevent subjects seeing more than one of the four versions of a given item, four lists were prepared containing one of each of the four conditions obtained from crossing Target Order and Proposition Type. The assignment of conditions was systematically varied so that the four lists contained an equal number of items in each condition. The same random ordering of true and false items was used for the four lists. Each list was given to an independent group of subjects. Thus, the subject design included a between-subjects factor (Group), as well as three within-subject factors (Context Order, Proposition Type and Target Order). As well as performing subject and item analyses of variance, minimum F’ was calculated in order to permit simultaneous generalization to new subject and new item populations (cf., Clark, 1973). The level of significance for all statistical decisions was set at a: = 0.05. Procedure All stimulus materials were presented on the oscilloscope terminal of a PDP11 computer. Subjects began each trial by pressing a button marked Go. A context paragraph appeared on the screen, which subjects had to read carefully, taking as much time as they needed. They then pressed the Go button again and a target sentence appeared on the screen. The subjects’ task was to decide as quickly and accurately as possible whether the target sentence was consistent or not consistent with the context, and then to press a Yes or and “inconsistent” were a No button accordingly. The terms “consistent” used in preference to “true” and “false” because of the logical problem that sentences with false presuppositions cannot themselves be false. The time taken to read the context and the time taken to verify the target sentence were measured to the nearest millisecond. Each subject received six practice trials during which the experimenter provided feedback about the correctness of each response.


Table 1.

369

Examples of contexts and targets for false items from Experiment I Cleft-pseudocleft

item

False Assertion Context Jane and Mary are flatmates. They get on well together but often in the evenings. They already have a radio but Mary would like well. False Presupposition Context Jane and Mary are flatmates. They get on well together but often in the evenings. They already have a television but Jane would like

find themselves bored to buy a television as

find themselves bored to buy a radio as well.

Target Sentencesa a) It is Jane who wants to get a television. b) The one who wants to get a television is Jane. Factive

complement

item

False Assertion Context Linda’s maths teacher is a defensive, discouraging person. He likes to prove his superiority by giving his students problems which are too hard for them. He was quite cross today when Linda, the brightest in the class, managed to solve the problem he set. False Presupposition Context Linda’s maths teacher is a defensive discouraging person. He likes to prove his superiority by giving his students problems which are too hard for them. He was delighted today when not even Linda, the brightest in the class, could solve the problem he set. Target Sentences a) Linda’s teacher was delighted that she could solve the problem. b) The fact that Linda could solve the problem delighted her teacher. aTarget

a) mentions

assertion

first and b) presupposition

first.

Subjects

Forty undergraduate students at the University of Melbourne were paid for participating in the experiment. All were native speakers of English.

Results and discussion The mean and standard deviation of each subject’s response distribution were calculated and, in order to minimize the influence of exceptionally long or short times, any observed verification time which exceeded two standard deviations from the mean was set at that value. This procedure affected 5.5% of verification times for false items and 3.8% of verification times when true

310

.I. I,angj?wd ard

Table 2.

V. M. Holmes

Mean l~erijkation times ill millisecorlds for j&e

___~ Assertion

Target First

items in Experiment Order Presupposition

Context

/

First

Order

Type of Proposition

Assertion Last

Presupposition Last

Assertion Last

Presupposition Last

False Assertion

259-I

2462

2629

2782

F&Z Presupposition

3047

2912

3039

3036

and false items were combined. Data for incorrect responses were excluded from the verification time analyses. Table 2 shows the means for the adjusted verification times for the test (inconsistent) items. In the analysis of variance on these means, the main effect of Proposition Type was highly significant, with F,( 1,36) = 47.34, F,(1,22) = 16.21 and min F’(l,37) = 12.08, showing that sentences with false assertions were verified significantly faster than sentences with false presuppositions. The main effect of Target Order was significant by subjects, with F,(l ,36) = 8.24. However, this effect was not significant in the item analysis, with F2( 1,22) = 1.33, and therefore this result cannot be considered typical of all items. From inspection of Table 2, it can be seen that false assertions were detected faster when they appeared first rather than second in the target sentence (a difference of 176 milliseconds) but false presuppositions were detected no faster when the presupposition was first (a reverse difference of 28 milliseconds). However, this interaction between Proposition Type and Target Order did not approach significance in either the subject or the item analyses. Neither the main effect of Context Order, nor any of the other possible interaction effects, approached significance in either the subject or the item analyses. In order to compare the two types of target sentence structure, an analysis contrasted the verification times for cleft-pseudocleft and for factive complement sentences. There was no evidence that these two structural types differed in overall verification time as the main effect of Sentence Type was not significant, with F,(1,36) = 2.96 and F, < 1. Nor did Sentence Type interact significantly with either of the other factors in the analysis, Proposition Type and Target Order. In a further analysis the means of the test items were compared with the means of the distractor items where the required response


Table 3.

