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RESEARCH ARTICLE |
a Department of Communication Disorders, Boston University, Massachusetts
b Neuropsychology Lab, Massachusetts General Hospital, Boston
Gloria Waters, 635 Commonwealth Ave., Boston, MA 02215 E-mail: gwaters{at}bu.edu.
Decision Editor: Margie E. Lachman, PhD
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Abstract |
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The term working memory refers to a limited-capacity specialized memory system in which small amounts of information are maintained for short periods of time while operations are performed on this information to achieve computational goals (Baddeley and Hitch 1974
). Behavioral and neurological evidence has suggested that different working memory systems are engaged by verbal and visual tasks (Shah and Miyake 1996; Smith and Jonides 1997). Whether there are further specializations within each of these systems is debated. Just and Carpenter 1992 have argued that there is a single verbal working memory system that is used in all aspects of language processing—the single resource (SR) theory. In contrast, we have argued that the verbal working memory system that is used in language processing can be fractionated, such that the verbal working memory system that is used to determine the literal, preferred, discourse-appropriate meaning of an utterance (interpretive processing) is distinct from that used in other aspects of language processing (e.g., Caplan and Waters 1999a, Caplan and Waters 1999b; Waters and Caplan 1996a)—the separate language interpretation resource (SLIR) theory. We have suggested that a specialized working memory system for interpretive processing may come into existence because the operations that make up interpretive processing (accessing lexical items, constructing syntactic structures, intonational contours, thematic roles, focus, topic, etc.) constitute a highly overpracticed set of computations that are always carried out together—precisely the conditions that would be expected to foster the development of a degree of independence of a mental skill. In contrast, postinterpretive processing—using the products of the comprehension process to accomplish tasks such as entering information in semantic memory, reasoning, and so forth—involves much more conscious, controlled, verbally mediated processing, and different postinterpretive operations occur in association with different utterances rather than as a uniformly functionally integrated process. We have argued that postinterpretive processing draws on a set of resources that is at least partially separate from that used for interpretive processing.
A variety of types of experimental evidence have been brought to bear on the issue of the nature of the resource system used in language processing. One approach has been to determine whether individual differences in verbal working memory capacity (usually measured on a task like the Daneman and Carpenter 1980 reading or sentence span task) are related to individual differences in language processing efficiency. At least moderately high correlations have been found between reading span and many verbal tasks, such as verbal Scholastic Aptitude Test scores or scores on the Nelson Denny reading comprehension test (see Daneman and Merikle 1996, for a review). We have argued that these correlations may be largely accounted for by the heavy postinterpretive demands of these verbal tasks (Waters and Caplan 1996c).
Far fewer studies have investigated the relationship between interpretive aspects of language processing and verbal working memory capacity. A number of studies that have focused on this issue have studied the ability to structure a sentence syntactically. We have found that participants with lower working memory capacity do not have increased difficulty in processing syntactically complex sentences, such as garden path sentences and sentences with object relativization (Waters and Caplan 1996c; Waters, Rochon, and Caplan 1998). This is true for several different offline measures including enactment, sentence–picture matching, and sentence acceptability judgment and for online measures such as word-by-word reading and continuous lexical decision (Caplan and Waters 1999a). Several studies with college students have claimed to find a relationship between online measures of syntactic processing efficiency and working memory capacity (e.g., King and Just 1991; Kluender and Kutas 1993, MacDonald, Just, and Carpenter 1992; Pearlmutter and MacDonald 1995), but we have argued that the evidence they present is not convincing. We critically reviewed the evidential basis for the SR theory and argued on the basis of detailed analyses of aspects of experimental controls, statistical analyses, and other features of the experiments reported in the literature that the data do not support this theory (see Caplan and Waters 1999a, Caplan and Waters 1999b; Waters and Caplan 1996a, for discussion).
Evidence bearing on the SR and SLIR theories also comes from neuropsychology. Many studies have shown that patients with neurological disease who have severely reduced short-term memory due to limited rehearsal or phonological storage capacities retain the ability to structure sentences syntactically (see Caplan and Waters 1990, for a review), supporting the SLIR model. Patients with Alzheimer's disease, whose reduced working memory capacity is due to limited central executive functions (Rochon, Waters, and Caplan 2000) have also shown normal syntactic processing in several studies. Rochon, Waters, and Caplan 1994 used a sentence–picture matching task with 10 sentence types. They found that Alzheimer's patients were less accurate than controls, but were not more affected than controls by syntactic complexity. Rochon and associates 2000 replicated this finding. Waters and colleagues 1998 extended these results to a sentence–video verification task and to a sentence–picture matching task using variable numbers of foils. Grossman and White-Devine 1998 reported that 22 Alzheimer's patients performed at the same level in answering questions about thematic roles in semantically reversible active and passive sentences (see A, Note 1). Waters, Caplan, and Rochon 1995 further found that a concurrent digit memory load equal to participants' spans did not cause performance to decline more on syntactically complex sentences than on syntactically simple sentences in sentence–picture matching, in both Alzheimer's patients and normal controls. Waters and Caplan 1997 reported a similar result, using a timed end-of-sentence measure of judgments about the well formedness of sentences related to reflexive pronoun–antecedent agreement, under no interference conditions and while engaged in a concurrent task consisting of continuously tracking dots or digits on a computer screen. All participants showed effects of syntactic complexity, which were not increased in either of the two concurrent task conditions in either patients or controls. All these results indicate that patients with Alzheimer's disease can structure sentences syntactically to derive their meaning and that loading their already reduced verbal working system does not affect this ability. A few studies have reported patterns of performance that indicate that there are impairments in syntactic comprehension in Alzheimer's patients (Grober and Bang 1995; Kempler, Almore, Tyler, Andersen, & MacDonald, 1998). In these cases, it is not clear that the patients' impairment in syntactic processing is related to their reduction in working memory. Grober and Bang 1995 concluded that the two functional deficits were unrelated because the patients' syntactic deficit in a sentence–picture matching task was present even when a written sentence and a picture were in view simultaneously. On the other hand, Kempler and colleagues 1998 argued that the two deficits were related because performance on a grammaticality judgment task correlated with a composite measure of working memory in their patients. Rochon and associates 2000, however, failed to find a significant correlation between the magnitude of a syntactic comprehension deficit in sentence–picture matching and any of the measures of working memory in their patients. Overall, the bulk of the literature has indicated that Alzheimer's patients do not have an impairment that affects their ability to construct complex syntactic structures, as determined by end-of-sentence measures.
