Processing complexity and sentence memory: Evidence from amnesia

BRAIN

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LANGUAGE

42, 431-453 (1992)

Processing Complexity and Sentence Memory: Evidence from Amnesia LEWISP. SHAPIRO,*PATRICK MCNAMARA,~ EDGAR ZURIF$$ SUSAN LANZONI,~ AND LAIRD CERMAK~ *Department of Psychology, Florida Atlantic University; TMemory Disorders Research Center, Boston University School of Medicine; $Aphasia Research Center, Boston University School of Medicine; and 8Brandeis University

This study examines sentence memory as a function of linguistic processing complexity in amnesic patients. Sentence length as well as lexical and syntactic complexity were manipulated in two sentence repetition experiments. It was found that the amnesic patients performed considerably worse than the control subjects and that performance decreased: (1) when sentence length was increased by the addition of adjuncts compared to arguments of the verb; (2) when the verb selected more thematic frames; and (3) when sentences involved “empty” argument positions that must be linked to antecedents, particularly across clausal boundaries. These data showed how linguistic complexity affects sentence memory and implied that the amnesic deficit did not involve a generalized difficulty for materials of similar length, rather, the deficit was specific to certain representational types and processing routines. 0 lW2 Academic Press, Inc.

We describe a sentence repetition study that explores the manner by which linguistic complexity affects sentence memory. By doing so we hope to shed light on the role and operation of sentence memory during language processing. As a starting point, consider what we mean by sentence memory: It is a memory system that we assume is required by the architecture and operation of the sentence processor; it acts on the representational vocabulary specific to on-line lexical, syntactic, and thematic computations. We use amnesic patients to investigate this system because these patients exhibit interesting short-term memory and verbal analytical The research reported here was supported by NIH Grants DCO0494, DCOOO81,and NS26985. We thank Chris Barry and two anonymous reviewers for their helpful suggestions. Address correspondence and reprint requests to Lewis P. Shapiro at the Department of Psychology, and Center for Complex Systems, Florida Atlanta University, Boca Raton, FL 33431. e-mail:ShapiroL@fauvax. 431 0093-934X/92 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

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deficits (Cermak, 1982) which, by hypothesis, could cause difficulties with processing complex linguistic structures. The paper is organized as follows: We begin with a summary of the memory requirements for sentence processing. We next review some neuropsychological evidence for a memory system specific to sentence processing. We then detail some of the relevant linguistic issues that underlie the sentence constructions used in our study. Finally, we describe two experiments using a sentence repetition paradigm. To forecast our results, we find that our amnesic patients perform worse than our control subjects and that performance decreases: (1) when sentence length is increased by the addition of adjuncts rather than arguments of the verb, all other variables being equal; (2) when the verb selects more possible thematic frames; and (3) when sentences involve “empty” argument positions that must be linked to antecedents. We interpret these data as indicating the operation of a sentence memory system; we can observe the limits of this system only when it is taxed by complex linguistic structures. MEMORY REQUIREMENTS FOR SENTENCE PROCESSING We begin with a review of the literature regarding the role of sentence memory. Our approach here uses evidence both from parsing research and from neuropsychological investigations. Parsing models usually assume a limited memory system where computations take place over the input. For example, Marcus’ deterministic parser (Marcus, 1980) uses a three- to five-cell “buffer” where linguistic constituents are placed in a “first-in, first-out” fashion. The parser uses this buffer as a workspace; it decides how to structure an active constituent only after examining the current information in the buffer. Similarly, Bet-wick and Weinberg (1984) assume a three-cell memory storage capacity with the ability both to “look ahead” over the content of these cells when disambiguation is necessary and to backtrack to search for the antecedent of a referent. The analogy to human working memory is clear: The buffer is a limited-capacity workspace; room in the buffer opens up for further input when work has been completed on the current representations placed within the buffer. Importantly, complex linguistic structures may place particular demands on the computations performed in the buffer. So, for example, in the sentence “It was the man with the hat that the girl kicked,” the direct object of kick-“the man with the hat”-has been displaced from its normal postverb direct object position to a position occurring well before the verb. When the unfilled direct object position is encountered in the string, a search backward over the input--some of which remains active (in a buffer)-must occur to link that position to its antecedent . These works on parsing come with an implicit assumption: Memory is viewed as domain-specific. That is, it is assumed that there must be mem-

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ory systems specific to each representational domain whereby particular kinds of input are held temporarily-r remain “activated’‘-while computations proceed. At the macro-level, language surely involves domainspecific representations and processing (Chomsky, 1981; Fodor, 1983). But even within the language domain a strong case can be made for separate memory processes. Klapp, Marshburn, and Lester (1983), for example, have suggested separate pools of memory rather than a unitary system (see also Monsell, 1984); there is a large body of evidence suggesting a “passive phonological store”; and there is evidence suggesting memory processes specific to syntactic computations (e.g., to the construction of noun phrases, verb phrases, etc.) and to the assignment of semantic/thematic roles (e.g., Agent of the action, Object of the action, etc.) (Caplan & Waters, 1990; Jackendoff, 1987a). Neuropsychological evidence also suggests a division of labor in memory. For example, Baddely and his colleagues (Baddely, 1986; Baddely, Vallar, & Wilson, 1987) have proposed that “working” memory is composed of a nonspecialized central executive as well as more specialized subsystems-the articulatory loop and visual-spatial scratch pad. The articulatory loop in turn is composed of a phonological store and a rehearsal process which ensures that the contents of the store remain active long enough to be entered in a long-term memory. Baddeley (1986) argued from neuropsychological data that the phonological store is crucial for sentence comprehension; that is, he demonstrated that sentence comprehension was impaired in span-impaired patients. However, our purpose in this study was not to investigate Baddeley’s notion of working memory. Although some patients with neurological impairment resulting in severely restricted memory spans sometimes evince an associated sentence comprehension deficit (see also Vallar & Shallice, 1990), others show relatively good comprehension. Caplan and Waters (1990), reviewing the literature on phonological memory and sentence processing, concluded that phonological representations maintained in a short-term memory play little if any role in language processing (see also Martin, 1990). Finally, although the passive phonological store appears to be preserved in amnesic patients, there is independent evidence that other short-term memory systems are not (Cermak, 1982; Parkinson, 1982). In his review of short-term memory in amnesia, Parkinson concluded that amnesics performed more poorly than controls on the Brown-Peterson distractor task, memory search, free recall, and supraspan serial recall. That is, although these patients are span-preserved in terms of their short-term memory, other aspects of their immediate memory performance are far from normal. Perhaps more to the present point, amnesic patients have been observed on the Boston Diagnostic Aphasia Exam to be deficient in their ability to repeat a sentence immediately following its presentation. Their overall language ability falls within normal limits, yet for some of

