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Sentence comprehension following agenesis of the corpus callosum
BRAIN
AND
LANGUAGE
37, 59-72 (1989)
Sentence Comprehension following Agenesis of the Corpus Callosum RICHARD University
J. SANDERS
of North Carolinrr
rrt Wilmington
The sentence comprehension skills of a h-year-old girl with callosal agenesis were compared to skills of three other children matched for age and verbal IQ. Sentence-picture matching and acting out tasks were used with reversible active, passive, and center-embedded relative clause sentences. The acallosal subject showed a deficit in syntactic comprehension. The difficulty was due to a failure to assign correct semantic roles to some sentence forms, not to a lack of ability to discriminate among the sentence forms. The data are consistent with a previous report by M. Dennis (1977, In Topics in child neurology. pp. 189-212) that in acallosal subjects syntactic comprehension can be adversely affected. Because this acallosal subject is only 6 years old, follow-up studies will be needed to determine whether she eventually acquires normal syntactic skills. e 19x9 Academic Prcas. Inc.
INTRODUCTION
Numerous studies have indicated that the left hemisphere of the brain is an area that is essential for normal language development and language functioning (see for example, Gazzaniga, 1970; Caramazza & Zurif, 1976; Dennis & Whitaker, 1976; and Lineberger, Schwartz, & Saffran, 1983). In particular, these studies have associated the left hemisphere with the ability to use the syntactic structure of sentences for the purposes of comprehension or production. The present study focuses instead on the role of the corpus callosum in language acquisition. A number of authors have suggested that in the acallosal brain verbal and nonverbal functions compete for resources, with the verbal functions generally dominating (Saul & Sperry, 1968; Gazzaniga, 1970; Hecaen, 1976). Dennis (1977, 1981), on the other hand, has speculated that agenesis of the corpus callosum often results in deficits in either syntactic abilities or visualspatial abilities, with concurrent sparing of the other function. In a detailed examination of these functions in two mature acallosal subjects, she Address correspondence and reprint requests to Dr. Richard J. Sanders, Department of Psychology, University of North Carolina at Wilmington, Wilmington, NC 28403-3297. 59 0093-934X/89 $3.00 Copyright 0 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.
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J. SANDERS
found that one was skilled at a visual-spatial task but extremely impaired at understanding the syntax of English; the other was linguistically skilled but visually-spatially impaired. Specific syntactic deficits have not yet been documented in other acallosal subjects. In the present investigation, the syntactic abilities of a 6-year-old girl with callosal agenesis was compared with the syntactic abilities of three normal 6-year-old children to see whether the agenesis had affected the development of language comprehension. Development of Syntactic Comprehension It has been suggested that adult sentence comprehension may involve the use of numerous heuristic strategies, each of which is conditional upon the occurrence of a particular type of syntactic structure (Fodor, Bever, & Garrett, 1974). Four sentence types were chosen for use in this study because they represent common English syntactic structures that have been used in previous studies of comprehension strategies. The sentence forms that were used in the study were actives, passives, and ‘sentences containing embedded relative clauses. Comprehension of these sentence forms by children has also been widely studied and may be summarized as follows: After an early period during which children seem to rely solely on a semantic strategy of guessing the most sensible meaning of a sentence based on the meanings of its words, children first learn to comprehend active sentences reliably. It has been widely suggested that they use a noun-verb-noun strategy in which the first NVN sequence in the sentence is taken to denote the agent, action, and object of the sentence, respectively (Bever, 1970). This strategy works well for simple active sentences, but is inadequate for passive sentences or complex sentences with multiple clauses. After an initial attempt to apply the NVN strategy too widely, children seem to recognize that the strategy cannot be applied to passive sentences, but they do not yet have an alternative strategy for interpreting passive sentences. The ability to comprehend passives correctly develops gradually, beginning around the age of 4 years (Bever, 1970; Maratsos, 1974; devilliers & devilliers, 1973). Children also have difficulty comprehending some sentences with a relative clause until they are well into their early school-age years. Two variables have been found to have an effect on the comprehension of sentences containing a relative clause. The first is the position of the relative clause in the sentence. Two positions that have been most often studied are center-embedded relative clauses that follow the noun phrase that is the subject of the matrix sentence as in sentence 1 and rightbranching relative clauses that follow the object of the matrix sentence as in sentence 2.
ACALLOSAL
SENTENCE COMPREHENSION
61
1. Center-embedded: The boy who liked the girl read the book. 2. Right-branching: The boy liked the girl who read the book. The second variable to affect comprehension in children is the syntactic role of the relativized noun phrase in the relative clause. The head noun phrase may be the subject of the relative clause as in sentence 3 or the object of the relative clause as in sentence 4. 3. Head noun as subject of the relative clause: The clown who lifted the balloons dropped the flowers. 4. Head noun as object of the relative clause: The balloons that the clown lifted frightened the baby. Children generally comprehend sentences with embedded relative clauses less well than sentences with right-branching relative clauses, and centerembedded relative sentences are generally more difficult to comprehend when the head noun is the subject of the matrix sentence and the object of the relative clause than when it is the subject of both clauses (Clancy, Lee, & Zoh, 1986; devilliers, Tager-Flusberg, Hakuta, & Cohen, 1979; Tavakolian, 1978; Sheldon, 1974). Part of the advantage of latter type of relative sentence has been attributed to the fact that the NVN strategy that is already familiar to children in their comprehension of simple active sentences also works with relative clauses in that type of relative sentence (devilliers et al., 1979). However, the NVN strategy does not provide a correct analysis of both clauses in these relative sentences. This means that the child must discover an additional comprehension strategy to deal with the second clause. The purpose of the present study was to examine the use of comprehension strategies by the acallosal and control subjects in response to four sentence types in order to determine whether agenesis of the corpus callosum had affected the development of comprehension strategies associated with these sentence types. METHOD Subjects. The acallosal subject was a 6-year-old female. At the age of I month she was
admitted to the Eugene Talmadge Memorial Hospital for examination because of an enlarged head (98th percentile). A CT scan suggested some cerebellar agenesis and agenesis of the corpus callosum. The scan did not indicate whether the agenesis was partial or complete. Despite the enlarged head circumference, the CT scan showed normal ventricle size and no diagnosis of hydrocephalus was indicated in the medical report. Her EEG was normal. Her verbal intelligence at the age 5 years and 6 months, as measured by the Wechsler Preschool and Primary Scale of Intelligence, was 91. Like other acallosal subjects (Saul & Sperry, 1968; Gazzaniga, 1970), the subject showed a lack of disconnection symptoms that are associated with commisurotomy patients. She was able to accurately name common objects presented to either visual field or presented haptically to either hand. The control subjects were two males and a female matched with the acallosal subject in age and verbal intelligence. The relevant characteristics of the subjects are presented in Table 1. All of
62
RICHARD J. SANDERS TABLE 1 DESCRIFTION
OF SUBJECTS
Age
Subject
(years, months)
Sex
Verbal IQ
Performance IQ
Condition
C.F. M.C. K.M. T.J.