Even fewer studies have addressed the possibility that patients with Alzheimer's disease have online syntactic processing impairments; that is, that they find it harder than control participants to assign and interpret syntactic structures as they construct such representations while the words of a sentence are being perceived. Kempler and colleagues 1998 tested Alzheimer's patients on a cross-modal naming task, in which participants heard an introductory sentence and a sentence fragment and then had to read aloud a visually displayed word that constituted either a grammatical or an ungrammatical continuation of the fragment. Alzheimer's patients showed the same increase in reading times for incorrect continuations as normal controls; moreover, the difference in reading times for the grammatical and ungrammatical continuations did not correlate with a composite measure of working memory. Almor, Kempler, MacDonald, Andersen, and Tyler 1999 reported that the pattern of no difference between Alzheimer's patients and controls in reading times for grammatical versus ungrammatical sentence-fragment continuations occurred for subject–verb agreement when the verb was separated from the sentence subject by a long intervening stretch of material. These results suggest that online syntactic processing is normal in Alzheimer's patients, at least as far as has been measured to date.
The purpose of this study was to further examine the relationship between working memory capacity and the ability to structure sentences syntactically online. The study used a paradigm developed by Ferreira and her colleagues (Ferreira and Anes 1994; Ferreira, Henderson, Anes, Weeks, & MacFarlane, 1996) to extend the study of online sentence processing to the auditory modality—the auditory moving windows task. This task requires participants to listen to a sentence one word or phrase at a time by pressing a button to receive successive words. The time required to call for the next segment is the dependent variable for each word or phrase. The assumption underlying this task is that, as in self-paced reading and eye fixation duration measurements, the time taken to listen to each segment reflects the time needed to recognize the words in the segment and to integrate them into the accruing syntactic and semantic representation of the sentence in the discourse. Ferreira and colleagues 1996 have shown that listeners are immediately sensitive to both lexical frequency and syntactic complexity during the auditory processing of sentences in this task, indicating that it can be used to examine the effect of syntactic variables on processing spoken sentences. We predicted that the auditory moving windows task would be sensitive to local increases in processing load, and, consistent with SLIR theory, we predicted that this effect would not be greater in patients with Alzheimer's disease than in matched control participants.
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Methods |
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All participants were pretested on a battery of neuropsychological tests to rule out any evidence of cognitive decline or dementia in the controls and to document cognitive impairment in the DAT patients. The background measures included the Mini-Mental State Exam (Folstein, Folstein, and McHugh 1975); the visual memory, mental control, and logical memory immediate (I) and delayed (II) subtests of the Wechsler Memory Scale–Revised (Wechsler 1987); the Boston Naming Test (Goodglass and Kaplan 1972); and the block design subtest of the Wechsler Adult Intelligence Scale–Revised (Wechsler 1981).
Table 1 shows the performance of the participants on the background measures. The DAT patients had a mean score of 21.4 on the Mini-Mental State Exam, which indicates that they were in the mild to moderate stages of the disease, and the controls obtained a mean score of 28.6, which is in the normal range. Scaled scores using Mayo's Older Americans Normative Studies norms (Ivnick et al. 1992) were in the average range for the controls on all of the background tests. Scaled scores for the DAT patients were in the impaired range on all of the subtests.
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Procedures and Materials
Working memory measures
Working memory capacity was tested using six operation span tasks. These tasks were taken from a large study investigating working memory and aging (Waters and Caplan 2001b; see A, Note 2) and included Alphabet Span (Craik 1986), Backward Digit Span (Botwinick and Storandt 1975), Subtract 2 Span (Salthouse 1988), and Sentence Span (Daneman and Carpenter 1980) for syntactically simple and complex sentences.
In the Alphabet Span task, participants were required to repeat a series of words after rearranging them in alphabetical order. The stimuli consisted of monosyllabic words of moderate frequency. The words presented on each trial were semantically and phonologically dissimilar, and no words were repeated within a trial.
In the Backward Digit Span task, participants were required on each trial to repeat a series of digits in reverse order of presentation. In the Missing Digit Span task, participants were read a string of digits. The experimenter then reread the string in a different random order with one item omitted. The participant was required to report the missing item. In the Subtract 2 Span task, participants were required to repeat a random sequence of digits after subtracting 2 from each. The stimuli for the three tasks involving digits were drawn from the digits 1 to 9 and presented randomly.
Finally, Sentence Span was measured using the methods and a subset of the materials from Waters and Caplan 1996b. Participants were presented with a series of sentences on the video screen of a computer. They were required to read each sentence silently and then to make a judgment about the acceptability of each sentence in the series. When the participant had made a decision about the last sentence in the series, an asterisk appeared to indicate recall of the last word of each of the sentences in the series in the correct serial order. Participants were instructed not to recall the last word presented first. In addition, they were told to perform the sentence task very accurately and then to perform as well as they could on the recall task. Participants were tested with two different sets of stimulus materials that varied in syntactic complexity and number of propositions. The syntactically simple sentences consisted of one-proposition sentences of the cleft subject form (e.g., It was the gangsters that broke into the warehouse), and the complex sentences consisted of subject–object relatives that contained two propositions (e.g., The meat that the butcher cut delighted the customer).
The order of presentation of the tasks was randomized across participants. For all tasks, testing began at Span Size 2. Controls were tested up to Span Size 8, and DAT patients were tested up to Span Size 5, other than for the Sentence Span task. Owing to time limitations, participants were only tested up to span on the Sentence Span tasks. There were five trials at each span size. Span was defined as the longest list length at which participants were correct on 3 of 5 trials. An additional 0.5 was added if participants were correct on 2 of 5 trials at the next span size. In a few cases, DAT patients correctly recalled the items on 3 of 5 trials at Span Size 5. In this case, testing was continued until they were correct on fewer than 2 of 5 trials. Because our previous research had shown that a composite measure based on performance on several different working memory tasks was more reliable and stable than working memory span scores based on a single task, performance across the six tasks was averaged for each participant to yield a single span score (Waters and Caplan 2001b).