371

Mean percentage of errors for false items in Experiment I Target Assertion

Presupposition

First Context

Type of Proposition False Assertion False Presupposition

Assertion Last

Order

Presupposition Last

First

Order Assertion Last

Presupposition Last

5.8

5.0

6.7

5.0

18.3

11.6

4.2

9.2

was consistent. The means for correct false and true items were 2826 milliseconds and 3387 milliseconds respectively. False items were verified significantly faster than true items, with F,(1,36) = 53.59, F,(1,46) = 25.95 and min F’(1,78) = 17.48. Analyses were also performed on the mean numbers of errors. Table 3 shows the mean percentage error for the test items. A trend was observed for there to be more errors when presuppositions were falsified than when assertions were falsified, although inspection of Table 3 reveals that this effect differed in magnitude for the four order conditions, being entirely absent when the assertion was last in the context and the presupposition first in the target. In the subject analysis there were significant main effects of Proposition Type, F,(1,36) = 12.25, and of Context Order, F,(1,36) = 7.20, and significant interactions between Proposition Type and Context Order, F,( 1,36) = 6.33, and between Proposition Type, Target Order and Context Order, F,(1,36) = 4.65. However, not one of these effects approached significance in the item analysis, suggesting that they would not be generalizable to another set of items. That the main effect of Proposition Type was not representative of the items used was confirmed by inspection of the individual item means; only 7 out of the 24 false items exhibited the effect. The means of the context inspection times for the two experimental conditions, i.e., for the false assertion and the false presupposition conditions, were 15,827 milliseconds and 15,669 milliseconds respectively. These were not significantly different in either the subject or the item analysis, with F, < 1 and F, < 1. The major finding of this experiment was that verification times for items with false assertions were significantly faster than verification times for items with false presuppositions. There was also a tendency for there to be more

372


errors on false presupposition items than on false assertion items, although this was only true of a subset of the items. The non-significant interaction between Sentence Type and Proposition Type suggests that the assertionpresupposition distinction was created just as strongly by the factive complement sentence structures as by the clefts and pseudoclefts. The absence of an interaction between which proposition was false and which was mentioned first in the target sentence rules out the possibility that subjects simply verified sentences in left to right sequence. The non-significant interaction between Proposition Type and Context Order indicates that verification times were no faster for items where the discrepant proposition was mentioned last in the context, suggesting that, at least in this task, recency of mention did not systematically affect the salience of contextual antecedents in working memory. A possible weakness of Experiment I is that the experimental contrast necessitated a comparison between verification times for quite different factcounterfact pairs. It is conceivable that the contradictions involved in the false presuppositions happened for some reason to be more difficult to detect than those involved in the false assertion condition. To determine whether or not this was the case, a control experiment was run where each target sentence was separated into two simple “component” sentences. For example, a factive complement sentence from Experiment I, Basil’s failing physics upset his parents was separated into Basil failed physics (the presupposition component) and BasiE’s parents were upset (the assertion component). Cleftpseudocleft sentences were separated by inserting indefinite pronouns. For example, It was the coffee that ruined our carpet was separated into The coffee ruined something (the assertion component) and Something ruined our carpet (the presupposition component). The procedure of the control experiment was identical to that of Experiment I. Each component sentence appeared with the Experiment I context which contradicted it. It was found that component sentences which had originally been presuppositions were no more difficult to verify than component sentences which had originally been assertions, with F,( 1 ,I 8) = 3.9 1 and F, < 1. In fact the difference between the means for the two conditions was in the opposite direction. The results of this control experiment therefore rule out the possibility that the outcome of Experiment I was simply due to a confounding of difficulty of contradiction with type of proposition contradicted. One other finding that deserves comment at this point is the fact that true items in Experiment I took significantly longer to verify than false items. This result is atypical of the general finding in verification tasks that true responses, at least for explicitly affirmative sentences, are faster than false


