Sentence comprehension in Parkinson's disease: The role of attention and memory

BRAIN

AND

LANGUAGE

42, 347-384 (1992)

Sentence Comprehension in Parkinson’s Disease: The Role of Attention and Memory MURRAY GROSSMAN,*'f' SUSAN CARVELL,* MATTHEW B. th3w,*‘j’ STEPHEN GoLLoMP,*'t AND HOWARD I. HURTIG*'j' *Department

of Neurology,

University of Pennsylvania; and TThe Graduate University of Pennsylvania

Hospital,

Sentence comprehension is a complex process involving at least attentional, memory, grammatical, and semantic components. We report three experiments designed to evaluate the impairments underlying sentence comprehension difficulties in nondemented patients with Parkinson’s disease (PD). In the first experiment, we asked patients to answer simple questions about sentences which varied in terms of grammatical complexity and semantic constraint. We found that PD patients are significantly compromised in their ability to perform this task. Their difficulties became more prominent as grammatical complexity increased, but they were significantly assisted by semantic constraints that limited possible interpretations of a sentence. Analyses of individual patient profiles revealed heterogeneous performance across the group of PD patients and somewhat inconsistent performance for patients across testing sessions. In the second experiment, we tested the possibility that patients’ heterogeneous performance on the sentence comprehension task is due to an impairment in memory or attention, cognitive domains known to be compromised in some PD patients. Although PD patients and control subjects differed on one memory measure, there were no significant correlations between attention and memory performance and the results of the sentence comprehension task. In the final experiment, we manipulated the sentences used in the first experiment in a fashion that stressed the need for memory and attention in a sentence. The results indicated that PD patients are significantly compromised in their ability to attend to certain critical grammatical features of a sentence. A regression analysis identified specific grammatical, semantic, and attentional mechanisms as significant contributors to PD patients’ overall sentence comprehension, accounting for over 97% of the variance in their performance. We conclude that there are multiple sources of cognitive difficulty Address correspondence to Murray Grossman, Cognitive Neurology Section, Department of Neurology, Hospital of the University of Pennsylvania, 3400 Spruce Street, Philadelphia PA 19104-4283.Fax: 215-349-5579; email: [email protected]. We express our appreciation to Guila Glosser and two anonymous reviewers for their helpful comments. This work was supported in part by grants from the National Institutes of Health (DC00039, AG09399) and the McCabe Foundation. 341 0093-934X/92 $5.00 Copyright D 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

348

GROSSMAN ET AL. underlying PD patients’ sentence comprehension impairment.

o 1992 Academic PRSS,

Inc.

INTRODUCTION Patients with Parkinson’s disease (PD) may exhibit impairments in memory, attention, and other cognitive domains (Albert, 1978; Cummings, 1990; Grossman, Carvell, Stern, Gollomp, Vernon, & Hurting, 1990). A language impairment is said to occur infrequently in PD (Cummings, 1990), but studies of language processing in PD have been rare (Bayles and Tomoeda, 1983; Cummings, Darkins, Mendez, Hill, & Benson, 1988; Dubois, Pillion, Sternic, Lhermitte, & Agid, 1990; Matison, Mayeux, Rosen, & Fahn, 1982; Pirozzolo, Hansch, Mortimer, Webster, & Kuskowski, 1982; Taylor, Saint-Cyr, & Lang, 1986). It has recently been demonstrated that nondemented patients with idiopathic PD in fact are significantly compromised in their comprehension of sentences and may be particularly impaired in the processing of a sentence’s grammatical features (Geyer and Grossman, submitted; Grossman and Carvell, submitted; Grossman, Carvell, Gollomp, Stern, Vernon, & Hurtig, 1991; Natsopoulos, Katsarou, Bostantzopoulos, Grouios, Mentemopoulos, & Logothetis, 1991). This difficulty with sentences may be a primary impairment in appreciating linguistic material that resembles the sentence processing deficits seen after insult to portions of the frontal lobe or the basal ganglia (Alexander, Benson, & Stuss, 1989; Damasio, Damasio, Rizzo, Varney, & Gersh, 1982; Masdeu, Schoene, & Funkenstein, 1978; Naeser, Alexander, Helm-Estabrooks, Levine, Laughlin, & Geschwind, 1982; Nadeau, 1988; Novoa and Ardila, 1987; Wallesch, Kornhuber, Brunner, Kunz, Hollerbach, & Suger, 1983), regions of the brain that have been implicated in the pathophysiology underlying PD (Alexander and Crutcher, 1990; DeLong, Georgopoulos, & Crutcher, 1983). Alternately, the sentence processing difficulty may be due in part to a coexisting deficit in memory or attention that can be seen in PD, that is, an impairment in paralinguistic or supportive functions that have been implicated in sentence processing (Bloxham, Dick, & Moore, 1987; Caramazza, Basili, Koller, & Berndt, 1981; Grossman et al., 1990; Lees and Smith, 1983; Levin, Llabre, & Weiner, 1989; Pirozzolo et al., 1982; Saffran, 1990; Sagar, Sullivan, Gabrieli, Corkin, & Growdon, 1988; Schwartz, Linebarger, Saffran, & Pate, 1987; Taylor et al., 1986). Indeed, Lieberman, Friedman, & Feldman, (1990) noted sentence comprehension difficulties in PD patients who were mildly demented. The purpose of this study was to assessthe role of memory and attention functioning in PD patients’ sentence comprehension impairments. Deficits in attention or memory have been shown to play a role in braindamaged patients’ sentence comprehension impairments (Caramazza et al., 1981; Grossman, in press; Kolk and van Grunsven, 1985; Ostrin and

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349

Schwartz, 1986; Saffran, 1990). Indeed, performance characteristics of a comprehension impairment due to an attention or memory disorder are said to include inconsistent patterns across subjects on a given type of sentence and inconsistent patterns across different testing sessions in a particular patient. The specific contributions made by an attention or memory mechanism to sentence comprehension are many. It may be necessary to attend to the stream of speech sounds in order to identify the presence and nature of a sentence’s grammatical morphemes, for example, as a prelude to computing the grammatically specified relationships among the sentence’s words. A patient with an attention disorder may not be able to perform this function and thus may have difficulty understanding a sentence. It may also be necessary to retain a transient mental representation of a sentence in order to be able to determine whether a word at the beginning of the sentence is related to a word at the end of the sentence. A patient with a short-term memory (STM) disorder may not be able to remember an entire sentence, and this may interfere with sentence comprehension. Thus, a highly abbreviated description of a processing model underlying sentence comprehension might include an attentional mechanism that identifies pertinent attributes of the stimulus material. This material may be held in a STM buffer mechanism while a processor supporting grammatical computations can interrelate words in a sentence, and a lexical semantic processor can establish the meanings of the individual words. It is through the interpretation of an appropriately arrayed set of words that the meaning of a sentence can be constructed. The level in this highly simplified model at which PD patients’ sentence comprehension breaks down is unclear. In order to address the role of memory and attention functioning in sentence comprehension in PD, we performed three experiments. In the first study, we examined individual and group performance profiles on a sentence comprehension task. This was done to establish the pattern of sentence processing impairment in PD patients and specifically to assess the role of a grammatical computation mechanism in their sentence comprehension. Several characterizations of agrammatism have been proposed (e.g., Caplan and Futter, 1986; Grodzinsky, 1986; Schwartz et al., 1987), and we wished to determine whether any of these could be applied to PD patients’ sentence comprehension difficulties. In the second experiment, we administered several clinical neuropsychological tests to patients. This was done to ascertain whether difficulties in the domains of attention and/or memory described in PD patients correlate with their sentence comprehension performance. We also assessedother aspects of sentence comprehension such as phonemic appreciation, semantic comprehension of sentence- and paragraph-length material, and repetition. In the third experiment, we manipulated several features of the sentence types used in the first experiment so that we could study the role of attention and

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memory in sentences more directly. We hoped to test whether PD patients are compromised in their ability to employ attentional and mnestic mechanisms which are specialized for sentence processing. All studies were performed on the same set of patients, allowing us to compare patient performance profiles across studies. To the extent that a sentence comprehension impairment can be related in part to an attention or memory deficit, we hypothesized that PD patients should have a sentence comprehension impairment, should exhibit inconsistent performance profiles across patients for the set of sentences and across sessions in a particular patient, should have correlative deficits on neuropsychological tests of memory and attention in conjunction with their sentence comprehension difficulties, and should be impaired following manipulations of sentential material that stress memory and attentional mechanisms. EXPERIMENT 1 Methods Putienfs. We evaluated sentence comprehension in 20 consecutively referred patients with idiopathic Parkinson’s disease (typically Hoehn and Yahr Stage 1 or 2). These right-handed, high school-educated, native English speakers were between 48 and 73 years of age. None of the patients were demented, according to the clinical examination and DSM-III criteria (American Psychiatric Association, 1980). The Mini Mental State Exam (Folstein, Folstein, & McHugh, 1975) was administered as a rapid clinical tool to rule out cooccurring Alzheimer’s disease in the PD patients. We adopted the widely used criterion of 24 or less as grounds for exclusion. Additional exclusionary criteria included the presence of another akinetic-rigid syndrome such as progressive supranuclear palsy or multiple system atrophy, a secondary cause of Parkinsonism such as neuroleptic medication use, the diagnosis of another neurodegenerative condition such as Alzheimer’s disease or a primary psychiatric disorder such as major depression, or any other medical condition that may have resulted in an encephalopathy or dementing process which would interfere with the performance of a mental task. Twelve age- and education-matched neurologically intact subjects recruited through advertisements in local newspapers served as a control group. Neurologists experienced in the evaluation of PD used the Unified Rating Scale for Parkinson’s Disease to assesspatients (Lang and Fahn, 1989). This instrument allows the standardized assessment of neurologic signs such as tremor and rigidity, activities of daily living, and affect. The clinical neurological assessment was performed within 2 weeks of the cognitive evaluation (typically it occurred on the same day), and the neurologists were blinded to the results of language and cognitive functioning. Age, education, duration of the disease, and medication regimen were also noted for each patient. The clinical characteristics of the two groups are summarized in Table 1. Materials. In this experiment, we studied PD patients’ ability to answer a simple question about a target sentence. Moreover, we performed a detailed analysis of individual patient performance profiles in order to define the pattern of sentence comprehension impairment across individual PD patients and across different types of sentences. The sentence comprehension task consisted of 96 target sentences, each of which was followed by a simple question (e.g., “The eagle chased the hawk that was fast. Which bird was chased?“). Subjects were trained to make a two-stage evaluation. The first decision involved judging whether the target sentence was well-formed. If the sentence was judged acceptable, the patients then answered a question about the sentence. Patients judged a very small number of wellformed sentences to be unacceptable (<2%), so these items will be ignored. Examples of

