Script generation as an indicator of knowledge representation in patients with Alzheimer's disease

BRAIN

AND

LANGUAGE

40,

344-3.58 (1991)

Script Generation as an Indicator of Knowledge Representation in Patients with Alzheimer’s Disease

JORDAN

GRAFMAN,* MARTINEZ,?

KAREN BRIAN

THOMPSON,?

A. LAWLOR,?

HERBERT

WEINGARTNER,~

RICK

AND TREY SUNDERLAND?

*Cognitive Neuroscience Section, Medical Neurology Branch, NINDS, Maryland 20892, $Department of Psychology, George Washington Washington, DC 20052, and ‘FUnit on Geriatric Psychopharmacology, Bethesda, Maryland 20892

NIH, Bethesda, University, NIMH,

We examined script and lexical retrieval in patients with probable DementiaAlzheimer’s Type (DAT), Depressed patients, and normal controls. DAT patient breakdown in script production was structurally similar to their impaired lexical retrieval such that script events of low frequency and low centrality value were lost first. DAT patients also produced more events that fell outside the script boundary as well as more event-order errors. Four cases with DAT were identified on the basis of 2 scores whose script production was at least 2 SDS greater than their lexical production or vice versa. This finding suggests that it may be possible to dissociate script and lexical knowledge and production processes. The findings lend partial support for a model of knowledge representation that includes parallel and partially redundant memory networks that are distinctly distributed in the brain. 0 1991 Academic Press, Inc. The authors express their gratitude to Mrs. Marjorie Risher for her administrative expertise and her invaluable assistance in preparing the manuscript. We thank the National Institute of Neurological Disorders and Stroke, and the Medical Neurology Branch for providing the facilities which allowed us to prepare the manuscript. We also thank two anonymous reviewers who provided very useful critiques of earlier versions of the manuscript. The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or necessarily reflecting the views of the National Institutes of Health, the United States Public Health Service, the Department of Health and Human Services, the Uniformed Services University of the Health Sciences, or the Department of Defense. Address all correspondence to Jordan Grafman at Cognitive Neuroscience Section, Medical Neurology Branch, NINDS/NIH, Building 10; Room X422, Bethesda MD 20892, Fax: (301) 480-2909. 344 0093-934x/91 $3.00 Copyright 0 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.

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INTRODUCTION Recent studies of patients with probable Dementia-Alzheimer’s Type (DAT) have indicated that these patients suffer from a semantic memory deficit (Weingartner, Grafman, Boutelle, & Martin, 1983; Martin & Fedio, 1983; Kertesz, Appell, & Fisman, 1986) although the exact nature of this deficit is still being debated (Grober, Buschke, Kawas, & Fuld, 1985; Huff, Corkin, & Growden, 1986; Nebes, Martin, & Horn, 1984; Schwartz, 1985; Nebes, 1989). The tasks typically used to evaluate semantic memory include word fluency/category fluency, naming pictures and naming to description, object recognition, sentence cloze tasks, and priming tasks (Huff et al., 1986). The most common model used to represent semantic memory (Chang, 1986) in these studies is the lexical network model with individual items associatively (e.g., dog-bark) and semantically (e.g., house-dwelling) linked to related items, linked to descriptive features about that item (e.g., canary-yellow), and organized by category (e.g, animal-lion) membership (Kroll & Potter, 1984). The most general claim made about the semantic memory failure of DAT patient is that they most often lose access to semantic category information about an item (e.g., is a canary a bird) and occasionally to item-specific distinctive features (e.g., does an elephant have a trunk) in the earlier stages of the disease. Nebes (Nebes et al., 1984) and others (Grober et al., 1985) have argued that only access to semantic memory is impaired since priming and lexical decision studies indicate that associative and semantic network links may be degraded but intact (Nebes et al., 1984; Grober et al., 1985). Longitudinal studies are lacking in this area of research so the description of the dissolution of this kind of semantic memory representation is absent. The study of semantic memory processes suggests at least one reason why DAT patients encounter such severe problems in learning contextdependent information. If a patient has difficulty in accessing information held in a semantic network (be it categorical, featural, or otherwise), then it would not be surprising that such difficulty would directly impair meaningful encoding of stimulus and context-dependent information (Diesfeldt, 1985). A semantic network composed of items, categories, and their features may not be the only kind of knowledge memory that exists or can be investigated using cognitive tasks. There are probably numerous kinds of knowledge representation networks distributed in topographically distinct regions of the brain. Evidence exists that indicates one kind of knowledge representation can be described as schema or script knowledge (Bower, Black, & Turner, 1979; Thorndyke & Hayes-Roth, 1979; Abbott, Black, & Smith, 1985; Mandler, 1984; Bower & Clark-Myers, 1980; Galambos & Rips, 1982; Galambos, 1983). Schema or script knowledge is defined