3’73

responses (e.g., Clark and Chase, 1972). A simple explanation of this finding is that the true items necessitated an exhaustive search of the context representation, whereas the false items permitted the search to be terminated as soon as a discrepancy was located. A further, artifactual reason for the finding may have been that subjects had difficulty in deciding whether some supposedly equivalent expressions were actually consistent or not. In fact, in some of the true items, the expression in the target sentence was more general than the corresponding expression in the context. Subjects reported that they sometimes found it difficult to decide whether terms such as “oyster” and “seafood”, “godfather” and “man”, and “relations with China” and “foreign policy” were meant to be consistent. Experiment

II

This experiment was designed to investigate whether the assertion-presupposition effect obtained in Experiment I would also be present when target sentences are processed in the absence of prior context. Presumably, any view based on the idea that people use the marking of assertion and presupposition as directions to new and given information would predict that the assertion-presupposition effect would not be present in such a situation. People would adjust to a situation where there is no prior context (and ipso facto, no given information) and would have no need to distinguish structurally between assertion and presupposition. A verification task was again employed but this time with the order of target sentence and context reversed. Accordingly, subjects were required to judge the relevance of a picture to a previously presented sentence. Picture, rather than paragraph contexts were used, on the assumption that a nonverbal context would provide minimal interference with memory for the target sentence. The sentences were all clefts and pseudoclefts, factive complements being excluded because their semantic content proved too difficult to depict unambiguously. Materials and design

There were 16 true and 16 false items. For each item there was one picture context and four possible sentence structures: cleft agent, cleft object, pseudocleft agent and pseudocleft object. Since all the target sentences were clefts or pseudoclefts, the inconsistency between picture and target always involved a noun. For any given false item, the discrepancy involved the same noun in all treatment conditions. As in Experiment I, the cleft and pseudocleft structures controlled for whether the target mentioned the assertion or

374

J. Langford and V. M. Holnws

the presupposition first. The agent and object sentences allowed the discrepant noun to be either focussed or presupposed. Table 4 gives examples of the target sentences used for two of the test items in Experiment II. Pictures were simple black line drawings. Half of the false items were constructed so that the discrepancy involved the logical subject of the action in the picture and the other half were constructed so that the discrepancy involved the logical object of the depicted action. To ensure that all lexical presuppositions were fulfilled, the discrepant noun was always present somewhere in the picture. For instance, in the first example in Fig. 1, the discrepant noun woman is depicted, but not in the appropriate relationship with the cupboard. In the pictures for the true distractor items there was always a third irrelevant object present, to prevent these items being noticeably different from the false items. Figure 1 shows the pictures which were used for the two false items exemplified in Table 4. To summarize, for the test items the design consisted of one betweenitems factor (Role of False Entity) and two within-item factors (Proposition Type and Target Order). Role of False Entity referred to whether the discrepant object was the logical subject or the logical object of the depicted action. As in Experiment I, Proposition Type referred to whether the target sentence asserted or presupposed the discrepant noun and Target Order referred to whether the target sentence mentioned the assertion or the presupposition first. Again, all four treatment conditions for each item (corresponding to the four sentence structures) were assigned to different lists and the assignment was varied over the items so that overall each list contained the same number of each sentence type. The same random ordering of true and false items was used for the four lists, which were given to four independent groups of subjects. The inspection times for all items (i.e., the time taken to read the target sentence) was classified according to two factors, Structure and Case of Clefted Noun. Structure referred to whether the sentence was a cleft or a pseudocleft structure and Case of Clefted Noun referred to whether the sentence asserted the logical subject or the logical object. Once again, subject and item means were analysed, and min F’ was calculated for all analyses. Procedure The stimuli,

which were black and white transparencies, were projected onto a light grey wall by a carousel projector. Subjects were seated at a response table. On each trial they pressed an Advance button to bring on the target sentence, and then, when ready, pressed it again to bring on the picture. Subjects then had to decide whether the picture and target were consistent or


Table 4.

375

Target sentences for two false items from Experiment II a) A false logical subject item Cleft agent: It’s the woman who is pushing the cupboard. Cleft object: It’s the cupboard that the woman is pushing. Pseudocleft agent: The one who is pushing the cupboard is the woman. Pseudocleft object: What the woman is pushing is the cupboard. b) A false logical object item Cleft agenrt It’s the man who is washing the floor. Cleft objecr: It’s the floor that the man is washing. Pseudocleft agent.’ The one who is washing the floor is the man. Pseudocleft object: What the man is washing is the floor.