SENTENCE

COMPREHENSION TABLE

351

IN PD

1

CLINICALCHARACTERISTICS OF CONTROLSUBJECTS AND PARKINSON’S DISEASEPATIENTS [MEAN (SD)] Clinical characteristic Size of group Age (years) Education (years) Duration (years) Medication (mg dose/day) Levodopa Bromocriptine Eldepryl Trihexyphenidyl Tremor laterahty (% patients) Right predominance Left predominance Symmetric Tremor severity (O-4) Rigidity laterality (% patients) Right predominance Left predominance Symmetric Rigidity severity (O-4) Hoehn and Yahr stage (% patients) Stage 1 Stage 2 Stage 3 Depression (O-4)

Control subjects

Parkinson patients

12

20 61.9 (7.04) 14.6 (3.02) 5.57 (3.75)

61.8 (5.53) 16.3 (2.09)

na 0 0 0 0

395.0 2.87 2.50 0.00

(418.6) (6.08) (4.44). (0.00)

na na na na

25.0% 25.0% 50.0% 0.89 (0.78)

na na na na

20.0% 25.0% 55.0%

na na na 0

0.91 (0.73) 25%

60% 15% 0.38 (0.58)

items illustrating the variables in these sentences are provided in Table 2, and each of the variables is discussed in more detail below. We manipulated three features in these target sentences, including the synlacric complexity of the target sentence, voice correspondence between the sentence and its probe, and semantic constraint within the target sentence. The variable of syntactic complexity involved the manipulation of increasingly demanding grammatical phrase structures in the target sentence (Fodor, Bever, & Garrett, 1974). Thus, one-third of the target sentences were simple declarative in form (such as “The eagle chased the hawk”). Target sentences were made more complex syntactically in one-third of the items by the addition of a relative clause at the end of a simple sentence (a so-called subordinate item, such as “The eagle chased the hawk that was fast”). The sentences were made even more complex syntactically in onethird of the target sentences by placing the relative clause in the center of the simple sentence (a so-called center-embedded item, such as “The eagle that chased the hawk was fast”). Despite the difference in syntactic complexity between subordinate and center-embedded types of sentences, it may be noted that they contain the same amount of information and that both of these sentence types contain more information than the simple sentences. We expected PD patients individually and as a group to encounter increasing difficulty as the target sentences became more complex syntactically, that is, the level of sentence difficulty is not expected to reflect only the length of a sentence or the amount of information that needs to be processed.

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GROSSMAN ET AL. TABLE 2

EXAMPLES

OF TARGET SENTENCES AND PROBES USED TO ASSESS SENTENCE COMPREHENSION

Simple Constrained Nonconstrained Subordinate Constrained

Nonconstrained

Center-embedded Constrained

Nonconstrained

Corresponding voice

Noncorresponding voice

The worm was chased by the eagle. What was chased? The eagle chased the hawk. What did the chasing?

The eagle chased the worm. What was chased? The eagle chased the hawk. What was chased?

The cat chased the balloon that was black. What did the chasing? The skunk was chased by the porcupine that was hungry. What was chased?

The cat chased the balloon that was black. What was chased? The skunk was chased by the porcupine that was hungry. What did the chasing?

The car that hit the tree was green. What did the hitting? The car that hit the truck was green. What did the hitting?

The car that hit the tree was green. What was hit? The car that the truck hit was green. What did the hitting?

For the voice correspondence variable, half of the target sentences used the active voice (or a subject-relative clause in a center-embedded sentence) such as “The eagle chased the hawk.” The remaining half of the sentences used the passive voice (or an object-relative clause in a center-embedded sentence) such as “The hawk was chased by the eagle.” In half of the items, the question was said to correspond to the target sentence since a similar voice was used in both, but half of the items were noncorresponding since the question was in a different voice (such as the noncorresponding item “The eagle chased the hawk. Which bird was chased?“). Since nonmatching items were thought to require some computational facility over the grammatical feature of voice, it was hypothesized that these would prove more d&cult for PD patients individually and as a group than items where the voices did match. Moreover, by evaluating the precise pattern of impairment on target sentences cast in a particular voice within a specific phrase structure, we could begin to determine whether any of the hypothesized models describing so-called agrammatic patients may also be applied to PD patients’ sentence comprehension difficulties (Caplan and Futter, 1986; Grodzinsky, 1986; Schwartz et al., 1987). The variable of semantic constraint concerned the use of reversible or nonreversible nouns in a target sentence. Half of the target sentences were considered nonconstrained since the nouns could exchange places without violating the semantic coherence of the target sentence (the above examples are said to be reversible, for instance, since an eagle and a hawk are equally capable of chasing each other). Half of the target sentences were constrained since exchanging the places of the nouns would have violated commonly held semantic notions (the target sentence “The worm was chased by the eagle” is said to be nonreversible since

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it is much more likely for an eagle to chase a worm than the reverse). If PD patients are compromised in an across-the-board fashion on all aspects of sentence appreciation, then they should also have difficulty accessing semantic information represented in a sentence, and the presence of semantic constraint should not facilitate sentence comprehension performance. The variables were counter-balanced and evenly distributed across different types of items so that there were eight instances of each type of item listed in Table 2. The entire set of items was arrayed in random order and read twice to patients in a naturalistic fashion which provided no external cues as to the correct response. Patients were given as many repeated exposures as they requested (each patient typically requested one additional repetition on about 5% of the items). Responses were untimed, and all patients responded to all items. A practice session preceded the administration of the sentence comprehension battery. The practice items were ordered so that they became progressively more difficult, eventually approximating items in the experimental battery. We also wished to obtain a measure of session-to-session reliability in the PD patients’ sentence comprehension performance. During a separate session, each of the PD patients was asked to answer a simple question about eight semantically nonconstrained sentences which varied in their syntactic complexity and voice correspondence.

Results An analysis of variance (ANOVA) using a group [2] x syntactic complexity [3] x semantic constraint [2] x voice correspondence [2] design revealed a significant effect for group [F(l, 30) = 10.51; p < .005]. Since performance on tasks such as these are often influenced by education level, we also performed an analysis of covariance that covaried for education, but this factor could not fully explain the differences between PD patients and control subjects [F(l, 29) = 7.88, p < .009]. These findings indicated that PD patients as a group encounter significantly more difficulty than control subjects in answering simple questions about sentences. A discriminant analysis (Dixon, 1988) was performed in order to determine the proportion of PD patients who are compromised in their sentence processing. Patients’ overall performance and their performance on each of the variables were used by the algorithm to partition the entire set of subjects involved in the study into two groups. The most effective variable at discriminating between control subjects and PD patients was the center-embedded syntactic phrase structure. On the basis of their performance on these items, 78.1% of subjects were classified correctly. Thus, 100% of control subjects were classified as control subjects, but 65% of PD patients were classified as different from control subjects [approximate F(1, 30) = 19.891. The next most effective factor at discriminating between control subjects and PD patients was the feature of semantic constraint within a sentence [75.0% classified correctly; approximate F(1, 30) = 9.381. Sixty percent of the PD patients were classified differently from the control subjects. In terms of overall performance on the set of sentences, 68.8% of subjects were classified correctly, and 55.0% of PD patients differed from control subjects in terms of their perfor-

354

GROSSMAN ET AL. 100

90

I-

80

H 8 *

70

6c

5C SIMPLE SUBORDINATE CENTER-EMBEDDED FIG. 1. Performance of control subjects and Parkinson’s disease patients on sentences

varying in their grammatical phrase structure.

mance. These findings indicate that a large number of individual PD patients are compromised in their appreciation of sentences, but that performance across the entire group of PD patients is not homogeneous. We assessedeach of the PD patients a second time using a brief battery composed of identical types of sentences. When we correlated PD patients’ overall performance on the larger battery of sentences with their performance on the abbreviated retest battery, we found a significant correlation [r(B) = 0.469; p < .05]. However, this is far from a one-to-one correspondence across sessions in the individual patient’s performance. Thus, there would appear to be some session-to-session variability in PD patients’ sentence comprehension. Further analysis of the group data derived from the ANOVA revealed a significant effect for syntactic complexity [F(2, 60) = 8.48; p < .OOl] and a group x syntactic complexity interaction [F(2, 60) = 5.63; p < .005]. As can be seen in Fig. 1 and demonstrated by planned t tests, PD patients were significantly more impaired than control subjects at answering questions about subordinate sentences [t(30) = 2.02; p < .05] and center-embedded sentences [t(30) = 4.46; p < .OOl], but there was no difference between PD patients and control subjects with simple sen-