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as memories for a typical sequence of events such as “visiting the doctor,” “going to a movie,” or “eating at a restaurant.” The memory is of the complete sequence. Two studies exist where script knowledge was examined in patient groups in order to more fully characterize deficits in knowledge representation. In the first study by Weingartner, Grafman and their colleagues (Weingartner et al. 1983), patients with Korsakoff’s Disease (KD) and DAT were tested on a word fluency task, a sentence completion (cloze) task, and a script-event discrimination task on which subjects had to determine whether two events came from the same script (e.g., eating at a typical restaurant) and, if they did, were they presented in the correct order (e.g., “be seated at a table” and “order food”). Next, a script generation task was administered which required subjects to generate all the events they could think of that occurred after they got up in the morning but before they left the house for work. KD patients performed similar to controls on all the semantic-script memory tasks, but DAT patients performed more poorly than either controls or KD patients on the fluency and sentence completion tasks. DAT patients also made more order errors than the KD or controls on the script discrimination task and, like these other groups, their script-event order errors tended to occur on those events that naturally occurred close together rather than far apart. Finally, DAT patients, as on the fluency task, produced fewer script events than the KD patients or controls. In a second study, Roman, Brownell, Potter, & Seibold, (1987) investigated script knowledge in older adults and right-hemisphere brain-damaged patients and found that both groups demonstrated relatively preserved script knowledge. Some right-hemisphere brain-damaged patients showed tangentiality in their scripts while others terminated their scripts rather early. Despite this relative intactness in accessing script knowledge, right-hemisphere brain-damaged patients often showed difficulty in understanding story narratives. Both of these studies attempted to assess script knowledge and retrieval because of the hypothesis that script knowledge is a distinct kind of knowledge representation. Yet, neither study was able to fully address this hypothesis. We decided to try and replicate the results of the first study, adding an error analysis of script generation performance which we suspected would help reveal the structural characteristics of individual scripts, and to plot the performance of subjects across lexical, sentence cloze, and script generation tasks to see whether we could identify single cases with dissociations in performance which would support the idea of distinct knowledge representation systems. Script structure can be defined by it’s sequential nature and the characteristics of the events contained within the script. A script is composed of a set of events that occur in a typical sequence (e.g., the actions or events that describe what one does when going to eat at a restaurant). Each event can be described in terms of it’s place within the temporal

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sequence of events, how central it is to defining the script, how distinctive the event is to the script, how frequently it is produced by subjects, and so on. Subjects may occasionally produce events that fall outside the boundaries of the script or are inappropriate. In the present study, we examined script knowledge and retrievability by testing DAT patients on a single script generation task and comparing their performance on this task to a group of depressed patients and age-matched controls. We determined how central an event was to a particular script and questioned whether DAT patients would be able to retrieve those events that were less central to that individual script, analogous to their problems in retrieving low frequency words on Word Fluency Tasks. In addition, when DAT patients produced events for a script that were not events produced by the normative sample (college students), we wondered whether such events were plausible. In the likelihood that DAT patients would not be able to produce the same number of events that the depressed and control subjects did, would they resort to repeating previously mentioned events in an effort to reactivate the script memory? These central questions were addressed in the following study. MATERIALS

