Context and distance-to-disambiguation effects in ambiguity resolution: Evidence from grammaticality judgments of garden path sentences

JOURNAL

OF MEMORY

AND

LANGUAGE

26, 714-738 (1987)

Context and Distance-to-Disambiguation Effects in Ambiguity Resolution: Evidence from Grammaticality Judgments of Garden Path Sentences JOHNWARNERANDARNOLD Rutgers

L. GLASS

University

This study investigated whether single or multiple structures are constructed at the level of syntactic analysis for ambiguous strings. To this end, three experiments were conducted to determine the effects of various types of snytactic and nonsyntactic information on grammaticality judgments for garden path sentences. The results of Experiment 1, using a speeded grammaticality judgment task, were that context and length of the ambiguous region influenced the probability of calling a garden path sentence grammatical. In Experiment 2, even when given unlimited time to make grammaticality judgments, 40% of all long garden path sentences that required a less preferred analysis were called ungrammatical. Experiment 3 extended the length effects found in Experiments 1 and 2 to some additional types of garden path sentences. The results support a parsing model that is influenced by context and that constructs only a single interpretation of an ambiguous string at a time. 0 1987 Academic Press, Inc

When a sentence is comprehended, several different representations are constructed, including phonological, syntactic, and semantic representations. Several important questions that have yet to be fully answered concern how these representations are constructed, how each exerts influence on the construction of the others, as well as when these influences become active. A very broad range of models assume that a grammatical structure is first constructed by the syntactic processor and is This study was originally presented as a Masters thesis by J. Warner, under the direction of A. Glass, whom he thanks along with the other members of his committee-Patricia Gildea and Elliot Noma-for their valuable suggestions, incisive criticism, and enthusiasm. Both authors additionally express their gratitude to Pat Carpenter, Scott Robertson, Laura Beck, Dirk Ruiz, Maria Slowiaczeck, and two anonymous reviewers for providing detailed and very helpful comments and criticisms of earlier drafts of this manuscript. Correspondence and reprint requests should be addressed to Arnold Glass at the Department of Psychology, Busch Campus, Rutgers University, New Brunswick, NJ 08903. 714 0749-596X187 $3.00 Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

then input to the semantic construction process (e.g., Ferreira & Clifton, 1986; Frazier & Fodor, 1978). A good overview of such syntactic processing systems is provided in Winograd (1983) and Harris (1985). This report will refer to the syntactic processor as the parser. Parsing and Ambiguity

The study reported here will examine sentences in which there are two possible interpretations arising from some structural ambiguity in the sentence, but one and only one interpretation of the ambiguous constituent produces a grammatical analysis. When people first encounter such a sentence, they may fail to recognize it and call the sentence ungrammatical. If people can spend some time reanalyzing the sentence, especially if additional information is made available, they often will be able to recognize the sentence. These sentences are called “garden path sentences.” To be clear, we will use this term to refer to any sentence in which such a potential ambiguity exists, regardless of whether the am-

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by the parser or biguity is “noticed” whether the occurrence of the ambiguity leads to a misanalysis (cf Frazier & Rayner, 1982). Determining how the misanalysis occurs (and, as importantly, why it doesn’t always occur) should provide important clues to the operation of the parser. Consider for a moment whether strings (a) through (d), below, are sentences: (a) When

the boys [strike]

the dog

kills.

(b) When the boys [strike] the dog the cat. (c) When the boys [strike] the dog kills. (d) When the boys [strike] the dog the cat kiffs. Each of these strings contains at least one ambiguous constituent whose interpretation determines the grammaticality of the entire string. The constituent in square brackets is the first ambiguous constituent, the words contained in angle brackets are the second ambiguous constituent, and the italicized constituent is the one which resolves the ambiguity. In all cases, the ambiguity arises out of the question of how to close the preposed temporal clause (these types of sentence structures have been termed “late closure ambiguities” by Frazier and Rayner (1982) and others who have studied them). In string (a), the ambiguous constituent is “strike.” If “strike” is encoded as intransitive, then “the dog” becomes the subject of the main clause and string (a) is grammatical. However, if “strike” is encoded as transitive, then “the dog” becomes its object and string (a) is ungrammatical. The word “strike” is also ambiguous in strings (b)-(d). However, encoding “strike” as intransitive in strings (b) and (d) leads to an ungrammatical string and encoding it as transitive in both strings makes a grammatical interpretation possible. In string (c), as in string (a), “strike” must be encoded as intransitive.

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In addition, each of the strings (b)-(d) has a second ambiguous constituent. In string (b), if “the horse kicks” is encoded as a relative clause modifying “the dog,” then this string will lack a main clause and be ungrammatical. But if “the horse kicks” is encoded as the main clause, then the string will be grammatical. In contrast, in strings (c) and (d), encoding “the horse kicks” as a relative clause leads to a grammatical interpretation, while interpreting it as the main clause makes the string ungrammatical. Models

of Parsing

for Ambiguous

input

We place the different possible models for ambiguity handling in the parser along a continuum whose end points are defined by a Parallel Multiple Path parser and a Single Path parser. Briefly, a Parallel Multiple Path parser makes use of all possible interpretations of an ambiguous constituent to construct multiple interpretations of the sentence in parallel (Kurtzman, 1985). A Single Path parser, on the other hand, orders the interpretations of the ambiguous constituent on the basis of syntactic and nonsyntactic information, but, in contrast to a Multiple Path parser, can only select one most highly valued interpretation to construct the remaining syntactic structure of the sentence. Neither of these extremes provides a satisfactory explanation of the abundant evidence of garden pathing and parsing preference principles in human performance. The former is too powerful to account for the difficulty observed with garden path sentences, while the latter is too weak to account for the degree to which people seem to be able to recover from such errors. Between the end points defined by the Parallel Multiple Path and Single Path models lies a range of models that are formed by either attenuating the assumptions of the former or augmenting the assumptions of the latter. We will propose here a model in which a Single Path parser is enhanced by assuming that, if the first

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interpretation of a constituent fails to produce a grammatical structure, then there is some probability that the output of the parser will be checked and a second interpretation of the ambiguous constituent will be tried (Frazier & Rayner, 1982). We will refer to this model as an Augmented Single Path parser. Figure 1 presents a flow-chart of the Augmented Single Path parser and the checking mechanism, beginning with the box labeled “access word.” Notice that the model delineates three critical steps in the processing of a sentence. First (Figure la), the syntactic representation of the word is retrieved from the lexicon. Second (Figure lb), an attempt is made to combine the constituent with the syntactic structure of the preceding portion of the sentence. If the constituent is combined with the syntactic structure of the rest of the sentence,

no

more

sentence Retrieve syntactic representation word -___I

then the next word is accessed. Third, if at the end of the sentence all constituents have been incorporated successfully into the sentence structure and there are no more words, then the sentence is perceived as grammatical. However, if a constituent can not be combined with the rest of the sentence structure, then the sentence is perceived as ungrammatical. In this case, as suggested by Milne (1982) and Rayner, Carlson, and Frazier, (1983), the subject may check the input and attempt to recompute the syntactic structure of the sentence. This stage is indicated by the dotted box and line (Figure lc). Such a recomputation may lead to a sentence structure and to a “grammatical” decision. Additional motivation for the checking mechanism comes from the study by Milne (1982). Sometimes, the ambiguity of the ambiguous constituent is not detected at all

words

structure

complete

attempted

structures

i I

sentence

ii

structure

I -

+

of all

(B)

GLASS

succeeds - - - -

rpz II II II

IC,

= = = -“r, Attempt to II + recompute II sentence II structure II

- - -I

LX-G _--# of

combinations

I

_ +

"UNGRAMMATICAL"

>1 b-1

FIG. I. Schematic flowchart of the Augmented SingIe Path parsing model, which contains three main processing steps: (a) the syntactic representations of words are retrieved from the lexicon: (b) an attempt is made to combine the representation of the word with the sentence structure; (c) there is a failure at (b); the resulting sentence structure may be checked.