6’3 68
F F M M
91 101 95 87
70 110 112 92
Acallosal Control Control Control
5,lO 5,lO
the control subjects had verbal intelligence scores within 10 points of the acallosal subject’s score, 10 points being the normal range of test-retest variability for the test. The subjects in the control group were students in local elementary school kindergarten classes, participating with their parents’ consent. Materials. Two comprehension tasks were devised to test comprehension of reversible sentences. These are sentences whose subject and object noun phrases could be exchanged without violating the selection restrictions of the language. The first comprehension task was a sentence-picture matching task, patterned after a test used by Caramazza and Zurif (1976). A basic set of eight reversible, active sentences with transitive verbs was constructed. Each sentence contained two nouns, the first one modified by an adjective. The eight sentences are shown in Table 2. Each of the eight sentences was also transformed into a passive sentence form and into two complex sentences with center-embedded relative clauses. These alternate sentence forms are also presented in Table 2. The relative clauses always modified the noun phrase that was the subject of the matrix sentence, but differed in terms of the syntactic role of that noun phrase in the relative clause. In one case the subject of the matrix sentence was also the subject of the relative clause. These were called SS relative sentences. In the other case the subject of the matrix sentence was the object of the relative clause. These were called SO relative sentences. These 32 sentences were each transformed once again by reversing the two nouns. Examples of the reversed sentence forms are also presented in Table 2. In all, 64 different sentences were constructed for use in the test: 8 different base sentences occurring in each of four syntactic forms with the two nouns presented in both forward and reversed orders of mention. For each of the eight basic sentences, two pictures were prepared: one showing the subject noun carrying out the action and another showing the object noun carrying out the action. Copies of these two pictures were mounted on a 5 x 7-in. index card with the correct picture on the left. A second card was also prepared with the correct picture on the right. One of the two picture cards was assigned to each of the eight transformations of each of the basic sentence in such a manner that the side of the card on which the correct picture was located was counterbalanced within each of the four sentence forms and within the eight transformations of each of the basic sentences. The 64 sentences were arranged in a list for presentation in a quasi-random order, with the restriction that two instances of the same syntactic form could not be presented successively, nor could two syntactic forms of the same basic sentence be presented successively. The second comprehension test required that a subject act out an action described in a reversible sentence. The actions described in the sentences involved an animal of one species either jumping over or bumping into an animal of another species. The animals used in the task were cows, horses, and pigs. A white and a brown plastic model of each of these animals were used. A set of 96 sentences was constructed describing various ways animals of one species could jump over or bump into an animal of another species. The sentences were evenly divided among the same four sentence types that had been
ACALLOSALSENTENCECOMPREHENSION
63
TABLE 2 MATERIALS FOR SENTENCE-PICTURE MATCHING TASK
Eight basic sentences The The The The
The The The The
unhappy boy is pushing the girl. funny cat is chasing the dog. nice boy is following the man. fat girl is tickling the boy.
yellow car is bumping the bus. friendly horse is kissing the cow. funny clown is lifting the balloons. happy boy is hugging the girl.
Four syntactic forms Active: Passive: SS relative: SO relative:
The The The The
unhappy boy is pushing the girl. unhappy girl is being pushed by the boy. boy who is pushing the girl is unhappy. girl who the boy is pushing is unhappy.
Active: Passive: SS relative: SO relative:
The The The The
unhappy girl is pushing the boy. unhappy boy is being pushed by the girl. girl who is pushing the boy is unhappy. boy who the girl is pushing is unhappy.
Reversed sentence forms
Note. Examples of alternate syntactic forms and reversed forms are given only for the first basic sentence. used in the sentence-picture matching task. The variables of species of the sentence subject, species of the sentence object, type of action, and stated color were counterbalanced within each sentence type and across the entire set of sentences. The color of only one animal was specified in each sentence. The color of the agent was specified in one half of the sentences and the color of the object in the other half. Examples of the sentences used in the acting out task are presented in Table 3. Procedure. Subjects were tested individually in weekly sessions of less than 45 min. It required three or four sessions to complete the two tasks. The sentence-picture task was administered first. Before beginning the task the child was shown each of the picture cards and asked to point to the picture that the experimenter described. The experimenter then TABLE 3 EXAMPLES OF MATERIALS FROM ACTING OUT TASK
Basic sentences with “cow” The The The The
brown cow jumps over the pig. white cow jumps over the horse. cow jumps over the white pig. cow jumps over the brown horse.
The The The The
white cow bumps into the pig. brown cow bumps into the horse. cow bumps into the brown pig. cow bumps into the white horse.
Alternative transformations of first basic sentence Active: Passive: SS relative: SO relative:
The The The The
brown cow jumps over the pig. brown cow is jumped over by the pig. cow that jumps over the pig is brown. cow that the pig jumps over is brown.
64
RICHARD .I. SANDERS
described one of the two pictures using a simple active sentence like The boy is pushing the girl. Every child was able to identify each of the pictures correctly. This was a precaution to ensure that all subjects were able to interpret the pictures correctly and to point accurately to the picture of their choice. The child was then told to continue pointing to the pictures described by the experimenter for the remainder of the trials. On each trial one picture card was placed on the table in front of the child and the experimenter read a sentence describing one of the pictures. The sentence was reread at intervals of approximately IO set until the child pointed to one of the pictures. The acting out task was begun at the start of the session that followed completion of the sentence-picture matching task. The child was first shown the six animals and asked to point to each one as the experimenter named an animal of a particular species and color. All of the children were able to do this. The children were then told that the experimenter would describe an action that the child should act out with the animals. The experimenter demonstrated the “jumping over” and “bumping into” actions and asked the child to demonstrate the actions as well before beginning the task itself. On each trial the six animals were placed on the table in front of the child. The animals were not arranged in any fixed order. The experimenter read a sentence to the child and repeated it approximately every IO set until the child completed an action.
RESULTS AND DISCUSSION The results from the sentence-picture matching task were examined for evidence of differences in the performance of the two groups of subjects. Figure 1 contains the mean percentage of trials with errors as a function of subject group and sentence type. A comparison of the errors by the acallosal subject versus the control subjects on each of the sentence types showed clearly that the control subjects did better than the acallosal subject on both active sentences (0% vs. 33% errors) and passive sentences (25% vs. 58% errors). In both cases there was no overlap between the scores of the two groups. On the SS relative sentences, the control subjects did better than the acallosal subject on the average (11% errors vs. 25% errors). However, one of the control subjects also erred on 25% of the trials. The performance of the groups on the SO relative sentences did not differ greatly. The acallosal subject made errors on 42% of the trials and the control subjects on the average made errors on 47% of the trials. The acallosal subject’s error rate on the SO relative sentences was equal to the performance of the best subject in the control group on this sentence type. To summarize, the control subjects did better overall than the acallosal subject on all sentence types except the SO relative sentences. The two groups did not differ on the SO relative sentences. In the acting out task, the control subjects again did better on the average than the acallosal subject on all sentence types except the SO relative sentences. The results are presented in Fig. 2. The mean error rates for the control subjects and the acallosal subject were 0 and 21%, respectively, on active sentences; 10 and 63%, respectively, on passive sentences; 31 and 63%, respectively, on SS relative sentences; and 57 and 58%, respectively, on SO relative sentences.
65
ACALLOSALSENTENCECOMPREHENSION 60-
Acallosal Normal
Active
Passive Sentence
SS Relative
SO Relative
Type
FIG. 1. Percentage of errors on sentence-picture matching task as a function of subject group and sentence type.
Several types of errors were possible on the acting out task. The most common type of error was the reversal of the agent and object of the action. For example, in sentence 5 the horse is the agent of the action “jump over” and the pig is the object. 80,
m z.; ‘;j E (I) 0 z a
60
40
20
0
Active
Passive Sentence
SS Relative
SO Relative
Type
FIG. 2. Percentage of errors on acting out task as a function of subject group and sentence type.