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the more complex sentences, errors in repetition do occur. Of course, this could be because of sentence length, in which case length of sentence would predict probability of being correct. However, sentence length alone does not appear to be the sole detriment since complexity (in this case probability level) seems to interact with length. Most theories of abnormal memory such as a consolidation deficit (Milner, 1968; Moscovitch, 1982) or retrieval deficit (Warrington & Weiskrantz, 1970, 1978) would not predict such an interaction. However, encoding theories (Cermak, 1979, 1989) would make such a prediction because the hypothesis of deficient verbal analysis has always been forwarded as the underlying cause of memory deficiency by this theory. Analytic deficits could play a role in determining clinically observed sentence repetition difficulty. Perhaps as sentences become more complex, and difficult to analyze, they become less memorable. Again, it is with this sense in mind that we use the term “sentence memory.” In order to investigate how linguistic complexity, analytical processing routines, and sentence memory interact, we decided to investigate amnesic’s ability to repeat several different types of linguistically complex sentences. But before describing our experiments, we need first to outline a current linguistic framework within which our experimental manipulations might be best understood-manipulations designed to tax sentence memory. LINGUISTIC ISSUES Verb Representation

We first consider the way in which verbs might be organized in the mental lexicon and how the complexity of that organization affects normal sentence processing. Lexical entries appear to consist of, at least, a word’s syntactic category, phonological form, subcategorization frame, argument structure (a-structure), and thematic information (Chomsky, 1986; Grimshaw, 1990; Jackendoff, 1987b; Levin, 1990; Pinker, 1989).’ These properties are idiosyncratic; each verb selects its own set of properties from among a limited class of each property. For present purposes we will only describe astructure and thematic information. Consider: 1. send,

a-structure: thematic grid:

(XYY) b? Yt 4 (Agent Theme) (Agent Theme Goal)

’ Recent linguistic work has suggested that thematic roles and complex semantic types are epiphenomenal descriptions taken from the lexical-conceptual structure of verbs (Grimshaw, 1990; Jackendoff, 1987b).

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2. [,,Joelle] [vr [v sent] [NPthe book]] 3. k.JJoeW 1vp [v sent] [NPthe book] [pp to [NpMitzi]]] Argument structure characterizes the number of arguments that is associated with a verb. Sentence 1 shows that send allows both a single, obligatory two-place a-structure (x,y)-as in sentence 2-and a threeplace structure (x,y,z) as well-as in sentence 3. In 3 the x-argument refers to the subject NP (or “external” argument), “Joelle,” the y-argument refers to the direct object NP, “the book,” and the z-argument refers to the indirect object NP, “Mitzi.” Also represented in the lexical entry is the thematic grid-a set of thematic roles (e.g., Agent, Theme, Goal) that must eventually be assigned onto the arguments in a sentence. So, for example, the subject NP-“Joelle’‘-in sentence 3 is assigned the Agent role specified by the verb, the object NP-“the book”-is assigned the Theme, and the indirect object NP-“Mitzi”-is assigned the Goal. These thematic roles give an approximation of a semantic description of the sentence in which the verb is embedded. Verbs select also for complements having more complex semantic realizations (Grimshaw, 1979, 1990; Pesetsky, 1983). Consider, for example: 4. We [vp [“knew] [sthat Joelle would be wild]] Proposition (P) 5. We [vp [“knew] [show wild Joelle would be]]! Exclamation (E) 6. (Only) we [vp [“knew] [show wild Joelle would be]] Interrogative (Q) Sentences 4-6 show that ~~ZOW, although allowing a sentential complement an S) in all cases, nevertheless allows different complex semantic i.e., ( realizations. We can thus partially represent know in the following way: 7. know,

argument structure: (x,y) thematic grids: x ‘knows’ y; y--+y

a P) ii a anQ?

Notice in this framework that know has several thematic possibilities or subentries, each corresponding to a different semantic realization of its single syntactic frame-a sentential clause. In summary, lexical entries for verbs include representations referring to a-structure and thematic information, thereby expressing the general syntactic and semantic character of a sentence in which a verb is embedded. Such are the facts about lexical representations. We now turn to some processing work that exploits these facts. In a series of psycholinguistic studies, Shapiro and colleagues (e.g.,

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Shapiro, Zurif, & Grimshaw, 1987; Shapiro, Brookins, Gordon, & Nagel, 1991) sought to explore whether thematic representations might have an affect on verb processing during on-line sentence comprehension. We exploited what we called the “representational complexity” of verbscomplexity defined in terms of the number of different thematic frames each verb allows. A verb with fewer frames or subentries was considered less complex than a verb with more frames or subentries. Our “pure transitive” verbs allowed only a single, two-place thematic entry (Agent Theme), and our “dative” verbs allowed two subentries, both a two-place (Agent Theme) and a three-place representation (Agent Theme Goal). Also, our “two-complement” verbs allowed two subentries: a simple twoplace representation (Agent Theme) and an additional subentry whereby the argument had a single complex semantic realization (a Proposition). Our “four-complement” verbs allowed four subentries: a simple two-place representation (Agent Theme) and three additional subentries, each selecting a different complex semantic realization (a Proposition, an Interrogative, and an Exclamation). We placed instances of these verb types into similar, simple NP-VNP-PP sentences and presented these sentences to normal listeners via headphones. Immediately after encountering the verb in the sentence, subjects were required to perform a complex secondary task (visual lexical decision), and reaction times (RTs) to this task were recorded. It was assumed that as the verb became more complex in terms of its number of different thematic subentries, the time taken to make the secondary lexical decision would increase. We found the following: pure transitive verbs, those that were the least complex in terms of thematic information, resulted in significantly faster RTs on the secondary task than datives. And two-complement verbs yielded significantly faster RTs than fourcomplement verbs. We interpreted this pattern of data as showing that as the verb becomes more complex in terms of its thematic information, processing load increases. In effect, all thematic information about a verb is momentarily and exhaustively activated, and this activation is computationally expensive. One explanation of these data involves processing resources: The longer RTs associated with verbs that have more thematic information may be due to having to hold-or activate-these thematic possibilities when the verb is accessed, thereby subsequently allowing the appropriate thematic roles to be assigned onto the argument positions in sentences. In the experiments to follow, we exploit this notion of thematic complexity and its role in sentence processing. Arguments versus Adjuncts Implicit in the organization of verb lexical entries is the distinction between an argument of the verb and an adjunct. The distinction is not