AND METHODS

Subjects Groups of probable DAT, elderly depressed, and normal controls were included in this study. Subjects were matched as closely as possible for age and educational background (see Table 1). There were no age differences between groups but controls were significantly more educated (p < .05) than either depressed or DAT patients (F(2, 49) = 3.91, p < .02). However, the magnitude of the performance differences between controls, depressed, and DAT patients on the measures used in this study diminishes the possibility of education serving as a potential confound in interpretation of the results. Moreover, the depressed patients were closely matched to the DAT group on educational achievement, providing for a more direct comparison of results. Note that some educational data was missing from all the groups. The DAT and depressed patients were tested as inpatients at the NIMHUnit on Geriatric Psychopharmacology. The diagnosis of Major Affective Disorder-depressed type was established clinically using DSM III-R criteria, whereas probable DAT was diagnosed using both DSM III-R (American Psychiatric Association, 1987) and NINCDS-ADRDA criteria (McKahn, Drachman, Folstein, Katzman, Price, & Stadlan, 1984). Patients were excluded if they had a history of significant cardiovascular disease, diabetes mellitus, cerebrovascular accident, epilepsy, or serious head injury. Elderly controls were also screened for evidence of cognitive dysfunction or positive psychiatric history and were excluded if they had a first-degree relative with either major affective disorder or dementia. Most elderly controls were at least partially retired although a few retained fulltime employment. All subjects were free of psychoactive medication for at least 3 weeks prior to testing.

Methods Controls. DAT, and depressed patients received all the experimental tests during the same testing session at the National Institute of Mental Health. Test administration was counterbalanced within groups. The tests reported below were part of a comprehensive neuropsychological evaluation and were embedded within a testing session. In order to take a test, subjects had to indicate an understanding of the instructions.

14.00 (2.60) 13.91 (3.39) 16.10 (3.22)

Education 34j66 42/58 67/3

93/7 9713

Sex (%) M/F

90/10

Handedness (%) R/L

1

CHARACTERISTICS

TABLE

71.79 (11.26) 103.79 (15.98) 124.74 (15.62)

WMS MQ 1.45 t.511

Hughes Dementia Scale

20.23 (8.74)

Hamilton Depression Scale

Note. This table presents basic data on subjects who participated in our study. Note that the controls are slightly more educated and have a higher percentage of females than the other groups. The Global Depression Scale indicates that the DAT patients were mildly depressed and the clinically depressed patients were moderately depressed. DAT patients were mildly to moderately demented on the basis of the Hughes Scale.

64.07 (10.79) 65.71 (9.48) 63.65 (8.45)

DAT (N = 29) Depressed (N = 14) Controls (N = 31)

Age

(years)

Group

SUBJECT

rp

3

% % z ii?

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Script Generation. Subjects were asked to tell the examiner or write “all the things that you do when you get up in the morning till you leave the house or have lunch.” Subjects were encouraged to generate as many events as they could. Scripts were scored for total number of events generated, centrality values (normative college student data provided by L. Barsalou, personal communication; averaged values ranged between 1 and 30 in accordance with the number of script events rated with low numbers representing high centrality and high numbers representing low centrality), which indicated a rating of how central or critical an event was to the definition of the script (e.g., taking a shower for a “getting up in the morning” script), plausibility of an event occurring within the script (dichotomous ratings representing plausibility or nonplausible), whether the event fell within the raterderived borders of the script (e.g., between waking up in the morning and leaving the house)-also, a dichotomous rating, and for events that were repeated within a script or perseverated. The script that was chosen is one that is commonly used in research studies and, in addition, we felt that all subjects would have a memory representation for “getting up in the morning.” Script Event Discrimination. A subset of DAT and depressed patients were shown two events at a time and asked to assert whether the events were presented in the correct order. All events came from the above script and were chosen from the set of events generated by college students (events provided by L. Barsalou, personal communication). For comparison purposes, we administered two other kinds of tests of knowledge retrieval: Word Generation and Sentence Clozure. For the Word Generation task, subjects had to produce as many words as they could in 90 set to a letter cue or freely associate as many words as possible to a word cue. For the sentence cloze task, subjects were required to produce the last word in a sentence that was highly contextually constrained (Bloom & Fischler, 1980). Forty-two sentences were presented. Statistical analyses included x2 ANOVAs, Kruskal-Wallis Nonparametric test, MannWhitney II test, rank order correlations and Post-Hoc Scheffe and Bonferroni/Dunn Tests. Pot-Hoc test results are presented only as p values.