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RICHARD J. SANDERS
5. The pig that the horse jumps over is brown. A noun reversal error would occur if a subject who was acting out sentence 5 made the pig jump over the horse. In active sentences and SS relative sentences, the first noun is the agent and the second noun is the object. The opposite is the case in passive sentences and SO relative sentences. The relative frequencies of noun reversal errors are presented in Fig. 3 by sentence type and subject group. The control subjects committed fewer noun reversal errors on the average than the acallosal subject on all sentence types. With the exception of the error rates on the SO relative sentences, there was no overlap between the error rates of the control group and the acallosal subject. The mean error rates for the control subjects and acallosal subject were 0 and 17%, respectively, on the active sentences; 7 and 54%, respectively, on the passive sentences; 8 and 29%, respectively, on the SS relative sentences; and 25 and 42%, respectively, on the SO relative sentences. A second type of error occurred when subjects failed to use an animal with the color specified in the matrix sentence. Color assignment errors were rare in active and passive sentences where the color term occurred immediately prior to the term that it modified. The error data are presented in Fig. 4. The control subjects made fewer color assignment errors than the acallosal subject on most sentence types except the SO relative sentences, where the frequency of color errors for the two groups did not differ.
Active
Passive Sentence
SS Relative
SO Relative
Type
FIG. 3. Percentage of reversal errors on acting out task as a function of subject group and sentence type.
ACALLOSAL
SENTENCE COMPREHENSION
67
40
m 30 z .; z E : b
20
n
10
0 Active
Passive
SO Relative
SS Relative
Sentence
Type
FIG. 4. Percentage of color assignment errors on acting out task as a function of subject group and sentence type.
The color assignment error rates for the control group and the acallosal subject were 0% for both groups on active sentences; 1 and 17%, respectively, on passive sentences; 18 and 38%, respectively, on SS relative sentences; and 32 and 33%, respectively, on SO relative sentences. The last, and least frequent, type of error in the acting out task occurred when subjects used an animal that was not mentioned in the sentence. The total number of substitution errors by sentence type and subject group is presented in Table 4. The acallosal subject made a total of six substitution errors, three of them on the SO relative sentences, and one on each of the remaining sentence types. Of the three subjects in the control group, one made no substitution errors, one made one substitution error, and one made two substitution errors. Two of the errors occurred on passive sentences and one on an SO relative sentence. The results were next examined for evidence of the ways the subjects assigned relational roles to the nouns in the sentences. With the exception of the copula be that was used in the matrix clause of sentences containing TABLE 4 TOTAL
NUMBER
OF SUBSTITUTION
ERRORS IN ACTING
OUT TASK
Sentence type Group
Active
Passive
SS relative
SO relative
Acallosal Control
I 0
I 2
1 0
3 1
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RICHARD
J. SANDERS
relative clauses, all of the verbs in the sentences used in this study were action verbs with noun phrases serving the roles of agent and object of the action. On the basis of previous studies reviewed in the introduction, it was expected that subjects would perform better on active sentences than on passive ones and that subjects might be observed doing one of the following: (1) using a semantic guessing strategy on both active and passive forms (which would produce chance performance on these reversible sentences); (2) using a NVN strategy for both active and passive sentences; (3) discriminating between active and passive forms, and guessing randomly on the passive forms while comprehending the active forms correctly; or (4) discriminating between active and passive forms, making progress toward the correct interpretation of passive forms while comprehending the active forms correctly. The results placed the control group in the last category. They made no errors comprehending the roles of the nouns in the active sentences and correctly comprehended the roles of the nouns in the passive sentences at an above-chance level, suggesting that they knew how to comprehend active sentences and were making progress toward the correct interpretation of passive forms. The acallosal subject tended to make the first noun of an active sentence serve as the subject of the sentence, but in passive sentences selected the subject randomly from the two nouns. These results would place the acallosal subject in the third category, representing an earlier point in linguistic development than the control subjects. Among the relative clause sentences, the SO relative sentences generally were more difficult for both groups. However, one control group subject (M.C.) made no noun assignment errors in the relative clause sentences and seemed to have mastered comprehension of these forms. Both groups tended to assign the role of agent of the relative clause to the noun located immediately before the verb of the relative clause. This tendency yielded the correct interpretation of both SS and SO relative clauses because the verb was in the active voice. This NV tendency was stronger in the control group, where the correct assignment was made 91% of the time on the average in SS relative sentences across tasks and 64% of the time on the average in SO relative sentences across tasks, than it was in the acallosal subject who made the correct assignment 73% of the time on the average in SS relative sentences across tasks and 58% of the time on the average in SO relative sentences across tasks. The difference between the relative frequency of correct assignments of agent and object roles in SS and SO relative clauses reflected a weak bias in both groups to assign the role of agent of the relative clause to the first noun in the sentence. This first noun bias favors the correct assignment in SS relative clauses, but favors the incorrect assignment in SO relative clauses. The first noun bias was especially strong among control subjects
ACALLOSAL
69
SENTENCE COMPREHENSION
in the sentence-picture matching task. In the sentence-picture matching task, the difference between the relative frequency of correct assignments of agent and object roles in SS and SO relative clauses was 36% for the control group and 17% for the acallosal subject. In the acting out task, the differences were 17 and 13%, respectively. These results indicate that the superior performance of the control group in interpreting SS relative clauses was due chiefly to their strong tendency to interpret the noun immediately in front of the verb as the agent of the action. The poorer performance of the control group on the SO relative clauses in the sentence-picture matching task was due to a strong competing tendency to make the first noun serve as the agent of the relative clause. This tendency was not as strong in the control group in the acting out task, nor was it strong in the acallosal subject. The fact that it was stronger in the sentence-picture matching task suggests that some aspect of the task led the control subjects to focus on the first noun. In the acting out task, the color of one of the nouns was always specified in the sentences and correct color assignment was necessary in order to act out the sentence correctly. Color assignment was generally correct in active and passive sentence conditions. In the relative clause conditions, the color was specified in the matrix sentence where it modified the subject noun. The role of subject of the matrix sentence was assigned to the first noun of the SS relative sentences 82% of the time by control subjects and 63% of the time by the acallosal subject. For the SO relative sentences, the relative frequencies were 68 and 67%, respectively. A more detailed analysis revealed that this tendency was affected in some cases by the perceived role that the first noun in the sentence played in the relative clause. These results are presented in Table 5. In SS relative sentences, the control group correctly assigned the first noun to the role “subject of the matrix sentence” without regard to the perceived role the first noun played in the relative clause. The correct assignment was TABLE 5 COMPREHENSION OF MATRIX SENTENCE OF SS AND SO RELATIVE SENTENCES IN ACTING TASK AS A FUNCTION OF PERCEIVED AGENT OF RELATIVE CLAUSE
OUT
Group
Sentence type
Perceived agent
Percentage correct
Acallosal
SS relative
First noun Second noun First noun Second noun First noun Second noun First noun Second noun
59 71 60 71
SO relative Control
SS relative SO relative
82 83 89 59
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RICHARD
J. SANDERS
