Working memory constraints on the processing of syntactic ambiguity

COGNITIVE

PSYCHOLOGY

Working

24, 56-98 (1992)

Memory Constraints on the Processing Syntactic Ambiguity

of

MARYELLEN C.MACDONALD Massachusetts

Institute

of Technology

AND MARCELADAMJUSTANDPATRICIA A. CARPENTER Carnegie Mellon

University

We propose a model that explains how the working-memory capacity of a comprehender can constrain syntactic parsing and thereby affect the processing of syntactic ambiguities. The model’s predictions are examined in four experiments that measure the reading times for two constructions that contain a temporary syntactic ambiguity. An example of the syntactic ambiguity is The soldiers warned about the dangers . . . ; the verb warned may either be the main verb, in which case soldiers is the agent; or the verb warned may introduce a relative clause, in which case soldiers is the patient of warned rather than the agent, as in The soldiers warned about the dangers conducted the midnight raid. The model proposes that both alternative interpretations of warned are initially activated. However, the duration for which both interpretations are maintained depends, in part, on the reader’s working-memory capacity, which can be assessed by the Reading Span task (Daneman & Carpenter, 1980). The word-by-word reading times indicate that all subjects do additional processing after encountering an ambiguity, suggesting that they generate both representations. Furthermore, readers with larger working-memory capacities maintain both representations for some period of time (several words), whereas readers with smaller working-memory capacities revert to maintaining only the more likely representation. 0 19%Academic press, IX.

This paper investigates the processes in comprehending sentences that contain a temporary syntactic ambiguity. Our approach focuses on individual differences in working-memory capacity, that is, differences between individuals who are better or poorer at manipulating language (Carpenter & Just, 1988). We present a model of sentence parsing in which the working-memory capacity of the comprehender influences the degree to which she/he maintains multiple syntactic representations during the processing of a syntactic ambiguity. This research was partially supported by NIMH Grant MH-29617 and MH-00662 to Marcel Just and NIMH Grant MH-00661 to Patricia Carpenter. Requests for reprints should be sent to Dr. Marcel Just, Department of Psychology, Carnegie Mellon University, Pittsburgh, PA 15213. 56 OOlO-0285/92$7.50 Copyright AII rights

Q 19!32 by Academic Press, Inc. of reproduction in any form reserved.

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Whether multiple syntactic representations are generated when a syntactically ambiguous string is encountered has been a prominent issue in the field. According to single representation models, only one syntactic representation is constructed for an ambiguous string (e.g., Frazier, 1979; Marcus, 1980). If later information in the sentence is incompatible with this interpretation, then the original interpretation may be abandoned and a different interpretation may be sought. By contrast, in multiple representation models, multiple syntactic representations are generated when a syntactic ambiguity is encountered (e.g., Kurtzman, 1985; Gorrell, 1987). However, the multiple representation models have to acknowledge a limit on the amount of information that can be maintained. Typically, multiple representations are assumed to be maintained only until some later information indicates which syntactic interpretation is correct, at which point the incorrect interpretations are abandoned. Sometimes an alternative representation may be abandoned before the definitive disambiguating information is reached. If the alternative interpretations are not equally likely, the rarer or more complex interpretation is assumed to be less available than a more frequent, simpler interpretation. In garden-path sentences, such as The horse raced past the barn fell, the correct interpretation is assumed to be so rare that it usually cannot be retrieved at the point of the disambiguation, fell, and so parsing fails. The model we propose transcends the single/multiple representation dichotomy in that it hypothesizes individual differences in the degree to which multiple representations are maintained for a syntactic ambiguity. We have called the model the Capacity Constrained Parsing Model to emphasize its central feature, namely, that the working-memory capacity of the comprehender constrains syntactic processes in general and the processing of syntactic ambiguity in particular. Elsewhere, we have developed a more complete theory and a computational model of the influence of working-memory capacity on syntactic processing, including the influence of capacity on syntactic modularity and the processing of syntactically complex constructions (Just & Carpenter, in press). A central argument is that individual differences in working-memory capacity influence syntactic processing. Individual differences in working-memory capacity for language are assessed with the Reading Span task (Daneman & Carpenter, 1980). In this task, a subject reads successive sentences while retaining the final word of each sentence. The number of sentence-final words that the person can subsequently recall (usually between two and five words) is their reading span. The rationale underlying the reading span task was to simultaneously measure both storage and computations in language processing. The measure might be specific to language processing rather than provide a generalized measure of working memory. Consequently, the

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phrase working-memory capacity in this paper refers to working-memory capacity for language. Reading span correlates quite highly with specific measures of language comprehension, including the ability to answer questions about a passage (Daneman & Carpenter, 1980), Verbal SAT scores (Daneman & Carpenter, 1980) and various standardized comprehension tests (e.g., Baddeley, Logie, Nimmo-Smith, & Bereton, 1985; Masson & Miller, 1983). However, the Capacity Constrained Parsing Model goes beyond predicting that high span subjects will routinely show better performance on both the comprehension and the reading time measures. The model’s predictions are both more complex and more interesting. THE CAPACITY CONSTRAINED PARSING MODEL According to the model, sentences without ambiguities are integrated as fully as possible into a running representation of the text (Just & Carpenter, 1980, 1987). When a reader encounters a syntactic ambiguity, he initially constructs multiple representations, corresponding to the alternative syntactic analyses of the ambiguous string. If one interpretation of the ambiguity is syntactically simpler, pragmatically more plausible, or more frequently encountered, then the representation corresponding to this preferred interpretation is maintained at a higher level of activation than the unpreferred interpretation (Carpenter & Daneman, 1981; Gorrell, 1987; Kurtzman, 1985). The preferred interpretation obtains its higher activation level by virtue of an integration mechanism that summates the weight of its supporting evidence, as specified by the READER model (Just & Carpenter, 1987; Thibadeau, Just, & Carpenter, 1982) or analogous mechanisms in constraint satisfaction models (e.g., Altmann & Steedman, 1988; Taraban & McClelland, 1988). It is only after the initial construction of multiple representations for an ambiguity that the processing differs for high and low span readers. Low span readers will be more quickly taxed by the burden of maintaining multiple representations as they continue the sentence. Thus, if the sentence is sufficiently long or complex, the representation with the lower activation level will be more likely to be abandoned and be inaccessible for the low span reader, even in the absence of any new disambiguating information. By contrast, a high span reader is more likely to maintain multiple representations for longer periods. If the disambiguating information is encountered sufficiently soon, the high span reader is likely to have both alternatives available to choose from. The nature of the processing that occurs at the point of disambiguation will depend on whether the ultimate resolution of the ambiguity corresponds to the preferred or unpreferred interpretation. If the ambiguity is resolved with a highly preferred interpretation, both the high and the low

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span reader are relatively well off. In the case of a high span reader, both representations are likely to be available, and the correct interpretation can be selected (by virtue of newly encountered disambiguating information), Nevertheless, the high capacity reader may pay a price for maintaining both representations. During the period in which multiple representations are maintained, there may be insufficient resources to execute other processes, such as the elaboration of the high level referential representation. These processes may be postponed in the ambiguous region and executed later, resulting in a net increase in the sentence processing time, compared to a condition in which there is no catching up to do. The delayed processing of the selected representation will result in increased processing time for an ambiguous sentence, even if it is resolved with the favored resolution. Thus, the maintenance of alternative syntactic representations can result in longer processing times for ambiguous sentences than their unambiguous counterparts. In the case of a low span reader, however, capacity limitations make it likely that only one interpretation (the preferred one) has been maintained for any length of time. By abandoning the unpreferred representation, the low span reader may free up working-memory resources to devote to normal processing and have only negligible catching up to do at or after the point of disambiguation. The single representation retained by the low span reader will have received more elaboration than either of the two representations that are maintained by the high span reader. Thus, when the ambiguity is resolved with a strongly preferred resolution, the Capacity Constrained Parsing Model makes the counterintuitive prediction that high span readers will show more effect of ambiguity than low span readers. The model makes a different set of predictions for the processing of a temporarily ambiguous sentence that is disambiguated in favor of a much less preferred resolution. First, the model predicts that low span readers should make more errors than high span readers on these sentences. According to the model, the low span readers are less likely than high span readers to maintain the unpreferred interpretation throughout a long ambiguous region. By the time the disambiguation is reached, the low span readers should be less likely to represent the correct meaning, leading to poorer comprehension. If a reader correctly comprehends an ambiguous sentence with an unpreferred resolution, the model predicts that response times will increase relative to the time for an unambiguous sentence. The increase in response time may reflect two types of processing penalties associated with the syntactic ambiguity. One penalty, as in the case of the preferred resolution, is incurred by virtue of maintaining multiple representations and thereby having insufficient capacity to execute other processes. This

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cost will fall more heavily on high span readers because they are more likely to maintain multiple representations. A second cost may be incurred by virtue of encountering a disambiguation that is incompatible with a much more active, preferred representation and having to select a less preferred representation. If there is no trace of the unpreferred representation at the point of disambiguation, readers may engage in error recovery processes; these processes are not specified here, although they have been investigated and described elsewhere (Carpenter & Daneman, 1981; Frazier & Rayner, 1982). In sum, the Capacity Constrained Parsing Model predicts a precise pattern of reading time and comprehension accuracy data as a function of both the subject’s reading span and the nature of the resolution for an ambiguity. For temporary ambiguities resolved with a highly preferred interpretation, both high and low span subjects should comprehend well because all subjects have carried the preferred interpretation. The difference between the span groups should instead appear in reading time; high span subjects will have carried two interpretations for some time, and so their reading times on the ambiguity should be slower, relative to their performance on an unambiguous sentence, for which there is only one possible interpretation. Low span subjects, however, will typically have carried only the preferred interpretation for much of the sentence; thus, their reading times should be similar to that for an unambiguous sentence. When the ambiguity is resolved with the unpreferred interpretation, the model predicts that the high span subjects’ comprehension should be better than the low span subjects’, because the high span subjects have the capacity to maintain unpreferred interpretation for longer periods; thus, they are more likely to have the correct representation available when the disambiguation is encountered. Correct comprehension of an ambiguous sentence with an unpreferred resolution should be accompanied by large increases in reading time for all subjects. EXPERIMENT

1

The goal of Experiment 1 was to assess how subjects with different working-memory capacities process syntactic ambiguities. The syntactic ambiguity chosen for this and the next several experiments should provide a strong test of the model because it has one interpretation that is strongly preferred over the other. Moreover, several words can intervene between the ambiguity and its resolution, so that working memory is more likely to be a constraining factor in processing. The model predicts that the ambiguity effect will be larger for high span readers than for low span readers when the ambiguity is resolved with its preferred interpretation. The syntactic ambiguity that was studied occurs at the first verb, such

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as warned, in sentences like (1) and (2). Either this verb can be the main verb, as in sentence (I), or it can introduce a relative clause, as in sentence (2). 1. The soldiers warned about the dangers before the midnight raid. 2. The soldiers warned about the dangers conducted the midnight raid.