Online measure of sentence processing efficiency
As noted above, online sentence processing efficiency was assessed using the auditory moving windows paradigm (Ferreira et al. 1996). The methods were taken from our previous studies with young and elderly participants (Waters and Caplan 2001a, Waters and Caplan 2002).
The stimuli consisted of 130 semantically acceptable and 130 semantically unacceptable sentences divided equally among the five sentence types—cleft subject (CS), cleft object (CO), subject–subject (SS), object–subject (OS), and subject–object (SO) sentences—as shown in Table 2 . The CS and CO sentences were developed in pairs, such that they had the same words but differed in terms of word order. The SS, OS, and SO sentences were developed in triplets and also contained the same words but differed in terms of word order.
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). Empirical studies have demonstrated this effect in the form of longer reading times at the verb of object-relativized sentences and, in some studies with SO and SS sentences, at the verb following the relative clause. We used both OS and SS sentences because they offer different advantages as the syntactically simple counterparts to SO sentences. OS sentences can be constructed to have identical meanings to SO sentences; verbs and noun phrases can be compared within the relative and the main clauses across SS and SO sentences. On half the trials the sentence was acceptable, and on the other half it was unacceptable. The stimuli were designed so that acceptability judgments did not require judgments based on detailed semantic knowledge. Sentences had verbs that required either animate objects or animate subjects (e.g., It was the girl that the food nourished). Unacceptable sentences violated these constraints. Detection of anomaly could thus be based on fairly accessible, general semantic features and did not require extensive searches through semantic memory for item-specific information. The animacy of the nouns not affected by these constraints was varied across sentences of each type and counterbalanced across acceptable and unacceptable sentences of each type to prevent the use of animacy order in determining acceptability. The point of anomaly was varied across clauses in the SS, OS, and SO sentences to ensure that participants paced their way through the entire sentence before making a decision regarding acceptability.
A male speaker read the stimulus sentences in a semianechoic chamber. The stimuli were recorded and digitized using SoundEdit (Dunn 1994) with a sampling rate of 44.1 kHz and 16-bit quantization, stored as a waveform file, and edited using SoundEdit. A marker (referred to as a tag) was placed in the waveform at the locations defining the boundaries of presentation of segments. Table 2 shows the location of tags (indicated by / ) for the five sentence types used in the experiment. To make segment-to-segment transitions smooth, tags were placed in the waveform at areas of low signal amplitude, as indicated by auditory and visual inspection, whenever possible. When word boundaries did not coincide with areas of low signal amplitude, the tags were placed so as to maximize the intelligibility of the words. Following Ferreira and associates 1996, a tone was appended to the waveform of every sentence immediately following the offset of visible and auditory activity associated with the sentence-final word to signal the end of the sentence.
The resulting waveform files were then entered into PsyScope (Cohen, MacWhinney, Flatt, and Provost 1993). The experiment was controlled by a Macintosh PowerPC computer equipped with an additional digital I/O board and button box for gathering interresponse times. The participants' task was to pace their way through the sentence as quickly as possible by pressing a button on a box interfaced with the computer for the successive presentation of each phrase, and then to make an acceptability judgment about the sentence they had just heard by responding yes or no into a voice key. To discourage participants from pressing the button before they had heard and processed each segment, the segment was truncated at the point of the button press (see A, Note 3) if a participant pressed the button before the end of a segment. Reaction times for each button press, as well as latency to initiate vocalization and accuracy on the acceptability judgment (as measured from the last button press), were recorded in PsyScope (Cohen et al. 1993).
Predictions
The critical prediction of the SR theory is that Alzheimer's patients will be more affected than control participants by the processing load imposed by syntactically complex constructions. In terms of this experiment, SR theory predicts that there will be a three-way interaction between group, phrase, and sentence type. In contrast, SLIR theory predicts that this three-way interaction will not occur. SLIR theory predicts a two-way interaction between phrase and sentence type (that there will be longer listening times at the most capacity-demanding phrases of syntactically complex sentences compared with syntactically simple sentences), and it predicts effects of group (that Alzheimer's patients will respond more slowly than controls). However, it predicts that Alzheimer's patients and control participants will be equally affected by syntactic load; that is, that there will be no interactions involving the group factor (see Caplan and Waters 1999b, for more detailed discussion).
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Results |
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). Fig. 1 shows the mean reaction time for the acceptability judgments for acceptable sentences of each type for which participants had made a correct acceptability judgment. The data for each pair of matched sentences (CS vs CO, SS vs SO, OS vs SO) were analyzed in 2 (group) x 2 (sentence type) analyses of variance (ANOVAs) using both participant and item means as units.
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Mean A's were .98, .92, .93, .91, and .91 for the elderly control participants and .89, .85, .86, .83, and .83 for the DAT patients for the CS, CO, SS, OS, and SO sentences, respectively. These data were analyzed in the same manner as the reaction time data, other than the fact that item statistics were not calculated as the A' measure is not amenable to item statistics. Analysis of these data also resulted in a significant effect of group for all three comparisons, with the judgments of the DAT patients being less accurate than those of the controls, CS versus CO, F1(1,38) = 11.9, MSE = .011, p < .05; SS versus SO, F1(1,38) = 17.2, MSE = .007, p < .05; OS versus SO, F1(1,38) = 17.7, MSE = .008, p < .05. In addition, there was an effect of sentence type in the CS versus CO and SS versus SO comparisons, CS versus CO, F1(1,38) = 40.6, MSE = .001, p < .05; SS versus SO, F1(1,38) = 7.7, MSE = .001, p < .05; OS versus SO, F1(1,38) = 0.26, MSE = .002, ns. The interaction between group and sentence type was not significant in any of the analyses, CS versus CO, F1(1,38) = 0.74, MSE = .001, ns; SS versus SO, F1(1,38) = 0.29, MSE = .001, ns; OS versus SO, F1(1,38) = 0.04, MSE = .002, ns (see A, Note 4).