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355

tences. Moreover, PD patients were significantly more impaired on subordinate sentences than their own performance on simple items [t(19) = 3.33; p < .005] and more impaired on center-embedded items than simple items [t(19) = 4.06; p < .OOl]. The difference between subordinate and center-embedded items approached significance [t(19) = 1.89; p < .07]. These findings suggest that PD patients’ sentence comprehension impairment may be related to the syntactic complexity of a sentence’s phrase structure. In order to determine whether group characteristics accurately reflected individual patient performance profiles, we assessed individual PD patients’ relative difficulty with the three types of sentences. As can be seen in Table 3, we found that the center-embedded > subordinate > simple order of sentence difficulty was present in 82% of individual PD patients with nonzero differences in their performance profiles on these items (9 of 11 patients; p < .Ol, according to the binomial test). We also found that 12 of 15 PD patients with nonzero differences had more difficulty on subordinate sentences than on simple sentences, 14 of 17 patients found center-embedded sentences more difficult than subordinate sentences, and 15 of 17 patients had more difficulty on center-embedded than simple sentence types (all differences significant at the p < .Ol level, according to the binomial test). These findings confirm that the length and the amount of information are not the only determinants of performance, but that increasingly complex phrase structures also prove increasingly difficult. Moreover, they indicate a fairly consistent pattern of impairment in appreciating sentences across individual PD patients. Although the voice correspondence variable was not significant, we analyzed the effects of the target sentence’s voice across correspondence conditions. This was done to obtain additional information about the nature of the grammatical impairment that may underlie PD patients’ sentence comprehension difficulties, particularly in light of the specific claims that have been made about the relative difficulty of sentences such as these. As can be seen in Fig. 2 and demonstrated by t tests, we found that PD patients were relatively unimpaired in their attempts to answer questions about active voice sentences with simple grammatical structures. They were somewhat more impaired in their appreciation of simple sentences in the passive voice (although not significantly so), subordinate items in the active voice [t(19) = 2.16; p < .05], and subject-relative center-embedded sentences [t(19) = 3.12; p < .Ol]. Subordinate sentences in the passive voice, the longest sentences that we used, were significantly more difficult for PD patients than simple sentences in the active voice [t(19) = 3.83; p < .OOl] or the passive voice [t(19) = 3.11; p < .Ol]. However, PD patients were most impaired in their appreciation of objectrelative center-embedded sentences, items that were shorter than the subordinate passives but more complex grammatically. This effect was

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TABLE 3 INDIVIDUAL PARKINSON’SDISEASE PATIENTS’ PERFORMANCEPROFILES

Pt No. 9001 9008 9012 9014 9018 9021 9022 9025 9026 9028 9030 9038 9041 9042 9043 9045 9046 9055 9056 9086

As 52 67 63 73 48 66 50 68 70 68 65 67 61 56 63 56 63 53 65 64

Educ (1)

Dura

13 14 14 14 12 18 20 14 20 12 17 12 12 19 14 12 12 12 12 19

Simpl

Subor

(4)

Overall camp (5)

(6)

(7)

3 2 2 2 1 1 3 1 2 3 1 2 2 2 2 2 2 2 2 1

92.71 97.91 71.43 89.58 82.29 loo 95.83 94.79 97.91 97.91 89.58 95.83 89.58 95.83 80.21 87.5 81.6 97.91 88.54 85.41

100 100 75 96.87 81.25 100 96.87 100 100 100 100 96.87 96.87 96.87 96.87 93.75 96.87 93.75 96.87 87.5

93.75 100 62.5 87.5 72.32 100 96.87 93.75 96.87 93.75 90.62 96.87 84.37 100 78.12 87.5 87.5 100 90.62 87.5

H and Y

(2)

Tremor (3)

6 10 6.5 11 3 3 2.5 4 9 7 3 2 2 2.5 3.5 3 10 14 1.5 2

R 0 R 0 L 0 L R R 0 B L L R B 0 L 0 B 0

Pt No.

Centr

Corr (9)

Ncorr 00)

Const (11)

Nconst

(8)

9001 9008 9012 9014 9018 9021 9022 9025 9026 9028 9030 9038 9041 9042 9043 9045 9046 9055 9056 9086

84.37 93.75 81.25 84.37 81.25 100 93.75 90.62 96.87 100 81.25 93.75 87.5 90.62 71.87 81.25 81.25 100 81.25 81.25

97.91 100 91.66 91.66 85.41 100 95.83 95.83 97.91 100 91.66 100 93.75 95.83 83.33 89.58 89.58 100 91.66 87.5

87.5 95.83 81.25 87.5 79.16 100 95.83 93.75 97.91 95.83 87.5 50 87.5 95.83 77.08 85.41 87.5 95.83 85.41 83.33

95.83 100 83.33 93.75 85.41 100 97.91 95.83 100 95.83 89.58 95.83 91.66 100 87.5 93.75 89.58 95.83 93.75 85.41

87.5 95.83 54.16 87.5 79.16 100 95.83 93.75 97.91 95.83 87.5 50 87.5 95.83 77.08 85.41 87.5 95.83 85.41 83.33

(12)

Adjacent (13) 100 100 50 100 100 87.5 100 87.5 87.5 87.5 62.5 loo 100 100 87.5 loo 100 100 100 100

Nonadj (14)

Missing gram morph (15)

87.5 100 87.5 100 87.5 100 62.5 87.5 100 100 loo 100 100 loo 62.5 75 100 loo 100 87.5

100 88.23 41.17 82.35 35.29 94.11 64.71 52.94 70.58 52.94 58.82 47.05 94.11 100 76.47 58.82 17.64 100 58.82 100

SENTENCE

COMPREHENSION

TABLE

Pt No. 9001 9008 9012 9014 9018 9021 9022 9025 9026 9028 9030 9038 9041 9042 9043 9045 9046 9055 9056 9086

Pt No. 9001 9008 9012 9014 9018 9021 9022 9025 9026 9028 9030 9038 9041 9042 9043 9045 9046 9055 9056 9086

Position change

Phono

(16)

(17)

100 100 47.82 100 91.3 100 82.6 95.65 100 100 95.65 82.6 100 95.65 86.95 95.65 47.82 100 91.3 100

100 93.75 43.75 7s 81.25 93.75 56.25 50 87.5 93.75 62.5 56.25 87.5 87.5 87.5 81.25 25 100 87.5 93.75

Regis (20)

Calcs

STM-1

STM-5

(18)

(21)

(22)

(23)

LTM (24)

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

7 7 5 7 7 7 7 5 6 3 5 5 3 7 7 7 5 7 7 7

1 1 1 1 1 1 1 1 1 1 3 3 1 1 1 1 1 1 1 1

5 10 3 8 10 10 5 5 10 8 10 8 10 8 10 5 10 5 8 5

4 3.5 2 0.5 4 3.5 4 2 2 1.5 3 4 1.5 1 4 1.5 3.5 3 0 3.5

4 4 3.5 1.5 4 4 4 4 3.5 4 4 4 2.5 3 4 2 4 4 2 3

7.5 9.15 7.5 8.33 10 8.33 9.15 8.33 8.33 8.33 10 8.33 9.15 8.33 9.15 5.83 9.15 10 8.33 5.83

Cat nam (30)

Oral camp (31)

Read camp (32)

Oral

Writ

(28)

Conf nam (29)

exp (33)

ew (34)

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

14 12 8 14 14 12 14 14 11 14 11 14 13 12 10 7 12 14 8 13

10 10 8 8 10 10 10 10 10 10 10 10 10 10 8 10 8 10 8 10

9 10 7 10 9 9 10 9 10 10 10 6 10 6 9 10 6 10 10 10

10 8 10 10 10 10 8 10 10 10 6 9 8 10 4 10 10 8 10 10

10 10 10 10 10 10 8 10 10 10 8 8 10 10 10 8 10 10 10 10

Repet

(26)

Phono (27)

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

10 10 10 10 10 10 5 10 10 10 8 8 10 10 10 10 8 10 5 10

Auto

(25) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

3-Continued

Digit span (19)

Orient

Sem

357

IN PD

Notes. (1) educ, education in years. (2) dura, duration of disease in years. (3) 0, minimal tremor; R, right predominance; L, left predominance; B, bilateral tremor. (4) H and Y,

358

GROSSMAN ET AL. TABLE 3-Continued

Hoehn and Yahr Stage of disease severity (1 = mild). (5) overall camp, overall sentence comprehension performance percentage correct. (6) simpl, simple phrase structure sentences percentage correct. (7) subor, subordinate phrase structure sentences percentage correct. (8) centr, center-embedded phrase structure sentences percentage correct. (9) corr, corresponding voice items percentage correct. (10) ncorr, noncorresponding voice items percentage correct. (11) const, semantically constrained sentences percentage correct. (12) nconst, semantically nonconstrained sentences percentage correct. (13) adjacent, items probing information from adjacent sentence segments percentage correct. (14) nonadj, items probing information from nonadjacent sentence segments percentage correct. (15) missing gram morph, detection of missing grammatical morpheme percentage correct. (16) position change, detection of changes in position of morpheme percentage correct. (17) phono change, detection of changes in phonological shape of grammatical morpheme percentage correct. (18) orient, orientation correct (maximum score = 10); the scores on many of the following tasks have been prorated to a maximum score of 10 in order to facilitate comparisons across tasks. (19) digit span, number of digits repeated correctly. (20) regis, number of repetitions necessary to repeat three words correctly in the right order. (21) talcs, number of calculation problems correct (maximum score = 10). (22) STM-1, number of words correctly recalled in the right order at 1 min following presentation (maximum score = 4). (23) STM5, number of words correctly recalled in the right order at 5 min following presentation (maximum score = 4). (24) LTM, number of 8 presidents correctly recalled in the right order (maximum score = 10). (25) sem, number of semantic category violations produced during category naming. (26) auto, number of errors produced in expressing automatic speech sequences. (27) phono, number of phonologic discriminations correctly made (maximum score = 10). (28) repet, number of sentences repeated correctly (maximum score = 10). (29) conf nam, objects named correctly (maximum score = 10). (30) cat nam, number of items produced on category naming tasks. (31) oral camp, number of sentences correctly answered (maximum score = 10). (32) read camp, number of questions correctly answered about a paragraph (maximum score = 10). (33) oral exp, production of a spontaneous oral sentence (maximum score = 10). (34) writ exp, production of a spontaneous written sentence (maximum score = 10).