RESULTS Word Generation and Sentence Cloze Tasks

DAT patients were impaired on both free-associates (F(2, 61) = 10.26, < .OOOl; Controls > DAT, p < .OS) and letter fluency (F(2, 61) = 10.30, p < .OOOl; Controls and Depressed > DAT, p < .05) word generation tasks (see Fig. 1); whereas the depressed patients were impaired only on the free-associate word generation task (Controls > Depressed, p < .05). On the sentence cloze test, DAT patients were again significantly impaired (Kruskal-Wallis: x2(2) = 17.156, p < .0002) compared to controls (U = 96.5 (Z = -3.95), p < .OOOl) and marginally impaired compared to depressed patients (U = 127 (Z = -2.207), p < .02) using the MannWhitney U test. There was no significant difference between the performance of controls and depressed patients on this test. Despite their relatively poor performance on this test, DAT patients appeared to benefit from the context imposed by the sentence structure in retrieving the correct word in contrast to their poor performance on the word fluency generation tasks. p

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Associates

ET AL.

Controls DAT Depressed

FlUWlCy

1. This figure shows that controls and depressed patients performed similarly on letter fluency (word generation tasks, left) and sentence completion tasks (right). On the effortful free associates fluency task, the performance of the depressed patients was more similar to that of the DAT patients. Note the improved performance of the DAT patients on the highly contextual sentence completion test. FIG.

Script Generation

DAT patients produced significantly fewer events that occurred between getting up in the morning and leaving the house than did depressed patients or controls [F(2, 61) = 11.12, p < .OOOl); Depressed > DAT (p < .05), Controls > DAT (p < .05) ( see Fig. 2)]. Nineteen percent of 31 DAT patients, but only 5% of 19 controls and no depressed patients

20 T

Error* in Generming script Event order

Erron in Pan-wise Verification 01 script Event Order

F

FIG. 2. DAT patients generated fewer script events and were more confused about the correct order of script events than controls and depressed patients. (left) Total number of events. (right) Order of events.

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0

2 SCRIPT

4 EVENTS

6

8 IN ORDER

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Controls DAT Depressed

10

12

14

GENERATED

FIG. 3. Although the overall number of script events generated differed between DAT patients and controls and depressed patients, the slopes indicating the percentage of subjects generating a specific number of script events were similar.

generated script events out of order. This difference, while suggestive, was not significant using a contingency analysis (x” (2) = 4.60, p = .lO). On a task that only included a subset of DAT (N = 28) and depressed (N = 12) patients, DAT patients (88% correct) were significantly less accurate than depressed patients (97% correct) when asked to identify whether pairs of script events shown to them were in their typical sequential order (F(1,38) = 6.81, p < .Ol). An example of this error would be to endorse as correct the event “putting on your clothes” as occurring prior to the event “taking a shower.” When we qualitatively examined the relationship between total number of script events generated and percentage of subjects within-group who generated a certain number of events, we noted that (see Fig. 3) depressed and controls had similar slopes, whereas more than half the DAT patients produced fewer than five appropriate script events. Script Event Characteristics

For the first six events produced, DAT patients, depressed patients, and controls had similar slopes for event mean centrality values (see Fig. 4). For all groups, the earliest events produced were rated the most central to the script. Centrality values for the few DAT patients producing more than six script events indicated that those events remained central to the script. Depressed patients produced similar or less central events compared to controls. The mean centrality value for controls was generally consistent after the sixth script event they produced. However, no be-