Online Measures of Sentence Processing Efficiency
Phrase-by-phrase listening time
The online dependent measure for the auditory moving windows task consisted of the response time for each phrase. Corrected response times (listening times) were calculated by subtracting the segment's tag-to-tag duration from the response time. The data were trimmed as outlined above for the end-of-sentence acceptability judgment measure. Inspection of the dataset as a whole showed that the degree of skewness and kurtosis did not warrant transformation. Moreover, inspection of the normal probability plots for each pair of sentences showed that the fit was not improved by transformation.
Separate analyses were carried out on listening times for CS versus CO sentences, SS versus SO sentences, and OS versus SO sentences. Listening times for acceptable sentences of each type for which participants had made a correct acceptability judgment were analyzed in 2 (group) x 2 (sentence type) x 4 (phrase) ANOVAs using both participant and item means as units.
Fig. 2 shows the mean listening time for CS and CO sentences at each phrase for the two participant groups. ANOVAs by participants and items showed that there were significant main effects of group, F1(1,38) = 6.5, MSE = 1,457,473.8, p < .05, F2(1,200) = 299.1, MSE = 3,519.2, p < .001; sentence type, F1(1,38) = 30.8, MSE = 32,808.2, p < .001; F2(1,200) = 8.7, MSE = 182,517.9, p < .01; and phrase, F1(3,114) = 28.9, MSE = 64,351.9, p < .001; F2(3,200) = 14.0, MSE = 182,517.9, p < .001; a significant Sentence Type x Phrase interaction, F1(3,114) = 59.6, MSE = 45,438.6, p < .001, F2(3,200) = 19.7, MSE = 182,517.9, p < .001; and a Group x Phrase interaction, F1(3,114) = 59.6, MSE = 64,351.9, p < .001; F2(3,200) = 4.4, MSE = 35,919.2, p < .01; and a Group x Sentence Type x Phrase interaction, F1(3,114) = 10.2, MSE = 45,438.6, p < .001, F2(3,200) = 11.4, MSE = 35,919.2, p < .001.
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As can be seen in Fig. 2, both groups of participants showed the pattern outlined above of longer listening times at V of CO sentences and NP2 of CS sentences. However, there was a significant Group x Sentence Type x Phrase interaction. To investigate this further, we calculated difference scores (listening times for CO–CS) at each phrase, for each participant group. These scores served as an index of the magnitude of the effect of increased complexity in the more complex sentence type at each phrase. The difference scores were analyzed in a 2 (group) x 5 (phrase) ANOVA. The Phrase x Group interaction was significant, F(4,152) = 8.0, MSE = 88,290.8, p < .001. Tukey's post hoc tests showed that the magnitude of the difference between listening times for V in CO and CS sentences was greater in the Alzheimer's patients than in the controls. However, the magnitude of the difference between listening times for NP2 in CS sentences and for V in CO sentences between the Alzheimer's patients and controls was not significant.
Fig. 3 shows the data from the SS and SO sentences. The ANOVA showed that there were significant main effects of sentence type, F1(1,38) = 14.7, MSE = 32,400.2, p < .001, F2(1,200) = 5.5, MSE = 90,120.8, p < .001; and phrase, F1(3,114) = 4.9, MSE = 46,868.4, p < .001; F2(3,200) = 2.9, MSE = 90,120.8, p < .001; and a significant Sentence Type x Phrase interaction, F1(3,114) = 11.4, MSE = 41,639.1, p < .001; F2(3,200) = 6.9, MSE = 90,120.8, p < .001. The group effect was significant in the item but not in the participant analyses, F1(1,38) = 3.5, MSE = 1,716,334.2, ns; F2(1,200) = 190.7, MSE = 36,691.7, p < .001. As can be seen in Fig. 3, the effect of sentence type was due to longer listening times for SO than for SS sentences. Post hoc tests showed that the effect of phrase was due to longer listening times at V1 than at either NP1 or NP2. The Sentence Type x Phrase interaction was due to significantly longer listening times at V1 in SO sentences compared with SS sentences. Furthermore, listening times at V1 in SO sentences were significantly longer than those at NP2 in SS sentences, showing that the effect at V1 was not simply due to its position at the point of possible thematic closure. There were no interactions involving the group factor.
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Correlation of working memory measures and online sentence processing measures
Table 4 shows the correlations between measures of working memory and difference scores for phrases in the syntactically more complex sentences (CO, SO) and syntactically less complex sentences (CS, SS, OS). These correlations were calculated separately for each participant group (elderly control and DAT) and for the group as a whole (all). For the SO–SS and SO–OS sentence pairs, virtually none of the correlations were significant. For the CO–CS sentence pair, the correlation between the composite working memory measure for all participants and the difference in listening times on the verb in CS and CO sentences was significant.
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Discussion |
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With respect to end-of-sentence measures, the patients with Alzheimer's disease were slower and made more errors than the matched controls in end-of-sentence acceptability judgments, but were affected to the same degree by the syntactic complexity of the sentences. As noted in our review of the literature in the introduction to this article, these end-of-sentence data replicate the majority of findings in the literature. Patients with Alzheimer's disease typically do not do as well as control participants on tasks such as sentence–picture matching, video verification, and so forth, but are not more affected than controls by syntactic complexity. The overall poorer performance of Alzheimer's patients could be due to any of a number of factors; the lack of a greater effect of syntactic complexity is evidence against the SR model of working memory.
This study provides new data regarding online processing of syntactic structure in Alzheimer's patients. The self-paced listening technique is sensitive to local increases in processing load. In the present study, this sensitivity was tested by constructing three matched pairs of sentences with object- and subject-relativized clauses: CO and CS sentences, SO and SS sentences; and SO and OS sentences. In the present study, the effects of increases in processing load were discernible in all three sentence-type comparisons in the form of longer listening times at the verb of the object-relativized clause than at either the verb or the object noun of the subject-relativized clause.