In sentence (2), the relative clause introduced by warned is a paraphrase of who were warned. Because the relative pronouns, such as who or that, are omitted, this construction is frequently called a reduced relative clause. The interpretation of a sentence with a reduced relative clause is often difficult, even for college students, and some subjects report that they can find no coherent interpretation for such sentences (Frazier, 1979; Kurtzman, 1985). The reduced relative construction is much less frequent than the main verb construction. Because the two alternative interpretations differ so sharply both in syntactic complexity and in frequency of usage (properties which are not necessarily independent), the model predicts that the representation of the relative clause interpretation will be activated to a considerably lower degree than the representation of the main verb interpretation. Examples of the ambiguous and unambiguous sentences that were used in the current experiments are shown in Table 1. In the two sentences with the Main Verb resolution (A and B), the first verb is the main verb. Sentence (A) is unambiguous because spoke can only be interpreted as the past tense of speak. By contrast, sentence (B) is temporarily ambiguous because warned can be either the past tense form of a main verb or a past participle that introduces a reduced relative clause. This ambiguity is not resolved until the end of the sentence. In fact, the ambiguity is resolved by the presence of the period that follows the last word. Until TABLE 1 Examples of Syntactically Ambiguous Sentences, Experiment 1 Main verb resolution-Unambiguous A. The experienced

soldiers spoke about the dangers before the midnight

Main verb resolution-Temporarily B. The experienced

raid.

ambiguous

soldiers warned about the dangers before the midnight

raid.

Comprehension question: Did someone tell the soldiers about dangers? Relative clause resolution-Unambiguous C. The experienced raid.

soldiers who were told about the dangers conducted

Relative clause resolution-Temporarily D. The experienced

the midnight

ambiguous

soldiers warned about the dangers conducted

Comprehension question: Did the soldiers speak about dangers?

the midnight

raid.

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that point, it is still possible that the ambiguous verb is introducing a relative clause, as in the sentence: The soldiers warned about the dangers before the midnight raid conducted the raid. Sentences (C) and (D) have a Relative Clause resolution. In the unambiguous sentence (C), the words who were make it clear that told is part of a relative clause; thus, the first noun, soldiers, has the thematic role of patient of told, rather than the role of agent. By contrast, sentence (D) has an ambiguous verb, warned, and no disambiguating relative pronoun. Thus, the initial portion of sentence (D), like the identical initial portion of sentence (B), is temporarily ambiguous between the main verb interpretation and the relative clause interpretation. The disambiguation in favor of the relative clause interpretation occurs four words after the ambiguity is introduced, with the second verb, conducted. The major purpose of this experiment was to determine whether the presence of a temporary ambiguity influences reading time, particularly in the case where the ambiguity is eventually resolved as a Main Verb sentence, which is the more frequent resolution. According to the model, high span readers should be more likely to represent both interpretations in the ambiguous region and postpone some higher level processing. The cost of maintaining multiple representations should manifest itself as additional processing time either before or at the disambiguation (which is the last word in the sentence in the Main Verb condition) to execute the postponed processes. Therefore, high span readers should take longer to process ambiguous Main Verb sentences than unambiguous Main Verb sentences. By contrast, low span readers are likely to maintain only the preferred interpretation and consequently, they will not have extra catching up to do when the disambiguation is reached. Thus, the low span readers should paradoxically pay less of a penalty if the temporarily ambiguous sentence is resolved as a Main Verb sentence. To assess processing time, the sentences were presented in a word-byword, self-paced reading paradigm. A comprehension question followed each sentence to check whether the sentence was correctly comprehended. Consequently, there were two dependent measures: the reading time per word for ambiguous sentences and their unambiguous counterparts, and the accuracy in answering the comprehension questions that followed ambiguous and unambiguous sentences. An ancillary experiment examined whether the processing of these sentences was specific to reading individual, isolated sentences, or whether the processing would be similar if the sentences were embedded in a passage context, as illustrated by the paragraph in Table 2. These paragraphs were thematically appropriate, but they were neutral with respect to whether the correct interpretation of the ambiguous verb was as a main verb or as a reduced relative. This paragraph experiment was intended to

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TABLE 2 An Example of a Paragraph Context-Experiment

lb

Even though the sun had set hours before, the sky was bright with flares and explosions. The troops huddled together to go over the new plans for the surprise attack. The experienced soldiers warned about the dangers before the midnight raid. Comprehension question: Did the soldiers talk about the dangers?

be a conceptual replication. We will refer to it as Experiment lb and it will be discussed along with the main experiment, Experiment la. METHOD Subjects. The subjects in this and subsequent experiments were native speakers of English with normal or corrected-to-normal vision. In Experiment la, the subjects were 50 students from Carnegie Mellon University who participated for course credit. One additional subject was tested and rejected because of an error rate over 25% on the comprehension questions. In Experiment lb, the subjects were 26 students, none of whom had participated in Experiment la. In Experiment lb, subjects were recruited from a pool in which the reading spans had been measured in previous, unrelated experiments. Only those students who had reading spans in the high or low range were recruited. Materials. Two versions of each ambiguous sentence were constructed such that one version resolved the ambiguity in favor of the main verb interpretation, and the other version in favor of the relative clause interpretation. For each of these two ambiguous sentences, there was an unambiguous sentence that was identical except for the verb or the verb and relative pronoun that eliminated the syntactic ambiguity. Consequently, there were four versions of each experimental sentence, reflecting the factorial manipulation of the two variables Resolution (Main Verb or Relative Clause resolution) and Ambiguity (unambiguous or temporarily ambiguous), as shown in Table 1. To construct the sentences, we selected eight verb triplets, such as spoke, told, and warned. One member of the triplet, such as spoke, allowed only a main verb continuation either because the past tense of the verb differs from the past participle or because the semantics of the verb disallowed a relative clause interpretation (e.g., cried, giggled). Another member of the triplet, such as to/d, appeared with a relative pronoun and auxiliary verb and so unambiguously introduced a relative clause. The third member, such as warned, was temporarily ambiguous between a main verb and relative clause interpretation because the past tense form of the verb is the same as the past participle. The lengths of the verbs in the three groups did not differ significantly, F(2,21) = 1.37, ns (The mean number of characters was 4.9, 5.6, and 6.3 for the verbs in the spoke, told, and warned categories, respectively). Frequency also did not vary significantly across the three groups, F < 1 (110, 120, and 108 occurrences per million; Kucera & Francis, 1967). For each triple, three unrelated sentences were written, producing 24 distinct sentence frames. In most cases, the three verbs in a triplet were semantically related. In some cases, they were less related, but an effort was made to keep all three verbs equally compatible with the sentence frames in which they occurred. The verbs and sentence frames are presented in Appendix A. Every subject saw 6 sentences in each of the four conditions, for a total of 24 experimental sentences. Four sets of materials were constructed. In each set, four of the eight ambiguous verbs occurred in only one type of sentence and four occurred in both a Main Verb sentence and a Relative Clause sentence, but in different sentence frames. Across the four sets, each

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ambiguous verb occurred an equal number of times. Subjects were randomly assigned to the four sets. A yes/no comprehension question was written for each sentence to assess which interpretation was assigned to the first verb. Examples are given in Table 1. The correct answer was yes for half of the questions. The questions for the Main Verb resolution differed slightly from those for the Relative Clause resolution, so that the correct answer remained either yes or no across both levels of this factor. In Experiment la, the 24 target sentences were presented along with 10 practice and 73 filler sentences that utilized a variety of syntactic structures. Twelve of the filler sentences contained syntactic category ambiguities as part of a conjoint study, which will be reported elsewhere. It is important to note that only 12 of the sentences that a subject encountered in the course of the experiment, or 11% of the total number of sentences, had a Relative Clause resolution. Experiment lb had the same target sentences as Experiment la, but each target was the last sentence of a three-sentence paragraph. The first two sentences were neutral and were designed to be equally compatible with all four versions of each target sentence. There were different paragraph contexts for each sentence frame. In addition, subjects in Experiment lb saw 3 practice paragraphs and 60 filler paragraphs, all containing between two to four sentences. The paragraphs were followed by a comprehension question. In the case of the target passages, these questions were the same as in Experiment la. In the case of the filler sentences, the question sometimes interrogated information presented in other sentences in the passage. Again, there were four groups of texts and subjects were randomly assigned to groups. Senrence presentation. The sentences were presented on a video monitor in a Moving Window display (Just, Carpenter, & Woolley, 1982), so that only one word was visible at any one point in time. At the start of each trial, the display contained several lines of dashes representing all nonspace characters of the stimulus sentence and the comprehension question. When an experimental sentence did not tit on one line of the display in one of its four conditions, all four versions of the sentence were displayed on two lines, with the line break at the same point for each version. Subjects held a switch in their left hand to advance the moving window to new words in the sentence. When the switch was pressed to initiate a trial, the first word of the sentence replaced the dashes corresponding to that word. When the switch was pressed a second time, the second word appeared and the first was replaced again by its dashes, and so on. After the display of the last word of the critical sentence, all of the words of the comprehension question were presented simultaneously on a separate line. Subjects answered the question using the right hand to press buttons marked YES and NO. Subjects were encouraged to read and respond quickly while maintaining good comprehension and response accuracy. Following the practice trials, the remaining trials were presented in random order. Reading span. In this task, which was conducted at the start of the testing session, subjects read a series of unrelated sentences aloud without pausing between sentences. At the end of a series, they were asked to recall the last word of each sentence in the set (Daneman and Carpenter, 1980). For example, consider the following set of two sentences: When at last his eyes opened, there was no gleam of triumph, no shade of anger. The taxi turned up Michigan Avenue where they hnd a clear view of the lake. After reading this set, the subject was to recall the words anger and lake. Subjects initially were given five sets with two sentences per set. If they correctly recalled both words for three of the five sets, they were presented with five three-sentence sets, followed by four- and five-, and sixsentence sets, if the subject was still recalling words accurately. A subject’s reading span score was defined as the largest set for which they correctly recalled all of the words from three of the five sets. An additional 0.5 was added to the subject’s score if they had recalled two of the five words in the next highest set. For

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example, if a subject recalled all final words on at least three of the three-sentence sets and then recalled all words on only two of the four-sentence sets, the subject would be assigned a score of 3.5. In this and all experiments reported here, subjects who recalled an average of 2.5 words or less were classified as low span subjects; subjects who recalled 3.5 or more words were classified as high span subjects. Design and analyses. In analyzing reading times, we will refer to three regions of the sentences. For the Main Verb resolution, these regions are indicated with brackets for the sample sentence: The experienced

soldiers [warned about the dangers] [before the midnight]

Region:

1

[raid.]

3

2

The first region introduces the verb, which may be ambiguous, and continues the ambiguity for three more words. The second region contains three words that maintain the ambiguity. The third region is the final one-word disambiguation. The major interest was in whether the time in one or more of these regions would be greater when the verb was ambiguous, such as warned, than when it was unambiguous, such as spoke. Notice that the exact same words (with the exception of the verbs) enter into each region for both the ambiguous and the unambiguous conditions. Consequently, differences between the ambiguous and unambiguous conditions do not entail differences in wording or length. For the Relative Clause resolution, the analysis also focuses on three regions of the sentence, as indicated by the following example: The experienced

soldiers [warned about the dangers] [conducted

Region:

1

the midnight]

[raid.]