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an easy one to make, but it appears to have processing implications. An argument is idiosyncratically selected by the verb; it forms part of the verb’s entry in the lexicon. An argument is thus assigned its thematic role by the verb. An adjunct is not selected by particular verbs; any verb can appear in a sentence with an adjunct and thus the adjunct does not need to be specified as part of a verb’s entry. Consider: 8. Mitzi sent the car to the garage 9. Mitzi fixed the car in the garage In 8 the verb send assigns three thematic roles to its arguments, an Agent to the NP “Mitzi,” a Theme to the NP “the car,” and a Goal to the NP “the garage” falling within the PP “to the garage.” The verb fix in 9 assigns only two thematic roles to its arguments, an Agent to the NP “Mitzi,” and a Theme to the NP “the car.” The PP “in the garage” is considered a locative adjunct (i.e., ‘the garage is where the car was fixed’); it’s “meaning” is not inherent in the verb’s representation. There is also a structural distinction between an argument PP and an adjunct. A PP that contains an argument of the verb is a sister to the head of the verb phrase, i.e., on the same phrasal level as the verb in a phrase structure configuration. An adjunct PP can be attached to a phrase structure level dominating the VP, that is, at a higher level in the phrase structure configuration of the sentence, or can be attached at a lower level than the V. Finally, an argument of the verb can be either obligatory or optional. For example, in 8 the third argument of send is optional; it can be left out of the sentence (as in “Mitzi sent the car”) yet the verb carries it implicitly (one has to send something to someone or somewhere). An argument can also be obligatory, as in “Mitzi put the car in the garage”; omitting the PP in this sentence will render it ungrammatical (*“Mitzi put the car”). The appearance of an adjunct, however, is always optional. There are processing implications to this argument/adjunct distinction. Abney (1989) has suggested that the human sentence processing device prefers an argument to an adjunct. Clifton, Speer, and Abney (1991) recently found that reading times are faster when a prepositional phrase is an argument rather than an adjunct. One interpretation of these data is that arguments are preferred and adjuncts are not because the former match the thematic frame of the theta (thematic role)-assigning verb and the latter do not. Our own work has also implicated this distinction: Shapiro, Nagel, and Levine (1991a) found a greater processing load in the immediate vicinity of a preposition that heads an adjunct PP than in the vicinity of a preposition that heads a PP that contains an argument of the verb. And Canseco-Gonzalez, Shapiro, Zurif, and Baker (1991) found that a severe Broca’s aphasic patient trained in an artificial language had significantly more difficulty learning symbols that depicted “verbs”

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embedded in “sentences” where the third referent was an adjunct rather than an argument. We exploit the argument/adjunct distinction in the experiments to follow. Sentence Structure In the following experiments we also manipulate the syntactic construction of the sentences that subjects are asked to repeat. The sentence types used in the experiments include actives, passives, subject-cleft relatives, and object-cleft relatives, much like the following: 10. The girl hit the boy: Active 11. The boy was hit by the girl: Passive 12. It was the girl that hit the boy: Subject-cleft 13. It was the boy that the girl hit: Object-cleft English, like most languages, allows NPs in a number of grammatical positions to each convey the same thematic role. Thus, although sentences lo-13 convey different syntax and different discourse-level semantic features such as focus, all four convey the same thematic roles linked to the same argument positions. This invariance among thematic roles set within different syntactic structures can be captured by Chomsky’s (1981) theory of D- and S-structure and the constituent movement rules that relate one to the other.2 The underlying form, or D-structure, of sentences lo-13 can be characterized roughly as follows: 10’. 11’. empty 12’. 13’.

The girl hit the boy [e] was hit the boy the girl ([e] stands for a phonologically null category) It was the girl [COMP] [the girl hit the boy] It was the boy [COMP] [the girl hit the boy]

In 11-13’ a transformation (i.e., “movement” rule) obligatorily moves the NP the boy to its final position in the surface realization of the sentence, leaving behind a “trace” of that movement that is coindexed with the moved NP. The “landing site” for the moved constituent in 11 is the empty category; in 12 and 13 it is the position marked COMP (for “complementizer,” lexically filled by “who” or “that,” for example). The effect ’ We are using the government-binding theory of Chomsky (e.g., 1981, 1986) as our descriptive framework. But of course there are several other theoretical frameworks. For example, Generalized Phrase Structure Grammar (GPSG) (see Gazdar, Klein, Pullum, & Sag, 1985) is a nontransformational theory; it does not assume that constituents move from one sentence position to another. However, GPSG does have a mechanism that directly connects an “empty” direct object position, for example, to its antecedent. For the present purposes, the differences among these theories can be ignored.