significant when compared with simple active sentences [t(19) = 4.64; p < .OOl], simple passive sentences [t(19) = 3.83; p < .OOl], subordinate active sentences [t(19) = 3.01; p < .Ol], and subject-relative centerembedded sentences [t(19) = 2.29; p < .04]. We also found that objectrelative center-embedded sentences are more difficult than subordinate passive sentences in 11 of 15 patients with nonzero differences. Group analyses with the ANOVA also indicated a significant effect for semantic constraint [F(l, 30) = 7.18; p < .02] and a significant group x semantic constraint interaction [F(l, 30) = 6.19; p < .02]. As can be seen in Fig. 3, PD patients as a group encountered significantly more difficulty than control subjects at appreciating sentences with semantic constraint [t(30) = 2.66; p < .02] and sentences without semantic constraint [t(30) = 3.29; p < .005]. Moreover, the PD patients were significantly more impaired in their responses to nonconstrained items than

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100 90 80 E 8 ze 70 60 50 subordinate

(:enter-embedded

FIG. 2. Performance of Parkinson’s disease patients on sentences varying in their voice (for the simple and subordinate types of sentence) or subject- or object-relative structure (for the center-embedded type of sentence).

their own performance on constrained items [t(19) = 3.56; p < .005], but this difference was not seen in control subjects. These group findings emphasize that the impairment in PD patients’ sentence comprehension primarily involves the appreciation of grammatically complex phrase structures. Analyses of individual patient performance profiles indicated that 81% of PD patients with nonzero differences between item types encounter more difficulty with semantically nonconstrained items than constrained items (13 of 16 patients; p < .Ol according to the binomial test), as shown in Table 3. This finding again demonstrates that many individual PD patients exhibit similar error patterns in their attempts to appreciate a sentence. In order to determine whether PD patients’ motor disorder played a role in their sentence comprehension, we correlated patients’ overall sentence comprehension performance with the clinical features of their movement disorder. There were no significant correlations, as can be seen in Table 4. We also compared the subgroups of PD patients with left hemiParkinsonism and with right hemi-Parkinsonism, but failed to find significant differences for overall performance or performance on grammatical aspects of the task.

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60

50 CONSTRAINED

NON-CONSTRAINED

FIG. 3. Performance of control subjects and Parkinson’s disease patients on sentences varying in the presence of semantic constraint.

Discussion These observations suggest that PD patients may have an impairment that interferes with their sentence comprehension. Several types of explanations have been forwarded in an attempt to explain PD patients’ sentence comprehension difficulty. These include a motor disorder, a deficit in memory or attention, or a language processing impairment. These are discussed in turn below from the perspectives of group and individual patient analyses. Previous studies of PD patients have found speech difficulties, and these have often been attributed to their motor disorder (Critchley, 1981; Darkins, Fromkin, & Benson, 1988; Darley, Aronson, & Brown, 1975; Illes, 1989; Metter and Hanson, 1986). A survey of PD patients’ speech and language with an aphasia battery has found dysarthria and reduced phrase length in spontaneous speech (Cummings et al., 1988; Illes, Metter, Hanson, & Iritani, 1988). While findings such as these implicate a motor component at some level of PD patients’ speech and language functioning, performance on a sentence comprehension task such as ours minimizes a motor component. Since our task was untimed, performance is unlikely to be due to bradykinesia. There was no correlation between sentence

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TABLE 4 CORRELATION OF SENTENCE COMPREHENSION PERFORMANCE AND CLINICAL FEATURES IN PARKINSON’S

DISEASE Clinical feature

Age

Education Duration Levodopa Bromocriptine Trihexyphenidyl Tremor severity Tremor laterality Rigidity severity Rigidity laterality Hoehn and Yahr stage Depression

Sentence comprehension” 0.079 0.483 0.286 0.471 - 0.008 - 0.066 -0.148 -0.107 -0.075 0.104 0.140 0.212

’ A Bonferroni type of correction was used to establish a level of significance that was appropriate for multiple correlations (i.e., p < .05/12 or p < .005).

comprehension performance and clinical features of the movement disorder. Thus, there are many reasons to believe that PD patients’ motor difficulties cannot fully explain their sentence comprehension performance. Several observations, however, are consistent with the claim that PD patients are compromised in their ability to compute grammatical relations among words in a sentence. PD patients as a group appeared significantly compromised in proportion to the grammatical complexity of the target sentence’s phrase structure. We found, for example, that center-embedded sentences are more difficult than subordinate sentences which are, in turn, more difficult than simple sentences. The direct correspondence between the grammatical complexity of a sentence and its difficulty for PD patients is consistent with the characterizations developed by some investigators in their analyses of agrammatic brain-damaged patients’ performance on sentences such as these (Heeschen, 1980). However, our data are less consistent with other characterizations of agrammatism. Grodzinsky (1986, 1989) has made the specific claim that so-called agrammatic patients should treat object-relative center-embedded sentences randomly since the location in deep structure of the trace of a shifted phrase is lost in their surface reconstructions of grammatically complex sentences. While sentences with object-relative center-embedded

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phrases are the most difficult for PD patients, these items are not treated randomly. Caplan and Futter (1986) suggest that some so-called agrammatic patients use a strategy that assigns thematic roles to nouns on the basis of their order in a sentence. We did not find that PD patients treat the first noun of a sentence in the passive voice as the agent of the verb. Given the variety of models which have been forwarded to characterize “agrammatism,” it is difficult to establish a criterion that PD patients ought to meet in order to qualify as “agrammatic.” In fact, we view a criteria1 approach to agrammatism as highly simplistic. It is probably the case that a grammatical processor is a multifaceted mechanism which can break down in any of several different fashions, and the identification of a single pattern of performance as the gold standard would probably be detrimental to furthering the study of sentence processing. The task ought to be for us to determine the specific way in which the appreciation of grammatical aspects of sentences can break down in individuals and in groups of patients such as those with Parkinson’s disease, rather than determining whether a PD patient is agrammatic. In sum, there may be support for the claim that PD patients are compromised to some extent in their ability to perform grammatical computations since their pattern of difficulty directly reflects the degree of grammatical complexity. Our patients may not be clinically demented, but a large proportion of PD patients are nevertheless said to be subtly compromised in domains such as memory and attention (Bloxham, Dick, & Moore, 1987; Dubois et al., 1990; Grossman et al., 1990; Lees and Smith, 1983; Levin et al., 1989; Pirozzolo et al., 1982; Sagar et al., 1988; Taylor et al., 1986). It is possible that a deficit in memory or attention can explain in part the sentence comprehension deficits of PD patients. Notably, deficits in memory and attention have been related to impaired sentence processing (Caramazza et al., 1981; Grossman, in press; Kolk and van Grunsven, 1985; Ostrin and Schwartz, 1986; Saffran, 1990). Observations of group performance indicated that subordinate and center-embedded sentences that contain more information are generally more difficult than simple sentences containing less information. This can be interpreted to support the claim that there is a limitation in the size of a STM buffer that retains a representation of a sentence while it is being processed mentally. However, the sentences also differed in their syntactic processing demands. When the sentences are matched in terms of the amount of information they contain, the center-embedded sentences which are syntactically more complex are more difficult than the syntactically less complex subordinate sentences. Similarly, grammatically complex object-relative center-embedded sentences contain fewer words but are more difficult than the passive subordinate sentences that contain more words. These observations diminish the likelihood that a limitation in the size of a short-term memory buffer can fully explain PD patients’ comprehension impairment, at least on this set of relatively short sentences. Thus, there may be a deficit at

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a performance level of functioning, but it does not appear to be related to the size of a STM buffer. Analyses of individual patient performance profiles do not provide a clear answer to the choice between a deficit in a grammatical computational mechanism or a compromised performance component such as reduced attention or memory. On the one hand, not all PD patients are impaired in their performance on this task. Such heterogeneity can be taken as an inconsistent performance profile across patients. Moreover, when PD patients were reevaluated a second time, the correlation coefficient was significant but only accounted for about 22% of the variance. Thus, there was some evidence for an inconsistent performance profile for individual patients across sessions as well. This patient-to-patient and session-to-session variability is said to be seen in the setting of a deficit at a performance level of processing, such as with an impairment in memory or attention. On the other hand, detailed analyses of error patterns revealed that a very large proportion of PD patients exhibited a similar order of difficulty across sentences couched in different phrase structures and across differing levels of semantic complexity. PD patients’ consistent error profiles, then, may be less than fully consistent with a deficit in attention or memory. It may also be noted at this point that there are alternate explanations for performance variability in the PD patients that are not likely to be related to a deficit in memory or attention. For example, there are individual differences in the distribution of pathology within brain regions that may play a role in PD patients’ intellectual impairments (German, Manaye, Smith, Woodward, & Saper, 1989; Rinne, Rummukainen, Paljarvi, & Rinne, 1989; Torack and Morris, 1988). CNS levels of dopamine may vary from session to session, even if the sessions are conducted at a standard amount of time following medication administration, because of a variety of factors like the amount of protein in the gastrointestinal tract competing with levodopa for uptake into the bloodstream (Brown, Marsden, Quinn, & Wyke, 1984; Girotti, Carella, Grassi, Soliveri, Marano, & Caraceti, 1986; Mohr, Fabbrini, Ruggieri, Fedio, & Chase, 1987). Thus, a cautious interpretation ought to be given to variability across patients and across sessions. The fact that PD patients exhibit somewhat homogeneous error patterns in their sentence comprehension performance or that group data are not fully consistent with a limitation in the size of a STM buffer in PD do not rule out a deficit in some other performance aspect of sentence processing. In the next experiment, we evaluated in more detail the possibility that PD patients’ difficulty with sentence comprehension is due at least in part to a memory or attention deficit. We specifically administered to these patients independent neuropsychological measures of memory, attention, and other features related to sentence processing.