made on 82% of the trials when the first noun was perceived as the agent of the relative clause and on 83% of the trials when the first noun was perceived as the object of the relative clause. In the SO relative sentences, the control group was correct 89% of the time when the first noun was perceived (incorrectly) to be the agent of the relative clause and was correct 59% of the time when the first noun was perceived (correctly) to be the object of the relative clause. This strong effect in SO relative sentences was due chiefly to one subject in the control group (T.J.). When the first noun in the SO relative sentence was perceived to be the subject of the relative clause, T.J. interpreted the matrix sentence correctly 100% of the time. When the first noun was perceived to be the object of the relative clause, T.J. interpreted the matrix sentence correctly only 38% of the time. Corresponding data for the control group without T.J. were 86 and 76%, respectively. This indicates a tendency to make the perceived agent of the SO relative clause and the subject of the matrix sentence be the same noun. The acallosal subject was also influenced by her perception of the first noun’s role in the relative clause, but in a different way. In SS relative sentences, she comprehended the matrix sentence correctly 71% of the time when the first noun was perceived to be the object of the relative clause, compared to 59% correct comprehension when the first noun was perceived to be the agent of the relative clause. In SO relative sentences, the corresponding relative frequencies were 71 and 60%, respectively. These results indicate a tendency to make the perceived object of both SS and SO relative clauses be the subject of the corresponding matrix sentences. The central issue to which this study was addressed was whether the acallosal subject’s comprehension strategies would differ from those of the normal control subjects. Consider first the performance of the acallosal subject. Three interesting features of her performance are relevant here: (1) that she did better when the content words were in an NVN order; (2) that she did better when the NVN sequence was the agent-actionobject of a clause; and (3) that she discriminated between the four sentence types, processing each one differently. Three of the sentence types began with an NVN sequence. In the active sentences and SS relative sentences, this sequence was also the agent-action-object of a clause and the acallosal subject interpreted the NVN sequence as an agent-action-object sequence about 75% of the time. This is what would occur if she were using an NVN strategy most of the time. Passive sentences also contain an NVN sequence, but it denotes the object-action-agent of the sentence, respectively. The NVN strategy would lead to the miscomprehension of passive sentences. The acallosal subject did not use the NVN strategy with the passive sentences as she had done with other sentences containing an initial NVN sequence. Nor did she regularly interpret the sequence as the object-action-agent of the sentence. She seemed to guess randomly
ACALLOSAL
SENTENCE
COMPREHENSION
71
about the semantic roles of the two nouns, selecting each as the agent of the action about 50% of the time. Similarly, in the SO relative condition where there was no initial NVN sequence and where it is likely that the initial NNV sequence blocked the use of the NVN strategy, the acallosal subject seemed to guess randomly which noun was the agent of the action, choosing each noun as agent roughly 50% of the time. To summarize, the acallosal subject seemed to use the NVN strategy unless its use was contraindicated by the was verbed by sequence in passive sentences or by the initial NNV sequence in the SO relative sentences. When use of the NVN strategy was blocked, she relied on random guessing to determine the agent and object of the action. Subjects in the control group recognized that the NVN strategy would not work on passive sentences, but did not guess randomly in place of using the NVN strategy. They assigned the correct object-action-agent interpretation to the NVN sequence in passive sentences 75% of the time or more. It is in the SO relative condition that the control subjects seemed to have the most trouble. There was a greater difference between their performance on the sentence-picture matching task and the acting out task when the sentences were SO relatives than when the other sentence types were used, suggesting that comprehension was more difficult, less automatic, and therefore more susceptible to interference from extraneous task variables. In the easier acting out task, two of the three control subjects were generally correct in their interpretation of the SO relative clauses, while the third seemed to have guessed randomly. To summarize these points, the control subjects generally seemed to be well on their way toward accurate comprehension of these four sentence types, although they were still making occasional errors. It is clear that the acallosal subject was approaching the comprehension tasks with a different set of strategies than were the normal control subjects. It is uncertain, however, whether the acallosal subject’s performance represents a delay or a deficit. The comprehension strategies that she used are comparable to those of a younger child, leaving open the possibility that her comprehension will reach a normal level at a later age. Finally, the analysis has shown that successful comprehension involves two distinct abilities that should be considered when diagnosing functional comprehension deficits. The first is the ability to recognize the form of the sentence-whether it is an active, passive, SO relative, or other form. According to results of studies of comprehension mentioned previously, in normal populations this ability seems to develop before the second, which is the ability to correctly assign semantic functions to the sentence constituents. The same developmental pattern was observed in the normal and the acallosal subjects in this study. The normal subjects were able to recognize that the four sentence forms were different and were doing a fairly good job of comprehending all but the SO relative
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sentences. The acallosal subject was also able to distinguish between the four sentence forms, but had greater difficulty than the control subjects assigning the correct semantic functions to nonactive sentence forms. A question that remains to be answered in future studies of other acallosal children is whether the development of both the ability to recognize various sentence forms and the ability to assign semantic functions are delayed in acallosal children. REFERENCES Bever, T. G. 1970. The cognitive basis for linguistic structure. In J. R. Hayes (Ed.), Cognition and the development of language. New York: Wiley. Caramazza, A., & Zurif, E. B. 1976. Dissociation of algorithmic and heuristic processes in language comprehension: Evidence from aphasia. Brain and Language, 3, 572-582. Clancy, P. M., Lee, H., & Zoh, M. 1986. Processing strategies in the acquisition of relative clauses: Universal principles and language-specific realizations. Cognition, 24, 22% 262. Dennis, M. 1977. Cerebral dominance in three forms of early brain disorder. In M. E. Blau, I. Rapin, & M. Kinsbourne (Eds.), Topics in child neurology. New York: Spectrum. Pp. 189-212. Dennis, M. 1981. Language in a congenitally acallosal brain. Brain and Language, 12, 3353. Dennis, M., & Whitaker, H. A. 1976. Language acquisition following hemidecortication: Linguistic superiority of the left over the right hemisphere. Brain and Language, 3, 404-433. devilliers, J. G., & devilhers, P. A. 1973. Development of the use of word order in comprehension. Journal of Psycholinguistic Research, 2, 331-341. devilhers, J. G., Tager-Flusberg, H. B., Hakuta, K., & Cohen, M. 1979. Children’s Research, 8, 499-518. comprehension of relative clauses. Journal ofPsycholinguistic Fodor, J. A., Bever, T. G., & Garrett, M. F. 1974. The psychology of language. New York: McGraw-Hill, Pp. 344-361. Gazzaniga, M. S. 1970. The bisected brain. New York: Appleton-Century-Crofts. Hecaen, H. 1976. Acquired aphasia in children and the ontogenesis of hemispheric functional specialization. Brain and Language, 3, 114-134. Lineberger, M. C., Schwartz, M. F., & Saffran, E. M. 1983. Sensitivity to grammatical structure in so-called agrammatic aphasics. Cognition. 13, 361-392. Maratsos, M. P. 1974. Children who get worse at understanding the passive: A replication of Bever. Journal of Psycholinguistic Research. 3, 65-74. Saul, R. E., & Sperry, R. W. 1968. Absence of commissurotomy symptoms with agenesis of the corpus callosum. Neurology, 18, 307. (Abstr.) Sheldon, A. 1974. The role of parallel function in the acquisition of relative clauses in English. Journal of Verbal Learning and Verbal Behavior, 13, 272-281. Tavakolian, S. L. 1978. The conjoined-clause analysis of relative clauses and other structures. In H. Goodluck & L. Solan (Eds.), Papers in the srructure and development of child language. University of Massachusetts Occasional Papers in Linguistics. Amherst, MA: Department of Linguistics. Vol. 4.