CONTEXT

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and its dominant interpretation is inconsistent with the syntactic structure of the sentence. For example, Milne found that subjects had trouble with sentences such as The sentry stands are green, even though the disambiguating constituent “are” is adjacent to the ambiguity. Apparently the preferred interpretations of the ambiguous words (in this case, noun and verb rather than adjective and noun) were often so dominant that they were not perceived as ambiguous and the syntactic disambiguation process was aborted. The existence of such a checking mechanism might explain why people sometimes notice a garden path or ambiguous construction and sometimes do not. It depends on whether the parser’s initial output is recomputed and whether that recomputation produces a different result. Since the Augmented Single Path parser described above is one of a number of Parallel and Single Path models whose predictions closely mimic each other, the following approach to presenting the logic, results, and conclusions of this study will be taken. Initially, predictions for the experiments will be derived only from the model described above. Then, after the results of each experiment are presented, it will be considered to what extent other augmentations or attenuations might also account for our results. After setting out these alterations, a judgment will be made about which of the revised models is more plausible for explaining the data presented here. Selection in the Augmented Path Model

Single

How might a Single Path parser select an interpretation of an ambiguous constituent? There are several kinds of nonsyntactic information that might influence the selection. First, there might be strong preferences built into the system. These preferences might be captured in the weighting of the rules or even in the lexical properties of the

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input, or both (this is similar to some of the proposals of Ford, Bresnan, & Kaplan (1982)). So, for instance, the parser might have its rules ordered in a hierarchy or weighted to make a transitive analysis of verbs when the input was ambiguous, or it could be that, when the constituent is ambiguous and immediately local information fails to resolve it, preferences are dictated by features assigned to the lexical item itself. Another possibility is that these preferences are expressed as strategies the parser itself uses to deal with ambiguities. For example, Frazier and Rayner (1982) have suggested two such strategies, Minimal Attachment and Late Closure, both founded upon the assumption that the parser structures input as soon as it is received. Second, there is the most recent interpretation of a similar nonambiguous constituent. For example, suppose string (a) (from (a) through (d), above) were preceded by string (e) as shown below: (e) When the girl sleeps the cat eats. (a) When the boys strike the dog kills. The nonambiguous sentence (e) might influence a similar parsing for string (a). Some evidence for this possibility is provided by an earlier study (Frazier, Taft, Roeper, Clifton, & Ehrlich, 1984) in which it was found that, for conjoined conjunction sentences (e.g., “Joshua hit the girl with a book and the boy with a bat”), subjects had less difficulty (in terms of reading time) when the second conjoined clause had the same structure as the first clause. We will refer to such influences as “syntactic context.” Third, there is the role of semantic plausibility. Semantic plausibility could influence the syntactic selection in two ways. There is the context independent plausibility of the sentence itself, that is, the plausibility of boys striking dogs. The other factor is the plausibility of the sentence within the context that it appears in. For instance, strings (a) through (d) might ap-

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pear in a paragraph about newsboys going on strike, or in a paragraph about boys hitting dogs. An example may again make this clearer. Consider what might happen if a person encountered string (f) just prior to encountering string (a) as shown below: (f) The dog becomes dangerous whenever boys attack. (a) When the boys strike the dog kills. As in the case of (a) and (e) before, where a recent nonambiguous syntactic analysis influenced the selection of an interpretation of the ambiguous constituent in (a), here the vaguer paraphrase may serve to prime an intransitive analysis for the ambiguous string which succeeds it. We will refer to this influence as “semantic context.” You will notice that the semantic context string (f) utilizes the same nouns as (a) with vaguer, more abstract verbs (e.g., instead of directly being described as killing, the dog is only said to be dangerous). Additionally, the clauses are reversed so that meaning remains constant while, in fact, the overall syntactic structure is altered. It should be pointed out that one would expect these effects to work in both directions, i.e., that any or all of these sources of information could be manipulated or exploited to increase the likelihood of an incorrect analysis of (a). For instance, imagine if (g), below, were presented just before (a) in place of(e): (g) If the girls pets the cat she sings. (a) When the boys strike the dog kills. Where (e) unambiguously “models” the grammatical interpretation of (a), (g) sets up an expectation for a transitive analysis which, if applied to (a), might result in an ungrammatical interpretation. Also consider if (h), below, were presented before (a) in place of(f): (h) Violence occurs because the boys attack the dog. (a) When the boys strike the dog kills.

GLASS

Like (f), (h) utilizes the same nouns as (a) with less concrete verbs. However, sentence (h) is altered so that there is an additional NP (i.e., Violence) and the two nouns from (a) are set incorrectly in an actor-patient relation. Thus, both the syntax and the meaning of the target string (a) have been distorted in (h). There are two other very obvious sources of information for resolving ambiguities which we will not address here in great detail because they are so obvious and because they do little to distinguish between possible parsing models as we have outlined them here. The most obvious is the possible effect of surface punctuation, in the form of marks in text and intonation (changes in vocal stress or pauses) in speech. Hence, a comma in string (a) between “strike” and “dog” makes “strike” intransitive and string (a) grammatical. Also, there is the frequency with which the different interpretations of the ambiguous constituent appear in nonambiguous contexts. For example, if “strike” were used as a transitive verb 75% of the time in nonambiguous contexts, the parser might select it that often in ambiguous contexts (or it might select that interpretation 100% of the time if the parser maximized rather than matched the more likely probability). This would lead to errors, but lacking better information, such a scheme would select the correct structure most of the time for structures that are not too complex. The purpose of this study was to examine the effect of syntactic and semantic context on grammaticality judgments for garden path sentences similar to strings (a)-(d). Subjects were presented with lists of strings, both grammatical and ungrammatical. In each list were embedded garden path sentences preceded by context strings that provided information that could be used for resolving the ambiguity in the garden path strings. Some conditions provided information which, if utilized, should

CONTEXT

AND

GRAMMATICALITY

result in a grammatical analysis while other conditions provided context which would make an ungrammatical analysis more likely. EXPERIMENT

1

In this first experiment, subjects were tested under conditions that limited the availability of the input and placed the subjects under pressure to respond quickly. This was done to try to get at very early, on-line stages of the parsing process. We wanted to observe the effects of context independent of later pragmatic or conceptual processes, and to establish whether such information were even available to the parser at such an early stage. For a Single Path model, even augmented as we have proposed, such context information should bias subjects so that the proportion of garden path sentences called grammatical would change significantly in the predicted direction. That is, for positive context, more garden path sentences should be called grammatical and, since fewer outputs would need to be checked, reaction time (RT) may be decreased. The opposite effects would be expected for negative bias. Additionally, we decided to manipulate the length of the ambiguous region in order to gain more insight into possible attenuations or augmentations that might be at work and to more clearly distinguish between possible Single Path and Multiple Path models. That is, if the parser has only limited flexibility in handling ambiguous input, it should further tax the system to increase the distance between the ambiguous and the disambiguating constituents. A similar manipulation was attempted by Rayner, Carlson, and Frazier (1983, Experiment 2) with mean reading time per character and mean eye-fixation duration before and after the ambiguous region as the measures. In both cases, they failed to find any effects. However, it did not seem impossible that length might affect the proportion of garden path sentences judged

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719

grammatical without affecting either of their measures. If subjects found fewer long garden path sentences grammatical compared to short garden path sentences, it would indicate limits on how much information the Attenuated Parallel model can maintain in parallel, or how far back our Augmented model can look to resolve the ambiguity. Such information could be useful in comparing the models’ relative merits. Method Stimuli. Three types of sentence/strings were used in this experiment, as shown in Table 1. In the following description, “garden path sentences” are the critical items of interest for our predictions; “context string” refers to the unambiguous sentence that immediately preceded each garden path sentence and provided information that could be used in interpreting the garden path sentences; “filler strings” refers to the remaining unambiguous strings that did not provide information that might be used in interpreting the garden path sentences. As shown in Table 1, two types of garden path sentences were used, those whose ambiguity was resolved by a transitive construction and those resolved by an intransitive construction. As also shown in Table 1, each type of garden path sentence was paired with five types of context. These context types were positive syntactic, in which the syntactic structure of the context string was unambiguously identical to the correct interpretation of the garden path sentence; negative syntactic, where the syntactic structure unambiguously modeled the ungrammatical interpretation; positive semantic, wherein the sentence gives a paraphrase of the meaning of the grammatical interpretation of the garden path sentence; negative semantic, where the paraphrase is designed to set up an expectation resulting in an ungrammatical analysis of the garden path sentence; and, finally, neutral context, which were simply

720

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EXAMPLES

OF STRING

AND

GLASS

TABLE I TYPES USED IN EXPERIMENTS

1 AND 2

Garden path sentences Intransitive Short WHEN THE BOYS STRIKE THE DOG KILLS Long BEFORE THE BOY KILLS THE MAN THE DOG BITES STRIKES Transitive Short AFTER THE DOG BITES THE MAN THE CAT KILLS Long WHEN THE HORSE KICKS THE BOY THE DOG BITES THE MAN Context strings (for short intransitive garden path sentence) Syntactic Positive BEFORE THE MAN SLEEPS THE CAT EATS Negative IF THE GIRLS PET THE CAT THEY SING Semantic Positive THE DOG BECOMES DANGEROUS WHILE THE BOYS ATTACK Negative VIOLENCE OCCURS BECAUSE THE BOYS ATTACK THE DOG Control Neutral context MEN HAVE TO BE PUT OUT Filler strings Grammatical THE MAN IS BELIEVED TO HAVE HAD THE DOG THE CAT JUMPS ACROSS THE FENCE WHEN STARTLED Ungrammatical RED IS TO BE UGLY TO RAPIDLY DOGS WHO IS STRONG KILLED THAT STRIKE MEN

additional filler strings. An example of each of the five types of context strings is given in Table 1. All context strings were constructed to be as unambiguously grammatical as possible. The two kinds of garden path sentences and the five kinds of context sentences were combined to form 10 basic subtypes of test list, each with a different garden path sentence-context string pairing. For each subtype, three different lists were constructed that were identical in design, but each of which contained completely different garden path sentences, context, and filler items from the other two. Thus, there were 30 test lists in all. Each test list contained 18 items: 4 garden path sentences, 4 ambiguously

grammatical context strings, 1 unambiguously grammatical filler string, and 9 unambiguously ungrammatical filler strings. Therefore, half the strings in each list were grammatical. Two of the garden path sentences in each list were short and 2 were long. In other words, as can be seen in Table 1, the number of intervening constituents in the ambiguous region between the ambiguous verb and the disambiguating constituent was varied. In the short garden path sentences, the ambiguous region contained two to four words; in the long garden path sentences, the ambiguous region contained five to six words. As also shown in Table 1, the intransitive short garden path sentences had only one level of ambiguity; it only had to be determined