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EXPERIMENT 2 Methods Patients. The same 20 patients with idiopathic PD and 12 control subjects examined in Experiment 1 were also evaluated in Experiment 2. Materials. PD patients were evaluated on several neuropsychological tasks, among which were measures of verbal attention, verbal memory, and other language functions. These tasks were administered within 2 weeks of the sentence comprehension task and in a fashion which was blinded to the results of the sentence comprehension task. A brief description of the pertinent neuropsychological tasks follows. The full battery of tasks and a detailed description of the scoring procedure are available from the first author. Measures of attention included: Orientation: Assessing the patient’s knowledge of personal facts, current location and permanent address, and current day, date, and season with a IO-item questionnaire. Digit span: Repeating digit sequences presented at a rate of one/set, with the sequence length beginning at three items and a failure criterion of missing two sequences at a given length. Word registration: The number of trials required to repeat three low imageability words in the correct order immediately after presentation. Culculutions: Responding correctly to four word problems that require some arithmetic computations. Measures of memory included: Word recall: Reproducing three low imageability words in the correct order at 1 and 5 min after presentation; correct repetition was the criterion for registration, and verbal interference material occupied the time between presentation and recall; full credit was given for reproducing a target word spontaneously and half credit was given for reproducing the correct word in response to a cue. Long-term memory: Recalling the current president and seven previous presidents, with full credit given for producing the name and half credit given for producing any other facts. Semantic memory: The number of items produced in response to a target superordinate semantic category that violated the semantic coherence of the target category. Other measures of language skills included: Automatic speech: Saying the days of the week in the correct order without prompting. Phonemic discrimination: Discriminating between eight pairs of CVCs which differed by one phonemic feature on half of the trials. Repetition: Repeating four sentences which varied in length (up to 13 words) and grammatical complexity. Category fluency naming: Producing the names of as many different items as possible from target semantic categories (vegetables, furniture) and target letter categories (C, L); 1 min was allowed for naming to each target category; Confrontation naming: Naming four black-and-white drawings and four objects, with full credit given for producing the name and half credit given for name production in response to a semantic or phonologic cue. Oral semantic comprehension: Understanding and responding appropriately to six orally presented grammatically simple requests that were increasingly complex semantically. Paragraph comprehension: Answering eight questions about information presented in a fifth grade level written paragraph that consisted of six grammatically simple sentences. Oral expression: Using spontaneous speech to describe the subject’s house; grammatical structure, semantic coherence, and speech clarity were scored. Written expression: Using spontaneous writing to describe the subject’s job or hobby; grammatical structure, semantic coherence, and writing mechanics were scored.

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Results A series of t tests, adjusted for multiple comparisons, was used to compare control subjects and PD patients on the set of neuropsychological tasks. The results of the neuropsychological evaluation are provided in Table 5. The only memory or attention task to prove difficult was recall of three nonimageable words at 5 min following presentation. Mild deficits were also seen on category fluency naming, semantic comprehension of grammatically simple sentences, spontaneous oral expression, and spontaneous written expression. In most of these tasks, the likelihood of obtaining a significant difference between control subjects and PD patients may have been increased by an apparent ceiling effect in the controls. These neuropsychological results confirm the clinical impression that there were, at worst, mild intellectual impairments on measures of memory and attention in these PD patients. The major reason for administering these tasks was to evaluate whether neuropsychological performance on measures of attention and memory plays a role in sentence comprehension. In order to begin determining the functional significance of these measures, a correlation analysis was performed between the results of the neuropsychological tasks and the sentence comprehension measures obtained in Experiment 1. We specifically assessedperformance on the features of sentences which differentiated significantly between control subjects and PD patients, namely their overall performance, their appreciation of subordinate and center-embedded phrase structures, and their appreciation of semantically nonconstrained sentences. Individual patient performance profiles can be seen in Table 3. It may be noted that most scores were prorated to a maximum score of 10 in this table in order to facilitate interpretation. The correlation analysis, shown in Table 6 and adjusted for multiple comparisons, demonstrated significant correlations only between oral semantic comprehension and the sentence comprehension measures. There were no statistically significant correlations between sentence comprehension performance and the measures of attention and memory administered for this study. Since many of the neuropsychological tasks contained few items, and in order to increase the variability in patients’ neuropsychological scores, we also combined conceptually similar tasks into overall scores of attention and memory. These correlations are also provided in Table 6, but they too were not significant. In order to determine whether other neuropsychological measures in addition to oral semantic comprehension contributed in a subsidiary fashion to sentence comprehension, stepwise linear regression analyses were performed (Dixon, 1988). When determining the contribution of performance on these neuropsychological tasks to overall sentence comprehension, the optimal regression formula that was identified explained 35.19%

(SD)

TABLE 5

a t Tests were adjusted for multiple comparisons.

(0.00) (1.56) (0.00) (13.0) (24.6) (7.6) (10.7) (0.00) (0.00) (4.9) (0.00) (0.00) (1.75) (00.0) (11.3) (0.00) (0.00)

10.0 6.54 1.00 87.5% 81.3% 96.9% 87.4% 0.00 100.0% 98.6% 100.0% 100.0% 10.53 100.0% 91.6% 100.0 100.0

Control subjects

100.0% 92.0% 100.0% 99.3% 8.99 95.5% 89.5% 88.5% 94.4%

64.9% 86.25% 84.5% 0.00

10.0 6.48 1.20 75.0%

(0.00) (16.1) (0.00) (1.0) (1.72) (8.0) (12.9) (16.3) (10.3)

(32.3) (26.6) (11.5) (0.00)

WV (1.64) (0.61) (24.25)

Parkinson’s patients

PERFORMANCE ON NEUROPSYCHOLOGICAL TASKS RELATED TO SENTENCE COMPREHENSION AND PARKINSON’S DISEASE PATIENTS

Attention Orientation Digit span Registration Calculations Memory Short-term-1-min recall Short-term-5-min recall Long-term Semantic Language Automatic speech Phonologic discrimination Repetition Visual confrontation naming Category fluency naming Oral comprehension Paragraph comprehension Oral expression Written expression

Neuropsychological task

MEAN

BY CONTROL

t Test”

(19.0) (23.0) (19.0) (25.9) (19.0) (19.0)

Kz; p 2.42; p 2.50; p 0.48; p 3.15; p 2.44; p

< < < > < <

.05

.lO .Ol

.02 .05

.lO

0.00 (24.3) 1.70; p > .lO

0.00

(28.1) 1.60; p > .lO (26.5) 2.07; p < .05 (24.3) 0.71; p > .I0

0.00 (24.2) 0.23; p I=- .lO (19.0) 1.45; p > .lO (29.8) 1.89; p < .07

SUBJECTS

F

4 b

E m

z

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TABLE 6 CORRELATION

MATRIX FOR NEUROPSYCHOLCGICAL TASKS AND SENTENCE COMPREHENSION PERFORMANCE IN PARKINSON’SDISEASE PATIENTS

Phrase structure0 Neuropsychological task Attention Orientation Digit span Registration Calculations Memory Short-term-l-min recall Short-term-5-min recall Long-term Semantic Language Automatic speech Phonologic discrimination Repetition Visual confrontation naming Category fluency naming Oral comprehension Paragraph comprehension Oral expression Written expression

Total camp” -0.068

0.000 -0.069

0.081 0.200

Subordinate -0.023

Center-embedded -0.060

Semantic’ Nonconstrained -0.078

0.000

0.000

0.000

0.155 0.483 0.224

-0.236 -0.013 0.040

-0.134 0.144 0.258 0.071 0.099 0.299 0.250

0.029

0.052

0.076

0.010

0.091

-0.011

0.221

0.198

0.300

0.184 0.000

0.183 0.000

0.173 0.000

0.028

0.055

0.094

0.045

0.000 -0.061 0.000

0.000 -0.135 0.000

0.000 0.071 0.000

0.000 -0.071 0.000

0.362

0.324

0.467

0.302

0.504 0.593%

0.483 0.606* 0.152 0.133

0.492 0.590: 0.138 0.380 -0.003

0.574 0.587* 0.202

0.176 0.221 -0.127

-0.198

0.000

0.110 -0.089

y A Bonferroni type of correction was used to establish a level of significance that was appropriate for multiple correlations (i.e., p < .05/48 or p < .OOl). An asterisk indicates significance at the adjusted p < .05 level.

of the variance in PD patients’ sentence comprehension performance [F( 1, 18) = 9.771. The formula included only oral semantic comprehension performance. When the formula was applied to individual PD patients, the actual score of only one patient (9012) deviated by more than 2 SD from his predicted score. His actual score (71.43% correct) may have been less than his predicted score (82.15% correct) due in part to his inferior performance on verbal memory tasks. We performed a similar regression analysis in an attempt to determine the relative role of neuropsychological task performance in PD patients’ comprehension of sentences with center-embedded phrase structures. The solution identified by the algorithm included only the oral semantic comprehension task [F(l, 18) = 9.611, explaining 34.80% of the variance in PD patients’ comprehension of center-embedded sentences. When the