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of this movement is to create an S-structure, roughly represented in lO”13” (we include only the information relevant to the present discussion): 10”. The girl hit the boy 11”. [NPthe bOy]i was hit [NPtrace]i by [NPthe girl] 12”. It was [NPthe girl]i [who], [NPtrace]i hit [NPthe boy] 13”. It was [NPthe bOy]i [who]i [NPthe girl] hit [NPtra~e]~ Sentences 11-13 are thus said to have a “moved constituent.” One focus of the present work is the effect of constituent movement on sentence repetition. That is, we will address the question of whether it is more difficult to repeat sentences with moved constituents-sentences that require linking the moved constituent with its antecedent-than sentences where constituents do not move between D- and S-structure. There is independent evidence for such a “complexity” effect: Most sentences with moved constituents are more “structurally dense”-have more phrasal nodes in their phrasal geometry-than sentences without moved constituents. And it turns out that a sentence with more phrasal nodes leads to more complicated analyses than a sentence with less phrasal nodes (Frazier, 1979; Frazier & Rayner, 1982). Relatedly, sentences with moved constituents are more difficult to comprehend than more “basic” forms for both normal subjects (King & Just, 1989; MacDonald, 1989) and some aphasic patients. For example, object-cleft sentences are more difficult to process than passives, and both are more difficult than active sentences (Caplan & Hildebrandt, 1988; but see Grodzinsky, 1990). Furthermore, Caplan and colleagues claim that their data are compatible with the possibility that some aphasic patients have an impairment in the use of a “working memory” system, since sentences with moved constituents involve searching back over the input for an antecedent to a trace. And finally, Berwick and Weinberg’s (1984) parser assumes that complex constructions such as relative clauses yield a greater load on processing resources. In summary, there is evidence from normal and disordered sentence processing that both lexical and sentence-level representations contribute to the overall complexity of producing, understanding, and perhaps repeating sentences. We now report on two experiments designed to investigate the thematic complexity of verbs, the syntactic complexity of sentences, and the argument/adjunct distinction on sentence repetition. Given both the analytical deficits shown by amnesic patientehypothesized to contribute to their memory disorder-and the clinical evidence that these patients have difficulty repeating some complex sentences, we predict that amnesic patients will have difficulty with sentence repetition when the sentences require complex processing routines.

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EXPERIMENT

1

Target sentences were manipulated by varying the sentences along three dimensions: a syntactic dimension where sentences in the active and passive voices are compared with their counterparts cast into cleft-subject and cleft-object forms; a predicate dimension where “pure transitive” verbs that allow one thematic possibility are compared with “dative” verbs that allow two possibilities; and a dimension we call “padding” whereby sentence length is manipulated by adding two or three adjectives as part of subject noun phrase or by adding a prepositional phrase appearing after the direct object noun phrase. This padding device allowed us to compare length effects where length is defined by sheer number of words forming different syntactic units. Method Subjects. The subjects in this study were all right-handed adults who were paid for their participation. There were six amnesics (mean age = 58.6), six alcoholic controls (mean age = 54.8), and six young (nonpaid volunteers) subjects (mean age = 22.3). All of these young subjects performed virtually flawlessly and so will not be discussed further. All but one of the amnesics carried diagnoses of alcoholic Korsakoff’s syndrome and lived in chronic care facilities. The non-Korsakoff amnesic had suffered an anoxic episode during a heart attack (no significant differences in repetition performance occurred between this anoxic amnesic and the other Korsakoff amnesic patients). All amnesic patients had severe anterograde amnesias as well as varying degrees of retrograde amnesia. They were unable to adequately recall day-to-day and current events. Some aspects of short-term memory were also impaired in all amnesic patients: None of these patients exhibited normal release from proactive interference (using a variant of Wicken’s procedure), and supra-span serial recall of word lists was abnormal. Forward digit span (mean = 7.2), backward digit span (mean = 6.1), confrontation naming (Boston Naming Test, mean = 74.6), and word associations (paradigmatic to syntagmatic responses = 74/13.5) were all within normal limits. The alcoholic controls digit spans were also within normal limits. As part of an initial neuropsychological work-up, all amnesic patients received a verbal WAIS-R (verbal = 95.7; performance = 89.6) and a WMS-R (verbal = 69.0; visual = 71.4; attention = 95.8; and delay = 54.5). Marerie&. Each verb (five each of pure transitive and four-complement verbs) was inserted into sentences of the following syntactic types: active voice, passive, subject-cleft, and objectcleft. Each sentence type varied in length along three dimensions: no padding, padding with adjectives before the first noun, and padding with prepositional phrases (generally in direct object position). Example sentences are shown in Table 1. For the four-complement verbs, there were 5 sentences of each of the padding types yielding 15 sentences per sentence type for a total of 60 sentences. For the transitive verbs, we took the sentences with the four-complement verbs and duplicated them, constructing 60 sentences identical to the four-complement sentences except for the verb. We also generated an additional 60 sentences for the transitive verbs that contained different lexical items than those in the sentences embedded with four-complement verbs, yielding a total of 120 sentences embedded with transitive verbs. Our purpose in constructing two lists for the transitive verbs was to assessthe role of specific lexical items in repetition performance. In our analyses of repetition performance, we compared sentences embedded with fourcomplement verbs to both the duplicated (and therefore identical) sentences embedded with pure transitives and the other sentences (containing different lexical items) with pure tran-

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TABLE 1 EXAMPLE SENTENCESFROM EXPERIMENT 1: SENTENCE TYPE (ACTIVES, PASSIVES,SUBJECT-, ANDPADDING(No, ADJECTIVE,PP) VARIABLESFORTHE PURETRANSITIVE AND OBJECT-CLEFTS) VERB “HIT”

Actives No padding Adjective padding PP padding

The girl hit the boy The tall handsome man hit the lady The monkey hit the lion in the face

Passives No padding Adjective padding PP padding

The singer was hit by the dancer The performer was hit by the loud angry man The doctor was hit by the patient in the stomach

Cleft-subject relatives No padding Adjective padding PP padding

It was the doctor that hit the child It was the old ugly cowboy that hit the Indian It was the clown that hit the midget in the eye

Cleft-object relatives No padding Adjective padding PP padding

It was the soldier that the captain hit It was the scared young policeman that the thief hit It was the politician in the hall that the reporter hit

sitives. No significant differences in performance were observed between the duplicated and nonduplicated transitive sentences in any of our analyses (that is, it did not matter whether all the lexical items were the same or different across the pure transitive and four-complement sentence groups), so we collapsed all subsequent analyses, yielding a total of 120 sentences with pure transitives and 60 sentences with the four-complement verbs. Procedure. Subjects were tested individually. After each sentence was presented verbally to a subject, the subject was required to count backward from 10 to 1. The subject was then required to repeat the sentence verbatim. There were several practice sentences but the subjects never required more than two. They were allowed breaks whenever they asked. All responses were tape recorded and transcribed. Transcriptions were done by a trained, paid assistant naive to the purpose of the experiment and to the subject’s identities. Scoring. Only exact repetitions of the target were scored as correct. Proportion of words correctly recalled were tallied for each sentence. Omissions, substitutions, and perseverations were counted as errors.