Figure 1.

Picture contexts for (a) a false logical subject item and (b) a false logical object item in Experiment II.

(a)

(b)

not, and to press the Yes or the No button accordingly. A digital printout timer, connected to a photocell in the projector, recorded to the nearest millisecond the times taken for inspection of the target sentence and the verification time from the onset of the picture. At the beginning of each session there were seven practice trials during which the experimenter provided feedback as to the correctness of the subjects’ responses. Subjects

Forty undergraduate students at the University of Melbourne were paid for participating in the experiment. All were native speakers of English.

376


Results and discussion

The cut-off procedure described above was used in each verification time analysis. This affected 4.6% of verification times for false items and 5.5% of times for true and false items combined. Table 5 shows the means of the adjusted verification times for false items in Experiment 11. In the analysis of variance, the main effect of Proposition Type was significant, with F,(1,36) = 19.58, F,(1,14) = 31.57 and min F’(1,48) = 12.09. Sentences with false assertions were verified significantly faster than sentences with false presuppositions. The main effects of Target Order and Role of False Entity were non-significant in both the subject and item analyses, as were all the possible interactions between the three factors. The mean verification times for false and true items were 1226 milliseconds and 1195 milliseconds respectively. These were not significantly different, with F,( 1,36) = 2.20 and F,( 1,30) < 1. Table 6 shows the means of the percentage error for false items. Analyses of the mean numbers of errors revealed that, although there was a tendency, once again, for there to be more errors on false presupposition items than on false assertion items, this difference was not significant, with F, (1,36) = 3.00 and F,( 1 ,14) = 2.74. None of the other main or interaction effects was significant in either the subject or the item analysis. A preliminary analysis revealed that the means of the target inspection times for true and false items, which were 2146 milliseconds and 2344 milliseconds respectively, were not significantly different in either the subject or the item analysis. Thus, true and false inspection times were combined, the means being presented in Table 7. In the analyses, the main effect of Structure was significant by subjects, with F,(l,36) = 6.54, and by items, with F2( 1,3 1) = 4.44, but min F’ failed to reach significance, with min F’(1,62) = 2.63. There was a strong trend, therefore, for cleft sentences to be processed faster than pseudocleft sentences. The interaction between Structure and Case of Clefted Noun was significant in the subject analysis, with F,( 1,36) = 5.68, but not by items, with F, < 1. This interaction was a cross-over, whereby cleft agent sentences were inspected faster than cleft object sentences, but pseudocleft agent sentences were inspected slower than pseudocleft object sentences. Experiment II has demonstrated that, even when sentences have no prior context, and therefore contain no given information, they are represented in a form which distinguishes between assertion and presupposition. Sentences which presupposed discrepant information took significantly longer to verify than sentences which asserted it. In contrast with Hornby’s findings,


Table 5.

377

Mean verification times in milliseconds for false items in Experiment II Role of False Entity Logical

Subject

Logical Target

Table 6.

Object

Order

Type of Proposition

Assertion First

Presupposition First

Assertion First


False Assertion

1208

1142

1145

1131

False Presupposition

1321

1267

1297

1356

Mean percentage error on false items in Experiment I1 Role of False Entity Logical

Subject

Logical Target

Table 7.

Object

Order

Type of Proposition

Assertion First


Assertion First


False Assertion

5.00

2.50

1.25

3.75

False Presupposition

7.50

3.75

5 .oo

5.00

Mean inspection times in milliseconds for all items in Experiment II Case of Clefted Noun

Agent Object

Sentence

Structure

Cleft

Pseudocleft

2100 2162

2244 2175

the overall error rate for false presupposition items was low, 5.3%. Furthermore, there was no significant difference between the numbers of errors made on items with false presuppositions and on items with false assertions.