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30-

z

H M M

25-

Controls DAT Depressed

: $

20-

2 58

15-

f 5

lo-

51”“““““’

1

2

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6

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IN ORDER

9

10

11 12

GENERATED

FIG. 4. The mean centrality values for script events in the order that they were generated is represented for DAT patients, depressed patients, and controls. Note that the centrality values for all groups are generally similar. Lower values indicate that an event is more central to the script.

tween-group differences emerged for mean centrality value across all events generated. When we broke script event centrality values into four ranges with range 1 representing events most central to the script (using a quartile procedure based on mean centrality values for each event derived from college student norms: see Fig. 5), we found significant effects for group (F(2, 61) = 29.71, p < .OOOl) and for centrality range (F(3, 61) = 13.34, p < .OOOl) but not for the group x centrality range interaction, indicating a similar pattern of results for all groups and that few events are seen as critical to the definition of a script (range l), whereas a larger number of typically retrieved events are deemed relatively unnecessary for the definition of a script (range 4). As reported above, DAT patients retrieved fewer script events than either controls or depressed patients (Bonferroni/Dunn comparisons, p < .OOOl). Events falling into ranges 2 and 4 were retrieved at a relatively greater frequency than events from ranges 1 and 3 (Bonferroni/Dunn comparisons, p < .0002). Script Event Appropriateness

Two examiners rated all the script event productions as falling within or outside the script boundary and whether the events produced were “plausible” or “implausible” given the context of the script. No interrater reliability was obtained, but the examiners were blind as to which group subjects belonged to and came to a consensus judgement regarding the

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1

0

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3

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1

2

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4

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1

2

3

4

Depressed RANGES

The figure demonstrates that script event production is distributed similarly across centrality ranges across the three groups. Depressed patients were less likely than controls to generate low frequency script events. Range 1 represents events most central to the script and ranges 2 through 4 represent script events of decreasing centrality.

ratings. As rated by the two examiners, more DAT patients produced at least one “beyond-script boundary” event (e.g., going shopping) compared to either depressed patients or controls [x2(2) = 5.94, p < .05 (see Fig. 6 for the mean number of within- and beyond-script boundary events produced per group)]. These “beyond-script boundary” events, although inappropriately produced given the task instructions, were plausible actions that generally continued the day’s activities beyond the activity confined to the script. Many script events (approximately 43% of the total) generated by our sample of aged subjects were not on the Barsalou (unpublished norms) lists which provides evidence that script event norms are likely to differ between older and younger subjects. When we qualitatively analyzed those “within-script boundary” events produced by subjects that were not contained on the normative lists for script plausibility, we found that more DAT patients produced at least one implausible event within the script boundary compared to controls or depressed patients [x2(2) = 5.94, p < .05]. Twenty-nine percent of DAT patients produced at least one perseverative script event compared to 7% of depressed patients and 5% of controls [x2(2) = 5.94, p < .05]. Dissociations in Knowledge Production

We plotted z scores for word fluency, sentence cloze, and script event generation tasks to try and qualitatively identify single cases that dem-

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ET AL.

-

,-. -L 2-

”

Within Script

Beyond Script

CONTROLS

Within Script

l-

Beyond Script

DAT

Within Script

Beyond Script

DEPRESSED

6. DAT patients produced fewer within-script events and more beyond-script events than either depressed patients or controls. FIG.

onstrated within-subject dissociations in task performance (see Fig. 7). We decided to focus on DAT cases since their relatively good performance on a task would place that score within normal limits, whereas relatively poor performance on a task would place that score in the impaired range. Even though a relative dissociation might occur in some subjects in the control and depressed groups, both scores would likely be within normal limits making the dissociation more difficult to intepret. DAT patients who performed poorly on sentence completion almost always performed poorly on script event production but the inverse did not hold. In contrast, we observed that three DAT patients showed relatively (greater than 2 z scores) better word generation performance than script generation performance, while one DAT patient had relatively better script event generation than word generation performance. DISCUSSION

The results of this study partially lend support to the idea that there are multiple representations of knowledge based on similar structural constraints. DAT patient breakdown in script production is quite similar to their impaired lexical production. DAT patients produce a few highfrequency script events, but then their overall production curtailed. Studies of Word Fluency in DAT patients show reduced access to low frequency

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DISEASE

b. .