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formula was applied to individual PD patient’s performance, the actual performance deviated from predicted performance by less than 2 SD in all of the PD patients. Semantic constraint was also a factor in sentence comprehension which differentiated significantly between control subjects and PD patients. A regression analysis was performed to determine the neuropsychological tasks which contributed to this sentence comprehension factor, and again only oral semantic comprehension contributed significantly to the regression solution [F(l, 18) = 9.421. When the formula was applied to PD patients, the predicted performance of only one PD patient (9012) deviated by greater than 2 SD from his actual performance-the same patient whose overall sentence comprehension performance was not accurately predicted. Discussion

At first glance, these findings would appear to indicate that measures of performance factors such as memory and attention do not contribute significantly to PD patients’ overall sentence comprehension performance or to the specific aspects of sentence comprehension which are most difficult for PD patients. Digit span performance and the ability to recall words, for example, were not related to sentence comprehension performance. This is despite the fact that PD patients were mildly impaired on measures such as word recall at 5 min following presentation and category fluency naming, a task that requires sustained attention for periods of 1 min. The only neuropsychological measure related to overall sentence comprehension and to specific features of sentences distinguishing between PD patients and control subjects was oral semantic comprehension. This task assessespatients’ ability, for example, to point to a window, to point to their right ear, and to determine whether they put on their shoes before their socks. This is a very different kind of sentence processing task when compared with the ability to answer a simple question about a target sentence that varies in grammatical complexity and semantic support, and indeed performance on the oral semantic comprehension task could explain only about 35% of the variance in PD patients’ performance on the task used in Experiment 1. More importantly, the overall pattern of results cautions us from associating sentence comprehension impairments too closely with nonspecific measures of neuropsychological functioning in the domains of memory and attention. These largely negative findings do not indicate, however, that there is no role for memory and attention in PD patients’ sentence comprehension. It can be argued that performance on general-purpose neuropsychological tasks measuring memory or attention may be inadequate since they may not be related closely enough to the particular, material-specific or taskspecific attention and memory mechanisms that may contribute to sentence

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comprehension. Indeed, some have suggested that there is a special-purpose STM mechanism that is dedicated to sentence comprehension and differs in some important ways from nonlinguistic or general-purpose mnestic mechanisms (Berwick and Weinberg, 1984; Marcus, 1980). In order to determine whether there are special-purpose memory or attention mechanisms in sentence processing that may play a role in PD patients’ sentence comprehension impairment, we developed another set of sentences for PD patients. Specifically, we manipulated various aspects of the sentences used in Experiment 1 in order to stress some of the specific memory and attention resources that may be needed during sentence processing. EXPERIMENT 3 Methods Patients. The same 20 patients with idiopathic PD and 12 control subjects examined in Experiments 1 and 2 were also evaluated in Experiment 3. Materials. In order to assessthe role of memory and attention more directly in sentence processing, we developed an additional set of 80 sentence-probe items similar in form to those used in Experiment 1. We again asked subjects to make a two-step judgment: They first judged the acceptability of the sentence and, if acceptable, they responded to the probe. In order to evaluate short-term memory, we manipulated a subset of 16 sentences with relative clauses that were well-formed and were equivalent in length to the sentences used in Experiment 1. Specifically, we manipulated the distance between probed words in the target sentence that form the basis for a correct response to the question. Thus, we probed information located in adjacent portions of a sentence (such as “The eagle chased the hawk that was fast. Which bird was fast?“) or information located in nonadjacent portions of a sentence (such as “The eagle that chased the hawk was fast. Which bird was fast?“). In a sense, this is analogous to the technique developed by Peterson and Peterson (1959). These investigators assessed memory functioning by presenting the stimulus material and then probing what could be recalled only after additional material had been interposed between presentation and recall. In our assessment of memory in sentence processing, we used as the intervening material a part of the sentence where the content was irrelevant to the probe. Patients nevertheless had to attend to the intervening material since they did not know whether this would be probed by the question that followed. The advantage of this technique is that we can use sentences of the same length as the stimulus material used in Experiment 1, since it was sentences of this length which proved difficult for PD patients. We reasoned that PD patients with STM impairments should have more difficulty responding to probes relating information from nonadjacent sentence segments than items relating information from adjacent sentence segments. Fifty-six of the remaining sentences had errors. For these types of items, we were not interested in PD patients’ ability to compute grammatical forms or appreciate semantic features in order to answer a specific question about information in the sentence. Instead, we wished to evaluate PD patients’ ability to attend to several specific aspects of a sentence that may contribute to sentence comprehension such as the sentential features that may play a role in a grammatical computation. We reasoned that the ability to detect an error restricted to a specific feature of a sentence would test an attentional mechanism that functions primarily to prepare for the actual grammatical computations. One of three possible types of manipulations was present in the sentences with errors, including a missing grammatical morpheme, a change in the phonologic shape of a gram-

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matical morpheme, or a change in the position of a grammatical morpheme. A missing grammatical morpheme occurred in 33% of the items with errors (such as *“The eagle chased by the hawk”). The omitted morpheme was equally distributed across three unbound grammatical morphemes found obligatorily in the sentences we used, including “that,” “was,” or “by.” Eight additional items were well-formed and were of equivalent grammatical complexity. These items differed in that “that” was legitimately omitted from a nonobligatory context, such as “The eagle the hawk chased was fast.” Difficulty attending to the absence of a grammatical morpheme that marks a sentence’s major phrase structure was expected to result in a deficit recognizing this type of error. In 33% of the items with errors, there was a change in the phonological shupe of a closed class word that rendered the word anomalous (such as *“The eagle gat chased the hawk was fast”). These phonemic transformations were distributed equally over the same three unbound grammatical morphemes used in our sentences. Difficulty with this type of error was expected in the setting of an impairment attending to the specific phonologic shape of a grammatical morpheme in a sentence. In 33% of the items with errors, there was a change in position of a closed class word. This transposition yielded an unacceptable sentence since the word was displaced from its proper location in a sentence (such as *“The eagle chased that the hawk was fast”). The displaced word was equally distributed over “that,” “was,” and “by.” An impaired ability to attend to the specific ordering of words in a sentence was expected to result in difficulty with this type of error. In contrast to a deficit in detecting one of these types of errors, difficulty detecting all types of errors would be consistent with several possible explanations, including a nonspecific attention disorder or difficulty appreciating phonologically unstressed material in a sentence. These items were embedded in a larger set of similarly constructed but well-formed sentences. The items were read one at a time to patients in a naturalistic fashion that did not provide any external cues as to the answer to the requested judgment. Each item was read twice, and patients were allowed to request as many repetitions as they wanted (typically 5% of the items were repeated an additional time for each patient). Responses were untimed. A brief training procedure preceded the presentation of the experimental items. During this procedure, patients were specifically made aware that some items would have errors, and they were exposed to the types of errors that would occur in the experimental session. An error was identified during the training procedure when it was not detected by the patients, and its nature was discussed. We also confirmed the nature of the errors that patients spontaneously identified.

Results In order to explore the possibility that a STM impairment interferes with sentence comprehension in PD, we compared patients’ ability to answer questions based on information in adjacent parts of a sentence as opposed to nonadjacent parts of a sentence. In fact, control subjects [98% correct; SD = 4.861 did not differ from PD patients [93% correct; SD = 13.691 in their ability to answer questions based on information from adjacent parts of a sentence [t(30) = 1.59; p > .lO]. Similarly, control subjects [98% correct, SD = 3.611 and PD patients [92% correct, SD = 12.351 did not differ in their ability to answer questions based on information from nonadjacent portions of a sentence [t(30) = 1.61; p > .lO]. Nor was there a significant difference seen in PD patient’s own ability to answer questions requiring information from adjacent or nonadjacent portions of a sentence. Thus, based on this measure of STM use in a sentence

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100

so

!P E 8 *

?O

60

50 OMllTED GRAMMATICAL MORPHEME

CHANGED PHONOLOGICAL SHAPE

CHANGED MORPHEME POSITION

FIG. 4. Performance of control subjects and Parkinson’s disease patients on the detection of errors in sentences.

of this length, this factor does not appear to play a significant role in PD patients’ sentence comprehension impairment. An ANOVA was performed for sentences with errors using a group [2] x type of error [3] design. A significant main effect for group was seen [F(l, 30) = 7.87; p < .Ol]. This finding demonstrates that PD patients encounter significantly more difficulty than control subjects at detecting an error in a sentence. There was also a significant group x type of error interaction [F(2, 60) = 3.17; p < .05]. As can be seen in Fig. 4, this indicates that PD patients are not equally compromised across the different types of errors. Difficulty in sentence processing thus may be related in part to a defect in a particular type of attentional mechanism which is specific for a certain feature of sentence processing. Further analysis of the interaction effect revealed that PD patients experience significantly more difficulty than control subjects at detecting a missing but obligatory closed class word in a sentence [t(30) = 3.14; p < .005]. However, they were as accurate as controls at judging the well-formedness of sentences where a nonobligatory “that” was omitted.