With one exception among six comparisons, which we discuss below, these differences in processing times were not greater in patients with Alzheimer's disease than in control participants. This pattern strongly suggests that these patients' end-of-sentence results do not mask greater than normal sentence-internal effects of processing load that are only seen locally and are gone by the time a sentence has ended. This in turn suggests that a reduction in working memory capacity, as measured by standard tests, is not associated with a decreased ability to devote processing resources to syntactic analysis and to the use of syntax to determine sentence meaning. This conclusion is bolstered by the fact that all but 1 of 24 correlations between a composite measure of working memory and difference scores in listening times for critical phrases in the matched sentence pairs were not significant.
One caveat to this conclusion concerns the fact that there were only 20 participants per group in the present study. It is possible that with a greater sample size and increased statistical power a different pattern of results might emerge. One argument against this is that the basic pattern of results found here has been found in several other studies of nondemented participants who nonetheless had reduced working memory capacity. These studies have included both elderly participants and college students with low working memory spans (Waters and Caplan 2001a; Waters and Caplan 2002).
In addition, one online finding could be taken to suggest that the Alzheimer's patients were more affected by syntactic complexity than the controls. This finding was the greater increase in listening times at the verb of CO compared with CS sentences in Alzheimer's patients than in controls. However, this finding is likely due to nonsyntactic processing that occurs when the thematic roles required by a verb are assigned, not syntactic processing per se. This conclusion is based on considering both the difference in listening times for V in CO sentences compared with V in CS sentences and the difference in listening times for V in CO sentences compared with NP2 in CS sentences. Both these comparisons showed longer listening times for V in CO sentences, but only the former (V in CO sentences compared with V in CS sentences) was greater in Alzheimer's patients than in controls. However, adopting Gibson 1998 model of syntactic processing, the difference in local syntactic processing load between V in CO sentences and V in CS sentences is the same as that between V in CO sentences and NP2 in CS sentences (see A, Note 5). Accordingly, assuming that Gibson's model provides an adequately accurate means of assessing local syntactic processing load, the greater increase in listening time in Alzheimer's patients at V of CO sentences than at V of CS sentences cannot be due to the increased syntactic processing load at these two points, because an identical increase in syntactic processing load at V of CO sentences compared with NP2 of CS sentences did not lead to disproportionate increase in listening times in the Alzheimer's patients. This explanation is consistent with the finding that Alzheimer's patients did not show a larger effect of increased processing load compared with controls at the capacity-demanding region of SO sentences when compared with either SS or OS sentences.
One source of increased load at V in CO sentences is related to discourse-level representations. In addition to constructing sentence-level semantic properties, such as thematic roles, listeners also construct discourse-level semantic properties, such as an inventory of entities in the universe of discourse, the entity in the focus of the discourse, and so forth. From the perspective of creating a discourse structure, sentences presented in isolation act as discourse-initial sentences (Altmann and Steedman 1988). Clefting and other markers of discourse focus are usually inappropriate in discourse-initial sentences, and therefore in sentences presented in isolation. CO sentences presented in isolation make additional demands on the construction of discourse focus. The focus of a discourse is usually the subject of a sentence and the agent of a verb (Kintsch and VanDijk 1978). In a CO sentence, this usual co-occurrence of agent and focus is violated, because the theme of the verb is put into the focused, clefted position. The verb of CO sentences is the first point at which the dissociation of the usual combination of agent and focus can be recognized. All these factors increase the processing load at the V of CO sentences. Because of the unnaturalness of the clefted construction in discourse-initial sentences, constructing the discourse representations associated with clefted structures in single-sentence presentations may lie outside the usual interpretive process and involve the putative "general purpose" pool of processing resources; the demands on this pool would be particularly high at the V of CO sentences. Increases in the processing load on the general pool of processing resources are expected to affect Alzheimer's patients more than controls, due to their reduced general pool of such resources (see A, Note 6).
This account is admittedly post hoc. However, it provides a possible explanation of the entire pattern of data, something that an account based on syntactic processing load cannot do, for the reasons we presented above. Furthermore, we note that there is much evidence that CO sentences are surprisingly difficult to process—more difficult than would be expected on the basis of their syntactic structure alone—an explanation of the particular difficulty associated with these sentences in terms of their discourse properties is attractive. The more speculative key claim that underlies the implication of this account for the SR and SLIR theories is that computing these discourse properties in the case of CO sentences presented in isolation involves the general pool of working memory resources. This claim is speculative, but is not a priori unreasonable, and it is testable. If the processing load associated with constructing a discourse focus uses a general pool of processing resources when these sentences are presented in isolation, reliance on this pool should disappear when these sentences are preceded by a context establishing the appropriateness of a CO structure. In that case, listening times should not be more elevated at V of CO sentences compared with V of CS sentences in Alzheimer's patients compared with controls.
The results of this experiment are consistent with online studies comparing processing of subject- and object-relativized sentences using other techniques, such as self-paced reading, in demonstrating that there is an increase in processing time at the most capacity-demanding portion of the syntactically more complex sentence (Berwick and Weinberg 1984; Haarman, Just, and Carpenter 1997; Caplan, Hildebrandt, and Waters 1994; King and Just 1991; Stine-Morrow, Ryan, and Leonard 2000). They are also consistent with our previous studies with normal participants in finding that the increase in listening time at the capacity-demanding portion of the more complex sentences was not larger in participants with lower working memory capacities or in older participants. Aside from our studies, two other papers in the literature have examined individual differences in processing these sentences using self-paced reading.