2

3

The first region includes the first verb, which may be ambiguous, and continues for three more words. The second region includes the main verb of the sentence, which constitutes the disambiguation for the ambiguous versions of this construction. This verb was always the first word of the disambiguating region and the fourth word from the end of the sentence. The third region is again the last word of the sentence, to maintain a consistent assignment to regions with the Main Verb sentences and also because the last word appears to be the site of much processing activity for these sentences. In the unambiguous version of the Relative Clause sentences, the words who were preceded the first verb and they were not included in any of the three regions that were analyzed. An example of the unambiguous Relative Clause sentence is: The experienced

soldiers who were [to/d about the dangers] [conducted the midnight]

Region:

1

2

[raid.]

3

Each region has the same number of words in the Main Verb and Relative Clause constructions. However, the two resolutions have different structures, and so the reading times for each resolution were analyzed separately. The main analysis concerned the reading times for sentences associated with correctly answered comprehension questions. First, reading times were trimmed by removing any reading time that was more than three standard deviations above the subject’s mean reading time for that word position across all of the sentences, and replacing this time with the cutoff value. This procedure affected 1.6% of the observations in Experiment la and 1.5% in Experiment lb. The same trimming procedure was used in all subsequent experiments, and approximately the same percentage of data was trimmed in each case. Next, the average reading time per word was calculated for the six or fewer sentences associated with correct responses in that condition for each subject. The average times were then aggregated into average reading times per word for the three regions for each construction. The reading times for the two resolutions were analyzed separately, with Region and Ambiguity as

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within-subject factors and Span as a between-subject factor. In Experiment lb, the statistical analysis of the reading times for the Relative Clause sentences omitted one high span subject who made errors answering the questions for all six exemplars of the ambiguous Relative Clause sentences. However, this subject did contribute to the analysis of Main Verb sentences and to the overall analysis of errors. Question-answering errors for both resolutions were analyzed together, with Resolution and Ambiguity as within-subjects factors and Reading Span as a between-subjects factor.

RESULTS AND DISCUSSION Reading span. The range of reading spans was from 1.5 to 5.5 words recalled. In Experiment la, there were 17 low span subjects (spans of 2.5 words and below), 23 medium span subjects (3.0 words), and 10 high span subjects (3.5 words and above). In Experiment lb, there were 11 low and 15 high span subjects. Reading times and comprehension errors. The data in these experiments address two important hypotheses of the model. The first is that at least some individuals construct multiple syntactic representations for syntactically ambiguous sentences. The result that could support this hypothesis is a finding of increased reading time in the ambiguous condition compared to the unambiguous condition. The Main Verb construction is particularly important for this prediction because it most clearly differentiates the predictions of the multiple and single representation models. Most single representation models would not predict any effect of ambiguity in this case, where the preferred resolution is so much more frequent than the unpreferred resolution. The second hypothesis that the data address is that working-memory capacity influences whether the multiple representations are maintained. The prediction is that the high span readers should show a larger effect of ambiguity than the low span readers. Both predictions were supported, as shown in the top of Fig. 1. Figure 1 presents the difference between the average reading times per word for the ambiguous and unambiguous sentences for each of three regions: the first region (the initial verb and the following three words), the second region (the next three words), and the third region (the last word of the sentence). When the curve is above 0, it means that the average reading time per word for the ambiguous sentences was longer than for the unambiguous sentences. The difference scores are used to graphically display the pattern of data. However, the statistical analysis was on the individual condition means and these are given in Appendix B. Main verb resolution. As shown in the top left quadrant of Fig. 1, both predictions of the model are supported in Experiment la for Main Verb sentences. First, the reading times were longer in the ambiguous condition than in the unambiguous condition, F(1,47) = 11.05, MS, = 5585, p

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in

Paragraphs

VERB

High

RELATIVE

CLAUSE

High

1

2 R+Xl

R+Xl

FIG. 1. The diffkences between the average reading time per word for the ambiguous ;I unambiguous sentences for Experiments la (isolated sentences) and lb (sentence in a p;~ graph) for the High, Medium, and Low span groups. The top graphs show that High \ readers have much longer reading times for ambiguous Main Verb sentences than fat unambiguous versions, whereas Low span readers show little or no ambiguity effect. bottom graphs show that all reading groups, but particularly the High span group, I longer reading times for the ambiguous Relative Clause sentences, both when the dr biguation is introduced (in Region 2) and on the last word of the sentence (Region 3).

< .Ol. Second, reading span did influence the ambiguity effect in predicted way. High span readers took relatively more time than low sp. readers on the ambiguous sentences with a Main Verb resolution, partic ularly at the last word of the sentence (Region 3). This pattern resulted in an interaction of Ambiguity x Region x Span, F(4,94) = 4.49, MS, = 3348, p < .Ol, as well as an interaction of Region x Span, F(4,94) = 3.01, MS, = 15,697, p < .05, and an interaction of Ambiguity X Region, F(2,94) = 4.15, MS, = 3348, p < .02. High span subjects spent increasing amounts of time for ambiguous Main Verb sentences (compared to unambiguous sentences) as they progressed through the sentence. Medium span subjects showed smaller increases, and low span subjects showed

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little effect of the ambiguity on reading time. We now examine these effects for the three regions. In the first region, where the ambiguity is introduced, ambiguous sentences took 14 ms longer than unambiguous sentences, but the effect was not statistically significant, F(1,47) = 2.06, MS, = 1496, p = .16. There were no significant differences among the reading times for the three span groups (F < l), which is consistent with the hypothesis that the individual differences among these college students are not general performance differences, but rather they are due to capacity differences which manifest themselves only when the task strains working-memory capacity. In the second region, subjects averaged 17 ms per word more for the ambiguous than the unambiguous Main Verb resolution, resulting in a main effect of Ambiguity, F(1,47) = 6.60, MS, = 1458, p < .05, and no interaction between Ambiguity and Span, F(2,47) = 1.56. This result is compatible with a multiple representation model that suggests that there is a greater burden associated with the ambiguous sentences, and at this point in the processing, the burden seems comparable across the groups. The presence of an ambiguity effect before the disambiguation is difficult to explain by a single representation model. Most single representation models would not predict an ambiguity effect because they would predict that the (ultimately correct) main verb interpretation would have been selected, given its greater frequency and simplicity. An ambiguity effect before the disambiguation is also difficult to accommodate within a probabilistic single representation model which postulates a less frequent interpretation is initially constructed on some proportion of trials. Such models typically attribute ambiguity effects to the error-recovery processes that are initiated on or after a disambiguation that conflicts with the initially selected interpretation. In the third region, where the sentence was disambiguated by the period, reading times on the last word were 46 ms longer in the ambiguous condition, F( 1,47) = 8.21, MS, = 9353, p < .Ol. The increase was largest for the high span readers, resulting in a significant interaction of Ambiguity x Span, F(2,47) = 4.38, MS, = 9353, p < .02. In this third region, both medium and high span readers spent more time in the ambiguous condition (medium: F(1,22) = 4.86, MS, = 6706, p < .05, highs: F(1,9) = 4.58, MS, = 21,931, p = .06). By contrast, low span subjects showed no increase in reading time in the ambiguous condition compared to the unambiguous condition, F( 1,16) < 1. The differences among the three span groups appear to be due to processes that are specifically associated with the ambiguity, rather than some general reading time difference that persists across all of the conditions. The differences among the groups in absolute reading time are minimal and not significant for straightforward sentences. The average

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reading times per word for the unambiguous Main Verb sentences were 411, 431, and 428 ms for low, medium, and high span subjects, respectively. However, as expected from the results reported this far, the average reading times per word for the three groups diverged for the ambiguous sentences: 418, 456, and 488 ms for the low, medium, and high span groups, respectively. There were no significant differences among the three groups for Main Verb sentences overall, F < 1. The major results for Experiment lb, in which the experimental sentences were embedded in paragraphs, replicate those of the primary experiment, and they are shown in the righthand side of Fig. 1. The ambiguous Main Verb sentences again took more time than their unambiguous counterparts, but the difference was significant only at the point of disambiguation, the third region, F(1,24) = 3.84, MS, = 15,439, p = .06, which was reflected in a significant interaction of Ambiguity and Region, F(2,48) = 5.12, MS, = 4455, p < .Ol. High span readers spent 118 ms longer on the last word in the ambiguous condition, F(1,14) = 4.65, MS, = 22,492, p < .05. In contrast, low span readers spent only 18 ms longer on the last word in the ambiguous condition, F < 1. Thus, whether the Main Verb sentences were read as part of a paragraph or in isolation, the high span readers consistently spent more time on the ambiguous sentences compared to the unambiguous sentences. One difference between the paragraph experiment and the isolated sentence experiment is that subjects read faster when the sentences were part of a paragraph (397 ms per word in Experiment lb compared to 437 ms in Experiment la). The fact that the additional time for an ambiguity persists and increases throughout the sentence for high span readers suggests that there is an increasing cost associated with maintaining two representations for the high span readers. The nature of the cost is addressed in more detail later in this section. The fact that the increased time is not particularly associated with the sector in which the ambiguous verb occurs suggests that the cost is not due to the particular linguistic or orthographic properties of the ambiguous verbs. However, this issue will be examined more directly in Experiment 2. An item analysis examined the consistency of the ambiguity effect for the last word (the third region) across sentences by collapsing across the reading times for all of the high span subjects who read the same versions in Experiments la and lb. The reading time for the last word of ambiguous sentences was significantly longer than that for unambiguous sentences, showing that the results do generalize to other sentences with linguistic properties similar to those of the current sentences, t(23) = 1.86, p < .05. We will postpone a more detailed presentation of the item analysis until after Experiment 3, where we present a meta-analysis of the data from all of the experiments. The meta-analysis shows the ambiguity

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effects in both the second and the third regions to be reliable for high span subjects both across Main Verb sentences and across subjects.’ The error rates for the comprehension questions are shown in Fig. 2. The comprehension questions were designed to distinguish the two interpretations of the first verb. According to the model, all groups of readers should tend to maintain the preferred main verb interpretation at a higher level of activation. Consequently, all readers should do fairly well at answering questions about the Main Verb sentences. Thus, for this construction, the three span groups should not be differentially affected by the ambiguity. This prediction was supported by the comprehension results. In both Experiment la and lb, the groups performed within 3% of one another on the comprehension questions for the ambiguous Main Verb sentences. Still, the ambiguous Main Verb sentences did have higher error rates than the unambiguous sentences for all three groups. This decline in comprehension performance with ambiguity is also consistent with the model. It suggests that generating two representations for the ambiguity was demanding enough for the subjects that it affected their ultimate comprehension. The higher error rate for ambiguous Main Verb sentences, even for the low span readers, suggests that they initially represent both interpretations, although the response time data indicate that they do not maintain both interpretations. Relative clause resolution. The model’s predictions for this interpretation of the ambiguity were that all subjects should make comprehension errors because the reduced relative interpretation is strongly unpreferred. However, high span subjects should make fewer errors than low span subjects because high span subjects are more likely to have maintained the correct interpretation, along with the more preferred interpretation. When a subject does correctly comprehend the sentence, reading times should be longer compared to the unambiguous condition. The additional reading time reflects the cost incurred by carrying multiple interpretations, and presumably having to defer other processes. Additionally, the increased reading time may reflect the cost of selecting an unpreferred representation or engaging in error-recovery processes. The comprehension errors, shown in Fig. 2, are consistent with the ’ In this and subsequent item analyses, we have treated sentences as the unit of analysis. However, there are likely constraints on the population of sentences to which the ambiguity effect is likely to generalize. First, the ambiguity effect may depend on properties of the verb. The current experiments have only eight sets of verbs, such as warned/spoke/was told and it is unlikely that the effect will be obtained with all verbs. It is unclear which verb properties may be crucial to obtaining the ambiguity effect, but one likely feature is that the verb not be obligatorily transitive. Second, not all nouns allow the ambiguity. In Experiment 2, we show that the ambiguity effect decreases with proper nouns, presumably because proper nouns exclude the reduced relative interpretation.