CONTEXT

AND

GRAMMATICALITY

whether the preposed temporal clause terminated in a transitive or intransitive construction. The short transitive and the longer garden path sentences had a second level of ambiguity; in order to resolve the transitive/intransitive ambiguity it had to be determined whether two adjacent nouns belonged together (wherein the second noun heads a relative clause modifying the first). The garden path sentences occurred at the 4th, 8th, 12th, and 16th list positions. Two of the lists of each subtype placed the garden path sentences in a short-longshort-long sequence while the third used a long-short-long-short sequence. The Appendix provides a complete list of all garden path sentences and context sentences used in this experiment. Finally, in addition to the experimental lists, a practice list of 40 strings was created. These were simple filler strings of which half were unambiguously grammatical and half were unambiguously ungrammatical. Design. There were five experimental factors in this experiment. Two of the factors were within-subjects. Each subject was tested on three lists of 18 strings each. All three lists contained the same kind of garden path sentences. On one list (called the “precritical” list), all of the context sentences preceding the garden path sentences were neutral, but on the other two lists (called the “critical” lists) the context sentences introduced some level of bias (the same level for both of these lists). Thus list type is one within-subjects factor. In addition, each fist contained garden path sentences with both short and long ambiguous regions. Thus length is also a withinsubjects factor. The remaining three factors are between-subjects factors. They include sentence structure (intransitive vs transitive), context (syntactic vs semantic), and bias (positive vs negative). Hence, each subject was exposed to only one level of each of these three factors. For example, a subject in the “intransitive positive syntactic context group” viewed three lists

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721

which contained both short and long garden path sentences that required an intransitive analysis and, in the last two lists, context strings that modeled unambiguously only the grammatical interpretation of each ambiguous garden path sentence. Finally, there were two additional groups that are not included under the above design that served as control conditions. These are the intransitive and transitive neutral context groups. All the context strings in these groups were unambiguously grammatical sentences that were unrelated to the succeeding garden path sentences so that there was no level of either “context” or “bias” in these conditions. These two groups provided, for both the intransitive and transitive garden path sentences, a baseline measure of grammaticality judgments in the absence of context. To summarize, the experimental design included both a within-subjects (list type) and a between-subjects (context) measure of the effect of context on grammaticality judgments for garden path sentences. Apparatus. For all subjects and groups, strings were presented on a 22.5cm diagonal video monitor that was controlled by an Apple II+ computer. Strings were presented one word at a time in uppercase and without punctuation in the center of the screen using the rapid serial visual presentation (RSVP) method. An asterisk pattern presented in the center of the screen was used both to mask the last word after presentation and to signal the subject to respond. The rate of presentation was approximately 120 words per minute. Subjects indicated whether a string was grammatical or ungrammatical by pressing one of two microswitches which were labeled “G” for grammatical and “U” for ungrammatical. “G” was always the right-hand microswitch and “U” was always the lefthand microswitch. The subject’s response was recorded by the control program as was the response latency, in milliseconds, from the onset of the asterisk pattern. Procedure. A total of 90 Rutgers Univer-

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sity undergraduates participated in this experiment as part of an introductory course requirement or for extra credit. All subjects were native speakers of English, meaning that English was the first language they learned. There were 10 groups of nine subjects in all. Five groups were presented with intransitive garden path sentences and 5 groups were given transitive garden path sentences. Each of the 5 groups in both the transitive and intransitive conditions was given a different type of context string (as indicated under Stimuli, above). The fifth group in both the intransitive and the transitive conditions was a control group in which all three trial lists contained filler strings in the list positions preceding the garden path sentences. These groups controlled for simple practice effects. All subjects in all conditions received 20 practice strings. On these practice trials the subject was provided with feedback on the screen about the accuracy of their judgments. The purpose of the practice trials was to allow the subject to get used to the task of making grammaticality judgments as well as to let the subject learn well which hand controlled the “G” microswitch and which hand controlled the “U” microswitch without having to look at the labels. No data was collected from the practice trials; however, if the subject made five or more errors and at least one error was made during the last 10 trials, the subject received a second block of 20 different practice trials with feedback. Out of 90 subjects, only 7 required the second block of practice trials. For each subject, after the practice trials, each of the three lists of 18 strings was presented for a total of 54 strings judged per subject. For all subjects, the first list was the “precritical” list containing only neutral context strings. For the eight experimental groups, the remaining two lists contained the context strings appropriate for that condition (the “critical lists”). In the two control conditions, all three lists con-

AND GLASS

tained neutral context strings. Between lists were rest periods, which were terminated by the subject. There were no practice trials between lists and there was no feedback to the subject about their accuracy. Subjects were instructed to respond as quickly as possible and encouraged to base their response on their initial intuition or gut reaction rather than spending time thinking further about it. This was done to attempt to tap as early a stage of processing as possible and to discourage possible rehearsal. Despite the accuracy feedback in the practice trials, the subjects were instructed to “respond as quickly as possible” once the asterisk pattern cue appeared. Data analysis. The data analyzed were the percentages of garden path sentences judged grammatical (response data) and the RT for garden path sentences judged grammatical for each subject. The response data were analyzed using a four-way mixed-design analysis of variance that did not include the neutral context control groups but included length (short/ long ambiguous region) as a within-subjects factor. For this analysis, the two critical lists were pooled together. The three between-subject factors, as indicated above, were garden path sentence structure (intransitive/transitive), context (syntactic/semantic), and bias (positive/negative). An additional four-way mixed Anova was calculated with list type (precriticah critical) as the within-subjects factor. The between-subjects factors were the same ones used above. The two critical lists were pooled together and compared to the precritical list and both levels of length were also pooled. The RT data were analyzed using a three-way analysis of variance with structure, context, and bias as between-subject factors (precritical RTs were not analyzed and the RTs were pooled across length for all subjects). The reaction times were summed across “grammatical” responses only.

CONTEXT

AND GRAMMATICALITY

Results The mean percentage agreement (i.e., percentage of strings subjects called grammatical when grammatical and ungrammatical when ungrammatical) when averaged across all subjects in all conditions was 93% for the context strings and 85% for the tiller strings. Table 2 lists, for each group, the mean percentage of garden path sentences judged grammatical across both critical lists, as well as the standard error. First, in the initial four-way Anova, there was clear evidence that a transitive analysis was the preferred interpretation for the verbs used in this experiment. This was shown by the significant main effect of structure, indicating that overall performance was higher (more garden path sentences were called grammatical) when a transitive construction resolved the ambiguity than when an intransitive structure was required (85% vs 66%; F(1,64) = 24.7, p < .OOl). There was a significant main effect of bias, indicating that subjects were more likely to judge a string grammatical in a positive context than a negative context (87% vs 65%; F(1,64) = 32.3, p < .OOl). The main effect of context was not significant (73% vs 79%), suggesting that the overall likelihood of a grammatical response was the same for both syntactic and semantic contexts. However, it may be that

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the present analysis lacks sufficient power to detect small effects and that even small differences are important. Significant interaction effects showed that, in the intransitive conditions, garden path sentences with short ambiguous regions were judged grammatical much more often (92%) than garden path sentences with long ambiguous regions (40%), but in the transitive conditions this difference disappeared (86% vs 85%; F(1,64) = 58.5, p < .OOl). This result is confirmed by a Newman-Keuls’ multiple range test that shows this was true for all four intransitive context groups (p < .Ol) but not for any of the transitive conditions. The purpose of the second four-way analysis (with list type as a factor) was to serve as a within-subjects control to confirm that context indeed changed performance and that bias determined the direction of that change. This was confirmed by the interaction effect showing that positive context increased (73% vs 87%) and negative context decreased (71% vs 65%) the number of garden path sentences called “grammatical” (F(1,64) = 30.0, p < .OOl). All between-subject effects exactly replicated the results of the previous analysis. The mean percentage of change (including the direction) for each mean from the corresponding precritical mean is also shown in Table 2.

TABLE 2 MEAN~ERCENTAGEOF GARDEN PATHSENTENCESJUDGEDGRAMMATICAL(+STANDARDERROROFTHE MEAN) FOR THE CRITICAL TRIAL LISTS (COMBINED) IN EXPERIMENT 1 Syntactic Bias: Length: Intransitive Mean

SE % Change from precritical Transitive Mean

Positive S

i-33

47 5 +41

100

100

0

0

SE % Change from precritical

Negative L

94 4

-6

Semantic

0

Positive

Control, Neutral

s

L

s

L

22 12

100 0

25 I1

0

+3

89 8

92 8

S

L

s

94 4

28 11

97 3

0

+8

61 11

97 3

97 3

89 6

83 9

-3

+8

0

0

-6 56 9 -22

-17

L

Negative

61

83

10

7

+28

0

-6

-11

-2

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AND GLASS

The analysis of the reaction time data revealed no significant main effects or interactions. The means are presented in Table 3.

tive and intransitive interpretations of all ambiguous sentences and hence predicts no preference for either interpretation.