A superior feature of Experiment II was that the two experimental conditions involved exactly the same contradiction between sentence and picture. Therefore the difference in verification times can only be attributed to differences between the target surface structures in the two conditions. The strength of the assertion-presupposition effect is quite remarkable in view of the unlimited inspection time, and of subjects’ own impressions that they were merely recoding the target sentences into a simple form. The fact that target inspection for true and false items did not differ suggests that subjects could not have anticipated whether an item would be true or false on the basis of the target sentence alone. When true and false inspection times were pooled and analysed in terms of surface structure features, it was found that cleft sentences were processed more rapidly than pseudocleft sentences. In addition, the interaction effect suggested that, at least for some items, sentences which mentioned the logical subject, verb and logical object in that order tended to be processed faster than the sentences with non S-V-O orders. Thus, cleft agent sentences, It is the S that is V-ing the 0 were inspected on average faster than cleft object sentences, It is the 0 that the S is Virlg and pseudocleft object sentences, What the S is V-irlg is the 0 were inspected faster than pseudocleft agent sentences, The OYIFthat is V-kg the 0 is the S. The fact that inspection times tended to be sensitive to the different ways of expressing the same basic meaning justifies the removal of time constraints from the processing of the target sentence. If only a limited amount of time is allowed, as was the case in Hornby’s experiment, then more complex surface structures may be encoded less adequately. In the present experiment, it may be fairly safely assumed that by the verification phase of the trial, the four surface structure types had been encoded in equivalent form. This is borne out by the absence of any effect of surface structure type per se on subsequent verification times and error rates. As in Experiment I, neither of the control factors was related to verification time. Thus there was no evidence either of a serial left-to-right verification strategy, nor of any primacy or recency effects on memory for the target sentence. Similarly, the results ruled out the possibility that subjects were using a systematic strategy to search the picture for logical subject before logical object. There was no difference between verification times for items with discrepant logical subjects and for items with discrepant logical objects. In contrast with Experiment I, verification times for true and false items were not significantly different. There are several aspects of the procedure in Experiment II which might explain this result. Firstly, the picture contexts were simpler than the paragraphs, and thus any exhaustive search in the true items would end much sooner. Secondly, the pictures were present in front of the subject, rather than being held in memory, so that any search involved

Syntactic presupposition in sentence cornprehension

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would be much more efficient. Finally, the confirming instances were less equivocal in Experiment II, where there were no problems with the intended equivalence of sentences and pictures. Thus a search of the picture could be terminated by either a confirming or a disconfirming instance. General discussion The major finding of the experiments reported was that sentence verification times were significantly longer when a discrepancy between target sentence and context was located in the syntactic presupposition than when the discrepancy was in the assertion, a result which has not been demonstrated before. As was pointed out above, previous studies of presupposition have either failed to make a direct comparison between the processing of assertion and presupposition, or have made the comparison but have confounded the syntactic distinction between assertion and presupposition with other nonsyntactic factors such as acoustic stress. The present experiments were not open to either of these criticisms. The result provides confirmation that once the surface structure of a sentence is processed, not only does it influence the memory representation of the sentence meaning, but it also serves to direct subsequent verification processes. Returning to the two psycholinguistic accounts of presupposition outlined above, the present findings reveal that the Given-New Strategy of Haviland and Clark (1974) is inadequate as a description of the processing of presupposition and assertion in sentence verification tasks. If subjects had been using this strategy to integrate target sentences with contexts, then they should have detected false presuppositions more rapidly than false assertions. It would probably be argued by Haviland and Clark that the present findings do not constitute a refutation of their model since the model was never intended as a description of verification tasks. However, it is surprising that a “procedure for relating the current sentence to.. . [a] knowledge base” was not evident at least in Experiment I, where subjects had to compare a target sentence to a previously assimilated paragraph context. The New-Given strategy, i.e., the model proposed by Hornby (1974) and by Clark and Clark (1977), is also unacceptable as an explanation of the present findings. In Clark and Clark’s formulation, the New-Given model was based on the assumption that the semantic representation of a target sentence contains not only propositional, or underlying logical information, but also thematic, or Given-New specifications (cf., Clark and Clark, 1977, p. 89). According to this model, subjects first encode the sentence’s thematic structure, and then adopt a search strategy based on their expectations about which parts of this thematic structure are likely to be true. To account for Hornby’s finding that subjects were less likely to detect false presuppositions