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FIG. 2. The error rates for the ambiguous and unambiguous sentences for each span group for Experiments la (isolated sentences) and lb (sentence in a paragraph). As the top graphs show, there is little difference in overall error rates among the three span groups for the highly preferred, Main Verb sentences. However, the ambiguous Main Verb sentences consistently have higher error rates than the unambiguous sentences. By contrast, as the bottom graphs show, all three reading groups, but particularly the Low span readers, make a high proportion of errors on the ambiguous Relative Clause sentences.

model’s predictions. In these two experiments (and in Experiment 3, reported below), the low span readers were close to the guessing rate (50% errors) for the ambiguous Relative Clause sentences (48% in Experiment la and 44% in Experiment lb). In contrast, high span subjects had error rates of 37 and 36% in Experiments la and lb, respectively. Medium span subjects, tested in Experiment la only, were intermediate between the low and high span groups, with error rates of 40% for the ambiguous Relative Clause sentences. The high error rate for ambiguous Relative Clause sentences, compared to the ambiguous Main Verb sentences, resulted in a significant interaction of Ambiguity and Resolution, F( 1,47) = 18.20, MS, = .027, p < .Ol, in Experiment la, and F(1,24) = 13.50, MS, = .025, p < .Ol, in Experiment lb. The three-way interaction, indicating that low span readers had disproportionate difficulty with ambiguous Rel-

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ative Clause sentences, was not significant in Experiment la, (F < I), nor in Experiment lb, F(1,24) = 2.39, MS, = .02, p = .13. Clearly, all three groups had difficulty with the ambiguous Relative Clause sentences, but the low span subjects are at a level that is close to guessing in these two studies and in Experiment 3, reported below. More comprehension errors occurred in conditions that the model predicts would be more capacity demanding. The Relative Clause sentences had more comprehension errors than the Main Verb sentences, F(1,47) = 65.82, MS, = .02, p < .Ol, in Experiment la, and F(1,24) = 33.67, MS, = .Ol, p < .Ol, in Experiment lb. Ambiguous sentences produced more comprehension errors than unambiguous sentences, F(1,47) = 55.36, MS, = .02,p < .Ol, in Experiment la, and F(1,24) = 37.31, MS, = .03, p < .Ol, in Experiment lb. Also, consistent with the model, low span readers made more errors on more difftcult sentences, but there was no reliable main effect of Span overall in either experiment, nor did Span interact with Resolution (F(2,47) = 1.56, MS, = .02, p < .22, in Experiment la, and F < 1, in Experiment lb). Thus, errors were more frequent in more demanding conditions, but typically for subjects at all span levels. On those trials for which a reader maintains the unpreferred reduced relative interpretation, and thus comprehends the ambiguous Relative Clause sentences, the model predicts that the reader should show large increases in reading time if both representations have been maintained. Also, additional time may be taken in selecting an unpreferred representation or engaging in error-recovery processes, if there is no longer any trace of the unpreferred interpretation. However, the entire analysis of the reading times for ambiguous sentences with the Relative Clause resolution must be viewed with caution; the high error rates for the comprehension questions make it uncertain how often the ambiguous sentences were correctly comprehended, particularly by low span readers. As the bottom half of Fig. 1 shows, the reading times were higher for the ambiguous Relative Clause sentences compared to their unambiguous counterparts, overall, F(1,47) = 21.18, MS, = 23,828, p < .Ol. The difference was significant in the second region (at the disambiguation) F(1,47) = 15.65, MS, = 5748, p < .Ol, and in the third region, F(1,47) = 20.21, MS, = 44,349, p < .Ol. The significant effect in the third region shows that there are processing effects at the end of the sentence, even though the disambiguation occurred three words earlier, at a clause boundary. The increasing effect of the ambiguity through the three regions was reflected in an interaction of Ambiguity x Region, F(2,94) = 16.72, MS, = 14,418, p < .Ol. This result did not interact with reading span, although high span subjects did show greater increases in reading time with ambiguity than low span subjects across experiments. The (nonsignificant) difference in reading times across span with the reduced rel-

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ative interpretation was not predicted by the model. The interaction is significant in Experiment lb. As with the Main Verb sentences, there was no indication of large, general differences in reading rate among the three span groups for the Relative Clause sentences (F < 1). In the unambiguous condition, low, medium, and high span subjects’ mean reading time per word was 417, 439, and 455 ms; in the ambiguous condition, these groups had mean reading times of 495, 515, and 561 ms per word, respectively. The results for Experiment lb, in which the Relative Clause sentences were embedded in paragraphs, replicate those of the main experiment. Reading times were significantly longer in the ambiguous condition than in the unambiguous condition, F(1,23) = 9.39, MS, = 18,777, p < .Ol, as shown in the bottom, righthand panel of Fig. 1. In the first region, where the ambiguity was introduced and before the disambiguation, reading times were 22 ms per word longer in the ambiguous condition, F( 1,23) = 6.02, MS, = 933, p < .05. The difference increased to 37 ms per word in the second region, where the disambiguation occurs, F( 1,23) = 7.67, MS, = 2269, p < .Ol, and the difference is maintained on the last word, four words after the disambiguation, F(1,23) = 7.13, MS, = 38,012, p < .05. The increasing effect of ambiguity as the sentence progressed was reflected in an interaction of Ambiguity and Region, F(2,46) = 5.24, MS, = 11,219, p < .Ol. The effect of ambiguity was greater in Region 3, and particularly so for the high span readers, so than Span significantly interacted with Ambiguity and Region, F(2,46) = 3.10, MS, = 11,219, p = .05. Both experiments revealed increased processing times for ambiguous Main Verb and Relative Clause sentences, sometimes before the point of disambiguation, and consistently on and after the disambiguation. In this section, we will consider several processes that might be the source of the increased processing time for ambiguous sentences. The model postulates that working-memory capacity is shared by the entire range of language comprehension processes. If working-memory capacity is sufficiently strained while the multiple representations are maintained, overall processing time could be slowed in those regions. In addition, if maintaining multiple representations of a syntactic ambiguity consumes computational resources that the other processes require, then the presence of an ambiguity could cause the postponement of other comprehension processes. The present research does not specify which processes are postponed. One possibility is that during the maintenance of multiple representations, some higher level processes, such as those that construct a referential representation, are postponed. Readers may postpone representing the objects and events that are being referred to or interrelating them to other preceding information while they are reading in

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the ambiguous region. These processes may be postponed either because there is some inherent limitation on representing mutually conflicting referential representations or because working-memory capacity limits the ability to construct and maintain a referential representation simultaneously with two syntactic representations. Alternatively, higher capacity readers could adopt a “wait-and-see” strategy to determine if one interpretation is more strongly supported by the subsequent context. In any case, the postponement of processing could cause the effects of the ambiguity to persist past the disambiguation because the reader would still have some catching up to do, executing the postponed processes. The catching up hypothesis is consistent with the result that high span readers showed the greatest effects of the ambiguity on the last word of the Main Verb sentences.2 This hypothesis can also be evaluated for the Relative Clause sentences, where the disambiguation occurred in the second region, several words before the end of the sentence. Reading times in the last region of these sentences indicate that the cost associated with processing an ambiguity is not confined to the disambiguating word and there are effects later in the sentence, particularly on the last word. This finding is consistent with the hypothesis that the price for maintaining two syntactic representations is paid when other processes are postponed or slowed because of capacity limitations. The additional processing time for an ambiguous sentence could also be (partially) due to the selection process itself. According to the model, the disambiguating information increases the activation of one interpretation. For the Main Verb sentences, the disambiguation occurs at the last word, so the reader must select the final interpretation at that point. The main verb representation already has a higher level of activation than the reduced relative representation, so the selection may occur when this already active representation reaches some threshold. The processing is somewhat different for the Relative Clause sentences because the relative clause interpretation is unpreferred and should have a much lower level of activation. Consequently, it should take more time after the disambiguation for the less active representation to reach threshold. Thus, the se* Stowe (1990) has recently argued for a capacity-modulated delay model very much in the spirit of the model proposed here. She reports a series of experiments on ambiguities that are resolved with the unfavored interpretation. Subjects showed increased garden pathing on the ambiguities as other processing demands increased (through manipulations of sentence length, as in our Experiment 3, and other manipulations). Stowe suggests that perceivers delay making decisions about ambiguities when they can, but that early decisions can be forced when processing is made more difficult, producing more garden paths for ambiguities that are resolved with the unfavored interpretation. Stowe favored a single representation model, but pointed out that her data were consistent with a multiple representation model like the one proposed here.