AND
LANGUAGE
37, 59-72 (1989)
Sentence Comprehension following Agenesis of the Corpus Callosum RICHARD University
J. SANDERS
of North Carolinrr
rrt Wilmington
The sentence comprehension skills of a h-year-old girl with callosal agenesis were compared to skills of three other children matched for age and verbal IQ. Sentence-picture matching and acting out tasks were used with reversible active, passive, and center-embedded relative clause sentences. The acallosal subject showed a deficit in syntactic comprehension. The difficulty was due to a failure to assign correct semantic roles to some sentence forms, not to a lack of ability to discriminate among the sentence forms. The data are consistent with a previous report by M. Dennis (1977, In Topics in child neurology. pp. 189-212) that in acallosal subjects syntactic comprehension can be adversely affected. Because this acallosal subject is only 6 years old, follow-up studies will be needed to determine whether she eventually acquires normal syntactic skills. e 19x9 Academic Prcas. Inc.
INTRODUCTION
Numerous studies have indicated that the left hemisphere of the brain is an area that is essential for normal language development and language functioning (see for example, Gazzaniga, 1970; Caramazza & Zurif, 1976; Dennis & Whitaker, 1976; and Lineberger, Schwartz, & Saffran, 1983). In particular, these studies have associated the left hemisphere with the ability to use the syntactic structure of sentences for the purposes of comprehension or production. The present study focuses instead on the role of the corpus callosum in language acquisition. A number of authors have suggested that in the acallosal brain verbal and nonverbal functions compete for resources, with the verbal functions generally dominating (Saul & Sperry, 1968; Gazzaniga, 1970; Hecaen, 1976). Dennis (1977, 1981), on the other hand, has speculated that agenesis of the corpus callosum often results in deficits in either syntactic abilities or visualspatial abilities, with concurrent sparing of the other function. In a detailed examination of these functions in two mature acallosal subjects, she Address correspondence and reprint requests to Dr. Richard J. Sanders, Department of Psychology, University of North Carolina at Wilmington, Wilmington, NC 28403-3297. 59 0093-934X/89 $3.00 Copyright 0 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.
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found that one was skilled at a visual-spatial task but extremely impaired at understanding the syntax of English; the other was linguistically skilled but visually-spatially impaired. Specific syntactic deficits have not yet been documented in other acallosal subjects. In the present investigation, the syntactic abilities of a 6-year-old girl with callosal agenesis was compared with the syntactic abilities of three normal 6-year-old children to see whether the agenesis had affected the development of language comprehension. Development of Syntactic Comprehension It has been suggested that adult sentence comprehension may involve the use of numerous heuristic strategies, each of which is conditional upon the occurrence of a particular type of syntactic structure (Fodor, Bever, & Garrett, 1974). Four sentence types were chosen for use in this study because they represent common English syntactic structures that have been used in previous studies of comprehension strategies. The sentence forms that were used in the study were actives, passives, and ‘sentences containing embedded relative clauses. Comprehension of these sentence forms by children has also been widely studied and may be summarized as follows: After an early period during which children seem to rely solely on a semantic strategy of guessing the most sensible meaning of a sentence based on the meanings of its words, children first learn to comprehend active sentences reliably. It has been widely suggested that they use a noun-verb-noun strategy in which the first NVN sequence in the sentence is taken to denote the agent, action, and object of the sentence, respectively (Bever, 1970). This strategy works well for simple active sentences, but is inadequate for passive sentences or complex sentences with multiple clauses. After an initial attempt to apply the NVN strategy too widely, children seem to recognize that the strategy cannot be applied to passive sentences, but they do not yet have an alternative strategy for interpreting passive sentences. The ability to comprehend passives correctly develops gradually, beginning around the age of 4 years (Bever, 1970; Maratsos, 1974; devilliers & devilliers, 1973). Children also have difficulty comprehending some sentences with a relative clause until they are well into their early school-age years. Two variables have been found to have an effect on the comprehension of sentences containing a relative clause. The first is the position of the relative clause in the sentence. Two positions that have been most often studied are center-embedded relative clauses that follow the noun phrase that is the subject of the matrix sentence as in sentence 1 and rightbranching relative clauses that follow the object of the matrix sentence as in sentence 2.
ACALLOSAL
SENTENCE COMPREHENSION
61
1. Center-embedded: The boy who liked the girl read the book. 2. Right-branching: The boy liked the girl who read the book. The second variable to affect comprehension in children is the syntactic role of the relativized noun phrase in the relative clause. The head noun phrase may be the subject of the relative clause as in sentence 3 or the object of the relative clause as in sentence 4. 3. Head noun as subject of the relative clause: The clown who lifted the balloons dropped the flowers. 4. Head noun as object of the relative clause: The balloons that the clown lifted frightened the baby. Children generally comprehend sentences with embedded relative clauses less well than sentences with right-branching relative clauses, and centerembedded relative sentences are generally more difficult to comprehend when the head noun is the subject of the matrix sentence and the object of the relative clause than when it is the subject of both clauses (Clancy, Lee, & Zoh, 1986; devilliers, Tager-Flusberg, Hakuta, & Cohen, 1979; Tavakolian, 1978; Sheldon, 1974). Part of the advantage of latter type of relative sentence has been attributed to the fact that the NVN strategy that is already familiar to children in their comprehension of simple active sentences also works with relative clauses in that type of relative sentence (devilliers et al., 1979). However, the NVN strategy does not provide a correct analysis of both clauses in these relative sentences. This means that the child must discover an additional comprehension strategy to deal with the second clause. The purpose of the present study was to examine the use of comprehension strategies by the acallosal and control subjects in response to four sentence types in order to determine whether agenesis of the corpus callosum had affected the development of comprehension strategies associated with these sentence types. METHOD Subjects. The acallosal subject was a 6-year-old female. At the age of I month she was
admitted to the Eugene Talmadge Memorial Hospital for examination because of an enlarged head (98th percentile). A CT scan suggested some cerebellar agenesis and agenesis of the corpus callosum. The scan did not indicate whether the agenesis was partial or complete. Despite the enlarged head circumference, the CT scan showed normal ventricle size and no diagnosis of hydrocephalus was indicated in the medical report. Her EEG was normal. Her verbal intelligence at the age 5 years and 6 months, as measured by the Wechsler Preschool and Primary Scale of Intelligence, was 91. Like other acallosal subjects (Saul & Sperry, 1968; Gazzaniga, 1970), the subject showed a lack of disconnection symptoms that are associated with commisurotomy patients. She was able to accurately name common objects presented to either visual field or presented haptically to either hand. The control subjects were two males and a female matched with the acallosal subject in age and verbal intelligence. The relevant characteristics of the subjects are presented in Table 1. All of
62
RICHARD J. SANDERS TABLE 1 DESCRIFTION
OF SUBJECTS
Age
Subject
(years, months)
Sex
Verbal IQ
Performance IQ
Condition
C.F. M.C. K.M. T.J.