Results Proportion of words correctly recalled across sentences. All statistical tests are based on the arcsine transformations of the proportions of words correctly recalled across sentences in a given condition or category. For example, there are 69 words in the 10 sentences that make up the active voice, transitive verb, adjectival padding category. If a subject recalled 65 words in this category his score would equal .95 for that category condition. A 2 (group) x 2 (verb) x 4 (sentence) x 3 (padding) mixed design

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SHAPIRO ET AL. TABLE 2 .JWRTION OF WORDY CORRECTLYRECALLED Across SENTENCES,EXPERIMENT 1

Verb Type Transitives

Four-complements Padding PP No

Sentence Type

No

Adjective

Control subjects Actives Passives Subject-clefts Object-clefts

.993 .968 .952 .95

.96 .%2 .955 .912

.968 .957 .958 ,967

Amnesic subjects Actives Passives Subject-clefts Object-clefts

.973 ,937 ,875 .758

.85 .90 .765 .72

,875 .907 .818 .885

Adjective

PP

,993 .945 ,965 ,955

.962 .96 .937 .963

.99 ,983 ,943 .97

,895 ,913 .802 .88

.85 .908 .783 .75

,928 .96 .748 .798

ANOVA was performed. Group was treated as a between-subjects factor and verb, sentence, and padding were within-subjects factors. Results of this analysis revealed a main effect of group. Alcoholic controls (mean = .96) performed significantly better than the amnesic patients (mean = .84), F(1, 10) = 7.25, p < .02. Main effects of sentence, F(3, 30) = 6.27, p < .002, and padding, F(2, 20) = 13.62, p < .0002, were also observed as well as a significant group x sentence interaction, F(3, 30) = 3.52, p < .02.

We performed separate sentence x verb x padding ANOVAs for each subject group based on our initial prediction that the amnesic group should show particular sensitivity to complex linguistic structures. Table 2 contains the data for this analysis. For the alcoholic control subjects, no significant effects or interactions were observed. Performance ranged from .91 to .99 on all variables. For the amnesic subjects, a main effect of sentence type, F(3, 15) = 5.15, p = .Ol, and padding, F(2, 10) = 12.25, p = .002, was observed. Tukey’s protected f tests were performed on the sentence variable data. Active sentences (mean = .895) yielded significantly better performance than both subject-clefts (mean = .799), p < .05, and object-clefts (mean = .798), p < .05. Passive sentences (.92) also yielded significantly better performance than both subject-clefts, p < .Ol, and object-clefts, p < .Ol. Protected t tests were also performed on the padding data. No padding (.879) and PP padding (.865) yielded significantly better performance than adjective padding (.816), p < .Ol in both cases. Proportion

of verbs correctly recalled in otherwise identical sentences.

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To what extent does repetition performance vary with verb type only? In order to control for local lexical effects on the verb, we exchanged sentence environments for each verb type. This gave us a set of 60 sentences for each verb type that were identical except for the verb (e.g., “The cat hit the mouse,” “ The cat discovered the mouse”). Are errors (e.g., substitutions, perseverations, lack of agreement, etc.) more likely with one verb versus the other? A group (amnesics, controls) x verb type (pure transitives, four-complements) mixed design ANOVA was performed. A main effect of group was observed: Alcoholic control subjects (.929) performed significantly better than the amnesic subjects (.826), 41, 10) = 35.04, p = .OOOl.A main effect of verb type was also observed: Pure transitive verbs (.913) yielded significantly better performance than the four-complement verbs (.84), F(1, 10) = 16.6, p = .002. Proportion of correct thematic role assignments. We examined the extent to which subjects made correct thematic role assignments onto argument positions. Errors involving the assignment of thematic roles were assumed The girl in cases of noun reversals (e.g., The boy kissed the girl w kissed the boy), verb substitutions where a new verb was substituted in place of the correct one and the new verb did not assign the same thematic roles as the correct one (e.g., the boy cherished the girl the boy sent the girl), and finally, omissions where a target noun in an argument position dropped out (e.g., the boy kissed the girl j kissed the girl). The number of possible thematic role assignments depended on the type of verb and sentence. For example, in the sentence “the boy cherished the girl,” two thematic roles need to be assigned, whereas in the sentence “the monkey hit the lion in the face” there are three possibilities for thematic assignment errors. We therefore took the proportions of the number of thematic assignment possibilities to the number of correctly assigned thematic roles. A group (controls, amnesics) x verb (pure transitives, four-complements) x sentence (actives, passives, subject-clefts, and object-clefts) x padding (none, adjective, and PP) mixed design ANOVA was performed on these thematic assignment data. Table 3 contains the data from this analysis. Results of this analysis revealed a main effect of group: Control subjects (.964) performed significantly better than amnesic subjects (.86), F(1, 10) = 12.51, p < .005. A main effect of sentence type, F(3, 30) = 17.36, p < .OOOl,padding, F(2, 20) = 6.73, p = .005, and a group by sentence type interaction, F(3, 30) = 6.11, p < .002, was also observed. We then performed separate verb x sentence type x padding ANOVAs for each subject group on the thematic assignment data. So far as the alcoholic control subject data were concerned, a main effect of sentence type was observed, F(3, 15) = 9.799, p < .OOl. Tukey’s protected t tests revealed that active sentences (.989) yielded significantly better performance than passives (.96), p < .Ol, subject-clefts (.967), p < .05,