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lection process could contribute to the additional processing time for an ambiguity, particularly for a Relative Clause sentence. Another process that could contribute to the ambiguity effect is mental backtracking and reparsing the ambiguous sentence. Overt backtracking to an ambiguity has been documented in eye fixation studies of lexical ambiguities (Carpenter & Daneman, 1981) and syntactic ambiguities (Frazier & Rayner, 1982, 1987). Readers sometimes overtly regressed to an earlier ambiguous word (and reinterpreted it) if a later word or phrase disambiguated it in a way that was inconsistent with the initial interpretation. Unlike an eye fixation paradigm, the Moving Window paradigm does not permit readers to overtly regress earlier words. Nevertheless, a subject could mentally retrieve earlier words and reparse them while viewing a word in the disambiguating region. The proposed model could incorporate backtracking and it seems like a plausible strategy that could be evoked if a low frequency syntactic representation has been abandoned too early (as in the ambiguous Relative Clause sentence). It may also be that backtracking is part of the process of reactivating an unpreferred interpretation. Even if backtracking turns out to be distant from reactivation, its role in processing is likely to be limited to the ambiguous Relative Clause sentences because it is unlikely that readers would prematurely abandon the strongly preferred main verb representation; consequently, there should be no backtracking for the Main Verb sentences. Backtracking plays a more central role in a probabilistic single representation model. In this sort of model, readers are assumed to construct a single representation of an ambiguity, but not always the simpler or more frequent one. It is assumed that on some occasions, readers construct the less preferred interpretation. If it is further assumed that high span readers more often choose the rare interpretation than low span readers, then high span readers would backtrack more often than low span readers if the ambiguity is resolved with the Main Verb interpretation, and high span readers would perform better than low span readers if the ambiguity is resolved with the unfavored, relative clause interpretation. This account can be made to fit the data, but it does so at some cost. First, there is no motivation for assuming that readers with high spans should pick the rare interpretation more often than readers with low spans. Second, backtracking cannot account for the effect of ambiguity before the disambiguation for the Main Verb sentences. This effect in Region 2 for Main Verb sentences was significant in Experiment la, and it is consistent across the experiments as a group. Thus, the probabilistic single representation model has difficulty accounting for this aspect of the data. At this point, we cannot draw strong conclusions about how the ambiguity effects for Relative Clause sentences might be apportioned among

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these various processes, namely, delayed processing, selection of a less preferred interpretation, and/or mental backtracking and reparsing. What these experiments do demonstrate is that there is a cost incurred in processing ambiguous sentences. Also, the cost occurs even for the high frequency interpretation, a result that is incompatible with most single representation models. More importantly for the theory, the additional time for an ambiguous sentence is systematically related to workingmemory capacity, such that people with greater capacity pay a higher cost. The next two experiments eliminate alternative explanations of the ambiguity effect and provide converging support for the interpretation of the effect in terms of working-memory capacity. EXPERIMENT

2

This experiment examined the extent to which the high span subjects’ sensitivity to the temporary ambiguity of Main Verb sentences might be due to a strategy that developed because they noticed that some sentences (11%) in Experiment 1 were resolved as Relative Clause sentences. Experiment 2 tried to eliminate this type of strategic influence by presenting only Main Verb sentences. If the ambiguous Main Verb sentences again produce longer reading times than their unambiguous counterparts, then an experiment-specific strategy is unlikely to be the source of the ambiguity effect in Experiment 1. This experiment also examined whether the ambiguity effect may have been due to some difference in the linguistic or perceptual properties of the different verbs used in the ambiguous and unambiguous Main Verb sentences. For example, it is known that different verbs can take different numbers of arguments (e.g., direct and indirect objects), and there is evidence that verbs with more argument options are more difficult to process than verbs that permit fewer possible argument structures (Shapiro, Zurif, & Grimshaw, 1987). The ambiguous verbs in Experiments la and lb, in addition to permitting reduced relative interpretations, also accept more arguments than their umambiguous counterparts. For example, compare the unambiguous/ambiguous pair went and pushed. The verb went is intransitive and can take no indirect or direct objects. By contrast, pushed can take several types of arguments; it can take a direct object (John pushed the ball), an indirect object following a direct object (John pushed the ball to Mary), and it can be used in a “dative shift” construction in which the indirect object precedes the direct object (John pushed Mary the ball). The ambiguous verbs were chosen because they allow a reduced relative interpretation. However, they could also permit more possible argument structures. Consequently, the increase in reading time in the ambiguous condition could be due to a different sort of ambiguity than was initially postulated.

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To address this second issue, we presented high span subjects with the ambiguous and unambiguous Main Verb sentences used in Experiment la. However, we manipulated the grammatical subjects of the sentences in a way that would distinguish the effect of permitting multiple argument structures from that of permitting a reduced relative interpretation. In half of the sentences, the subject noun phrase was a common noun (e.g., the experienced soldiers), as in Experiments la and lb; in the other half, it was a proper noun (e.g., Colonel Wilson). Sentences with proper noun subjects cannot have a reduced relative interpretation because the proper nouns cannot be modified by relative clauses as common nouns can. Therefore, Colonel Wilson warned about the dangers . . . cannot mean that Colonel Wilson was being warned. If the presence of a reduced relative interpretation is the source of the increased reading times for ambiguous Main Verb sentences, then this effect should be replicated only in the common noun condition, where the reduced relative reading is permissible. The proper noun condition should not produce systematically longer reading time for the ambiguous verb because the proper noun rules out the reduced relative interpretation.3 METHOD Subjects. Because high span subjects showed the most reliable evidence of maintaining multiple representations in Experiments la and lb, only high span subjects were tested in Experiment 2. Fifteen college students were paid $5.00 or given course credit for their participation. Mnrerials and procedure. New versions of the ambiguous and unambiguous Main Verb sentences were developed by replacing the common noun subjects with proper nouns, for example, replacing the experienced soldiers with Co/one/ Wilson. The same sentences were used as in Experiment la, with one exception. The sentences using the verbs fell/dropped were replaced by sentences using forgot/asked because fell/dropped used inanimate subjects (such as the yellow frisbee) that could not be changed easily to proper nouns. Two groups of sentences were composed by randomly assigning six of the common noun versions to one group and their proper noun versions to the other. An additional 48 filler sentences were intermixed, as in Experiment la. Subjects were randomly assigned to the two groups. The procedure was the same as in Experiment la. The data analysis was also the same. Extreme observations were trimmed as in Experiment la and lb; this procedure affected 1.6% of the observations.

3 There is an appositive interpretation of the proper noun sentences, but this interpretation must be indicated with a comma after the proper noun, for example, Colonel Wilson, warned about the dangers, was very upset. The common noun condition could also have this interpretation if the requisite comma were present. Even if subjects failed to note the omission of the comma and considered the appositive interpretation, they should do so equally often for the common and proper noun conditions. Ambiguity effects that are due to the possible appositive interpretation should not differentially affect the proper noun condition and the common noun condition.

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3. The differences between the average reading time per word for the ambiguous and unambiguous sentences for each of the three regions of the Main Verb sentences with common nouns and proper nouns. Only high span readers were in this experiment. Sentences with common nouns as grammatical subjects show a larger ambiguity effect than the sentences with proper nouns, which do not permit a relative clause reading. FIG.

RESULTS AND DISCUSSION

Reading times for the unambiguous and ambiguous sentences with their common and proper noun subjects are shown in Fig. 3. Overall, for sentences with common nouns, the sentences with ambiguous verbs took more time than those with unambiguous verbs, even though there were no relative clause sentences in the experiment, F(1,14) = 12.68, MS, = 4309, p < .Ol. As in Experiments la and lb, the Ambiguity effect increased through the sentence, resulting in an interaction of Ambiguity and Region, F(2,28) = 5.31, MS, = 2812, p = .Ol. This replication suggests that the ambiguity effect for Main Verb sentences in Experiment la and lb is not attributable to a strategy that high span subjects developed as a consequence of encountering the reduced relative clause sentences in the experiment. The ambiguity effect was larger for the sentences with common nouns as grammatical subjects than sentences with proper nouns. The common noun sentences permitted the possibility of a reduced relative clause interpretation. By contrast, for the proper noun sentences, there was no possibility of a reduced relative clause interpretation and the reading times for the ambiguous verb condition did not differ significantly from those for the unambiguous verb condition, F < 1. This pattern of reading times produced a significant interaction of Ambiguity and Type of subject noun, F(1,14) = 5.48, MS, = 4568, p < .05, when both sentence types are analyzed together. This attenuation in the ambiguity effect is what would be expected if the ambiguity effect for Main Verb sentences in the previous experiments were due to the possibility of the reduced relative interpretation. Consequently, this experiment replicates the previous

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studies and indicates the ambiguity effect is not due to some extraneous linguistic property of the ambiguous verbs. To examine the robustness of the effects across the sentences, we collapsed across subjects. In this analysis, ambiguous sentences took significantly longer than unambiguous ones, F(1,22) = 4.73, MS, = 27,900, p < .Ol, and the ambiguity effect was larger in the third region, resulting in an interaction of Ambiguity x Region, F(2,44) = 3.60, MS, = 21,826, p < .05.4 When the ambiguity effect was tested in each of the three regions individually, it was not significant in Region 1 (t < l), nor in Region 2 (t(22) = 1.46), but it was in Region 3 (t(22) = 2.44). Sentences with common nouns had larger ambiguity effects than the sentences with proper nouns; this was reflected in an interaction of Ambiguity x Type x Region, F(2,44) = 1.81, MS, = 11,286, p = .18; however, the difference was only significant for the third region, the last word in the sentence t(22) = 1.91, p < .05. This item analysis shows that the ambiguity effect generalizes across sentences and that the decrease in the ambiguity effect when proper nouns replaced the common nouns was general across sentences. Because of the lack of relative clause sentences, the overall error rate was low, only 6%. There was no significant effect of the Type of noun (F < l), nor was there any interaction between Type of noun and Ambiguity (F(1,14) = 1.92, ns); ambiguous sentences did have a higher error rate (8%) than unambiguous sentences (4%), F(1,14) = 5.79, MS, = .005, p < .05.

In summary, the ambiguity effect in Main Verb sentences appears to be due to the possibility of the reduced relative interpretation, rather than some extraneous difference between ambiguous and unambiguous verbs. The ambiguity effect for verbs like “warned” is lessened if other aspects of the sentence, such as the subject noun phrase, eliminate the ambiguity. Moreover, the study suggests that the Ambiguity effect in Experiments la and lb did not reflect some experiment-specific strategy that arose from encountering reduced relative clause sentences in that particular context. EXPERIMENT

3

This experiment examined whether multiple representations are maintained by the high span subjects when the distance between the ambiguous verb and the disambiguation is increased by several words. If a long ambiguous region allows the high span readers to abandon the unpreferred interpretation of a syntactic ambiguity, then their reading time patterns should begin to resemble those of the low span readers. By 4 We were able to analyze only 23 of the 24 sentences because all of the subjects in one condition missed the comprehension question for 1 of the sentences.

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contrast, in the case of a short ambiguous region, high span readers should show an effect of ambiguity, replicating the results in Experiments la, lb, and 2 for the Main Verb sentences. To test this prediction, we created slightly longer versions of the sentences used in Experiment la by increasing the number of words in the ambiguous region that immediately followed the ambiguous verb (by two words). This long sentence condition was compared to a short sentence condition, in which the ambiguous region had one fewer word than that in Experiment la. Examples of the longer and shorter versions for both the Main Verb sentences and Relative Clause sentences are given in Table 3. The prediction for the Relative Clause sentences remains essentially unchanged from Experiment la. It is predicted that both groups, but particularly the low span readers, will have difficulty correctly interpreting ambiguous Relative Clause sentences. Given the high rate of errors, it is unlikely that the length manipulation can further decrease the accuracy with which readers answer the questions for these sentences. METHOD Subjects. Thirty-nine undergraduates enrolled in psychology courses participated for course credit. Of these, 24 were classified as low span and 15 as high span. Materials and procedure. The stimuli were identical to the sentences used in Experiment la, with the exception of the length manipulation. Region 1 had three words in the short condition (e.g., warned about attacks) and six words in the long condition (e.g., warned about surprise enemy guerrilla attacks) for both Main Verb and Relative Clause sentences. Regions 2 and 3 remained the same as in Experiment la. Because of the addition of another variable (sentence length), there were only three exemplars in each condition rather than the six that were used in Experiment la. To balance all of the conditions, we constructed eight sets of sentences, with each sentence frame occurring once in each set in a different condition. The same number and type of tiller sentences were used as in Experiment la. Subjects were randomly assigned to the eight sets. The procedure within an experimental session was identical to that of Experiment la. All outlier observations were trimmed as in the previous experiments, which affected 1.7% of the observations. One low span subject did not enter into the statistical analysis for

TABLE 3 Examples of Short and Long Sentence Versions-Experiment

3

Main verb resolution-Unambiguous Short version: The experienced soldiers spoke about attacks before the midnight raid. Long version: The experienced soldiers spoke about surprise enemy guerrilla attacks before the midnight

raid.