than false assertions, the New-Given model proposes that subjects assume that given information is true, and only search for facts relating to the new information. This account founders when confronted with the results of the present experiment: given enough time, subjects rarely failed to detect false presuppositions. This difficulty may be dealt with by modifying the NewGiven model, and postulating that subjects do eventually search the context for information corresponding to the given part of the sentence, but only after they have searched for, and failed to detect discrepant new information. This revised version of the New-Given strategy is superficially more consistent with the results of the present experiments. However, closer scrutiny reveals that there is a major problem with the idea that subjects may search in serial order first for new and then for given information. In Hornby’s experiment, where the discrepancy involved an unfulfilled lexical presupposition in either the focus or the presupposition of the target sentence, it was possible for subjects to search for new information before searching for given. However, for the cleft and pseudocleft sentences in the present experiment, where the lexical presuppositions for focussed words were always fulfilled, it was logically impossible to detect new information without reference to the content of the presupposition. In the experiments reported above, the discrepant new information involved not the focus, but the combination of focus and presupposition, i.e., the assertion of the target sentence. Thus, the idea that subjects search first for new information, and then for given, cannot account for the findings of the present experiments. Clearly, an alternative explanation is called for. In what follows, we will outline an alternative account of our results which we shall designate the Structural hypothesis. A basic premise of the Stuctural hypothesis is that the encoding of the target sentence contains no specific marking of given and new information, but simply retains some representation of the hierarchical organization already present in the sentence’s surface structure. A second feature of the hypothesis is that it does not explain the assertion-presupposition effect in terms of ordered search strategies, and hence makes no assumptions about the order in which the context is searched. Instead, it assumes that the effect is attributable to the number of mental operations required to express, or to make explicit the discrepancy. Finally, the hypothesis assumes that the expression of a discrepancy involves the construction of either a negated proposition or of a Yes/No question, and that this process is necessary before a subject can be confident of a No response. When subjects attempt to construct a negated proposition, the simplest procedure is simply to predicate a negative to the sentence representation as it stands. Because the target sentence is represented according to its surface structure form, the first constructed negation will correspond to a denial of


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the assertion but not of the presupposition. If it is the assertion which is discrepant with the context, then a No response will be appropriate and immediate. However, if the discrepancy is in the presupposition then the first constructed denial, which does not extend to the presupposition, will not correspond to the mismatch exactly. In this case, subjects will not be able to respond No immediately but will have to reformulate the target sentence. This reformulation will involve stripping off the main clause and isolating the subordinate clause so that the presupposition may be directly negated. If, on the other hand, subjects verify sentences by generating Yes/No questions, then a similar explanation can be provided for why they take longer to verify sentences with false presuppositions. On the assumption that surface structure form determines the representation of the target sentence, the most easily generated question is one which manipulates elements of the main clause. For example, It is the boy who is chasing the cow is most simply transformed into Is it the boy who is chasing the cow? As with sentential negation, then, the first-generated question interrogates only the assertion. If the discrepant fact is asserted, then the question will be appropriate and an immediate No response possible. However, in order to locate a discrepancy in the presupposition, another question will be required which specifically interrogates the subordinate clause. Once again, the need to generate a second question accounts for the longer times associated with verifying false presuppositions. Clark and Clark (1977) have already pointed to the similarity between verifying a sentence and answering a Yes/No question. Their reason for making this comparison was that they wished to establish, by analogy, a plausible reason for why subjects should assume the presupposition is true and selectively search for New information. However, our proposal is that subjects may actually generate Yes/No questions, or alternatively, negated propositions, in the course of verifying sentences. It is possible to make some tentative suggestions as to the stage at which these structural manipulations occur within the overall comprehension process. In this regard, the distinction drawn by Cutler (1976) between Stage A and Stage B processing is pertinent. In her formulation, Stage A processing involves the parsing-plus-lexical-look-up activity necessary to construct a literal interpretation of the sentence, while Stage B processing involves the subsequent enrichment and modification of this interpretation in the light of extra-sentential factors. In Experiment I, where the verification times included the time taken to read the target sentence, it was not possible to isolate Stage A and Stage B processing. However, in Experiment II, where target inspection times were recorded separately, the two types of processing were distinguishable. The inspection times presumably reflected Stage A processing; they were sensitive to differences in the structural complexity of the four