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SHAPIRO ET AL. TABLE 3 PROPORTION CORRECT THEMATICASSIGNMENT,EXPERIMENT1 Verb Type Transitives

Four-complements Padding PP No

No

Adjective

Control subjects Actives Passives Subject-clefts Object-clefts

.983 1.0 ,967 .967

.983 .90 .983 908

.98.5 ,913 ,942 .973

,983 ,967 1.0 .967

1.0 ,983 ,983 .917

1.0 .937 ,927 ,905

Amnesic subjects Actives Passives Subject-clefts Object-clefts

.983 ,942 ,942 .85

,875 ,867 ,825 .642

.87 .823 .86 .822

.933 .933 .912 .883

.933 .95 .817 .683

.958 .947 .815 ,787

Sentence Type

Adjective

PP

and object-clefts (.939), p < .Ol; and both passives and subject-clefts yielded better performance than object-relatives, p < .05 and p < .Ol, respectively. So far as the amnesic subject data were concerned, a main effect of sentence type, F(3, 15) = 12.11, p < .OOl, and padding, F(2, 10) = 4.778, p < .05, was observed. There was also a significant sentence type x padding interaction, F(6, 30) = 2.935, p < .05. We then tested for effects of padding at each level of the sentence type variable. There were no significant differences among the padding conditions for either active or passive sentences. There were differences among the padding types within the subject-cleft sentences, F(2, 10) = 3.207, p = .08. For this sentence type, protected t tests found that no padding (.93) yielded significantly better performance than adjective padding (.821), p < .05; the PP padding condition (.84) did not yield significant differences. There were also significant differences among the padding types within the object-cleft sentences, F(2, 10) = 5.778, p = .02. Protected t tests found that both the no padding condition (.867), p < .Ol, and the PP condition (.804), p < .05, yielded significantly better performance than adjective padding ( .662). We also tested the effect of sentence type at each level of the padding condition. Within the no padding and PP padding conditions, active sentences (.958 and .914, respectively) yielded significantly better performance than object-clefts (.867 and .804, respectively), p < .05 in both cases. Within the adjective padding condition, actives (.904), passives

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(.908), and subject-clefts (.821) yielded significantly poorer performance than object-clefts (.662), p < .Ol in all cases. Discussion

Results of Experiment 1 demonstrated that: (1) Both subject groups were affected by verb complexity: When proportion correct was tallied at the verb, performance declined with the more complex four-complement verbs relative to the less complex transitives; (2) Alcoholic control subjects showed little effect of padding (due, perhaps, to ceiling effects), while the amnesic subjects were more affected by increases of length from the adjective condition relative to both the no padding and the PP padding conditions; and (3) Both subject groups showed an effect of sentence type: Object-clefts proved to be the most difficult sentence type. However, amnesic subjects were more affected by the sentence type variable, and as the complexity of the sentence increased (from actives to subject and object clefts), so to did the effect of padding. The difficulty subjects demonstrated with sentences in the relativized form suggests that the sentence processing mechanism handles these syntactic types differently from those in the passive or active voice. In the general discussion to follow our second experiment we entertain the possibility that the syntactic structure of relative clauses and their corresponding analysis might contribute to an increased processing load that subsequently presents greater difficulty for amnesic patients. The evidence that adjective padding yielded more difficulty than PP padding shows that it is not just the sheer number of words that affects repetition performance. Note, however, that the PPs in the sentences of Experiment 1 were adjuncts; they did not form an inherent part of the verb representation. The following experiment addresses whether adjunct and argument PPs affect sentence processing differently. EXPERIMENT

2

In this experiment we directly compared argument processing with nonargument (i.e., adjunct) phrasal processing by comparing dative with pure transitive verb types in active sentences. Again, the addition of a PP to a sentence with a dative verb is equivalent to adding an argument (of the verb) to the sentence. When a PP is added to a sentence with a pure transitive verb in it, however, the PP functions as an adjunct. So, if an effect of verb type is noted in this experiment it will be equivalent to an effect of the argument/adjunct distinction. The same subjects, methods, and procedures used in Experiment 1 were used here. Method Materials. Five dative verbs were compared with five transitive verbs in active sentences. There were also no padding and PP padding conditions. For the dative verbs, a total of 10

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PROPORTION OF

TABLE 4 ACROSSSENTENCES,

WORDS RECALLED

EXPERIMENT

Control subjects

Amnesic subjects

Verb type

None

PP

Transitive Dative Average

.99 997 .993

,953 .958 .956

Padding Average None .97 .977

.917 ,943 .93

2

PP

Average

,758 .882 .82

.837 .912

sentences (5 for each padding type) were construCted. Like the materials of Experiment 1, we then duplicated these 10 sentences but embedded them with pure transitive verbs. We also constructed another set of 10 sentences with pure transitive verbs, this time with different lexical items than those in the four-complement set. This manipulation yielded a total of 20 sentences embedded with the pure transitives.

Results Proportion of words correctly recalled across sentences. A 2 (group) x 2 (verb) x 2 (padding) mixed design ANOVA was performed on the transformed scores as in Experiment 1. Group was treated as a betweensubjects factor, with verb and padding as within-subjects factors. The data used in this analysis are shown in Table 4. Results of this analysis revealed a main effect of group: The proportion of sentences correctly recalled by control subjects (.97) was significantly better than that by amnesics (.87), F(1, 10) = 11.03, p < .005. An effect of verb type was observed: Sentences embedded with dative verbs (.945) yielded significantly better performance than sentences with pure transitive verbs (.90), F(1, 10) = 8.19, p < .02. An effect of padding was also observed: Sentences with no padding (.96) yielded significantly better performance than sentences with PP padding (.887), F(1, 10) = 18.06, p = .OOl. A significant group by verb interaction was observed, F(1, 10) = 5.99, p = .03, and the group by padding interaction approached significance, F(1, 10) = 4.36, p = .06. We next performed a verb x padding ANOVA for each subject group. For the control subject data, a main effect of padding was observed: Sentences with no padding (.993) yielded significantly better performance than sentences with PP padding (.956), F(1, 5) = 16.62, p < .OOl. No effect of verb type was observed. Since we were interested in the effect of adding the PP as an argument or as an adjunct, we directly compared performance on sentences containing PPs either embedded with pure transitive verbs or dative verbs; no differences occurred. For the amnesic subject data, an effect of verb type was observed: Sentences with dative verbs (.92) yielded significantly better performance than sentences with pure transitives (.837), F(1, 5) = 9.67, p < .05. A