Relative clause resolution-Unambiguous Short version: The experienced soldiers who were told about attacks conducted the midnight raid.

Long version: The experienced attacks conducted

soldiers who were told about surprise enemy guerrilla the midnight raid.

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the reading times for the Main Verb sentences because of errors on all three exemplars in a condition; similarly, six low span and two high span readers did not enter into the statistical analysis for the reading times for the Relative Clause sentences because of errors on all three exemplars in a condition. However, the main reading time effects are similar whether or not these subjects are included. Also, all of the subjects were included in the statistical analysis of error rates.

RESULTS AND DISCUSSION

As shown in the upper left quadrant of Fig. 4, the major results replicate Experiments la, lb, and 2. The reading times for ambiguous Main Verb sentences were significantly longer for the high span readers, but not for the low span readers, resulting in an interaction of Ambiguity x Span, F(1,36) = 4.09, MS, = 13,564, p = .05. Also, as predicted, the longer sentences produced different results from the shorter sentences. The longer ambiguous region was intended to make it more difficult for the high span readers to maintain both interpretations of the ambiguity. The high span readers did not show as large an ambiguity effect for the third region of the longer sentences, suggesting that the two interpretations were not maintained for the full duration; however, the interaction of Ambiguity and Length was not reliable, F(1,36) = 1.70, MS, = 20,974, p = .20. The ambiguous Main Verb sentences did not take significantly longer overall, F(1,36) = 1.33. No other effects were significant, except for the obvious differences among the reading times in the three regions, F(2,72) = 54.18, MS, = 28,657, p < .Ol. To examine whether the increase in sentence length decreased the ambiguity effect if sentences are considered as the unit of analysis, we analyzed the reading times for the long and short sentences, collapsing across high span subjects who read the same stimulus sentences. In this analysis, the decrease in the ambiguity effect (particularly for Region 3) with an increase in the sentence length was marginally significant, as reflected in the interaction of Ambiguity X Length x Region, F(2,42) = 2.73, MS, = 16,430, p = .07.5 With the exception of the effect of differences in reading times among the three regions, no other main effects or interactions approached significance. Thus, the decrease in the ambiguity effect with sentence length was consistent with the hypothesis, but not statistically robust. Both reading groups took more time for the ambiguous Relative Clause sentences, so that there was a main effect of Ambiguity, F(1,29) = 10.60, MS, = 38,683, p < .Ol, and no significant interaction with Span (F(1,29) 5 We were able to analyze only 22 of the 24 sentences; there were very few subjects per condition and it so happened that all of the subjects in one condition made errors on 1 sentence in the short sentence condition and a different sentence in the long sentence condition.

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FIG. 4. The differences between the average reading time per word for the ambiguous and unambiguous sentences for High and Low span readers. High span readers have longer reading times for the short ambiguous Main Verb sentences (shown on the left), but not for the long versions (shown on the right). This result indicates that the High span readers do not indefinitely maintain the unpreferred interpretation of the ambiguous verb. The bottom graphs show that all reading groups have longer reading times for the ambiguous Relative Clause sentences. For short sentences, the ambiguity effect is present when the disambiguation is introduced (in Region 2), but it is even more pronounced three words later at the end of the sentence (Region 3).

< 1). This result replicates the earlier studies and it is consistent with the hypothesis that it takes additional time to sufficiently activate the unpreferred relative clause representation to a threshold so that it is selected as the interpretation of the sentence. Moreover, this prediction applies to both high and low span readers for those trials on which Relative Clause sentences are correctly comprehended. High span readers took a particularly long time at the end of short, ambiguous Relative Clause sentences, resulting in a significant interaction of Ambiguity X Region, F(2,58) =

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3.33, MS, = 27,554, p = &I. This result is consistent with the idea that the capacity of the high span readers is strained even four words after the disambiguation. However, the high span readers did not show a comparable increase for the longer sentences, which resulted in an interaction of Ambiguity x Length x Region, F(2,58) = 3.69, MS, = 19,569, p < .03, and a marginal four-way interaction with Span, F(2,58) = 2.74, MS, = 19,569, p = .07. Overall, the high span subjects made fewer errors than the low span subjects, as shown in Fig. 5. Across all the conditions, there was a main effect of Span, F( 1,37) = 6.21, MS, = .033, p < .02. The low span readers made many comprehension errors for the relative clause sentences, although the interaction of Resolution x Span was not significant, F(1,37) = 1.51. Both groups made more errors on the ambiguous Relative Clause Short

Long Sentences

Sentences MAIN

Unambig

VERB

Unambig

Ambig RELATIVE

Ambig

CLAUSE

60 LOW

60 40 e e ; 30

High

High

6” 20 10

I

I Unambig

I Ambig

I

I Unarnbig

I Ambig

FIG. 5. The error rates for the ambiguous and unambiguous sentences for the High and Low span groups. The top graphs show that there is relatively little difference in overall error rates between the two span groups for the Main Verb sentences. However, the ambiguous sentences do consistently have higher error rates than the unambiguous sentences. The bottom graphs show that all reading groups, but particularly the Low span readers, make a large proportion of errors on the ambiguous Relative Clause sentences.

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sentences compared to the ambiguous Main Verb sentences, resulting in an interaction of Ambiguity and Resolution, F(1,37) = 5.94, MS, = .053, p < .02. Length had no significant effect on question-answering accuracy, nor did it interact with the other factors (F < 1). However, the lack of effect is difficult to interpret because error rates are a less sensitive measure than reading times. In summary, the results for high span readers show a robust ambiguity effect for shorter Main Verb sentences and, as predicted, less of an ambiguity effect for longer sentences. However, the interaction is not statistically reliable. The addition of two words in the longer sentences may be too minimal a change to reliably cause high span readers to abandon the second interpretation. Thus, further research is needed to adequately test the hypothesis that high span readers will abandon the second interpretation if their capacity is strained (and the intervening material is not biased toward the less favored interpretation). Meta-analysis. To gauge the reliability of the ambiguity effect and to take advantage of the conceptual replications across experiments, we examined all of the reading time data in a series of meta-analyses. These analyses evaluated the consistency of the response time patterns across subjects (within each span group) and across stimulus sentences, by including Experiments la, lb, 2, and 3 (the short sentence condition) as levels of a factor in a series of ANOVAs. One analysis examined the results for all three sentence regions and follow-up analyses evaluated each region individually. Tables 4 and 5 report the statistical results for the Ambiguity factor and any other effect that was statistically reliable across sentences or subjects. Any main effect or interaction that is not listed was not statistically reliable. A summary of the results for the Main Verb sentences for high span subjects, shown in Table 4, is that both the second and third region of ambiguous sentences take reliably longer to process than their unambiguous counterparts. The effect is reliable across sentences as well as across high span individuals.6 Thus, both maintaining and wrapping-up or resolving the temporarily ambiguous Main Verb sentence take more time than that needed for its unambiguous counterpart. The presence of a significant ambiguity effect in the second region, before the disambiguation, is consistent with the model we have presented. By contrast, it is much less compatible with a single interpretation model that postulates reinterpretation on or after the disambiguation. The main effect of region, ’ The differing degrees of freedom in the meta-analysis reflect the differing amounts of missing data due to comprehension errors. For the high span subjects, for the fell/dropped sentences, we estimated the reading times in the common noun condition of Experiment 2, with the times obtained for correctly comprehended forgot/asked sentences.

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TABLE 4 Main Verb Sentences Analysis

df

Factor

F-ratio

MS,

P

Sentence analyses: High span subjects“ All regions

Region 1 Region 2 Region 3

Ambiguity Ambiguity Region Ambiguity Ambiguity Ambiguity

1,22 x

region

2344 244 1,22 1,22 1,22

4.15 4.11 71.42
66,633 32,565 35,668

= .05 C.05 <.Ol

5,953 114,855

C.05 C.05

12,458 7,146 23,950 1,488 1,346 23,915

C.01 <.oi <.Ol C.15 <.Ol c.01

5,712 7,013

C.12 <.Ol

6,450 8,039 8,214

C.02 ‘C.17 <.06

25,547 2,153

C.12 C.13

Subject analyses: High span subjects0 All regions Region 1 Region 2 Region 3

Ambiguity Ambiguity x region Region Ambiguity Ambiguity Ambiguity

1,Sl 2,102 2,102 1.51 1,51 1,51

18.32 13.69 83.48 2.21 13.72 16.81

Sentence analyses: Low span subjectd’ All regions

Region 1 Region 2 Region 3

Ambiguity Ambiguity x region Region Ambiguity Experiment Ambiguity Experiment Ambiguity

1,22

2744 244 1,22 1,22 1.22 1,22 1,22

-Cl 2.27 81.65 il 7.91 2.03 4.30
Subject analyses: Low span subjectsb All regions Region 1 Region 2 Region 3

Ambiguity Ambiguity Experiment Ambiguity Ambiguity

I,48 1.48 1,48 1,48 1,48

-Cl
u Experiments la, lb, 2 (common noun), 3 (short sentences). b Experiments la, lb, 3 (short sentences).

noted throughout these experiments, reflects the fact that the reading time on the last word is generally longer overall than the reading times on the other regions of the sentence. In contrast to the analysis for the high span subjects, the low span subjects did not show any reliable ambiguity effects either when the data were analyzed across sentences or across subjects. Of less theoretical interest are the significant differences in the mean reading times for the three experiments, primarily due to the point, noted earlier, that subjects

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5

Relative Clause Sentences Analysis

Factor

df

F-ratio

MS,

P

Sentence analyses: High span subject@ All regions

Region I Region 2 Region 3

Ambiguity Ambiguity x region Region Ambiguity Ambiguity Ambiguity

1,16 2,32 2,32 1,16 I,16 1,16

22.58 25.45 45.19 3.89 11.10 25.99

67,280 27,829 77,917 6,412 7,924 108,602

c.01

34,628 26,336 45,738

<.Ol <.Ol <.Ol

5,752 78,043

c.01 <.Ol

27,836 10,724 13,413 85,877

<.03 <.07 c.01 <.03

15,232 8,446 14,407 32,111

<.Ol ‘Z.04 <.Ol C.04

24,621 14,965 19,913 2,056 7,984 44,512

<.Ol <.12 <.Ol < .02 <.Ol <.03

<.Ol <.Ol <.07 <.Ol c.01

Subject analyses: High span subjects’ All regions

Region 1 Region 2 Region 3

Ambiguity Ambiguity x region Region Ambiguity Ambiguity Ambiguity

1,35 2,70 2,lO 1,35 1,35 1,35

21.09 16.25 55.12
Sentence analyses: Low span subject? All regions

Region 1 Region 2 Region 3

Ambiguity Ambiguity X region Region Experiment Ambiguity Experiment Ambiguity Experiment Ambiguity