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Significant difficulty was also encountered in PD patients’ attempts to identify an error in the phonological shape of a closed class word [t(30) = 2.33; p < .05]. In contrast to their difficulty detecting the absence of a grammatical morpheme or a change in its phonological shape, PD patients did not differ from control subjects in their ability to detect a change in the order of a grammatical morpheme among the words of a sentence [t(30) = 1.88; p > .lO]. Moreover, in comparison to their own ability to detect order errors, PD patients encountered significantly more difficulty detecting phonological shape errors [t(19) = 2.37; p < .Ol] and missing grammatical morphemes [t(19) = 2.73; p < .Ol]. Thus, PD patients’ sentence comprehension deficits do not seem directly attributable to difficulty appreciating order information, although they may be somewhat compromised in their ability to detect an omitted grammatical morpheme or an anomalous phonological shape. A discriminant analysis (Dixon, 1988) was performed in order to determine the proportion of PD patients encountering difficulty with these memory and attention factors. The most effective variable at classifying subjects was the ability to detect a missing grammatical morpheme [71.9% of subjects classified correctly; approximate F(1, 30) = 9.871. On the basis of their performance on this feature, 60.0% of PD patients differed from control subjects. A discriminant analysis also indicated that 62.5% of subjects were correctly classified on the basis of their ability to detect a phonologically anomalous grammatical morpheme [approximate F( 1, 30) = 5.431. In this case, 45% of PD patients differed from control subjects. Thus, it can be seen that, at least for some attentional factors in a sentence, a large proportion of PD patients are compromised. A correlation analysis was performed in order to determine which of the variables used in Experiments 1 and 3 were significantly associated with overall sentence comprehension performance. Individual patient performance profiles can be seen in Table 3. The correlations between overall sentence comprehension performance and each of these sentence processing factors is presented in Table 7. As can be seen, essentially all of the variables correlated with overall sentence comprehension performance . A stepwise linear regression analysis (Dixon, 1988) was performed in order to determine the relative contribution of attentional, mnestic, grammatical, and semantic variables to PD patients’ overall sentence comprehension performance. The regression analysis algorithm identified a configuration of variables that explained 97.74% of the variance in PD patients’ overall sentence comprehension performance. These variables (and their partial correlations) were the grammatical factors of subordinate phrase structure (0.658) and center-embedded phrase structure (0.694), the semantic variable of nonconstrained nouns (-0.486), the attentional variables of detecting the presence of a grammatical morpheme (0.819)

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TABLE I CORRELATIONBETWEENOVERALL SENTENCECOMPREHENSIONPERFORMANCEAND SENTENCE PROCESSING VARIABLES IN PARKINSON’SDISEASE -

Variable

-

Phrase structure Simple Subordinate Center-embedded Semantic Constrained Nonconstrained Voice Corresponding Noncorresponding Memory Adjacent segments Nonadjacent segments Attention Presence of grammatical morpheme Anomalous grammatical morpheme Order of grammatical morpheme

Overall sentence comprehension” 0.545* 0.804* 0.680* 0.755* 0.766* 0.665* 0.617* 0.374 0.468* 0.749* 0.722* og47*

a A Bonferroni type of correction was used to correct for multiple comparisons in the correlation matrix (i.e., p < .05/12 or p < ,005). An asterisk indicates significance at the adjusted p < .05 level.

and detecting the order of a grammatical morpheme (O&59), and the memory factor of relating information from nonadjacent sentence segments (0.585). When this regression formula was used to predict each PD patient’s overall sentence comprehension performance, the prediction was always within 2 SD of the patient’s actual performance. It may be recalled that the grammatical factors of subordinate and center-embedded phrase structure, the semantic factor of nonconstrained nouns, and the attentional factor of detecting the presence of a grammatical morpheme also distinguished between control subjects and PD patients. Thus, information from multiple processing domains may contribute to sentence appreciation in PD patients, and several of these factors also distinguished between control subjects and PD patients. It is these latter features of sentence pro-

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cessing which we feel are most likely to underlie the deficits seen in PD patient’s sentence comprehension. Discussion These data are consistent with the claim that there are attentional deficits that contribute to PD patients’ sentence comprehension difficulty. Rather than a general-purpose attentional mechanism that may be measured by a traditional neuropsychological task, however, we hypothesize that there may be several special-purpose attentional mechanisms which are compromised in PD. One of these may be a language-specific attentional device that contributes to PD patients’ sentence comprehension. This appears to be specifically important for detecting the presence and phonological shape of grammatical morphemes in a sentence. Bradyphrenia apparently did not prevent PD patients from appreciating the rapid speech sound transitions of phonemic errors in grammatical morphemes. This explanation is unlikely since the items were repeated twice (and could have been repeated additional times if requested by the patients), although a definitive study would require patients to judge slowed speech samples. Moreover, patients’ CVC discrimination judgments in Experiment 2 were essentially normal, suggesting that poor auditory acuity or phonemic discrimination could not fully account for these findings. Another possible explanation is that PD patients were unable to detect unstressed grammatical morphemes in a sentence or were otherwise impaired in their ability to appreciate grammatical information in a sentence. However, these patients apparently were able to detect a change in the position of a grammatical morpheme, that is, where an unstresssed item occurred in the wrong location in a sentence. It could also be argued that a defect in a nonspecific attentional mechanism can account for patients’ difficulty identifying a missing grammatical morpheme. We did not find evidence for such an attentional impairment in Experiment* 2, although this negative finding must be confirmed. Moreover, we observed in this experiment that PD patients are selectively compromised in their ability to detect particular types of errors within a sentence. Our hypothesis, then, is that there may be a special-purpose attentional mechanism which is compromised in PD. This may represent one of many such special-purpose attentional mechanisms. Our data must be interpreted cautiously, however, since the assessmentswere not performed in an on-line fashion. Is it possible, for example, that patients developed other-albeit inappropriate-strategies for sentence processing. It is also beyond the scope of this study to discuss the parallel assessments in a nonverbal domain that would evaluate the hypothesized materialspecific nature of attentional mechanisms that may be disrupted in PD. Several studies (Sagar et al., 1988) have suggested that PD patients are compromised in their appreciation of ordered material, and indeed this

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“ordering deficit” hypothesis has been forwarded in other contexts as an attempt to explain some aphasic patients’ sentence processing difficulties (De Renzi and Nichelli, 1975; Tzortzis and Albert, 1974). It is interesting in this context that PD patients are able to detect errors in the ordering of words within a sentence. The sentences with positional errors were read to patients in a manner which was naturalistic and did not assign undo stress to the reordered material. We explicitly attempted to avoid gross external phonological cues that might mark an error, although we cannot rule out subtle cues in the sentence’s phonological features that might have indicated an abnormality within the sentence. In contrast to their difficulty with some attention-demanding features of sentence processing, PD patients performed well on the portion of the experiment intended to assessmemory functioning. PD patients’ success on this type of task simply may reflect that the items were not long enough to stress a memory component. Certainly this is possible, and indeed anyone can be forced to make a sentence comprehension error if the sentence is long enough. But using longer sentences that contain more verbiage would not explain PD patients’ deficit on sentences which are less than 10 words in length. The results of Experiment 1 indicated, moreover, that PD patients are not most compromised on the longest sentences (i.e., passive subordinates). It is unlikely that the intervening information in nonadjacent items was not demanding enough to occupy PD patients during on-line processing. These items were intermixed with items that probed patients’ knowledge of the intervening information, therefore forcing patients to attend to the entire sentence to the best of their abilities. These items may have been particularly simple for PD patients since we probed only the single adjective in the sentence rather than two thematic roles. This is unlikely to be the sole explanation of the patients’ performance since the items required the adjective to be related to one of the two nouns. Clearly additional work is necessary to help specify the nature of a STM mechanism in PD patients’ sentence comprehension. It is interesting to consider the role that an attentional component may play in a sentence comprehension mechanism. Several different components of attention have been identified (Posner and Petersen, 1990). Although not specifically developed to account for sentence comprehension, these attentional mechanisms include an orienting component, a selecting component, and a vigilance component. We do not feel that vigilance is particularly compromised in these patients. A deficit at this level of attention would have resulted in indiscriminant errors across all types of sentences, and PD patients performed as well as controls on the grammatically simple sentences. Difficulty orienting to the task of sentence comprehension similarly would have resulted in a relatively homogeneous pattern of errors across all sentence types, a performance profile which

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we did not observe. We believe that the pattern of impaired comprehension is most consistent with a deficit in a selectional mechanism. Within the context of a grammatical processor, this processing component may actively attend to the presence and nature of critical grammatical morphemes that mark the structure of a sentence under the guidance of a grammatical computation device. Additional evidence to support this attentional hypothesis comes from the observations of Geyer and Grossman (submitted for publication). We asked another group of PD patients to perform truth verification judgments of sentences containing two types of verbs with different rules for mapping syntactic roles onto thematic roles: the more typical simple transitive verbs such as “The boy ate the chicken. The boy ate-true or false,” and the less canonical lexical causative verbs such as “The boy drowned the swimmer. The swimmer drowned-true or false.” The sentences were cast in an active, passive, or periphrastic voice. We found that PD patients are more compromised in their judgments of sentences with lexical causative verbs than simple transitive verbs. However, this difference was neutralized through the use of the periphrastic voice, such as “The boy made the swimmer drown.” We hypothesized that an important factor in PD patients’ grammatical comprehension is the transparency with which thematic roles are stated in a sentence, a process that is facilitated by the periphrastic voice. It is noteworthy that this beneficial effect was evident despite the grammatical complexity of the periphrastic voice. This type of hypothesis was also forwarded by Schwartz et al. (1987) in their account of agrammatic aphasics’ sentence judgment difficulties. A defect in a performance factor such as this, however, does not fully explain PD patients’ sentence comprehension impairment. If a selective attentional deficit was the entire explanation for PD patients’ grammatical processing difficulties, then there ought not to have been a difference between subordinate and center-embedded items since PD patients would not have detected “that,” “was,” or “by” effectively enough to discriminate between these sentence types. Our findings seem more consistent with the possibility that there is an interactive effect of grammatical complexity and limited selective attention that underlies PD patients’ sentence comprehension difficulties. Indeed, the regression analysis suggested that there may be some contribution made by a grammatical processor and a semantic processor, that is, sentence processing factors identified in Experiment 1 that also distinguish between PD patients and control subjects. The regression analysis is also noteworthy since increasing grammatical complexity appears to detract from sentence comprehension performance, but the sign of the partial correlation for the semantic factor indicates that this may be an aid to PD patients’ attempts to understand sentences. We hypothesize that all of the factors in the regression analysis may contribute to sentence comprehension in PD, but that the factors in the