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SENTENCE MEMORY TABLE 5 PROPORTIONCORRECTTHEMATIC ASSIGNMENT, EXPERIMENT 2

Control subjects

Amnesic subjects Padding

Verb type Pure transitive Dative Average

None

PP

None

PP

1.0 1.0 1.0

.97 ,952 .961

.975 .967 .971

.813 ,783 ,798

main effect of padding was also observed: Sentences with no padding (.93) yielded significantly better performance than sentences with PP padding (.828), F(1, 5) = 8.12, p < .05. A direct comparison of performance on sentences containing PPs and with transitive verbs (.758) to the same sentences with dative verbs (.898) yielded a significant difference, F(1, 5) = 9.93, p = .02. Proportion of verbs recalled in otherwise identical sentences. We per-

formed a group x verb type ANOVA on the proportion of verbs correctly recalled. A main effect of group was observed: Control subjects (.912) performed significantly better than the amnesic subjects (.729), F(1, 10) = 7.744, p = .02. For the control subjects, transitive verbs (.925) resulted in better performance than dative verbs (.90), and for the amnesic subjects, transitives (.758) resulted in better performance than datives (.70); these differences were not significant, however. Proportion of correct thematic role assignments. We performed a group by verb type (pure transitives, datives) by padding (none, PP) mixed design ANOVA on the proportion correct thematic assignments. The data for this analysis are shown in Table 5. A main effect of group was observed: Alcoholic control subjects (.98) performed significantly better than the amnesic subjects (.894), F(1, 10) = 10.97, p < .Ol. An effect of padding was also observed: Sentences with no padding (.985) yielded significantly better performance than sentences with PP padding (.883). A significant group by padding interaction was observed, F(1, 10) = 9.56, p = .Ol. The source of this effect was the amnesic subjects: Sentences without padding (.97) yielded significantly better performance than sentences with PP padding (.798), F(1, 5) = 27.31, p < .003. Discussion

Results from Experiment 2 revealed that: (1) amnesic subjects performed worse overall than the alcoholic control subjects; (2) sentences containing a PP yielded worse performance than sentences without any increase in length; and (3) sentences embedded with pure transitive verbs-those in which a PP is an adjunct-yielded poorer performance

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than sentences embedded with dative verbs-those in which a PP is an argument of the verb.3 This distinction was particularly evident for the amnesic subjects. These results show that although both subject groups were affected by increases in length, amnesic subjects differentially benefitted when this increase involved a prepositional phrase that could be construed as an argument of the verb (i.e., when the sentence contained a dative verb) rather than as an adjunct (i.e., when the sentence contained a pure transitive verb). This distinction suggests once again that the amnesic deficit is not a generalized difficulty for materials of similar length that perhaps exceeds their capacity, rather, it appears that their deficit is specific to certain information types or processing routines--in this case, all other things being equal, adjuncts are more difficult to analyze than arguments. GENERAL DISCUSSION Summarizing both experiments, we find that: (1) amnesic subjects performed more poorly than alcoholic control subjects; (2) both subject groups were affected by increases in length (particularly the nonargument kind), by the cleft sentence type, and by verb complexity; (3) amnesic performance was particularly affected when complex sentences were involved, and performance significantly decreased when these sentences contained nonargument length increases; and (4) amnesic subjects showed considerable sensitivity to the argument/adjunct distinction, with adjuncts posing more difficulty. These results demonstrate a sentence memory deficit in severe anterograde amnesics. The deficit involves the memory system that mediates, at the very least, sentence repetition and perhaps extends to more common postures of sentence comprehension and production. Our data, however, allow us to provide some details concerning the role and operation of this system. To provide a context for these details we need first to consider the general requirements of a sentence processor: It must assign syntactic structure to incoming strings of words; it must assign thematic roles (e.g., agent, theme, goal); it must relate anaphoric elements (including empty categories) to their antecedents; and it must convert syntactic representations into representations appropriate for semantic interpretation. Although many of these analyses seem to be accomplished automatically, 3 At first blush, the fact that sentences containing dative verbs increase performance relative to sentences containing pure transitives appears at odds with the verb complexity hypothesis-that datives are more complex than pure transitives. We point out, however, that the verb complexity hypothesis is concerned with local effects at the verb, while the argument-adjunct distinction is concerned with more globally defined sentence-level effects. Indeed, when examining performance solely at the verb, datives yielded decreased performance compared to transitives-and of course four-complement verbs yielded worse performance relative to transitives in Experiment 1.