-1.5-lb.5

0 SCRIPT

.5

1

1.5

2

2.5

-.-

3

-1.5-l-.5

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SCRIPT

SCRIPT

0

.5 1 1.5 EVENTS

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COMPLETION

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3

..-1.5-l-.5 ASSOCIATION

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.5

1 1.5 FLUENCY

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-.5

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COMPLETION

.5

1 1.5

-2.5-Z-1.5-1

-.5

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.5

1

1.5

FIG. 7. This figure suggests that performance on lexical and script retrieval tasks can be used to argue for at least a partial dissociation in their representational systems for each kind of knowledge. For example, scatterplot b reveals a few patients who perform relatively better in producing script events than lexical items and vice versa. Scatterplot c indicates that patients who perform poorly generating sentence cloze items also do not perform well generating script events although the reverse holds true. Other scatterplots demonstrate the occasional individual cases that support further dissociation between sentence clozure and lexical retrieval. Correlations indicated that letter and association fluencies (r = .60) were more related than all other compared measures (r < .26) in DAT patients. The same pattern of correlations held true for the controls (letter and association fluencies: r = .48; other compared measures: r < .32) but not for the depressed patients. For the depressed patients, the script event measure was more highly correlated with the fluency and sentence cloze measures (r > .38) than were the comparisons among those measures (r < .25).

members of the class of items being retrieved (Huff et al., 1986). They also tended to produce more events that fell outside the boundaries of the script (i.e, similar to a lexical semantic error) or were inappropriate (i.e., similar to a lexical random error). Script events were occasionally inappropriately repeated by DAT patients (i.e., similar to a lexical perseverative error). These findings suggest that the internal structure of a script is similar to that of a lexical network (Barsalou & Sewell, 1985; Barsalou, 1988). Impaired script event production could be due either to a structural degeneration or to a processing problem. The results do not allow us to distinguish between the two explanations. There are some differences in characteristic properties of the knowledge represented in script and lexical networks. For example, there is no obvious parallel to production (or verification) errors in ordering script events (Yekovich & Walker, 1986) in a lexical production task (a phonological error is not an obvious candidate). Another script event characteristic property not assessed in our study but presumably unique is the repre-

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sentation of the time duration of an event within a script or of the entire script. Our study was unable to determine whether script representations are differentially located in the brain from lexical representations. One DAT patient performed relatively well in producing script events compared to category items-three others performed relatively well producing category items compared to script events. Performance on the sentence cloze task appeared independent of performance on the word fluency and script event retrieval tasks. These types of dissociations must be interpreted cautiously since they could be due to a chance occurrence or withinsubject variability that preceded the onset of their dementia. Furthermore, DAT patients are not the optimal group to obtain clear-cut dissociations with. For that, focal lesion patients with damage to the frontal or temporal-pariental lobes must be compared. Scripts have been described as a schema-like representation of knowledge (Gibbs & Tenny, 1980; Bellezza & Bower, 1981; Kaminska-Feldman, 1982; Light & Anderson, 1983) composed of a personally or socially relevant/typical sequence of events (Schank, 1979). There is some debate as to whether scripts and similar representations (e.g., schemas) have a distinct psychological representation (e.g., distinct from lexical or sentential representation) or are unique by virtue of an “assembly rule” that forms each script from a more primitive set of events or lexical items (Byrne, 1981). Script-like structures called Managerial Knowledge Units (MKUs) have been assigned an important role in a recent theory (Grafman, 1989) that suggested such knowledge-based representational systems are specifically subserved by prefrontal cortex. In that model, MKUs are activated in parallel with knowledge from other representational networks (e.g., lexical and propositional networks) located in more posterior cortex during the processing of information. The MKU model predicts that the structure of an MKU network should be basically similar to a lexical network, but by virtue of their distinctive knowledge and locations in the brain, the two representational networks should be dissociable and that the characteristic properties of the knowledge represented in the two networks should be different. The findings obtained in this study are compatible with some aspects of the MKU model. It is occasionally observed that DAT and other central nervous system impaired patients with poor fluency are able to appear oriented in social situations. The MKU model predicts that DAT patients with intact access to scripts and schema-like representations may be able to orient in lieu of access to simple linguistic structures or visual forms (Rabbitt, 1984). On the other hand, problems in retrieving the appropriate scripts could result in a DAT patient behaving inappropriately or in a disoriented manner in typical or atypical social situations despite relatively intact lexical or visual form access. Impaired script access, according to the MKU model, may imply relatively more pathological involvement of pre-