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sentence types, and resulted in a sema,rtic interpretation adequate for the accurate verification of the sentence. On the other hand, the verification times may be assumed to reflect Stage B processing. Thus, the structural reformulations needed to apprehend a discrepancy between presupposition and context may be considered a part of Stage B activity. Finally, the major conclusion which may be drawn from the experiments described here is that the structural distinction between assertion and presupposition has a real effect on the processes by which sentences are integrated with their contexts. Although this effect is normally undetected, it becomes strikingly apparent when there is a discrepancy between a sentence and its context. In seeking an explanation for this effect we have rejected the proposal that it is due to strategies based on subjects’ expectations about where Given and New information are normally located in the sentence. Instead, it has been proposed that the effect is primarily due to the position of the presupposition within the subordinate clause which renders it inaccessible to negative or interrogative predicates, both of which are implicated in the apprehension of discrepant information. In their discussion of Hornby’s findings, Clark and Haviland (1977) have noted that English has only clumsy and indirect devices for qualifying presuppositions. However, they conclude that the major reason for subjects having failed to detect false presuppositions was their assumption that the speaker was adhering to the Given-New contract. The difference between their account and ours lies in the emphasis, not so much on the listener’s assumptions about adherence to a Given-New contract, as on the unavoidable consequences of processing surface structure. Although we have not embraced the notion that listeners use assertion and presupposition in a deliberate, strategic way, it is clear that the structure in which a message is conveyed may facilitate the process by which the listener can reconstruct that message. Conversely, it is apparent that the same structure may obstruct the processing which attempts to integrate the sentence with contextual knowledge. Whether a structure will be facilitative or disruptive depends on the nature of the information placed in its presupposition. It appears that the content of the presupposition must be that information which requires minimal processing if sentence comprehension is not to be obstructed. It should be noted that the preceding discussion is not inconsistent with the idea that assertion and presupposition serve a crucial function in the communicative process. However, the emphasis of our Structural hypothesis is that this function is determined by the effect of sentence structure on the language processing mechanisms, rather than by strategies based on the listener’s pragmatic expectations.


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References Bock, J. K. (1977) The effect of pragmatic presupposition on syntactic structure in question answering. J. verb. Learn. verb. Behav., 16, 723-735. Clark, H. H. (1973) The language-as-fixedeffect-fallacy: a critique of language statistics in psychological research. J. verb. Learn. verb. Behav., 12, 335-359. Clark, H. H. and W. G. Chase (1972) On the process of comparing sentences against pictures. Cog. Psychol., 2, 101-111. Clark, H. H. and E. V. Clark (1977) Psychology and Language. New York, Harcourt Brace Jovanovich, Inc. Clark, H. H. and S. E. Haviland (1977) Comprehension and the Given-New contract. In R. 0. Freedle (ed.), Discourse Production and Comprehension. Norwood, N.J., Ablex. Cutler, A. (1976) Beyond parsing and lexical look-up; an enriched description of auditory sentence comprehension. In R. J. Wales and E. Walker (eds.). New Approaches to Language Mechanisms. Amsterdam, North Holland. Haviland, S. E. and H. H. Clark (1974) What’s new? Acquiring new information as a process in comprehension. J. verb. Learn. verb. Behav., 13, 512-521. Hornby, P. A. (1974) Surface structure and presupposition. J. verb. Learn. verb. Behav., 13, 530-538. Hupet, M. and B. Le Bouedec (1977) The Given-New contract and the constructive aspect of memory for ideas. J. verb. Learn. verb. Behav., 16, 69-75. Offii, C. E. (1973) Recognition memory for presuppositions in relative clause sentences. J. verb. Learn. verb. Behav., 12, 636-643. Osgood, C. E. (1971) Where do sentences come from? In D. D. Steinberg and L. A. Jakobovits (eds.), Semantics, Cambridge, Cambridge University Press. Singer, M. (1976) Thematic structure and the integration of linguistic information. J. verb. Learn. verb. Behav., 15, 549-558.

Resume On a fait deux experiences pour Studier le role de la presupposition syntaxioue dam la comprehension des phrases. Dans la premiere experience les sujets doivent verifier, en fonction de contextes present& avant les phrases, des phrases clivkes, des pseudoclivees et des phrases avec des complements factitifs. Les contextes peuvent contredire soit l’assertion soit la presupposition de la phrase cible. Les sujets mettent significativement plus de temps pour verifier les phrases avec des presuppositions fausses que pour verifier les phrases avec des assertions fausses. Dans l’expirience II, les sujets verifient les phrases clivees et pseudoclivees en fonction d’images present&es apres les phrases. Les temps de verification pour les phrases avec des presuppositions fausses sont ici aussi significativement plus longs que les temps de verification pour les phrases avec des assertions fausscs. On rend mieux compte de ces donnees avec une hypothese “structurale” qu’en termes de strategies ayant pour but de localiser les informations don&es ou nouvelles.