Discussion

tenuations would be necessary for a Parallel Multiple Path model to explain our results. For example, if it is assumed that all possible interpretations are not always carried to completion by the Parallel Multiple Path model but rather that some lesser valued paths are terminated after some number of constituents beyond the ambiguous one are processed, then the model no longer predicts that a grammatical interpretation will be found for every word string for which one is possible. A model attenuated in this way will be called an Attenuated Parallel Multiple Path model. If, for the verbs used in this experiment, the transitive interpretation is more highly valued than the intransitive interpretation, and hence more likely to be carried to completion, then this would explain the advantage for the transitive interpretation. Furthermore, if all structures are begun but not updated indefinitely, then it explains why the difference in grammatical judgments for transitive and intransitive structures was greater for long than for short constituents. However, in order to fit the quantitative results, a Parallel Multiple Path model must be so severely attenuated that it remains only nominally parallel in its underlying assumptions; in fact, it is functionally a Single Path model beyond about three constituents since after this point only a single

The results of this experiment clearly support the Augmented Single Path parsing model with the checking mechanism that we have proposed here. First, context was shown to significantly affect the proportions of garden path sentences called grammatical. Additionally, these shifts in proportion were in the direction predicted by the bias. Thus, presenting context that provided information about the grammatical interpretations of the garden path sentences increased the likelihood that subjects would call them grammatical, while presenting context that provided information about the ungrammatical interpretations of the garden path sentences decreased the likelihood of their being called grammatical. These differences were reflected both in changes of level of performance across lists from precritical to critical and, overall, in comparisons between performance in the critical lists of the positive context conditions and corresponding performance in the negative conditions. There was also a very powerful preference for a transitive construction. In four out of five conditions, intransitive sentences were more likely to be misinterpreted as transitive than the reverse. This bias is inconsistent with the Multiple Path model, which would construct both transi-

The Attenuated Parallel Multiple Path model. Let us consider a moment what at-

TABLE MEAN

REACTION

3

TIME (ms) FOR MAKING A CORRECT JUDGMENT OF “GRAMMATICAL” PATH SENTENCES IN THE CRITICAL LISTS (COMBINED) FOR EXPERIMENT

Syntactic Bias: Intransitive Mean SE Transitive Mean SE

Positive

FOR THE GARDEN 1

Semantic Negative

Positive

Negative

Control. Neutral

600 101

439 68

483 76

548 Ill

463 127

344 94

511 107

352 52

516 IO5

415 93

CONTEXT

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interpretation of the sentence is available. Also, there were significant differences (to be specified below) among the RT distributions that are not explained by an Attenuated Parallel Multiple Path model. As will be seen below, the tails of the RT distributions for those strings that elicited high percentages of ungrammatical responses were consistently longer than for RT distributions that did not elicit high percentages of ungrammatical responses. This model does not predict such a finding. The checking mechanism. Our results are most consistent with the Augmented Single Path parser with a checking mechanism that acts on the output of the parser to select an alternative interpretation of the ambiguous constituent. An important property of this checking mechanism is suggested in our results. So long as the disambiguating constituent occurs within three constituents after the ambiguity, it seems relatively easy for the checking mechanism to find the alternate structure. If more than three constituents have intervened, it becomes more difficult to tear down the erroneous structure and resegment the input. In fact, such an attempt may not succeed. Let us first consider how these predictions emerge from the model we have proposed (above, and in Figure I), and then we will show how each prediction is confirmed in the data. First, notice that, as shown in Figure lb, when an ambiguous word that has more than one possible way of being combined into the sentence structure (for example, the verb in the preposed clause in our garden path sentences) is accessed, an initial resolution is selected on the basis of whatever preferences are used by the parser (in this case, a transitive analysis is selected). If, as we have suggested, it is also the case that within three constituents, recomputing an alternative structure remains relatively easy, then we would expect both the short intransitive and the short transitive garden path sentences in this study to be judged grammatical equally

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often, as often as unambiguous grammatical sentences. In fact, this was the result obtained on the precritical lists for all subjects in the experiment. For the short strings, the proportion of “grammatical” responses was nearly the same for both intransitive and transitive garden path sentences, 87 and 92%, respectively. Also, the proportions of the unambiguous grammatical sentences included in the intransitive and transitive conditions that were called grammatical were 92 and 94%, respectively. Additional evidence comes from the two critical lists in the neutral conditions. This time, the proportions of “grammatical” responses for short intransitive and transitive garden path sentences were 100 and 89%, respectively. The corresponding proportions of “grammatical” responses to the unambiguous grammatical sentences used in these conditions were 93 and 90%, respectively. What about the situation where the distance between the ambiguity and the disambiguating constituent is greater than three? We have suggested that, in the absence of sufficient additional information, when a misanalysis of the ambiguity occurs, it may be difficult to recompute an alternate structure before a judgment is made. This would result in most of the long intransitive garden path sentences being judged “ungrammatical” while most of the long transitive garden path sentences would be judged “grammatical.” The results of the precritical list were that long intransitive garden path sentences were called grammatical only slightly more often than unambiguous ungrammatical strings (23% vs 16%) and long transitive garden path sentences were called “grammatical” as often as unambiguous grammatical sentences (89% vs 93%). Once again, this evidence is confirmed by looking at the critical lists of the neutral conditions. This time, the proportions of grammatical responses to the long intransitive and transitive garden path sentences were 25 and 92%, respectively. The proportion of

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“grammatical” responses to the unambiguous ungrammatical strings in the intransitive neutral condition was 25% and the proportion of “grammatical” responses to unambiguous grammatical sentences in the transitive neutral condition was 90%. Although the RT analysis revealed no significant effects, there are a number of potential problems with the analysis, given the relatively small number of observations per subject, that require caution in interpreting these results. First, there were far fewer “grammatical” responses in the intransitive conditions than the transitive conditions, making it likely the intransitive means are less stable. Second, because so few intransitive long garden path sentences were judged grammatical, length had to be pooled for each group of subjects. Thus, this Anova is likely to be quite insensitive. Nonetheless, we may be able to obtain some valuable information about the model from the distributions of RT. Figures 2 and 3 show the RT distributions for all unambiguous grammatical sentences and all unambiguous ungrammatical strings for the critical lists in all 10 conditions, thus providing a large number of observations on which to base conclusions.

AND GLASS

Recall that, as shown in Figure lc, if a grammatical structure is not found for an input string, then there is some probability that the structure will be checked before a response is made. The assumption of a possible checking process predicts a longer tail for the RT distributions for ungrammatical strings than for grammatical sentences because the latter are usually not checked. In fact, this is confirmed by Figures 2 and 3. The tail of the RT distribution for responses to ungrammatical strings (Figure 3) is much thicker. For Figure 2 (pooling all responses), only 1% of the distribution lay beyond 2000 ms, whereas, for Figure 3,6% of the distribution (all responses pooled) was located beyond 2000 ms. This difference in proportions (Bruning & Kintz, 1977) was significant (z = -5.90, p < .OOl). Also important is the relationship of the correct to error RTs shown in the figures. For the grammatical sentences in Figure 2, for responses greater than 1000 ms, an error was virtually never made. In contrast, for ungrammatical strings, as shown in Figure 3, the frequency of an incorrect “grammatical” response remained nearly constant throughout the distribution. This pattern can be explained by as-

400

RT hterval

UMisecds

x 11100)

FIG. 2. RT distribution of responses to the unambiguous grammatical sentences for all conditions combined. 0, “Grammatical” response + “ungrammatical” response.

CONTEXT

AND GRAMMATICALITY

RT Interval

(Milliseconds

x I/ 100)

FIG. 3. RT distribution of responses to the unambiguous ungrammatical combined. 0, “Grammatical” response vs “ungrammatical” response.

suming that there is some probability of checking an “ungrammatical” decision and of changing the response so that the tails of the RT distributions for responses to ungrammatical strings will consist of those decisions that have been checked. Thus the tails of the distributions of “grammatical” and “ungrammatical” responses to ungrammatical strings will be equally long, as shown in Figure 3. This general prediction is also supported in the patterns observed for the intransitive and transitive neutral conditions. Although the number of observations here are much fewer, rendering the RT distributions quite noisy and thus requiring caution in interpretation, the prediction we have made above would suggest that, for the long intransitive garden path sentences, there should be a longer tail since there will be more “ungrammatical” responses that have some probability of being checked and possibly changed. Such a tail would be evidenced by a further positive skewing of the distribution, resulting in an increase in both the mean and the distance between the mean and the median of the distribution. However, for both types of short garden path sentences (intransitive and

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strings for all conditions

transitive) as well as for the long transitive garden path sentences, we would expect the distance of the mean from the median to be much smaller. In fact, these are the patterns observed in the data. For the short intransitive, short transitive, and long transitive garden path sentences in the neutral conditions (all responses pooled), the distances between the mean and median were 85, 160, and 127 ms, respectively. The distance between the mean and the median in the RT distribution for the long intransitive garden path sentences in the neutral condition (all responses pooled) was 326 ms. Thus, in this respect, the RT distributions for the short intransitive, short transitive, and long transitive garden path sentences all resemble the distribution of RT (all responses pooled) for the unambiguously grammatical strings in the neutral condition (distance between mean and median = 119 ms) and the distribution for the long intransitive garden path sentences resembles the RT distribution (all responses pooled) for the unambiguously ungrammatical strings in the neutral conditions (distance between the mean and the median = 182 ms; the differences for the unambiguous strings