6’3 68
F F M M
91 101 95 87
70 110 112 92
Acallosal Control Control Control
5,lO 5,lO
the control subjects had verbal intelligence scores within 10 points of the acallosal subject’s score, 10 points being the normal range of test-retest variability for the test. The subjects in the control group were students in local elementary school kindergarten classes, participating with their parents’ consent. Materials. Two comprehension tasks were devised to test comprehension of reversible sentences. These are sentences whose subject and object noun phrases could be exchanged without violating the selection restrictions of the language. The first comprehension task was a sentence-picture matching task, patterned after a test used by Caramazza and Zurif (1976). A basic set of eight reversible, active sentences with transitive verbs was constructed. Each sentence contained two nouns, the first one modified by an adjective. The eight sentences are shown in Table 2. Each of the eight sentences was also transformed into a passive sentence form and into two complex sentences with center-embedded relative clauses. These alternate sentence forms are also presented in Table 2. The relative clauses always modified the noun phrase that was the subject of the matrix sentence, but differed in terms of the syntactic role of that noun phrase in the relative clause. In one case the subject of the matrix sentence was also the subject of the relative clause. These were called SS relative sentences. In the other case the subject of the matrix sentence was the object of the relative clause. These were called SO relative sentences. These 32 sentences were each transformed once again by reversing the two nouns. Examples of the reversed sentence forms are also presented in Table 2. In all, 64 different sentences were constructed for use in the test: 8 different base sentences occurring in each of four syntactic forms with the two nouns presented in both forward and reversed orders of mention. For each of the eight basic sentences, two pictures were prepared: one showing the subject noun carrying out the action and another showing the object noun carrying out the action. Copies of these two pictures were mounted on a 5 x 7-in. index card with the correct picture on the left. A second card was also prepared with the correct picture on the right. One of the two picture cards was assigned to each of the eight transformations of each of the basic sentence in such a manner that the side of the card on which the correct picture was located was counterbalanced within each of the four sentence forms and within the eight transformations of each of the basic sentences. The 64 sentences were arranged in a list for presentation in a quasi-random order, with the restriction that two instances of the same syntactic form could not be presented successively, nor could two syntactic forms of the same basic sentence be presented successively. The second comprehension test required that a subject act out an action described in a reversible sentence. The actions described in the sentences involved an animal of one species either jumping over or bumping into an animal of another species. The animals used in the task were cows, horses, and pigs. A white and a brown plastic model of each of these animals were used. A set of 96 sentences was constructed describing various ways animals of one species could jump over or bump into an animal of another species. The sentences were evenly divided among the same four sentence types that had been
ACALLOSALSENTENCECOMPREHENSION
63
TABLE 2 MATERIALS FOR SENTENCE-PICTURE MATCHING TASK
Eight basic sentences The The The The
The The The The
unhappy boy is pushing the girl. funny cat is chasing the dog. nice boy is following the man. fat girl is tickling the boy.
yellow car is bumping the bus. friendly horse is kissing the cow. funny clown is lifting the balloons. happy boy is hugging the girl.
Four syntactic forms Active: Passive: SS relative: SO relative:
The The The The
unhappy boy is pushing the girl. unhappy girl is being pushed by the boy. boy who is pushing the girl is unhappy. girl who the boy is pushing is unhappy.
Active: Passive: SS relative: SO relative:
The The The The
unhappy girl is pushing the boy. unhappy boy is being pushed by the girl. girl who is pushing the boy is unhappy. boy who the girl is pushing is unhappy.
Reversed sentence forms
Note. Examples of alternate syntactic forms and reversed forms are given only for the first basic sentence. used in the sentence-picture matching task. The variables of species of the sentence subject, species of the sentence object, type of action, and stated color were counterbalanced within each sentence type and across the entire set of sentences. The color of only one animal was specified in each sentence. The color of the agent was specified in one half of the sentences and the color of the object in the other half. Examples of the sentences used in the acting out task are presented in Table 3. Procedure. Subjects were tested individually in weekly sessions of less than 45 min. It required three or four sessions to complete the two tasks. The sentence-picture task was administered first. Before beginning the task the child was shown each of the picture cards and asked to point to the picture that the experimenter described. The experimenter then TABLE 3 EXAMPLES OF MATERIALS FROM ACTING OUT TASK
Basic sentences with “cow” The The The The
brown cow jumps over the pig. white cow jumps over the horse. cow jumps over the white pig. cow jumps over the brown horse.
The The The The
white cow bumps into the pig. brown cow bumps into the horse. cow bumps into the brown pig. cow bumps into the white horse.
Alternative transformations of first basic sentence Active: Passive: SS relative: SO relative:
The The The The
brown cow jumps over the pig. brown cow is jumped over by the pig. cow that jumps over the pig is brown. cow that the pig jumps over is brown.
64
RICHARD .I. SANDERS
described one of the two pictures using a simple active sentence like The boy is pushing the girl. Every child was able to identify each of the pictures correctly. This was a precaution to ensure that all subjects were able to interpret the pictures correctly and to point accurately to the picture of their choice. The child was then told to continue pointing to the pictures described by the experimenter for the remainder of the trials. On each trial one picture card was placed on the table in front of the child and the experimenter read a sentence describing one of the pictures. The sentence was reread at intervals of approximately IO set until the child pointed to one of the pictures. The acting out task was begun at the start of the session that followed completion of the sentence-picture matching task. The child was first shown the six animals and asked to point to each one as the experimenter named an animal of a particular species and color. All of the children were able to do this. The children were then told that the experimenter would describe an action that the child should act out with the animals. The experimenter demonstrated the “jumping over” and “bumping into” actions and asked the child to demonstrate the actions as well before beginning the task itself. On each trial the six animals were placed on the table in front of the child. The animals were not arranged in any fixed order. The experimenter read a sentence to the child and repeated it approximately every IO set until the child completed an action.
RESULTS AND DISCUSSION The results from the sentence-picture matching task were examined for evidence of differences in the performance of the two groups of subjects. Figure 1 contains the mean percentage of trials with errors as a function of subject group and sentence type. A comparison of the errors by the acallosal subject versus the control subjects on each of the sentence types showed clearly that the control subjects did better than the acallosal subject on both active sentences (0% vs. 33% errors) and passive sentences (25% vs. 58% errors). In both cases there was no overlap between the scores of the two groups. On the SS relative sentences, the control subjects did better than the acallosal subject on the average (11% errors vs. 25% errors). However, one of the control subjects also erred on 25% of the trials. The performance of the groups on the SO relative sentences did not differ greatly. The acallosal subject made errors on 42% of the trials and the control subjects on the average made errors on 47% of the trials. The acallosal subject’s error rate on the SO relative sentences was equal to the performance of the best subject in the control group on this sentence type. To summarize, the control subjects did better overall than the acallosal subject on all sentence types except the SO relative sentences. The two groups did not differ on the SO relative sentences. In the acting out task, the control subjects again did better on the average than the acallosal subject on all sentence types except the SO relative sentences. The results are presented in Fig. 2. The mean error rates for the control subjects and the acallosal subject were 0 and 21%, respectively, on active sentences; 10 and 63%, respectively, on passive sentences; 31 and 63%, respectively, on SS relative sentences; and 57 and 58%, respectively, on SO relative sentences.
65
ACALLOSALSENTENCECOMPREHENSION 60-
Acallosal Normal
Active
Passive Sentence
SS Relative
SO Relative
Type
FIG. 1. Percentage of errors on sentence-picture matching task as a function of subject group and sentence type.
Several types of errors were possible on the acting out task. The most common type of error was the reversal of the agent and object of the action. For example, in sentence 5 the horse is the agent of the action “jump over” and the pig is the object. 80,
m z.; ‘;j E (I) 0 z a
60
40
20
0
Active
Passive Sentence
SS Relative
SO Relative
Type
FIG. 2. Percentage of errors on acting out task as a function of subject group and sentence type.