King and Just 1991 claimed to have found a greater increase for high- than low-span college students in reading times for the main verb of SO sentences compared with SS sentences. Stine-Morrow and colleagues 2000 reported a complex pattern of results with these sentences: Low span (older) participants made more errors in answering questions about thematic roles in object-relativized than in subject-relativized sentences but showed smaller online effects of complexity. As noted in the introduction to this article, we have argued that the data in the King and Just article do not support their conclusions (Caplan and Waters 1999a, Caplan and Waters 1999b; Caplan and Waters 2002; Waters and Caplan 1996a). The results reported by Stine-Morrow and colleagues 2000 deserve comment. First, Stine-Morrow and her colleagues did not report Group x Sentence Type interaction in the question accuracy data, or a three-way interaction of group, sentence type, and region in the reading time data, leaving some uncertainty regarding whether the effects they describe truly differed in the older and younger participants. This aside, the reading time data are the opposite of those said to have been found by King and Just for the low-span group as a whole, where low-span participants showed a greater effect of object relativization at the verb. Stine-Morrow and her colleagues suggested that the low-span, older participants in their study stopped processing the complex portion of the object-relativized sentences earlier than the high-span, young participants. This would be consistent with King and Just's finding that low-comprehending, low-span participants showed less of an effect of object relativization than the better comprehending low-span participants. These data call into question the near universal assumption in psycholinguistic research that increased difficulty integrating a phrase into the accruing representation of a sentence leads to longer, not shorter, reaction times. In addition, they point to the need for more complex models of the effect of resource limitations on language processing.
An issue that needs to be addressed regarding the present study regards the auditory moving windows paradigm. A concern that has been raised about this technique is that splicing the speech signal into chunks for self-paced presentation may affect low-level word recognition processes and the use of prosodic information. However, Ferreira and colleagues 1996 reported that participants who rated the naturalness and intelligibility of sentences presented in their study found that the stimuli sounded reasonably natural and were judged to be intelligible. The same was true of informal judgments of the naturalness of the stimuli in our experiment made by lab personnel (both naïve and non-naïve to the goals of the experiment) at the materials development stage. In addition, although not a response to concern about the naturalness of auditory stimuli presented phrase by phrase, we note that the auditory moving windows task is not obviously less natural than many other tasks used in psycholinguistic research, in particular, self-paced reading, in which participants are denied parafoveal preview and are unable to make regressive eye movements (Ferreira et al. 1996). Finally, in defense of the auditory moving windows task, we note that it has the advantage of presenting the stimuli in the primary modality of language reception, which is particularly important when studying pathological populations, such as DAT patients, who may strongly prefer this input modality and who may have impairments in reading. Moreover, this paradigm has proven a useful method of investigating age differences in auditory processing time (Titone, Prentice, and Wingfield 2000).
In summary, the results of this study provide evidence that patients with early Alzheimer's disease are able to construct the syntactic structure of a sentence and use that structure to determine the meaning of a sentence, despite stable reductions in verbal working memory. This result supports the view that the verbal working memory system is fractionated, with some operations relying on a working memory system not well measured by standard tests of this function. The results are consistent with the hypothesis that one such specialization involves computing the syntactic structure of a sentence.
An alternative interpretation of these results is that syntactic processing uses the same working memory system that is measured on standard tests but is so automatic that it makes very few demands on this system, and even the reduced working memory capacity of the DAT patients tested here is sufficient to assign syntactic structure. There are several arguments against this account. First, the complex sentences used in this study are so resource demanding as to be at the limit of most individuals' ability to structure. Adding one more embedding to the SO sentences produces doubly center-embedded sentences (e.g., The woman who the bodyguard who the trainer encouraged attracted closed the drapes) that most people cannot comprehend unless they rehearse them or see them in written form. Arguably, it requires all, or almost all, of the working memory that a normal individual has available for syntactic processing to structure the sentences presented here. It therefore seems unlikely that working memory could be reduced to the extent seen in the DAT partients tested in this study (an average reduction of 38% across all span measures) without any effect on a measure of online syntactic processing, if such processing used the working memory system being measured. Second, working memory was not correlated with online measures of syntactic processing. This suggests that the availability of working memory resources for syntactic processing does not depend on the size of the general purpose working memory resource pool. Admittedly, such arguments cannot rule out this proposed analysis. Further empirical research, in which the processing load exerted by parsing is measured and compared with that exerted by working memory tasks, could address the question of how much resource is required by parsing. Comparing the effects of parsing and other working memory tasks on a common third task is one way to investigate this question (see Waters, Komoda, and Arbuckle 1985, for an example).
This discussion raises two related questions: How many resource pools are there, and can we delineate those aspects of comprehension that use the hypothesized special pool(s) and those that use the general pool? The discrepancy between performance on working memory tests and syntactic processing points to a specialized working memory pool for the latter. We have suggested that the processes that use this specialized system are online, first pass, obligatory, and unconscious. In the domain of syntactic processing, this includes the assignment of syntactic structure on the basis of grammatical categorical information (Frazier and Clifton 1996) as well as the utilization of lexical information (frequency of co-occurrence data as well as pragmatic information) to determine preferred syntactic structures (MacDonald, Pearlmutter, and Seidenberg 1994; Pearlmutter and MacDonald 1995). Conscious revision of syntactic analyses, as occurs in deliberate recovery from garden path situations, is not part of the syntactic processing we associate with this specialized resource pool. As noted in the introduction, we have suggested that this resource system supports all the online, first-pass, obligatory, unconscious processes that are devoted to assignment of the literal, preferred, discourse-congruent meaning of utterances, not just syntactic operations (Caplan and Waters 1999a, Caplan and Waters 2002; Waters and Caplan 1996c, Waters and Caplan 1997). We therefore hypothesize that there are two verbal working memory systems: one devoted to online, first-pass interpretive processing and one devoted to postinterpretive processing. Further studies are needed to determine whether this is the best way to characterize the differences in the types of processes that each of these two hypothesized working memory systems supports, or whether some other characterization of these domains, such as that between unconscious and conscious verbally mediated processes, is empirically supported.
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Acknowledgments |
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Received for publication June 26, 2000. Accepted for publication August 7, 2001.