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frontal cortex in DAT. Thus, studying scripts, schemas, and like-structures in central nervous system impaired patients may be both theoretically and practically useful (Giola & Poole, 1984). This study was mainly concerned with script event production but other ways to study scripts and script-event representation exist. For example, subjects could decide whether events belonging to two different scripts had similar actions or roles within their respective scripts or perform divided attention tasks requiring simultaneous processing of two or more scripts, etc. In summary, the results of this study support the notion that script event production, like category-based lexical production, is impaired in DAT. The character of impaired script event production is similar to that of impaired lexical production. Furthermore, we speculate that scripts are indicative of a separate representational system stored in prefrontal cortex that is devoted to the memory of sequential events that have a typical thematic structure (Shallice, 1989). Further research regarding script knowledge in central nervous system impaired patients should prove useful in testing theories of how types of knowledge are topographically represented in the brain. REFERENCES Abbott, V., Black, J. B., & Smith, E. E. 1985. The representation of scripts in memory. Journal of Memory and Language, 24, 179-199. American Psychiatric Association. 1987. Diagnostic and statistical manual of mental disorders, (3rd ed., revised) (DSM-III-R). Washington, DC: American Psychiatric Association. Barsalou, L. W., & Sewell, D. R. 1985. Contrasting the representation of scripts and categories. Journal of Memory and Language, 24, 664-665. Barsalou, L. W. 1988. The instability of graded structure: Implications for the nature of concepts. In U. Neisser (Ed.), Concepts reconsidered: The ecological and intellectual bases of categories. Cambridge: Cambridge Univ. Press. Bellezza, F. S., & Bower, G. H. 1981. The representational and processing characteristics of scripts. Bulletin of the Psychonomic Society, 18, l-4. Bloom, P. A., & Fischler, I. 1980. Completion norms for 329 sentence contexts. Memory and Cognition, 8, 631-642. Bower, G. H., Black, J. B., & Turner, T. J. 1979. Scripts in memory for text. Cognitive Psychology, 11, 177-220. Bower, G. H., & Clark-Myers, G. 1980. Memory for scripts with organized versus randomized presentations. British Journal of Psychology, 71, 369-377. Byrne, R. 1981. Mental cookery: An illustration of fact retrieval from plans. Quarterly Journal of Experimental Psychology, 33A, 31-37. Chang, T. M. 1986. Semantic memory: Facts and models. Psychological Bulletin, 99, 199220. Diesfeldt, H. F. A. 1985. Verbal fluency in senile dementia: An analysis of search and knowledge. Archives of Gerontology and Geriatrics, 4, 231-239. Galambos, J. A. 1983. Normative studies of six characteristics of our knowledge of common activities. Behavior Research Methods and Instrumentation, 15, 327-340. Galambos, J. A., & Rips, L. .I. 1982. Memory for routines. Journal of Verbal Learning and Verbal Behavior, 21, 260-281.

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Gibbs, R. W., & Tenny, Y. J. 1980. The concepts of scripts in understanding stories. Journal of Psycholinguistic

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Giola, D. A., & Poole, P. P. 1984. Scripts in organizational agement

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