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RICHARD J. SANDERS
5. The pig that the horse jumps over is brown. A noun reversal error would occur if a subject who was acting out sentence 5 made the pig jump over the horse. In active sentences and SS relative sentences, the first noun is the agent and the second noun is the object. The opposite is the case in passive sentences and SO relative sentences. The relative frequencies of noun reversal errors are presented in Fig. 3 by sentence type and subject group. The control subjects committed fewer noun reversal errors on the average than the acallosal subject on all sentence types. With the exception of the error rates on the SO relative sentences, there was no overlap between the error rates of the control group and the acallosal subject. The mean error rates for the control subjects and acallosal subject were 0 and 17%, respectively, on the active sentences; 7 and 54%, respectively, on the passive sentences; 8 and 29%, respectively, on the SS relative sentences; and 25 and 42%, respectively, on the SO relative sentences. A second type of error occurred when subjects failed to use an animal with the color specified in the matrix sentence. Color assignment errors were rare in active and passive sentences where the color term occurred immediately prior to the term that it modified. The error data are presented in Fig. 4. The control subjects made fewer color assignment errors than the acallosal subject on most sentence types except the SO relative sentences, where the frequency of color errors for the two groups did not differ.
Active
Passive Sentence
SS Relative
SO Relative
Type
FIG. 3. Percentage of reversal errors on acting out task as a function of subject group and sentence type.
ACALLOSAL
SENTENCE COMPREHENSION
67
40
m 30 z .; z E : b
20
n
10
0 Active
Passive
SO Relative
SS Relative
Sentence
Type
FIG. 4. Percentage of color assignment errors on acting out task as a function of subject group and sentence type.
The color assignment error rates for the control group and the acallosal subject were 0% for both groups on active sentences; 1 and 17%, respectively, on passive sentences; 18 and 38%, respectively, on SS relative sentences; and 32 and 33%, respectively, on SO relative sentences. The last, and least frequent, type of error in the acting out task occurred when subjects used an animal that was not mentioned in the sentence. The total number of substitution errors by sentence type and subject group is presented in Table 4. The acallosal subject made a total of six substitution errors, three of them on the SO relative sentences, and one on each of the remaining sentence types. Of the three subjects in the control group, one made no substitution errors, one made one substitution error, and one made two substitution errors. Two of the errors occurred on passive sentences and one on an SO relative sentence. The results were next examined for evidence of the ways the subjects assigned relational roles to the nouns in the sentences. With the exception of the copula be that was used in the matrix clause of sentences containing TABLE 4 TOTAL
NUMBER
OF SUBSTITUTION
ERRORS IN ACTING
OUT TASK
Sentence type Group
Active
Passive
SS relative
SO relative
Acallosal Control
I 0
I 2
1 0
3 1
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RICHARD
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relative clauses, all of the verbs in the sentences used in this study were action verbs with noun phrases serving the roles of agent and object of the action. On the basis of previous studies reviewed in the introduction, it was expected that subjects would perform better on active sentences than on passive ones and that subjects might be observed doing one of the following: (1) using a semantic guessing strategy on both active and passive forms (which would produce chance performance on these reversible sentences); (2) using a NVN strategy for both active and passive sentences; (3) discriminating between active and passive forms, and guessing randomly on the passive forms while comprehending the active forms correctly; or (4) discriminating between active and passive forms, making progress toward the correct interpretation of passive forms while comprehending the active forms correctly. The results placed the control group in the last category. They made no errors comprehending the roles of the nouns in the active sentences and correctly comprehended the roles of the nouns in the passive sentences at an above-chance level, suggesting that they knew how to comprehend active sentences and were making progress toward the correct interpretation of passive forms. The acallosal subject tended to make the first noun of an active sentence serve as the subject of the sentence, but in passive sentences selected the subject randomly from the two nouns. These results would place the acallosal subject in the third category, representing an earlier point in linguistic development than the control subjects. Among the relative clause sentences, the SO relative sentences generally were more difficult for both groups. However, one control group subject (M.C.) made no noun assignment errors in the relative clause sentences and seemed to have mastered comprehension of these forms. Both groups tended to assign the role of agent of the relative clause to the noun located immediately before the verb of the relative clause. This tendency yielded the correct interpretation of both SS and SO relative clauses because the verb was in the active voice. This NV tendency was stronger in the control group, where the correct assignment was made 91% of the time on the average in SS relative sentences across tasks and 64% of the time on the average in SO relative sentences across tasks, than it was in the acallosal subject who made the correct assignment 73% of the time on the average in SS relative sentences across tasks and 58% of the time on the average in SO relative sentences across tasks. The difference between the relative frequency of correct assignments of agent and object roles in SS and SO relative clauses reflected a weak bias in both groups to assign the role of agent of the relative clause to the first noun in the sentence. This first noun bias favors the correct assignment in SS relative clauses, but favors the incorrect assignment in SO relative clauses. The first noun bias was especially strong among control subjects
ACALLOSAL
69
SENTENCE COMPREHENSION
in the sentence-picture matching task. In the sentence-picture matching task, the difference between the relative frequency of correct assignments of agent and object roles in SS and SO relative clauses was 36% for the control group and 17% for the acallosal subject. In the acting out task, the differences were 17 and 13%, respectively. These results indicate that the superior performance of the control group in interpreting SS relative clauses was due chiefly to their strong tendency to interpret the noun immediately in front of the verb as the agent of the action. The poorer performance of the control group on the SO relative clauses in the sentence-picture matching task was due to a strong competing tendency to make the first noun serve as the agent of the relative clause. This tendency was not as strong in the control group in the acting out task, nor was it strong in the acallosal subject. The fact that it was stronger in the sentence-picture matching task suggests that some aspect of the task led the control subjects to focus on the first noun. In the acting out task, the color of one of the nouns was always specified in the sentences and correct color assignment was necessary in order to act out the sentence correctly. Color assignment was generally correct in active and passive sentence conditions. In the relative clause conditions, the color was specified in the matrix sentence where it modified the subject noun. The role of subject of the matrix sentence was assigned to the first noun of the SS relative sentences 82% of the time by control subjects and 63% of the time by the acallosal subject. For the SO relative sentences, the relative frequencies were 68 and 67%, respectively. A more detailed analysis revealed that this tendency was affected in some cases by the perceived role that the first noun in the sentence played in the relative clause. These results are presented in Table 5. In SS relative sentences, the control group correctly assigned the first noun to the role “subject of the matrix sentence” without regard to the perceived role the first noun played in the relative clause. The correct assignment was TABLE 5 COMPREHENSION OF MATRIX SENTENCE OF SS AND SO RELATIVE SENTENCES IN ACTING TASK AS A FUNCTION OF PERCEIVED AGENT OF RELATIVE CLAUSE
OUT
Group
Sentence type
Perceived agent
Percentage correct
Acallosal
SS relative
First noun Second noun First noun Second noun First noun Second noun First noun Second noun
59 71 60 71
SO relative Control
SS relative SO relative
82 83 89 59