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the parser nevertheless has a limited processing capacity and is therefore vulnerable to stresses including neurological damage. One model of a limited capacity natural language parser is that proposed by Berwick and Weinberg (1984). Roughly, the model contains an input buffer composed of three cells into which words from the input stream are fed. When the cells are filled the parser accessesthe word in the first position of the buffer and if necessary “looks ahead” to see the words in the other cells. This look ahead capacity allows resolution of cases of local ambiguity so that structural assignment can be made instantaneously. If ambiguity resolution is not possible on the basis of a three-cell look ahead the parser must initiate some kind of second-passreanalysis routine. Once a structure is assigned to a phrase, the parser temporarily pushes away the constituent (roughly the current elements in the buffer) into a “pushdown stack” and then begins work on the next phrase. When two or more phrases have been constructed theta roles can be assigned. When the NPs in a set of constituents have received theta roles, the phrase is removed from the pushdown stack and placed into a propositional list. This propositional list is a semantic representation of the predicate-argument structures of the sentence, corresponding in government-binding theory to the level of logical form (Chomsky, 1981). When anaphoric or referential dependencies need to be coindexed with their antecedents, the parser searches for a structurally appropriate antecedent by looking back over the structure it has created up to that point. This “looking back” or search process may require expensive analytical resources since the referential element needs to be “held” or remain active while the search for an antecedent proceeds. In this study we utilized sentences whose “referential” dependencies result from the movement of NPs out of their canonical positions in Dstructure into various positions in the S-structure of the sentence. When such movement takes place, the moved constituents leave behind a “trace” (a phonologically empty abstract element or category) that must then be coindexed with its antecedent (the moved element). Linguistic theory distinguishes between various types of movement and referential dependencies. Two of the most studied types of movement are “NP-movemerit”-as in passive sentences-and “Wh- movement”-as in relative clauses and wh-questions. NP-movement is always to a non-theta marked argument position (i.e., a structurally defined logical position usually occupied by such semantically defined elements as agent, patient, goal, etc.), whereas Wh-movement can involve movement of the NP into a nonargument position (usually specifier position of a “camp” phrase, lexically filled as “that” or wh-words, for example). As detailed in the introduction, both types of movement leave behind empty elements or traces that must eventually be coindexed or linked with an antecedent. The look back or search process performed by the parser may proceed

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differently for these different types of movement. For example, consider, again, the (partial) S-structure for the following passive and object-relative cleft sentences (we have simplified these representations, keeping only the relevant information): 14. [s [Nr the boy]i was hit [NPtruceli by [NPthe girl]] 1.5. It was [NPthe bOy]i [s [WhO]i [s [NPthe girl] hit [NPtruceli]] Note that 14 shows that only one S-node is required for the passive, yet in the object-cleft 15 both a more dominant S (called an S-bar) and an embedded S are required. Just in terms of structural density, then, 15 is more complex than 14. Indeed, Berwick and Weinberg (1984) point out that the S-bar in sentences like 15 causes the parser to wait for various structural attachments to occur before the parser can coindex a Whelement to its trace. On the other hand, the passive trace (from NPmovement) can be coindexed immediately since no S-bar is required.4 We suggest that this extra parsing complexity associated with Wh-movement contributed to the difficulty amnesics experienced when trying to repeat these types of (subject-cleft and object-cleft) sentences. A complementary explanation for our results relates the amnesic difficulty with relativized sentences and adjuncts to the extra analyses a thematic processor must perform to assign thematic roles to these structural types. Basically, the thematic processor suggested in most sentence processing models (see, for example, Ferriera & Henderson, 1990; Friederici & Frazier, 1990; Shapiro et al., 1992; Tanenhaus & Carlson, 1989; etc.) must identify those argument positions that require thematic role assignment by the verb and those that do not, must activate all thematic frames for a verb, must evaluate and choose, one of these frames, and then must assign thematic roles onto argument positions. We suggest that the thematic processor is of a limited analytical capacity and is taxed when: (1) theta-assigners (e.g., verbs) appear later rather than earlier in the sentence since arguments need to be held in memory if the thematic frame-activated when the verb is encountered-is not available early on; (2) when nonlocal unmarked arguments (i.e., those referring to moved constituents) are involved since the theta-assigning verb is not adjacent to its argument thus requiring memory-like resources (see also Friederici & Frazier, 1990, for similar assumptions); (3) when adjuncts (nonarguments) are involved since their interpretation is not 4 More specifically, Berwick and Weinberg point out that before the parser can bind a Wh-element to its trace, all the constituents dominated by the S-node that also dominates the trace would first need to attach to the S-node before this S-node could be attached to the S-bar node. Only after the S-node is attached to the S-bar node could the Wh-element be properly bound with its trace. Thus, for the relativized sentences involving Wh-movement, the parser must wait for the S to attach to the S-bar before binding to the S-bar is possible.

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linked to the verb, i.e., they do not match up to any thematic frame associated with the verb; and (4) when more rather than less thematic frames are associated with a verb since each frame must be temporarily activated until one is chosen.’ Our data also offer some support for the Capacity Constrained Parsing Model (CCPM) of MacDonald, Just, and Carpenter (in press). The model proposes that individual differences in working memory capacity influence the operation of a wide range of language-specific processes, such as generating multiple parses, activating multiple lexical information, and searching for antecedents. More to the point of the present study, the CCPM also predicts that computational resources consumed for one language process will affect performance in other language processes. We do find some evidence for such “additive” effects in the present study. For example, our amnesic subjects performed more poorly when adjunct length increases were contained in more complex syntactic constructions than in simpler sentences. And, consider once again object-clefts: The theta-assigning verb occurs late in the surface string of the sentence, and the need to both identify a nonargument COMP position and relate the filler to it at a clause boundary might create a substantial processing load. The limited capacity processor would have no particular difficulty with passive sentences since the landing site for the moved constituent is another argument position+asier to find than nonargument positions-and the antecedent and the gap are linked within the same clause.6 From the perspective we have just developed, we conclude that amnesic patients fail to normally process certain sentences because of their analytical requirements, rendering the sentences less memorable. And to observe these requirements, complex linguistic material is essential. One final note: It is not so surprising to us that linguistic notions such as “thematic frames,” “thematic assignment,” “arguments and adjuncts,” and “traces and antecedents”-notions assembled from linguistic theoryappear to have an effect on sentence repetition. After all, there is now a rich psycholinguistic literature on the processing implications of such abstract constructs. What is somewhat surprising, however, is how little of this information has been exploited in the investigations of memory ’ There is much to ask about the operation of the thematic processor, questions that our study was not designed to address. For example, does this processor operate in parallel with a phrase structure parser; does it use extrasentential knowledge when evaluating possible thematic frames; what about languages with relatively free word order whereby the verb can appear last in the surface string, thus posing difficulties for a limited capacity processor; etc? These questions have been the focus of several recent psycholinguistic investigations. ’ See also Frazier and Friederici (1992) for a similar perspective involving agrammatic aphasic comprehension deficits. They basically argue that structures involving longer “inferential chains” suffer greater disruption than structures involving shorter chains because of a reduction in the computational capacity of a language processor.

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