1,15 2,30 2.30 l,l5 1.15 1,15 1,15 1,15 1,15

6.12 2.94 53.52 5.95
Subject analyses: Low span subjectsb All regions Region 1 Region 2 Region 3

Ambiguity Ambiguity x region Region Ambiguity Ambiguity Ambiguity

1,46 2,92 2,92 1,46 1,46 1,46

11.08 2.18 49.13 6.25 12.64 5.04

0 Experiments la, lb, 2 (common noun), 3 (short sentences). b Experiments la, lb, 3 (short sentences).

read the sentences faster when they were embedded in paragraphs. (Note that the analyses for low span subjects involve data from Experiment la, lb, and 3 because Experiment 2 involved no low span subjects.) The analyses of Relative Clause sentences, in Table 5, demonstrate that the ambiguity effect is robust across sentences and across subjects (within span grouping) in the second region (where the sentence is disambigu-

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ated), and in the third region (where the interpretation is wrapped up) for both high span and low span subjects. There is some indication of the effect in the first region (where the ambiguity is introduced). However, as we emphasized earlier, the high error rate associated with these sentences makes their interpretation more tentative. The overall conclusion from these meta-analyses, is that the ambiguity effect is robust for both the Main Verb and the Relative Clause sentences. GENERAL

DISCUSSION

These experiments support the hypothesis that a reader’s workingmemory capacity, as assessedby the reading span task, can influence the processes that are executed to understand a syntactically ambiguous sentence. The model emphasizes two aspects of processing that are outside the scope of most syntactic processing models, namely, working-memory capacity and individual differences. The model proposes that multiple representations are constructed for syntactic ambiguities. Consequently, even a construction as common as a Main Verb sentence can reveal ambiguity effects in the reading times and error rates. Several converging experiments support the model. The ambiguity effects were present when the sentences were presented individually (Experiments la, 3) and when they were part of a paragraph (Experiment lb). Moreover, the ambiguity effect was found for Main Verb sentences when there were no relative clause sentences in the experiment (Experiment 2), suggesting that the effect is not due to a strategy that developed in response to the experimental task. The ambiguity effect appears to be due to the possibility of a reduced relative interpretation because the effect disappears if the ambiguity is eliminated by using a proper noun as the grammatical subject of the sentence (Experiment 2). Second, the model proposes that working-memory capacity is a major determinant of whether multiple representations are maintained. Thus, readers with high reading spans can maintain multiple representations for longer periods of time than low span readers. The strongest support for this aspect of the model comes from the consistent individual differences in reading times and errors in Experiments la, lb, and 3. The fact that both high span readers and low span readers have more errors for ambiguous sentences, even ambiguous Main Verb sentences, suggests that both groups are sensitive to the ambiguity and at least initially represent both interpretations. However, the low frequency interpretation may quickly become inactive for the low span reader. To be sure, even high span readers do not maintain multiple interpretations indefinitely. The reading times for long sentences (in Experiment 3) suggest that high span readers do not maintain the alternative interpretation throughout the nineword ambiguous region of the long Main Verb sentences; however, the

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lack of statistical reliability of the interaction means that this result requires further empirical support. The model also proposes that the maintenance of multiple representations interacts with other processes that draw on working-memory resources, so that capacity consuming processes, such as maintaining multiple syntactic representations, extract cost. This explanation accounts for the counterintuitive result that high span readers show longer reading times than low span readers (Experiment la, lb, and 3). Finally, if capacity limitations disallow the maintenance of multiple interpretations, it is the less frequent, less simple, and/or less contextually supported interpretation that is likely to be abandoned. Consequently, the relative clause interpretation of ambiguous sentences is less likely to be maintained if the demands of sentence processing exceed working-memory capacity. This aspect of the model accounts for the consistently higher error rates for the ambiguous Relative Clause sentences than for the ambiguous Main Verb sentences (in Experiments la, lb, and 3). The research highlights the importance of individual differences in accounts of language comprehension and, in particular, the role of workingmemory capacity. In earlier work, we have shown that individual differences in working-memory capacity are systematically related to language processes at a variety of levels, such as the computation of pronominal reference (Daneman & Carpenter, 1980), the reinterpretation of lexical ambiguities (Daneman & Carpenter, 1983), lexical access (Carpenter & Just, 1988), and the comprehension of syntactically complex sentences, such as center-embedded sentences (King & Just, 1991). The current studies support and extend this analysis by demonstrating that individual differences in working-memory capacity can produce different parsing outcomes for syntactically ambiguous sentences, which in turn can lead to differences in comprehension. Previous syntactic ambiguity research. The model proposed here unifies the previously disparate single and multiple representation models of parsing and points to the adaptability of the parsing mechanisms to the availability of memory resources. This section compares the Capacity Constrained Parsing Model to other models that have been postulated for the processing of syntactic ambiguity, such as single representation models, delay models, and multiple representation models. There have been several different single representation models (e.g., Frazier, 1979; Marcus, 1980). In one such model, it was proposed that the selection of the single representation was made on the basis of syntactic simplicity, with no input from nonsyntactic sources, such as the semantics of the verbs or pragmatic plausibility of the sentence (Frazier, 1979). Although there has been some controversy over this work, the disagreement has primarily focused on whether different ambiguous verbs could

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differ in their preferred resolutions (e.g., Ford, Bresnan, & Kaplan, 1982; Holmes, 1984), but it did not challenge the single representation aspect of the model. The predictions of the single representation model were supported in several eye fixation studies that found that the duration of on-line processing was influenced by syntactic heuristics, such as minimal attachment (Frazier & Rayner, 1982; Ferreira & Clifton, 1986). In the current view, such findings may partially result from sampling a large proportion of medium and low span individuals. Such readers are more capacity limited and, consequently, will conform more closely to the predictions of a single representation model. When individual differences are systematically examined, it becomes clear that such models have difficulty accommodating the range of parsing procedures that different individuals use. On the other hand, Frazier’s finding that syntactic factors (such as structural simplicity) influence the processing of a syntactic ambiguity is also consistent with the current model. In the current model, syntactic factors, as well as frequency of usage, semantic factors, and pragmatic factors, can influence the relative activation of the alternative interpretations of an ambiguity. In addition to single representation models, another option is a delay model, in which a syntactic ambiguity is noted, but neither representation is constructed until disambiguating information is encountered. Such a model has been proposed for other types of ambiguities (e.g., Chodorow, Slutsky, & Loring, 1988; Weinberg, 1988). These models do not claim that multiple syntactic representations are actually constructed for the ambiguity. The challenge for such models is to specify how the disambiguating information is identified and used if no syntactic representations have been constructed. Empirically, a pure delay model is likely to have difficulty accounting for the individual differences that we have documented in the current series of studies. Also, such models cannot easily explain the presence of an ambiguity effect before the point of disambiguation. It is not obvious that delaying a parsing decision should take more time than parsing an unambiguous sentence. Some researchers have even suggested that delaying the parsing process would speed processing in the ambiguous region relative to an unambiguous control (Frazier & Rayner, 1987). A delay model can be made more compatible by altering it to hypothesize that information at the point of the ambiguity is buffered for a limited time, but not necessarily until the point of disambiguation. Then the model is transformed into a version of the wait-and-see strategy, as described under Experiment 1. The current model can be compared with an earlier model that two of us proposed for the processing of lexical and syntactic ambiguities (Just & Carpenter, 1980, 1987). In that model, like the current one, multiple representations are initially constructed and each is weighed according to

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factors such as frequency and contextual support. However, in contrast to the current theory, the earlier model proposed that subjects immediately selected and retained only a single representation, namely, the most highly weighted one. The model was supported by eye fixation data that showed both the relative frequency of two interpretations and the prior semantic context influenced by the interpretation of lexical and syntactic ambiguities (Carpenter & Daneman, 1981; Daneman & Carpenter, 1983). Moreover, readers showed clear evidence of being “garden pathed” if later information in the sentence was inconsistent with the prior bias. The previous model corresponds approximately to the low-span version of the Capacity Constrained Parsing Model. Although single representation models of syntactic ambiguity resolution have predominated in recent years, early ambiguity research produced a variety of evidence for multiple representation models. It was pointed out by Garrett (1970) that studies which supported a multiple representation model tended to measure the effect close to the time that the ambiguity was presented. By contrast, studies that did not support the multiple representation model tended to assess the effects later, often at the end of the sentence. This observation is also accounted for by the Capacity Constrained Parsing Model, which postulates that multiple representations are initially represented, but the activation level of the unpreferred interpretation may be abandoned as capacity constraints take effect. Several early studies compared unambiguous sentences and easily resolved ambiguities, using the same logic advocated here, namely, that increases in response times in the presence of ambiguity are more compatible with a multiple representation model. One study found that subjects required longer to complete ambiguous sentence fragments than unambiguous ones (MacKay, 1966) and longer to detect a target phoneme immediately after an ambiguous phrase (Foss, 1970).7 In a task in which subjects arranged words on cards to form sentences, solution time increased when the verb in the sentence was syntactically ambiguous (Bever, 1970; Fodor, Garrett, & Bever, 1968). Similar results have been found with a sentence recall task (Chodorow, 1979; Holmes & Forster, 1972)and more recently with a processing load measure immediately after the verb (Shapiro et al., 1987; but see Hakes, 1971, who found some evidence of reduced paraphrase accuracy for temporarily ambiguous sen’ Newman and Dell (1978) have pointed out that some studies of ambiguity using the phoneme monitoring task contained a confound, so that the effects attributed to ambiguity in these studies were probably artifactual. However, Newman and Dell note that Foss (1970) used a different design that did not produce the confound that was present in other phoneme monitoring studies.