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made on 82% of the trials when the first noun was perceived as the agent of the relative clause and on 83% of the trials when the first noun was perceived as the object of the relative clause. In the SO relative sentences, the control group was correct 89% of the time when the first noun was perceived (incorrectly) to be the agent of the relative clause and was correct 59% of the time when the first noun was perceived (correctly) to be the object of the relative clause. This strong effect in SO relative sentences was due chiefly to one subject in the control group (T.J.). When the first noun in the SO relative sentence was perceived to be the subject of the relative clause, T.J. interpreted the matrix sentence correctly 100% of the time. When the first noun was perceived to be the object of the relative clause, T.J. interpreted the matrix sentence correctly only 38% of the time. Corresponding data for the control group without T.J. were 86 and 76%, respectively. This indicates a tendency to make the perceived agent of the SO relative clause and the subject of the matrix sentence be the same noun. The acallosal subject was also influenced by her perception of the first noun’s role in the relative clause, but in a different way. In SS relative sentences, she comprehended the matrix sentence correctly 71% of the time when the first noun was perceived to be the object of the relative clause, compared to 59% correct comprehension when the first noun was perceived to be the agent of the relative clause. In SO relative sentences, the corresponding relative frequencies were 71 and 60%, respectively. These results indicate a tendency to make the perceived object of both SS and SO relative clauses be the subject of the corresponding matrix sentences. The central issue to which this study was addressed was whether the acallosal subject’s comprehension strategies would differ from those of the normal control subjects. Consider first the performance of the acallosal subject. Three interesting features of her performance are relevant here: (1) that she did better when the content words were in an NVN order; (2) that she did better when the NVN sequence was the agent-actionobject of a clause; and (3) that she discriminated between the four sentence types, processing each one differently. Three of the sentence types began with an NVN sequence. In the active sentences and SS relative sentences, this sequence was also the agent-action-object of a clause and the acallosal subject interpreted the NVN sequence as an agent-action-object sequence about 75% of the time. This is what would occur if she were using an NVN strategy most of the time. Passive sentences also contain an NVN sequence, but it denotes the object-action-agent of the sentence, respectively. The NVN strategy would lead to the miscomprehension of passive sentences. The acallosal subject did not use the NVN strategy with the passive sentences as she had done with other sentences containing an initial NVN sequence. Nor did she regularly interpret the sequence as the object-action-agent of the sentence. She seemed to guess randomly
ACALLOSAL
SENTENCE
COMPREHENSION
71
about the semantic roles of the two nouns, selecting each as the agent of the action about 50% of the time. Similarly, in the SO relative condition where there was no initial NVN sequence and where it is likely that the initial NNV sequence blocked the use of the NVN strategy, the acallosal subject seemed to guess randomly which noun was the agent of the action, choosing each noun as agent roughly 50% of the time. To summarize, the acallosal subject seemed to use the NVN strategy unless its use was contraindicated by the was verbed by sequence in passive sentences or by the initial NNV sequence in the SO relative sentences. When use of the NVN strategy was blocked, she relied on random guessing to determine the agent and object of the action. Subjects in the control group recognized that the NVN strategy would not work on passive sentences, but did not guess randomly in place of using the NVN strategy. They assigned the correct object-action-agent interpretation to the NVN sequence in passive sentences 75% of the time or more. It is in the SO relative condition that the control subjects seemed to have the most trouble. There was a greater difference between their performance on the sentence-picture matching task and the acting out task when the sentences were SO relatives than when the other sentence types were used, suggesting that comprehension was more difficult, less automatic, and therefore more susceptible to interference from extraneous task variables. In the easier acting out task, two of the three control subjects were generally correct in their interpretation of the SO relative clauses, while the third seemed to have guessed randomly. To summarize these points, the control subjects generally seemed to be well on their way toward accurate comprehension of these four sentence types, although they were still making occasional errors. It is clear that the acallosal subject was approaching the comprehension tasks with a different set of strategies than were the normal control subjects. It is uncertain, however, whether the acallosal subject’s performance represents a delay or a deficit. The comprehension strategies that she used are comparable to those of a younger child, leaving open the possibility that her comprehension will reach a normal level at a later age. Finally, the analysis has shown that successful comprehension involves two distinct abilities that should be considered when diagnosing functional comprehension deficits. The first is the ability to recognize the form of the sentence-whether it is an active, passive, SO relative, or other form. According to results of studies of comprehension mentioned previously, in normal populations this ability seems to develop before the second, which is the ability to correctly assign semantic functions to the sentence constituents. The same developmental pattern was observed in the normal and the acallosal subjects in this study. The normal subjects were able to recognize that the four sentence forms were different and were doing a fairly good job of comprehending all but the SO relative
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sentences. The acallosal subject was also able to distinguish between the four sentence forms, but had greater difficulty than the control subjects assigning the correct semantic functions to nonactive sentence forms. A question that remains to be answered in future studies of other acallosal children is whether the development of both the ability to recognize various sentence forms and the ability to assign semantic functions are delayed in acallosal children. REFERENCES Bever, T. G. 1970. The cognitive basis for linguistic structure. In J. R. Hayes (Ed.), Cognition and the development of language. New York: Wiley. Caramazza, A., & Zurif, E. B. 1976. Dissociation of algorithmic and heuristic processes in language comprehension: Evidence from aphasia. Brain and Language, 3, 572-582. Clancy, P. M., Lee, H., & Zoh, M. 1986. Processing strategies in the acquisition of relative clauses: Universal principles and language-specific realizations. Cognition, 24, 22% 262. Dennis, M. 1977. Cerebral dominance in three forms of early brain disorder. In M. E. Blau, I. Rapin, & M. Kinsbourne (Eds.), Topics in child neurology. New York: Spectrum. Pp. 189-212. Dennis, M. 1981. Language in a congenitally acallosal brain. Brain and Language, 12, 3353. Dennis, M., & Whitaker, H. A. 1976. Language acquisition following hemidecortication: Linguistic superiority of the left over the right hemisphere. Brain and Language, 3, 404-433. devilliers, J. G., & devilhers, P. A. 1973. Development of the use of word order in comprehension. Journal of Psycholinguistic Research, 2, 331-341. devilhers, J. G., Tager-Flusberg, H. B., Hakuta, K., & Cohen, M. 1979. Children’s Research, 8, 499-518. comprehension of relative clauses. Journal ofPsycholinguistic Fodor, J. A., Bever, T. G., & Garrett, M. F. 1974. The psychology of language. New York: McGraw-Hill, Pp. 344-361. Gazzaniga, M. S. 1970. The bisected brain. New York: Appleton-Century-Crofts. Hecaen, H. 1976. Acquired aphasia in children and the ontogenesis of hemispheric functional specialization. Brain and Language, 3, 114-134. Lineberger, M. C., Schwartz, M. F., & Saffran, E. M. 1983. Sensitivity to grammatical structure in so-called agrammatic aphasics. Cognition. 13, 361-392. Maratsos, M. P. 1974. Children who get worse at understanding the passive: A replication of Bever. Journal of Psycholinguistic Research. 3, 65-74. Saul, R. E., & Sperry, R. W. 1968. Absence of commissurotomy symptoms with agenesis of the corpus callosum. Neurology, 18, 307. (Abstr.) Sheldon, A. 1974. The role of parallel function in the acquisition of relative clauses in English. Journal of Verbal Learning and Verbal Behavior, 13, 272-281. Tavakolian, S. L. 1978. The conjoined-clause analysis of relative clauses and other structures. In H. Goodluck & L. Solan (Eds.), Papers in the srructure and development of child language. University of Massachusetts Occasional Papers in Linguistics. Amherst, MA: Department of Linguistics. Vol. 4.