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seem less dramatic because the greater number of observations stabilize the means somewhat). The important point to be made here is that the results observed here (particularly in Figures 2 and 3), predicted by the Augmented Single Path model with a checking mechanism, would not be predicted by an Attenuated Parallel Multiple Path model. This model would predict that, while a less valued interpretation of an ambiguous constituent might take longer to process, once the parser has moved some number of constituents beyond the ambiguity and those less valued paths are terminated, there would be no changing of the output before making a decision. Thus, for the unambiguously ungrammatical strings as well as the long intransitive garden path sentences, the RT distribution would move to the right, increasing the mean, but the degree of skew would remain similar to the RT distribution for the unambiguously grammatical strings. The model of the checking mechanism that is shown in Figure 1 is a probabilistic model; depending on various factors (e.g., the relative strength of an alternative or the length of an ambiguous region), it may or may not compute an alternative structure for a string judged ungrammatical. Hence, the complete model of explaining decisions about garden path sentences is a hybrid containing both Single Path and Multiple Path components. However, the conceptualization of these components is quite different. The Single Path component models the activity of the parser itself, i.e., the process by which constituents are concatenated and the syntactic structure is built. The Multiple Path component models the activity of a process that checks the output of the parser and which may influence the processing of the parser by affecting its input, but it is not the parser itself (cf Rayner, Carlson, and Frazier’s (1983) “Thematic processor”). We assume that the check and recomputation process that we have inferred from

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the long tails of the RT distributions is a conscious process that is an expression of the subject’s general problem solving ability. The results in this study can be explained by assuming that what a subject does in the recomputation of a sentence is simply to impose a segmentation pattern on the input string different than was implied by the preferred interpretations of the lexical items. Finally, we are left with the question of how context exerted its influence on sentence processing. There are two possibilities within the framework of the Augmented Single Path model. Either context could influence the initial selection of a verb’s interpretation or it could influence the probability that the resulting sentence structure would be checked and recomputed. These alternatives predict different patterns in the RT data. Unfortunately, due in part to the low power of the betweensubjects design, the differences for the critical comparisons were not significant. Thus, further data collection will be required to establish the loci of context effects. This will be an important issue to address, particularly in resolving the differences between the Augmented Single Path model and the Attenuated Parallel Multiple Path model. If context influences the initial selection, context could in effect cause the Multiple Path model to terminate all but the primed interpretation, resulting in a Single Path model but with more complexity without more explanatory power. One of the assumptions that seems to motivate many parallel models is that context does not have an influence of initial selection, that all interpretations of an ambiguous word are activated initially even if the duration of these multiple activations are short, and context then affects which interpretation(s) are maintained. More recently, even this assumption has been called into question, at least for ambiguous words in naming and lexical decision tasks (Kiger & Glass, 1983; Van Petten & Kutas, 1987).

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729

In summary, the Augmented Single Path task constraints turned it into an Single Path parser by pressing for a response bemodel with a checking mechanism is.supfore processing was completed. Even if the ported by four main findings in this experiment: (1) Garden path sentences in which appropriate model is the Attenuated Parallel Multiple Path model, the degree of atthe disambiguating constituent followed tenuation might be far less severe than rewithin three constituents of the ambiguous verb were perceived as grammatical as flected in the previous experiment. To examine this issue, a second experioften as unambiguous grammatical sentences. (2) In contrast, for these garden ment presented the lists used in Experiment 1 in a paper-and-pencil subject-paced path sentences, when the disambiguating constituent followed the ambiguous verb task in which time constraints were elimiby more than three constituents, the verb nated. If the subjects now called all the was always interpreted as transitive, so garden path sentences grammatical, the that long garden path sentences requiring a Multiple Path parsing model would be suptransitive interpretation were called “gramported and doubt would be cast on the rematical” as often as unambiguous sen- sults of Experiment 1. If, however, the retences but long garden path sentences re- sults followed the same general pattern as quiring an intransitive interpretation were for the speeded task, it would suggest called “ungrammatical” as often as un- grammaticality judgment was measured in grammatical strings. (3) The probability of the first experiment. a response to a garden path sentence was influenced by both its syntactic and se- Method mantic context. (4) The tails of the RT disStimuli, design, and apparatus. Ten lists tributions for those strings that elicited high of 54 strings were generated, one for each percentages of ungrammatical responses of the intransitive and transitive context were consistently longer than for RT districonditions, and printed on paper. To the far butions that did not elicit high percentages right of each string on the list were printed of ungrammatical responses. An Attenthe symbols “G” (for grammatical), “7” uated Parallel Multiple Path model may be (for uncertain), and “U” (for ungrammatable to account for the first three findings, ical). These printed lists were identical to but it can not account for the fourth. the material presented to subjects over the course of three trial lists in Experiment 1 EXPERIMENT 2 except that there was no “precritical” list. The purpose of using a rapid, on-line The strings were printed in all uppercase form of presentation in Experiment 1 was letters, as they had been when presented to make the syntactic judgments as free as on the video monitor in Experiment 1. No possible of the more time-and-resourcepunctuation was indicated. The design was consuming top-down conceptual processes identical to Experiment 1. which are brought into play in normal Procedure. The subjects were 100 reading and language comprehension. It Rutgers University undergraduates who has been said that the results obtained here participated in the experiment to fulfill a relend support to an Augmented Single Path quirement for Introductory Psychology or parsing mechanism. However, it might be for extra credit. Additionally, some subsuggested that the real effect of using a jects had previously participated in Experirapid, on-line task wherein subjects were ment 1 or a similar task. Some (25) were encouraged to respond quickly was to arti- actually subjects in Experiment 1 who were ficially terminate syntactic processing. randomly selected to carry out the addiThus, while the human parsing mechanism tional task immediately after completing might well be a Multiple Path model, the the RSVP on-line grammaticality judgment

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task. Some other subjects (25) had previously participated in a pilot experiment very similar to Experiment 1, approximately 1 month before. All subjects were randomly assigned to conditions. No attempt was made to assign the subjects who had already participated in Experiment 1 to the same condition they were in previously. Subjects were asked to look over each string and to circle “G” if they were certain it was a grammatical sentence, “?” if they were not sure, and “U” if they were certain it was an ungrammatical string. Additionally, if subjects circled “?” or “II,” they were requested to make some simple indication of what might make the string grammatical. All subjects were encouraged to take as much time as they needed. Results and Discussion Table 4 shows the mean percentages of grammaticality judgments for all groups. Across the whole experiment, the proportion of agreement (strings judged grammatical when grammatical and ungrammatical when ungrammatical) was 95% for the context strings and 93% for the tiller strings. A four-way mixed Anova was performed on the data. As in Experiment 1, length (short/long) was a within-subjects factor and the three between-subjects factors were structure (intransitive/transitive), context (syntactic/semantic), and bias (positive/negative). First, the differences in level of perfor-

GLASS

mance on the garden path sentences between the intransitive and transitive lists did not diminish (70% vs 96%), as indicated by a strong main effect of structure (F( 1,74) = 43.7, p < .OOl). Second, there was a significant four-way within-subjects interaction that indicated, for long intransitive garden path sentences with syntactic context, that bias had a significant effect (58% vs 18%, p < .Ol, Newman-Keuls’ test) whereas for long intransitive sentences with semantic context and all transitive path sentences there were no significant effects (F(1,74) = 110.3, p < .OOl). Finally, other within-subjects interaction effects replicated the finding from Experiment 1 that for the four intransitive context conditions, more short garden path sentences were judged grammatical than long garden path sentences (99% vs 42%, p < .Ol, Newman-Keuls’ test), while in the transitive conditions there was no effect of length (96% vs 97%). Clearly, the structure effect remained quite robust, demonstrating that there is a marked preference for a transitive analysis even when there is no time constraint. This result provides strong support for a Single Path parsing model over a Multiple Path model. In spite of reflection, including reflection in which considering alternate structures was encouraged (i.e., when asked to consider how to make the string grammatical), without context the grammatical interpretation in the intransitive

TABLE

4

MEAN PERCENTAGE OF GARDEN PATH SENTENCES JUDGED GRAMMATICAL FOR EXPERIMENT 2 Syntactic

Intransitive Mean

SE Transitive Mean

SE

Semantic Negative

Control, Neutral

Negative

Bias:

Positive

Positive

Length:

S

L

S

L

S

L

S

L

S

L

100

0

58 10

100 0

18 10

98 2

48 11

98 2

43 14

95 3

33 9

97 3

93 5

96 4

96 3

100 0

100

92

0

6

97 2

98 2

90 4

CONTEXTANDGRAMMATICALITYJUDGMENTS

condition was available much less often than for the transitive garden path sentences. In a pattern similar to that found for Experiment 1, in the neutral conditions we find that for the short garden path sentences, both intransitive and transitive, and for the transitive long garden path sentences, the proportions of “grammatical” responses were 95, 98, and 90%, respectively. For the long intransitive garden path sentences, however, the proportion of “grammatical” responses was only 33%. This prompted a closer look at the raw data. When the actual forms were reexamined to see what corrections the subjects had made, of the 40 subjects in the intransitive conditions who called two or more of the garden path sentences ungrammatical, 37 made corrections that converted the structure to a transitive one. One subject made corrections to the long intransitive garden path sentences by deleting the subordinate NP, but maintaining an intransitive interpretation of the verb. Two subjects failed to clearly indicate their changes. What seems to be operating here is that, despite the time allowed to bring in pragmatic information and other processes, there is such a strong preference for a pure transitive construction when presented with direct action verbs that the preferred analysis takes over. This supports the Augmented Single Path parsing model and also suggests that, even when the stimulus is available for reparsing, the decision to do so may be highly influenced by the degree of preference or some other measure of confidence attached to that structural assignment that resulted in an ungrammatical analysis. Bias (with the exception of syntactic context on intransitives) did not have an effect in this experiment. This may be because the unlimited reaction time allowed was sufficient to reduce or eliminate any initial context-induced bias. It might also be that the semantic effects were less stable than syntactic effects because the subject perceives the task as a syntactic task-that

731

is, the subject was given no reason to believe that the meaning of any two pairs of strings might be connected, or that one string might imply another. EXPERIMENT 3

We considered it extremely significant that subjects failed to find the grammatical interpretation of a garden path sentence even when they were given unlimited time to ponder its structure. This result suggests that the parser does not have the capability to exhaustively construct all possible structures for an ambiguous input. The purpose of Experiment 3 was to generalize this result to other types of garden path sentences, not used in Experiments 1 and 2. Three types of garden path sentences were tested, as shown in Table 5. For each type, both a long and a short version were generated for both of two possible resolutions of the ambiguity. Thus, for each sentence set, versions (a) and (c) are resolved by the same analysis of an ambiguous constituent while (b) and (d) versions are resolved by the alternate resolution of the ambiguous constituent. For sentence set (1) in Figure 5, the ambiguity is simply the tense assignment for put our, which can only be resolved by checking for agreement with the main verb of the second clause (strikes in (la) and (lc) or struck in (lb) and (Id). Sentence set (2) presents a reduced relative ambiguity, similar to the minimal/ nonminimal attachment ambiguities studied by Rayner, Carlson, and Frazier (1983) and Ferreira and Clifton (1986), in which the men believed to strike . . . can either be interpreted as a main verb followed by a phrase describing what the men believed or, alternately, it may be analyzed as a reduced relative phrase describing what was believed about the men. This ambiguity is resolved by determining agreement between the men and the final verb (is in (2a) and (2~) and are in (2b) and (2d)). Finally, in sentence set (3), it must be determined whether the man who . . . begins a imbedded relative clause (as in (3a) and (3~))

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GLASS 5

THEGARDENPATHSENTENCESUSEDINEXPERIMENT~ Long (1) a. THE BOYS PUT OUT UGLY STRIKES THE b. THE BOYS PUT OUT UGLY STRUCK THE (2) a. THE MEN BELIEVED b. THE MEN BELIEVED (3) a. THE GIRLS BELIEVE STRUCK THE DOGS b. THE GIRLS BELIEVE STRUCK THE DOGS Short (1) c. THE BOYS PUT OUT d. THE BOYS PUT OUT (2) c. THE MEN BELIEVED d. THE MEN BELIEVED (3) c. THE GIRLS BELIEVE d. THE GIRLS BELIEVE

THE DOGS THAT ARE STRONG WHEN THE MAN WHO IS VERY CLOCK THE DOGS THAT ARE STRONG WHEN THE MAN WHO IS VERY CLOCK TO STRIKE THE VERY RED DOG THAT KILLED THE CAT IS UGLY TO STRIKE THE VERY RED DOG THAT KILLED THE CAT ARE UGLY THE MAN WHO BELIEVES THE VERY STRONG UGLY BOYS KILLED THE CATS THE MAN WHO BELIEVES THE VERY STRONG UGLY BOYS WHO WHO KILLED THE CATS THE THE TO TO THE THE

DOGS WHEN THE MAN STRIKES DOGS WHEN THE MAN STRUCK STRIKE THE DOG IS UGLY STRIKE THE DOG ARE UGLY MAN WHO STRUCK THE DOGS MAN WHO STRUCK THE DOGS

or whether it is the beginning of a recursive right-branching structure (as in (3~) and W)). Method Four lists of 24 strings were generated. Each list was identical except that each had a different version of the three types of garden path sentences shown in Table 5. One list had the three (a) sentences, another the (b) sentences, the third the (c) sentences, and, finally, the fourth list had the three (d) sentences. Each list also had three strings which were identical to the three garden path sentences on that list except that the ambiguous constituent was replaced with a constituent that made them unambiguous. For garden path sentences (la-Id) in Table 5, this meant substituting kill (in la) and (1~)) and killed (in (1 b) and (Id) for put out (present tense) and put out (past tense). For sentences (2a)-(2d), believed to strike in (2a) and (2~) was changed to believed that to strike, and the men believed in (2b) and (2d) was changed to the men who are believed. Finally, for garden path sentences (3a)-(3d), believe in (3a) and (3~) was changed to believe that, and believe in (3b) and (3d) was changed to struck. Because of the results of the pre-

THE CLOCK THE CLOCK

KILLED THE WHO KILLED

CATS THE

CATS

vious two experiments showing context effects, all unambiguous versions of the garden path sentences were placed at the very end of the list, after the ambiguous strings. The filler strings, which were the same in all four lists, were strings generated to be unambiguously either grammatical or ungrammatical. These lists were of the same form as the printed lists used in Experiment 2. To the far right of each string on the list were printed the symbols “G” (for grammatical) , “?” (for uncertain), and “U” (for ungrammatical). These strings were printed in all uppercase letters and no punctuation was indicated. Each list was completed by a different group of 14 Rutgers University undergraduates who were native speakers of English, for a total of 56 subjects. Each subject received extra credit toward their grade and none were told the purpose of the ratings beyond the fact that we were interested in looking at people’s “grammatical intuitions.” They were asked to look over each string and to circle “G” if they were certain it was a grammatical sentence, “?” if they were not sure, and “U” if they were certain it was an ungrammatical string. Additionally, if they circled “?” or “U,” they

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were requested to make some simple indi- 2, there seemed to be a strong preference cation of what might make the string gram- for one resolution of the ambiguity over anmatical. They were encouraged to take as other as reflected by a greater percentage of grammatical judgments for the (b) long much time as they needed. garden path sentences compared to the (a) Results and Discussion versions, although this only emerges The ratings for the strings on each list clearly in sets (1) and (2) (the difference in were tabulated and the ratings for each am- set (3) is nonsignificant). Sentence set (2) presents unexpected rebiguous garden path sentence and its unambiguous version were collected. The per- sults, although they are not a problem for centages of subjects calling each garden the model we have discussed. There seems path sentence and its unambiguous version to be a powerful preference on the part of grammatical were compared by obtaining a subjects to make a reduced relative analz score for the difference between proporysis, even with short garden path sentions and determining whether the differtences, despite the expectation from preence was significant (Bruning & Kintz, vious studies (e.g., Ferreira & Clifton, 1977). Table 6 lists the percentage of sub- 1986) that a simple main verb analysis is A close examination of subjects calling each garden path sentence and preferable. its unambiguous equivalent version gram- jects’ corrections confirmed this. In most matical. cases, subjects replaced is with are (they The predictions of the model were again had not seen the other version). However, supported, although the results for sen- we would hesitate to suggest that this item tence set (2) were somewhat unexpected. presents counter-evidence to the minimal We will first discuss sentence sets (1) and attachment principle since this was not the (3). For both sets, the percentages of sub- intent of the experiment. Further, on rejects calling short garden path sentences flection, it seems quite possible that an (the (c) and (d) sentences in Table 5) gram- event (i.e., striking a dog) may not be a matical did not differ significantly between pragmatically acceptable NP to be dethe sets or with their unambiguous ver- scribed as “ugly,” in which case the subsions. However, in both cases, the long jects were constrained to favor the nongarden path sentences were judged gram- minimal attachment structure. matical significantly less often than the GENERALDISCUSSION short garden path sentences (p < .02 for set (1); p < .OOl for set (3)) or their unambigAll three experiments showed that there uous versions (JJ < .02 for set (1); p < .002 is a low frequency of detecting the gramfor set (3)). Also, as in Experiments 1 and matical interpretation of long garden path sentences when it requires the less preferred interpretation of an ambiguous conTABLE6 stituent. These results demonstrate that PERCENTAGE OF SUBJECTS CALLING EACH OF THE only a single interpretation of the sentence THREE GARDEN PATH SENTENCES FROM TABLE 5 is available beyond about three constitAND ITS UNAMBIGUOUS VERSION GRAMMATICAL uents. The long tails of the RT distributions Garden path Unambiguous indicate that a checking mechanism is at Long Short Long Short work when the structure produced is un___ ____ grammatical. Together, these results proa b c d a b c d vide evidence that an Augmented Single 1. 64 100 100 100 100 100 100 100 Path parser with a checking mechanism is 2. 43 93 36 100 100 100 100 100 the most plausible model of the parsing 3. 43 71 loo loo 93 93 loo loo process beyond about three constituents.