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regression analysis which also distinguish statistically between PD patients and control subjects may represent the major contributors to PD patients’ sentence comprehension impairment. Thus, we believe that performance factors such as the ability to attend to specific grammatical features of a sentence as well as computational factors such as the ability to appreciate a sentence’s grammatical phrase structure and the ability to take advantage of a sentence’s semantic attributes all play a role in PD patients’ sentence comprehension difficulties. GENERAL DISCUSSION Sentence comprehension is a highly complex process. Briefly, it involves the conversion of an auditory input stream of information into a series of phonologic shapes that represent specific words. The words contain grammatical information that helps specify the relationships among the sentence’s constituents, and there are some words-grammatical morphemes-whose principal function is to organize the words and phrases of a sentence. The words also contain semantic informtion that, when taken together and in an appropriately organized array, convey the core meaning of the sentence. Performance factors such as attention and memory may be necessary to support the appropriate processing of these sentential relations. The value of assessing a sentence comprehension model such as this in PD is that their impairments are relatively mild. An obvious aphasia following a stroke appears to result from relatively severe deficits in several of these processing components, so it can become quite difficult to identify the precise source of a sentence comprehension impairment. By comparison, we may be more able to define the underlying deficit in PD since the processing impairments and their consequences may be more discrete in nature. Before continuing our discussion, it is important to remember that a considerable proportion of PD patients are said to have coexisting histopathologic evidence for Alzheimer’s disease (Boller, Mitzutani, Rossemann, & Gambetti, 1980; Hakim and Mathieson, 1979; Jellinger, 1987; Whitehouse, Hedreen, White, & Price, 1983) another neurodegenerative condition where language impairments are said to occur very frequently (Cummings, Benson, Hill, & Read, 1985; Faber-Langendoen, Morris, Knesevich, LaBarge, Miller, & Berg, 1988; Selnes, Carson, Rovner, & Gordon, 1988). There are several lines of evidence which suggest that these PD patients are unlikely to have Alzheimer’s disease. Clinically, our patients did not appear demented, and this was confirmed by MMSE scores. Moreover, the profile of language impairment described in Alzheimer’s disease differs in important ways from the pattern of deficits apparently seen in PD patients. Alzheimer’s patients are said to resemble stroke patients with anemic, transcortical sensory, or Wernicke’s types of aphasias (Kertesz, Appell, & Fisman, 1986). More detailed analyses of

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Alzheimer’s patients’ language functioning have noted prominent impairments in lexical semantics and naming (Grober, Buschke, Kawas, & Fuld, 1985; Huff, Corkin, & Growdon, 1986; Martin and Fedio, 1983), but have generally claimed that grammatical impairments are less prominent or nonexisting (Appell et al., 1982; Hier, Hagenlocker, & Shindler, 1985; Nebes, Brady, & Jackson, 1989). By comparison, PD patients seem most compromised in their appreciation of grammatical information and are able to take advantage of semantic information to help their understanding of a sentence. It is unlikely, then, that PD patients’ sentence comprehension difficulties are due to a second underlying disease such as Alzheimer’s disease, although this can be confirmed only through followup evaluation and histopathologic evidence obtained in the future. The concept of a working memory (Baddeley, 1986) is clearly important to our understanding of human intelligence and has been applied successfully to many cognitive domains. While not developed to account for sentence processing phenomena, attempts have been made to use the working memory model as a vehicle for explaining deficits in the sentence processing domain (Allport, 1984; Caramazza et al., 1981; Kolk and van Grunsven, 1985; Shallice and Warrington, 1977; Warrington and Shallice, 1969). Alternately, there may be a primary defect in a grammatical computational mechanism that subserves the various aspects of constructing a mental representation of a sentence’s structure and meaning (e.g. Berndt and Caramazza, 1980; Caplan and Futter, 1986; Caramazza and Zurif, 1976; Grodzinsky, 1989). There have been several explicit attempts to test the grammatical vs. STM explanations for these sentence comprehension impairments. Unfortunately, this work has not been definitive in its outcome. Kolk and van Grunsven (1985) for example, found that Broca’s aphasics’ comprehension deficits were more closely correlated with the length of a sentence rather than its grammatical complexity. By comparison, Martin (1987; Martin et al, 1989) found that Broca’s aphasics were more compromised in their attempts to understand grammatically complex sentences than longer sentences. We have taken observations such as these as a starting point in our attempts to analyze the sentence processing impairments of PD patients. The results of the experiments reported above are readily summarized. In the first experiment, PD patients were asked simple questions about sentences which varied in terms of their syntactic and semantic complexity. We found that PD patients encounter significant difficulty answering questions about the target sentences. They were particularly compromised when the sentences were more complex grammatically, and this difficulty was ameliorated when semantic constraints were provided to limit the number of possible interpretations of the target sentence. These findings confirmed the existence of subtle but widespread sentence comprehension impairments in patients with idiopathic PD. There was some evidence

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that a performance component may have contributed to PD patients’ impairment in that they exhibited somewhat inconsistent patterns across patients for the set of sentences and an inconsistent pattern across sessions for a given patient. In fact, some patients with PD are said to have difficulty in the domains of memory and attention (Bloxham et al., 1987; Dubois et al., 1990; Grossman et al., 1990; Lees and Smith, 1983; Levin et al., 1989; Pirozzolo et al., 1982; Sagar et al., 1988; Taylor et al., 1986). It is thus possible that subtle deficits in these domains may explain PD patients’ sentence processing difficulties. However, attributes of sentences such as their length or the amount of information they contained-characteristics that stress short-term memory mechanisms in sentence processing-were not found to be the most potent determiners of successful sentence interpretation. This set of findings left us with the impression that grammatical factors contributed to PD patients’ sentence comprehension difficulties, but did not rule out a role for a compromised performance component. We investigated the role of memory and attention in PD patients’ sentence comprehension more explicitly in the second experiment by administering some routine measures of these skills. We found that general purpose measures of memory and attention were not statistically related to PD patients’ sentence comprehension performance, even though one memory measure differed significantly between control subjects and PD patients. This finding does not necessarily lead to the conclusion that memory and attention do not contribute to sentence comprehension in PD, although this does diminish the likelihood that there is a “subcortical dementia” underlying these sentence comprehension difficulties. Similarly, a general-purpose “working memory”mode1 may not adequately account for divergent performances across neuropsychological tasks and specific domains of functioning such as sentence processing. An alternate explanation is that there may be a special-purpose mechanism that mediates attention and short-term memory as needed to support the idiosyncrasies of a sentence comprehension processor, just as there may be specialpurpose performance mechanisms dedicated to processing within other cognitive domains. In the third experiment, we attempted to manipulate some attributes of the sentences in order to stress just such special-purpose memory and attentional mechanisms that may play a specific role in sentence processing. We found that certain aspects of attention mediating particular features of a sentence in fact are significantly compromised in PD patients. Thus, these patients encountered significant difficulty detecting the presence of unbound grammatical morphemes and determining their phonological shape. Findings such as these can be interpreted to support the claim that there may be dedicated memory and attentional mechanisms that contribute to sentence comprehension and that these may be selec-

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tively compromised in states of central nervous system disease. This conclusion should be viewed with caution for the many reasons listed above. We would specifically like to emphasize that we have not demonstrated a dissociation between the particular attentional mechanisms compromised in these patients and a parallel set of mechanisms subserving performance in other cognitive domains, that additional neuropsychological measures must be administered to confirm the absence of a role for a generalpurpose performance mechanism, and that we cannot rule out the development of inappropriate heuristics for sentence comprehension given that our assessment was not on-line. There has been some debate in the literature whether a performance deficit can fully explain sentence comprehension difficulties in brain-damaged patients. Parkinson’s disease seemed to be an ideal setting for evaluating such a proposal. Some PD patients are said to exhibit memory and attention impairments. If these patients can be shown to have sentence comprehension difficulty, this might be due to a deficit in these domains. An alternate explanation might be that these patients have a primary, albeit subtle, deficit in appreciating the grammatical features of a sentence. After all, language impairments have been seen after insult to portions of the frontal lobe or the basal ganglia (Alexander et al., 1989; Damasio et al., 1982; Masdeu et al., 1978; Naeser et al., 1982; Nadeau, 1988; Novoa and Ardila, 1987; Wallesch et al., 1983), regions of the brain that have been implicated in the pathophysiology underlying PD (Alexander and Crutcher, 1990; DeLong et al., 1983). Based on the data we presented above, it would appear that both sources of error contribute to PD patients’ sentence comprehension difficulties. The apparent performance level deficits and the grammatical and semantic factors together explained a very large proportion of the variance in PD patients’ sentence comprehension performance. We thus conclude that performance factors such as the ability to attend to specific grammatical features of a sentence as well as computational factors such as the ability to appreciate a sentence’s grammatical phrase structure and the ability to take advantage of a sentence’s semantic attributes all contribute to PD patients’ sentence comprehension profile. Certainly this first-pass attempt to characterize the nature of sentence comprehension difficulties in PD is well shy of a computationally adequate description of sentence processing in a brain-damaged group. Nevertheless, it sets forth some initial hypotheses that attempt to provide an overall architecture of a sentence processing system and the ways in which it may be compromised in a common neurodegenerative condition. REFERENCES Albert, M. L. 1978. Subcortical dementia. In R. Katzman, R. D. Terry, & K. L. Bick (Eds.), Alzheimer’s disease: Senile dementia and related dkorders. New York: Raven Press.

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