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tences, but no effect on phoneme monitoring times immediately after the verb). Many early findings led researchers to propose the first versions of multiple representation models of ambiguity resolution, on the view that a single representation model could not account for added difficulty in the face of a syntactically ambiguous verb. Further evidence for a multiple representation model was garnered with an experimental’ paradigm that tried to influence the interpretation of an ambiguity during processing (Lackner & Garrett, 1972). Subjects attended to sentences that contained syntactic ambiguities presented in one channel of a headphone, while disambiguating information was presented in the unattended channel. The results showed that the disambiguating information biased the subjects’ interpretations of the ambiguity, even when the disambiguating information was presented after the ambiguous phrase. Lackner and Garrett argued that this result supports a multiple representation model of syntactic ambiguity resolution, because it is difficult to explain how biasing information presented after the ambiguity could affect the ambiguity’s interpretation unless multiple representations of the ambiguity were initially constructed and evaluated against the context. However, these results were not replicated in a similar paradigm by MacKay (1973), who found biasing effects from the unattended channel only for lexical ambiguities, not syntactic ambiguities. In contrast to this work, experiments using the sentence verification paradigm have not lent support to a multiple representation model. These studies have found increased reaction times to ambiguous sentences compared to unambiguous ones only when a picture of the unfavored meaning of the ambiguity was presented (Foss, Bever, & Silver, 1968) or when subjects were aware of the ambiguities (Carey, Mehler, & Bever, 1970). Such results suggest that ambiguous sentences are processed in the same manner as unambiguous sentences, as would be predicted by a single representation model. However, such results are also consistent with the idea that the multiple representations are somehow resolved by or at the end of the sentence (Garrett, 1970). This latter interpretation of the data led Bever, Garrett, and Hurtig (1973) to propose a model that shares several features of the one advocated here. They suggested that comprehenders initially construct multiple representations of syntactically ambiguous phrases and later abandon all but one interpretation. In a fragment completion task, they tested their hypothesis by manipulating the point at which subjects had to begin their completion. Completion time increased for some types of ambiguous fragments when the completion began within the same clause as the ambiguity, but not when the completions began the following clause, even though the sentence had not been disambiguated at that late point. Bever et al. argued that these results indicate that multiple representations are initially

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constructed for the ambiguity, resulting in added difftculty at the early completion position. They suggested that when a clause boundary is reached, all but one representation of the ambiguity are abandoned, so that the processing of ambiguous sentences does not differ from that of unambiguous sentences after this point. Like the current model, Bever et al. suggest that there are initially multiple representations. However, the two models differ on how they conceptualize the constraints on the maintenance of the multiple representations.8 In the Bever et al. model, reaching a clause boundary was the trigger to abandon representations. By contrast, the current model proposes that the individual’s working-memory capacity, bias information, and the capacity demands of the ongoing comprehension process determine which alternative interpretations are maintained and for how long. Our own explanation captures the Bever et al. results as well as the additional findings that we have presented in this paper. We would expect that by the late completion point in the Bever et al. study, most of their subjects had abandoned multiple representations because of memory load, not necessarily because they had reached a clause boundary. The current experiments have furthermore demonstrated that processing difficulties from ambiguity can continue after a clause boundary (in the Relative Clause sentences in Experiments la, lb, and 3). These data indicate that the clause boundary is not the absolute trigger that Bever et al. hypothesized. Nevertheless, clause and sentence boundaries do appear to be the site of considerable processing (Just & Carpenter, 1980; Daneman & Carpenter, 1983), which may decrease the ability to maintain multiple representations. Finally, the strongest evidence in favor of the Capacity Constrained Parsing Model is the consistent relation between reading span and the effects of ambiguity. Recently, difficulties with a strict single representation view have led to new proposals for multiple representation models. Kurtzman (1985) argued for a model in which multiple representations were constructed for a syntactically ambiguous structure until pragmatic information guided the choice of one representation (see also Crain & Steedman, 1985; Altmann & Steedman, 1988). Gorrell (1987) proposed a similar model with s Bever et al. (1973) classified ambiguities into several broad groups. In the category of “bracketing ambiguities,” they included main verb/reduced relative ambiguities, along with modification ambiguities such as o/d men and women. They did not find significant increases in completion time for bracketing ambiguities, but it is impossible to tell whether all types of ambiguities in this diverse class produced the same performance. They did find significant increases in completion time for a class they called “underlying ambiguities.” These included items such as, the shooting of the Indians . and John is quick to please. . In these, the ambiguity hinges on whether the noun is the agent or patient of the action. This is a feature which these ambiguities share with the main verb/reduced relative ambiguities.

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syntactic complexity as the major guide in the choice of representations, while Tanenhaus and Carlson (1989) have suggested that a limited form of multiple representations are generated through the access of the various thematic structures allowed by ambiguous verbs. Our own model is in the same spirit as these recent proposals, but it differs in its greater attention to the claim (common to all multiple representation models) that limitations on working-memory capacity cause the alternative representations to lose activation. the model proposed here has made much more precise predictions about the causes of increased difficulty with ambiguity and about the time at which these effects will be manifested, and it makes a number of interesting claims about the relationship between available memory capacity and language comprehension procedures. Another more minor difference between the current model and some of the other multiple representation models is that we have suggested that a variety of factors determine the degree to which one representation is favored over others, including verb based preferences and general plausibility of the alternate representations. The issue of whether different types of information influence interpretation is still an active area of research (e.g., Ferreira & Clifton, 1986; Taraban & McClelland, 1988). The present model is not the first to propose that multiple representations are constructed for a syntactic ambiguity, and our model joins many other multiple representation models in proposing a limit to the number of alternatives that can be maintained concurrently. Where the present model differs from these precursors is the verification of the central role of working-memory capacity in limiting multiple representations, the processing costs resulting from carrying multiple representations, and the demonstration of differences among individuals in their sensitivity to ambiguity as a function of their working-memory capacity for language. APPENDIX Sentences

A

Used in Experiments

1 a and 1 b

Verb: warned, spoke, who were told l The experienced soldiers warned/spoke about the dangers before the midnight raid. l The experienced soldiers warned/who were told about the dangers conducted the midnight raid. l The cotton farmers warned/spoke about bad floods just before harvest time. l The cotton farmers warned/who were told about bad floods had no other crops.

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l Several angry workers warned/spoke about low wages during the holiday season. l Several angry workers warned/who were told about low wages decided to file complaints.

Verb: pushed, went, who was shoved l The frightened kid pushed/went through the crowd to the front row. l The frightened kid pushed/who was shoved through the crowd got separated from Jane. l A small dog pushed/went through the fence into the chicken coop. l A small dog pushed/who was shoved through the fence hurt his hind leg. l An impatient shopper pushed/went through the doors to the sales racks. l An impatient shopper pushed/who was shoved through the doors complained to the manager. Verb: served, ate, who was fed l The evil genie served/ate the golden figs in the ancient temple. l The evil genie served/who was fed the golden figs went into a trance. l The kitchen staff served/ate in the cafeteria after the executives finished. l The kitchen staff served/who were fed in the cafeteria soon got very sleepy. l The sunburned boys served/ate the hot dogs at the football stadium. l The sunburned boys served/who were fed the hot dogs got a stomach ache. Verb: called, giggled, who were reprimanded l The silly boys called/giggled during the play until the teacher arrived. l The silly boys called/who were reprimanded during the play quickly left the auditorium. l The thoughtless secretaries called/giggled on the balcony when the parade passed. l The thoughtless secretaries called/who were reprimanded on the balcony returned to their desks. l The teenage girls called/giggled in the hallway while the principal watched. l The teenage girls called/who were reprimanded in the hallway answered the principal rudely. Verb: washed, cried, who was bathed l The sick child washed/cried early every morning in the hospital room.

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l The sick child washed/who was bathed early every morning wanted her rubber duck. l The calico cat washed/cried in the alley after drinking the milk. l The calico cat washed/who was bathed in the alley ran into the street. l The Indian children washed/cried in the stream after the mothers left. l The Indian children washed/who were bathed in the stream splashed and shouted loudly. Verb: watched, sang, who were seen l The young children watched/sang in the hallway while the adults argued. l The young children watched/who were seen in the hallway were following the adults. l The brown sparrow watched/sang on a branch high above the cat. l The brown sparrow watched/who was seen on a branch pecked at an insect. l The convicted criminal watched/sang in the cell during the parole hearing. l The convicted criminal watched/who was seen in the cell plotted a daring escape. Verb: dropped, fell, that was thrown l A yellow frisbee dropped/fell from the roof onto the narrow driveway. l A yellow frisbee dropped/that was thrown from the roof landed in the ditch. l The large package dropped/fell from the plane into the dark jungle. l The large package dropped/that was thrown from the plane hit several tall trees. l Many small stones dropped/fell from the cliff during the fierce storm. l Many small stones dropped/that were thrown from the cliff damaged passing cars below.

Verb: taught, learned, who was shown l The older kids taught/learned all the dances for the spring recital. l The older kids taught/who were shown all the dances were in the recital. l The young technician taught/learned the computer program from the thick manual. l The young technician taught/who was shown the computer program caught on right away. l The six volunteers taught/learned the complicated procedure without very much trouble. l The six volunteers taught/who were shown the complicated procedure became very good students.

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APPENDIX B Average Reading Time per Word (msec) Reading span

Main verb Region:

1

2

Relative clause 3

1

2

3

381 372 370 378 371 382

458 385 449 390 461 402

646 494 728 548 852 581

325 299 344 328

369 321 380 353

571 507 786 554

420 386 345 350

485 397 425 367

629 538 826 498

419 393 362 372

481 399 431 381

607 546 605 599

Sentences in isolation (Experiment la) Low Med. High

Ambiguous Unambiguous Ambiguous Unambiguous Ambiguous Unambiguous

377 366 394 374 378 374

383 356 375 373 369 336

493 512 599 546 715 574

Sentences in paragraph context (Experiment lb) Low High

Ambiguous Unambiguous Ambiguous Unambiguous

293 309 333 342

306 309 361 347

500 482 626 508

Short sentences (Experiment 3) Low High

Ambiguous Unambiguous Ambiguous Unambiguous

381 395 376 345

394 372 361 332

509 522 630 499

Long sentences (Experiment 3) Low High

Ambiguous Unambiguous Ambiguous Unambiguous

374 400 373 362

391 385 367 349

554 588 551 558

Proper nouns (Experiment 2) High

Ambiguous Unambiguous

351 366

366 372

580 553

Common nouns (Experiment 2) High

Ambiguous Unambiguous

362 343

378 349

644 544

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Bever, T. G., Garrett, M. F., & Hurtig, R. (1973). The interaction of perceptual processes and ambiguous sentences. Memory & Cognition, 1, 277-286. Carey, P. W., Mehler, .I., & Bever, T. G. (1970). When do we compute all the interpretations of an ambiguous sentence? In G. B. Flares D’Arcais & W. J. M. Levelt (Eds.), Advances in psycholinguistics. Amsterdam: North-Holland. Carpenter, P. A., & Daneman, M. (1981). Lexical retrieval and error recovery in reading: A model based on eye fixations. Journal of Verbal Learning and Verbal Behavior, 20, 137-160. Carpenter, P. A., & Just, M. A. (1988). The role of working memory in language comprehension. In D. Klahr & K. Kotovsky (Eds.), Complex information processing: The impact of Herbert A. Simon. Hillsdale, NJ: Erlbaum. Chodorow, M. (1979). Time compressed speech and the study of lexical and syntactic processing. In W. Cooper & E. Walker, (Eds.), Sentence processing. Hillsdale, NJ: Erlbaum. Chodorow, M., Slutsky, H., & Loring, A. (1988). Parsing non-deterministic verb phrases. Paper presented at the First Annual CUNY Conference on Human Sentence Processing, City University of New York. Crain, S., & Steedman, M. (1985). On not being led up to the garden path: The use of context by the psychological syntax processor. In D. R. Dowty, L. Karttunen, &A. M. Zwicky (Eds.) Natural language processing. Cambridge: Cambridge Univ. Press. Daneman, M., & Carpenter, P. A. (1980). Individual differences in working memory and reading. Journal of Verbal Learning and Verbal Behavior, 19, 45u66. Daneman, M., & Carpenter, P. A. (1983). Individual differences in integrating information between and within sentences. Journal of Experimental Psychology: Learning, Memory and Cognition, 9, 561-584. Ferreira, F., & Clifton, C. (1986). The independence of syntactic processing. Journal of Memory

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