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This leaves open the question of what occurs from when the ambiguous constituent is first encountered to the point at which the parser must restrict its processing capacity to a single interpretation. We now turn to this issue. Augmented

Single Path Models

There are at least three variations on the Single Path model that differ in what is assumed to occur at the onset of an ambiguous constituent, the crucial difference among them being the degree of augmentation (and, thus, the level of complexity) required to predict the same results. The simplest version is the Single Path model with no augmentation other than the checking mechanism. According to this model, the sooner the misinterpretation of an ambiguous constituent is detected, the more likely that the sentence structure will be recomputed. Hence, a grammatical interpretation is more likely to be made for a short garden path sentence than for a long one. If the misinterpretation is detected within three constituents, the correct interpretation is nearly always found. One alternative is the Buffer model, a Single Path parser with a three constituent buffer. When an ambiguous constituent occurs, it and the succeeding constituents are placed in a buffer until either the buffer is filled or a disambiguating constituent is encountered. In the former case, the path is selected by the listener’s preference as biased by the context. In the latter case the path is selected by the disambiguating constituent. This model is similar to the deterministic parser, PARSIFAL, of Marcus (1980) that utilizes a three constituent buffer. Although this model can account for our data, since the previous model requires no buffers it seems simpler and is, thus, more immediately compelling. On the other hand, a buffer model would require less structure rebuilding since it searches forward for the disambiguating constituent before committing itself to an interpretation of the ambiguous constituent.

AND

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Another alternative is to simply combine the previously considered Attenuated Parallel Multiple Path model with the checking mechanism that we have proposed. Because of the severity of attenuation needed to account for our length effects, this is really a hybrid Parallel Multiple Path1 Single Path parser in that it very quickly terminates all but one, the most highly valued or activated path. Thus, such a model may be equally well considered an Attenuated Parallel Multiple Path model or an Augmented Single Path model. In this model, whenever an ambiguous constituent is encountered, multiple structures may be begun in parallel for more than one possible interpretation. These are all continued until either the limit of the system to maintain parallel structures is reached or the disambiguating constituent is encountered. In the former case, which appears to be about three constituents, the path is selected by the listener’s preference as biased by context. This is then some probability that the output of the parser will be checked before an ungrammatical decision is made. This model is close to a proposal made by Kurtzman (1985). The hybrid model is both more complex and requires more computation than either of the other two models to perform the same function. The additional processing required by the model appears even less plausible when it is considered how rarely syntactic ambiguities occur that are not resolvable by intonation and/or semantic context. If persisting ambiguous syntactic structures are rare, then it becomes less plausible that a powerful ability to compute multiple syntactic structures or to store constituents in buffers has evolved just for the rare occasion when an ambiguity is created in the input. Constituent Size and Hierarchical Processing

For the model proposed here, it was suggested that the more syntactic constituents between the point of ambiguity and the de-

CONTEXT

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GRAMMATICALITY

tection of the syntactic mismatch, the less the likelihood that a new syntactic structure would be computed. This raises the question of what counts as a constituent. If a constituent is defined as a word, then the Single Path model predicts that the phrase The very red . . . will exhaust the parser’s scope. However, if a constituent is defined as a noun phrase or verb, then the single long noun phrase Twelve very important looking tuxedoed and top-hatted gentlemen . . * will not exceed that scope. Neither of these alternatives seem intuitively plausible, as has been noted elsewhere (e.g., Frazier (1985), footnote 14). However, these problems may be surmountable. Glass and Warner (1987) have proposed a hierarchical parser in which syntactic representations of contiguous words that form phrasal constituents are reduced to those phrasal constituents before the phrasal constituents are concatenated. Hence, for the word string The very red . . . , “very red” would form a single constituent and the phrase would not exhaust the scope of the parser’s structure building. On the other hand, the word string Twelve very important looking tuxedoed and top-hatted gentlemen . . . would form at least four constituents--“twelve,” “very important looking,” “tuxedoed,” “and”--0 it would exceed the parser’s scope. This may in fact explain why the effects of the length manipulation were significant in all three experiments when a previous study, mentioned above, attempting the same manipulation, failed to find any effects (Rayner, Carlson, & Frazier, 1983). An examination of the materials utilized in the Rayner et. al. study reveals that the ambiguous regions of their sentences were extended primarily with simple noun modifiers which would be easily collapsed into a single constituent like “the very red . . .” above by a hierarchical processing model. Thus, the length manipulation of the Rayner et. al. study may have been too small to show an effect. In summary, a hierarchical processing

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735

model produces plausible constituents that may vary from individual words to whole phrases. Together, the Augmented Single Path and hierarchical processing models provide a persuastive description of human parsing. APPENDIX

Garden Path Sentences Used for Experiments I and 2, with Their Matched Context Strings Short and long versions are alternated in the same pattern as the subject would encounter them and are designated by ‘s’ or ‘1’ after the number. Context strings are designated as follows: a = positive syntactic; b = negative syntactic; c = positive semantic; d = negative semantic; e = neutral context. Intransitive

Garden Path Sentences

Is. BEFORE THE MEN KILL THE BOY STRIKES la. WHEN THE GIRL SLEEPS THE CAT EATS Ib. WHILE THE WOMAN PETS THE CAT SHE SINGS lc. MEN BECOME DANGEROUS BECAUSE THE BOY ATTACKS Id. VIOLENCE OCCURS BECAUSE THE MEN ATTACK THE BOYS le. IS THE DOG BELIEVED TO BE UGLY 21. IF THE HORSE KICKS THE BOY THE DOG BITES KILLS 2a. AFTER THE MAN GOES THE DOG THE NEIGHBOR OWNS BARKS 2b. AFTER THE MAN FEEDS THE COW THE GIRL SIGHS LOUDLY 2c. THE HORSE REACTS BEFORE THE BOY THAT THE DOG HURT ATTACKS 2d. THE HORSE HURTS THE BOY BEFORE THE DOG ATTACKS 2e. THE GIRL HAS TO BE OUT OF THE HOUSE BEFORE THE CAT 3s WHEN THE BOYS STRIKE THE DOG KILLS 3a. BEFORE THE MAN SLEEPS THE CAT EATS 3b. IF THE GIRLS PET THE CAT THEY SING 3c. THE DOG BECOMES DANGEROUS WHILE THE BOYS ATTACK 3d. VIOLENCE OCCURS BECAUSE THE BOYS ATTACK THE DOG 3e. MEN HAVE TO BE OUT

736

WARNER

41. AFTER THE MEN KILL THE BOY THE COW KICKS STRIKES THE GIRL GOES THE DOG 4a. WHILE THE NEIGHBOR OWNS BARKS 4b. WHEN THE GIRL FEEDS THE HORSE THE DOG BARKS LOUDLY 4c. MEN ARE VIOLENT BEFORE THE BOY THAT THE COW HURT ATTACKS 4d. THE MEN ATTACK THE BOY BEFORE THE COW IS VIOLENT 4e. THE DOG THAT CHASES CATS IS FED BY THE WOMAN

AND

GLASS

8d. VIOLENCE OCCURS BECAUSE HORSE ATTACKS THE BOY 8e. THE GIRL PUT THE DOG OUT

THE

9s. WHILE THE COW KICKS THE MAN KILLS 9a. WHEN THE DOG SLEEPS THE BOY PLAYS 9b. IF THE BOY PETS THE CAT HE SINGS 9c. THE MAN ATTACKS BECAUSE THE COW IS VIOLENT 9d. VIOLENCE OCCURS BECAUSE THE COW ATTACKS THE MAN 9e. TWELVE OUT OF TWELVE MEN ARE UGLY

51. WHILE THE BOYS STRIKE THE DOG THE HORSE KICKS KILLS 5a. BEFORE THE MAN GOES THE DOG THE NEIGHBOR OWNS BARKS THE MAN FEEDS THE CAT Sb. BEFORE THE GIRL SIGHS LOUDLY SC. THE BOYS ARE VIOLENT WHEN THE DOG THAT THE HORSE HURTS ATTACKS Sd. BOYS ATTACK THE DOG WHENEVER HORSES ARE VIOLENT 5e. THE CLOCK VERY AWKWARDLY STRUCK TWELVE

101. WHEN THE DOG BITES THE MAN THE CAT SCRATCHES KILLS 10a. BEFORE THE WOMAN GOES THE DOG THE NEIGHBOR OWNS BARKS lob. AFTER THE BOY FEEDS THE CAT THE GIRL SIGHS LOUDLY 10~. THE DOG IS VIOLENT WHILE THE MAN THAT THE CAT HURT ATTACKS 10d. THE DOG HURTS THE MAN WHILE THE CAT ATTACKS 10e. THE WOMAN IN THE RED CAR DRIVES RAPIDLY

6s. WHEN THE DOG BITES THE MAN KILLS 6a. AFTER THE BOY SLEEPS THE CAT EATS 6b. BEFORE THE BOY PETS THE CAT HE SINGS 6c. THE MAN ATTACKS BECAUSE THE DOG IS VIOLENT OCCURS BECAUSE THE 6d. VIOLENCE DOG ATTACKS THE MAN 6e. THE GIRL HAS BOYS HAVE CATS