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TOPAbstractMethodsResultsDiscussionAppendixAppendix BReferences |
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also reported that Alzheimer's patients were disproportionally impaired on "periphrastic" sentences with transitive verbs (e.g., The boy made the girl kiss) than on such sentences with causative verbs (e.g., The mother made the baby awaken). However, this result is hard to interpret for two reasons: (a) The periphrastic sentences with transitive verbs were all semantically unconstrained, and those with causative verbs were both constrained and unconstrained (e.g., The boy made the vase break), and (b) at least some of the periphrastic sentences with transitive verbs seem to be ungrammatical, as with the example The boy made the girl kiss, taken from Grossman and White-Devine's illustration of their stimuli (kiss cannot be an intransitive verb). ). However, the data from our previous study of aging using these tasks showed that this task did not share a significant amount of variance with the other variables. In addition, the assessment of reliability, stability, and internal consistency showed that this measure had extremely low test–retest reliability, resulted in unstable classification of participants, and had low internal consistency. model assigns local processing load costs as a result of integration and storage demands. Integration costs are due to the distance (in number of words) between a word and the item that it is related to (e.g., a verb and a noun playing a thematic role around that verb), the number of new discourse referents introduced between the word and the item that it is related to, and the number of integrations that are performed at the word. Storage costs are the number of constituents that are necessary to complete any obligatory structures associated with a word that has occurred in a sentence (these constituents are said to be predicted and thus maintained in storage until they are recognized). Applying these measures, integration costs are 6 at V of CO sentences and 2 at V and NP2 of CS sentences. Storage costs are 1 at V of CS sentences and NP2 of CO sentences and 0 at NP2 of CS sentences and V of CO sentences. ). Though Balogh and associates suggested that this process occurs at the end of a sentence, their data are compatible with the equally (if not more) plausible view that it occurs when the argument structure of a verb is satisfied, not (or not only) at the end of a clause or a sentence. A process such as this could underlie the additional listening time at V of CO sentences compared with V of CS sentences in Alzheimer's patients. However, such a checking process would also be expected to operate in comparing V1 in SO sentences against V1 in both SS and OS sentences, and in neither case did the Alzheimer's patients show disproportionately longer listening times. Therefore, we reject this possible explanation of the greater increase in listening times for Alzheimer's patients than for controls at V of CO sentences compared with CS sentences. |
Appendix B |
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2. It was/ the water/ that/ appealed to/ the boy/ because it was hot.
3. It was/ the toy/ that/ interested/ the man/ because it was red.
4. It was/ the rain/ that/ woke up/ the father/ because of the leak.
5. It was/ the fire/ that/ injured/ the policeman/ on the highway.
6. It was/ the store/ that/ delighted/ the secretary/ yet it was closed.
7. It was/ the box/ that/ distracted/ the dog/ for it held a cat.
8. It was/ the food/ that/ nourished/ the child/ because it was plentiful.
9. It was/ the coffee/ that/ pleased/ the secretary/ because it was caffeinated.
10. It was/ the camera/ that/ interested/ the girl/ for it was automatic.
11. It was/ the car/ that/ amazed/ the boy/ although it was old.
12. It was/ the playground/ that/ pleased/ the mother/ for it was safe.
13. It was/ the painting/ that/ delighted/ the woman/ because it was beautiful.
14. It was/ the food/ that/ distracted/ the dog/ for he was hungry.
15. It was/ the book/ that/ interested/ the teenager/ because it was a romance.
16. It was/ the radio/ that/ scared/ the child/ because it was too loud.
17. It was/ the sweater/ that/ warmed/ the child/ for she was cold.
18. It was/ the girl/ that/ wrote/ the letter/ that was sent to the wrong address.
19. It was/ the girl scout/ that/ sold/ the cookies/ for charity.
20. It was/ the janitor/ that/ mopped up/ the spill/ although it was sticky.
21. It was/ the child/ that/ carved/ the pumpkin/ even though it wasn't Halloween.
22. It was/ the boyfriend/ that/ bought/ the flowers/ that cheered the girlfriend.
23. It was/ the announcer/ that/ read/ the message/ as a warning to all.
24. It was/ the swimmer/ that/ heard/ the whistle/ before he started.
25. It was/ the monkey/ that/ ate/ the banana/ before the show.
26. It was/ the man/ that/ read/ the map/ but still got lost.
CO Acceptable Sentences
1. It was/ the child/ that/ the movie/ terrified/ because it showed a monster.
2. It was/ the boy/ that/ the water/ appealed to/ because it was hot.
3. It was/ the man/ that/ the toy/ interested/ because it was red.
4. It was/ the father/ that/ the rain/ woke up/ because of the leak.
5. It was/ the policeman/ that/ the fire/ injured/ on the highway.
6. It was/ the secretary/ that/ the store/ delighted/ yet it was closed.
7. It was/ the dog/ that/ the box/ distracted/ for it held a cat.
8. It was/ the child/ that/ the food/ nourished/ because it was plentiful.
9. It was/ the secretary/ that/ the coffee/ pleased/ because it was caffeinated.
10. It was/ the girl/ that/ the camera/ interested/ for it was automatic.
11. It was/ the boy/ that/ the car/ amazed/ although it was old.
12. It was/ the mother/ that/ the playground/ pleased/ because it was safe.
13. It was/ the woman/ that/ the painting/ delighted/ because it was beautiful.
14. It was/ the dog/ that/ the food/ distracted/ for he was hungry.
15. It was/ the teenager/ that/ the book/ interested/ because it was a romance.
16. It was/ the child/ that/ the radio/ scared/ because it was too loud.
17. It was/ the child/ that/ the sweater/ warmed/ for she was cold.
18. It was/ the letter/ that/ the girl/ wrote/ that was sent to the wrong address.
19. It was/ the cookies/ that/ the girl scout/ sold/ for charity.
20. It was/ the spill/ that/ the janitor/ mopped up/although it was sticky.
21. It was/ the pumpkin/ that/ the child/ carved/ even though it wasn't Halloween.
22. It was/ the flowers/ that/ the boyfriend/ bought/ that cheered the girlfriend.
23. It was/ the message/ that/ the announcer/ read/ as a warning to all.
24. It was/ the whistle/ that/ the swimmer/ heard/ before he started.
25. It was/ the banana/ that/ the monkey/ ate/ before the show.
26. It was/ the map/ that/ the man/ read but/ still got lost.
OS Acceptable Sentences
1. The advertisement/ included/ the picture/ that/ depicted/ the product.
2. The mechanic/ transported/ the car/ that/ passed/ the inspection.
3. The activist/ protected/ the tree/ that/ sheltered/ the squirrel.
4. The report/ omitted/ the biographer/ who/ insulted/ the publisher.
5. The bodyguard/ attracted/ the woman/ who/ closed/ the drapes.
6. The audience/ bored/ the child/ who/ embarrassed/ the mother.
7. The coach/ selected/ the jockey/ who/ rode/ the horse.
8. The evidence/ defended/ the man/ who/ broke/ the window.
9. The mallet/ hit/ the man/ who/ pierced/ the pipe.
10. The soldier/ protected/ the sailor/ who/ piloted/ the boat.
11. The chemical/ burned/ the man/ who/ instructed/ the student.
12. The neighbors/ harassed/ the tenant/ who/ irritated/ the landlord.
13. The secret/ identified/ the aide/ who/ infuriated/ the senator.
14. The jeweler/ liked/ the customer/ who/ bought/ the bracelet.
15. The millionaire/ favored/ the law/ that/ frustrated/ the workers.
16. The yard/ surrounded/ the fence/ that/ pleased/ the carpenter.
17. The rocks/ encircled/ the wolf/ that/ watched/ the campfire.
18. The procedure/ used/ the computer/ that/ recorded/ the data.
19. The soldiers/ protected/ the fort/ that/ stored/ the ammunition.
20. The writer/ reviewed/ the article/ that/ satisfied/ the editor.
21. The captain/ towed/ the boat/ that/ won/ the race.
22. The scout/ warmed/ the cabin/ that/ contained/ the firewood.
23. The stocks/ devalued/ the investment/ that/ displeased/ the banker.
24. The performance/ disturbed/ the man/ who/ demanded/ the refund.
25. The table/ scratched/ the butcher/ who/ summoned/ the doctor.
26. The inflation/ slowed/ the recession/ that/ disturbed/ the consumers.
SS Acceptable Sentences
1. The picture/ that/ included/ the advertisement/ depicted/ the product.
2. The car/ that/ transported/ the mechanic/ passed/ the inspection.
3. The tree/ that/ protected/ the activist/ sheltered/ the squirrel.
4. The biographer/ who/ omitted/ the report/ insulted/ the publisher.
5. The woman/ who/ attracted/ the bodyguard/ closed/ the drapes.
6. The child/ who/ bored/ the audience/ embarrassed/ the mother.
7. The jockey/ who/ selected/ the coach/ rode/ the horse.
8. The man/ who/ defended/ the evidence/ broke/ the window.
9. The man/ who/ hit/ the mallet/ pierced/ the pipe.
10. The sailor/ who/ protected/ the soldier/ piloted/ the boat.
11. The man/ who/ burned/ the chemical/ instructed/ the student.
12. The tenant/ who/ harassed/ the neighbors/ irritated/ the landlord.
13. The aide/ who/ identified/ the secret/ infuriated/ the senator.
14. The customer/ who/ liked/ the jeweler/ bought/ the bracelet.
15. The law/ that/ favored/ the millionaire/ frustrated/ the workers.
16. The fence/ that/ surrounded/ the yard/ pleased/ the carpenter.
17. The wolf/ that/ encircled/ the rocks/ watched/ the campfire.
18. The computer/ that/ used/ the procedure/ recorded/ the data.
19. The fort/ that/ protected/ the soldiers/ stored/ the ammunition.
20. The article/ that/ reviewed/ the writer/ satisfied/ the editor.
21. The boat/ that/ towed/ the captain/ won/ the race.
22. The cabin/ that/ warmed/ the scout/ contained/ the firewood.
23. The stocks/ that/ devalued/ the investment/ displeased/ the banker.
24. The man/ who/ disturbed/ the performance/ demanded/ the refund.
25. The butcher/ who/ scratched/ the table/ summoned/ the doctor.
26. The recession/ that/ slowed/ the inflation/ disturbed/ the consumers.
SO Acceptable Sentences
1. The picture/ that/ the advertisement/ included/ depicted/ the product.
2. The car/ that/ the mechanic/ transported/ passed/ the inspection.
3. The tree/ that/ the activist/ protected/ sheltered/ the squirrel.
4. The biographer/ who/ the report/ omitted/ insulted/ the publisher.
5. The woman/ who/ the bodyguard/ attracted/ closed/ the drapes.
6. The child/ who/ the audience/ bored/ embarrassed/ the mother.
7. The jockey/ who/ the coach/ selected/ rode/ the horse.
8. The man/ who/ the evidence/ defended/ broke/ the window.
9. The man/ who/ the mallet/ hit/ pierced/ the pipe.
10. The sailor/ who/ the soldier/ protected/ piloted/ the boat.
11. The man/ who/ the chemical/ burned/ instructed/ the student.
12. The tenant/ who/ the neighbors/ harassed/ irritated/ the landlord.
13. The aide/ who/ the secret/ identified/ infuriated/ the senator.
14. The customer/ who/ the jeweler/ liked/ bought/ the bracelet.
15. The law/ that/ the millionaire/ favored/ frustrated/ the workers.
16. The fence/ that/ the yard/ surrounded/ pleased/ the carpenter.
17. The wolf/ that/ the rocks/ encircled/ watched/ the campfire.
18. The computer/ that/ the procedure/ used/ recorded/ the data.
19. The fort/ that/ the soldiers/ protected/ stored/ the ammunition.
20. The article/ that/ the writer/ reviewed/ satisfied/ the editor.
21. The boat/ that/ the captain/ towed/ won/ the race.
22. The cabin/ that/ the scout/ warmed/ contained/ the firewood.
23. The investment/ that/ the stocks/ devalued/ displeased/ the banker.
24. The man/ who/ the performance/ disturbed/ demanded/ the refund.
25. The butcher/ who/ the table/ scratched/ summoned/ the doctor.
26. The recession/ that/ the inflation/ slowed/ disturbed/ the consumers.
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TOPAbstractMethodsResultsDiscussionAppendixAppendix BReferences |
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