11s. IF THE BOY KILLS THE MAN STRIKES lla. WHILE THE CAT SLEEPS THE GIRL PLAYS 11 b. WHILE THE GIRL PETS THE CAT SHE SINGS llc. THE MAN ATTACKS WHEN THE BOY IS VIOLENT lid. VIOLENCE OCCURS WHEN THE BOY ATTACKS THE MAN 1 le. THAT VERY GIRL OWNS THE HORSE

71. BEFORE THE BOY KILLS THE MAN THE DOG BITES STRIKES 7a. WHEN THE GIRL GOES THE DOG THE NEIGHBOR OWNS BARKS 7b. IF THE GIRL FEEDS THE CAT THE WOMAN SIGHS LOUDLY BOY IS VIOLENT AFTER THE 7c. THE MAN THAT THE DOG HURTS ATTACKS 7d. THE BOY ATTACKS THE MAN AFTER THE DOG IS VIOLENT 7e. TO BE BELIEVED THE WOMAN MUST FIND THE LETTER 8s. AFTER THE HORSE KICKS THE BOY STRIKES 8a. IF THE GIRL SLEEPS THE CAT EATS 8b. WHEN THE GIRL PETS THE CAT SHE SINGS 8c. THE HORSE IS VIOLENT BEFORE THE BOY ATTACKS

121. BEFORE THE BOY STRIKES THE MAN THE HORSE KICKS KILLS 12a. IF THE GIRL GOES THE DOG THE NEIGHBOR OWNS BARKS THE WOMAN FEEDS THE CAT 12b. WHEN THE MAN SIGHS LOUDLY BOY IS VIOLENT WHEN THE 12c. THE MAN THAT THE HORSE HURTS ATTACKS BOY ATTACKS THE MAN BE12d. THE CAUSE THE HORSE IS VIOLENT 12e. THAT CAT IS THE CAT THE GIRL WAS TO PUT OUT

Transitive

Garden

Path Sentences

1s. AFTER THE DOG BITES THE MAN CAT KILLS la. IF THE STUDENT COMPLETES WORK THE CLOCK CHIMES lb. IF THE BOY SWIMS THE CAT THE MAN OWNS PLAYS

THE HER THAT

CONTEXT

AND

GRAMMATICALITY

THE CAT IS VIOLENT BECAUSE THE DOG ATTACKS THE MAN Id. THE DOG IS VIOLENT BEFORE THE MAN IS ATTACKED BY THE CAT le. IS THE DOG BELIEVED TO BE UGLY

IC.

21. WHILE THE COW KICKS THE MAN THE BOY STRIKES THE GIRL 2a. AFTER THE WOMAN CONTACTS THE POLICE THE COP ARRESTS THE SUSPECT 2b. BEFORE THE GIRL SLEEPS THE CAT THAT THE MAN BROUGHT CRIES 2c. THE BOY ATTACKS THE GIRL WHEN THE COW HURTS THE MAN 2d. THE GIRL ACTS BECAUSE THE COW HARMS THE MAN THAT THE BOY ATTACKS 2e. THE GIRL HAS TO BE OUT OF THE HOUSE BEFORE THE CAT 3s. IF THE BOYS STRIKE THE DOG THE HORSE KICKS 3a. WHEN THE MEN COMPLETE THEIR WORK THE CLOCK CHIMES 3b. WHEN THE MEN SWIM THE DOGS THAT THE GIRLS OWN PLAY 3c. THE HORSE IS VIOLENT WHEN THE BOYS ATTACK THE DOG 3d. BOYS ARE VIOLENT WHEN THE DOG IS ATTACKED BY THE HORSE 3e. MEN HAVE TO BE OUT 41. AFTER THE BOY KILLS THE MAN THE CAT STRIKES THE DOG 4a. IF THE GIRL CONTACTS THE POLICE THE COP ARRESTS THE SUSPECT 4b. IF THE MAN SLEEPS THE DOG THAT THE BOY BROUGHT CRIES 4c. THE CAT ATTACKS THE DOG BECAUSE THE BOY HARMS THE MAN 4d. THE DOG ACTS WHEN THE BOY HARMS THE MAN THAT THE CAT ATTACKS 4e. THE DOG THAT CHASES CATS IS FED BY THE WOMAN 51. WHILE THE MEN KILL THE BOY THE COW KICKS THE DOG 5a. BEFORE THE WOMAN CONTACTS THE POLICE THE COP ARRESTS THE SUSPECT 5b. IF THE GIRL SLEEPS THE CAT THE MAN BROUGHT CRIES SC. THE COW IS PANICKED BY THE DOG WHEN THE MEN ATTACK THE BOY Sd. THE DOG ACTS WHEN THE MEN ATTACK THE BOY THAT THE COW HARMS 5e. THE CLOCK VERY AWKWARDLY STRUCK TWELVE

737

JUDGMENTS

6s. WHEN THE GIRLS STRIKE THE CAT THE HORSE KICKS 6a. AFTER THE WOMEN COMPLETE THEIR WORK THE CLOCK CHIMES 6b. BEFORE THE MAN SWIMS THE DOG THAT THE BOY OWNS BARKS 6c. THE HORSE LASHES OUT BECAUSE THE GIRLS ATTACK THE CAT 6d. THE GIRLS ARE VIOLENT WHEN THE CAT IS HURT BY THE HORSE 6e. THE GIRL HAS BOYS HAVE CATS 71. BEFORE THE BOYS STRIKE THE DOG THE HORSE KICKS THE CAT 7a. AFTER THE MAN CONTACTS THE POLICE THE COP ARRESTS THE SUSPECT 7b. WHILE THE MAN SLEEPS THE CAT THAT THE GIRL BROUGHT CRIES 7c. THE HORSE ATTACKS THE CAT WHILE THE BOYS PLAN TO HIT THE DOG 7d. THE CAT ACTS AFTER THE BOYS ATTACK THE DOG THAT THE HORSE HURTS 7e. TO BE BELIEVED THE WOMAN MUST FIND THE LETTER 8s. AFTER THE HORSE KICKS THE BOY THE DOG BITES 8a. WHILE THE MAN COMPLETES HIS WORK THE CLOCK CHIMES 8b. WHEN THE WOMAN SWIMS THE CAT THAT THE MAN OWNS PLAYS 8c. THE DOG IS VIOLENT BECAUSE THE HORSE ATTACKS THE BOY 8d. THE HORSE IS VIOLENT BEFORE THE BOY IS ATTACKED BY THE DOG 8e. THE GIRL PUT THE DOG OUT 9s. AFTER THE COW KICKS THE GIRL THE HORSE BITES 9a. WHILE THE BOY COMPLETES HIS WORK THE CLOCK CHIMES 9b. WHILE THE BOY SWIMS THE DOG THAT THE MAN OWNS PLAYS 9c. THE HORSE IS VIOLENT BECAUSE THE COW HARMS THE GIRL 9d. THE COW IS VIOLENT BEFORE THE GIRL IS ATTACKED BY THE HORSE 9e. TWELVE OUT OF TWELVE MEN ARE UGLY 101. IF THE DOG BITES THE MAN KILLS THE HORSE 1Oa. WHEN THE BOY CONTACTS LICE THE COP ARRESTS PECT lob. WHEN THE BOY SLEEPS THAT THE MAN BROUGHT

THE

BOY

THE POTHE SUSTHE CRIES

CAT

WARNER AND GLASS

738

10~. THE BOY ATTACKS THE HORSE WHEN THE DOG HURTS THE MAN 1Od. THE HORSE ACTS BECAUSE THE DOG HARMS THE MAN THAT THE BOY ATTACKS 10e. THE WOMAN IN THE RED CAR DRIVES RAPIDLY 11s. WHILE THE BOY KILLS THE MAN THE CAT STRIKES lla. BEFORE THE GIRL COMPLETES HER WORK THE CLOCK CHIMES llb. BEFORE THE GIRL SWIMS THE HORSE THAT THE BOY OWNS TROTS Ilc. THE CAT ATTACKS BECAUSE THE BOY HARMS THE MAN lld. THE BOY ATTACKS WHEN THE MAN IS HURT BY THE CAT 1le. THAT VERY GIRL OWNS THE HORSE 121. WHEN THE HORSE KICKS THE BOY THE DOG BITES THE MAN 12a. WHILE THE WOMAN CONTACTS THE POLICE THE COP ARRESTS THE SUSPECT 12b. WHILE THE WOMAN SLEEPS THE CAT THAT THE GIRL OWNS CRIES 12c. THE DOG ATTACKS THE MAN BECAUSE THE HORSE INJURES THE BOY 12d. THE MAN ACTS WHEN THE HORSE HURTS THE BOY THAT THE DOG ATTACKS 12e. THAT CAT IS THE CAT THE GIRL WAS TO PUT OUT REFERENCES BRUNING, J. L., & KINTZ, B. L. (1977). Computational handbook of statistics. Glenview, IL: Scott, Foresman. CLIFTON, C., FRAZIER, L., & CONNINE, C. (1984). Lexical expectations in sentence comprehension. Journal

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