ERROR PATTERNS IN AUDITORY COMPREHENSION OF ADULT APHASICS Cynthia M. Shewan (Queens College of the City University of New York, Flushing, New York)
Research studies and data reported from clinical tests for aphasia have demonstrated that adult aphasics have difficulty with auditory comprehension of sentence materials (Schuell, Jenkins and Jimenez-Pab6n, 1964; Goodglass and Kaplan, 1972). The severity of this comprehension deficit varies with the type of aphasia (Shewan and Canter, 1971); and aphasics, in general, show impaired performance when compared with normal speakers. These consistent differences have been found using quantitative measures. However, reports concerning qualitative differences in auditory comprehension between normals and aphasics and among types of aphasics present conflicting data. Several studies have examined subjects' correct response patterns. Parisi and Pizzamiglio (1970), studying the syntactic comprehension of four groups of aphasics, found quantitative differences among the groups; but when the Broca's and Wernicke's groups were compared, similar qualitative patterns emerged. Qualitative similarity among three aphasics groups was corroborated by Shewan and Canter (1971) who measured accuracy of comprehension for sentences varying in length, vocabulary difficulty, and syntactic complexity. By contrast, Goodglass, Gleason and Hyde (1970) found different patterns of performance among types of aphasics on comprehension tests which measured breadth of vocabulary, auditory sequential pointing-span, directional prepositions, and recognition of correct usage of prepositions. Perhaps this contrast among results should not be surprising, since the nature of the tasks was very different. Comprehending sentences is not a unitary task and is the result certainly of processing at many levels of the linguistic system, not to mention the contribution of contextual situational variables. The tasks used by Goodglass et al. (1970) did not tap comprehension of grammatical sentences, but seemed to be directed at some factors which might contribute to such an auditory comprehension task. These factors, in addition to others, are no doubt involved in comprehending sentence materials, but the nature of the interaction of these and other factors is not yet understood. Cortex (1976) 12, 325·336.
C. M. Shewan
326
That these studies examined only correct responses and that the tests measured different aspects of comprehension might have influenced the findings regarding qualitative similarity. To examine the qualitative aspects of sentence comprehension more closely, the present study examined the error patterns of aphasics and normals using a test for which quantitative (accuracy) data were known. The test used was the auditory comprehension test for sentences reported by Shewan and Canter (1971). Two specific questions were posed: ( 1) Are the error patterns of normal speakers and adult aphasics consistent with the expected error pattern, computed by statistical probability? (2) Do normal speakers and adult aphasics show qualitatively similar patterns of error? That is, when a normal speaker and an aphasic make errors, are they the same kind of errors?
MATERIAL AND METHOD
Subjects
The clinical group consisted of twenty-seven aphasics evenly distributed among the types of Broca's, Wernicke's, and amnesic aphasia. Each group contained patients who varied in severity of language impairment, being classified as mild, moderate, or severe. The aphasics were neurologically stable at the time of testing and were native speakers of English. The aphasics ranged in age from 20 to 83 years, with a mean age of 53.5 years. Their educational level ranged from 8 to 19 years of formal education, with a mean of 13.0 years. Etiology in the majority of patients was cerebrovascular disorder. The remaining patients were divided between traumatic and surgicalfor-tumor etiologies, with one brain abscess. Nine normal adults served as the control group. They were comparable with the aphasic patients for chronological age and educational level. They ranged in age from 22 to 71 years, with a mean age of 53.7 years. Education ranged from 6 to 20 years, with a mean of 12.6 years. Test Procedures The auditory comprehension test
A detailed description of the auditory comprehension test is available elsewhere (Shewan and Canter, 1971). Briefly, the test contained forty-two sentences which varied systematically in the parameters of length, vocabulary difficulty, and syntactic complexity. There were seven types of sentences (six sentences of each type) designed by altering each of the three parameters independently to a moderate or a hard degree of difficulty. The subject's task was to demonstrate comprehension of the orally presented sentences by pointing to its corresponding visual picture on a response plate. Each plate contained the correct pictorial representation of the sentence presented and three "dummy" pictures (error responses). Each of the dummy pictures contrasted with the presented sentence in only one critical item. For example, in the sentence The girl is reading a book, the contrasted critical items would be
Error patterns in auditory comprehension of aphasics
327
girl, read, and book. This was not possible for the longer sentences because it necessitated including more than four alternatives. For these, the selected contrasting items were noted. Procedures
Each subject was screened for adequate auditory sensitivity, auditory discrimination, visual perception, and minimal language comprehension. After completion of these screening tests, the auditory comprehension test was presented. The subject's responses were recorded and each error was classified according to both position and type as described below. Each dummy picture was examined and coded for position and type of error. The position of the error referred to whether the error occurred in the first half or the second half of the sentence. The type of error was coded for grammatical form class (noun, verb, etc.) and for surface structure linguistic constituent (noun phrase, verb phrase, and prepositional phrase). For example, for the presented sentence The girl is reading a book, one dummy picture represented The boy is reading a book. When a subject selected the latter picture the error would be coded as occurring in the first half of the sentence and an error on the grammatical class of noun and within a noun phrase linguistic constituent. Because each error position and type had different possibilities of occurrence, error frequencies were converted to error proportions for analysis of the results. For example, 58 errors were possible in the first sentence half position while 68 errors were possible in the second sentence half. Therefore, the proportions of possible errors for these frequencies were .460 and .540, respectively. The frequency data for grammatical form class and linguistic constituent were converted to proportions in the same manner.
RESULTS
To determine whether the aphasic group and the normal group differed from the expected error proportion for position of error, the data were analysed by two-tailed z-tests for differences between proportions. As shown in Table I the total aphasic group did not differ from the expected error proportion and the z-test confirmed this (p < .05). The normal group tended to make a smaller proportion of errors than expected in the first sentence half and the difference approached statistical significance (p = .06 ). When the total aphasic group was compared with the normals, a significant difference was found (p < .01), due to the smaller error proportion in the first sentence half made by the normal group. Whether the three aphasic groups demonstrated patterns similar to the expected error pattern was examined. As Table I shows none of the comparisons was significant (p > .05), suggesting that the aphasics, regardless of type, performed similarly with respect to error position on the sentence comprehension task. The grammatical form classes of critical items on which errors could occur were noun, verb, adjective, adverb, pronoun, and preposition. Here,
C. M. Shewan
328
TABLE I
Error Proportions for Error Position in Aphasic and Normal Groups and Expected Error Proportions
Sentence half First
Second
Expected error proportion
.460
.540
Normal group
.309
.691
Broca's
.527**
.473**
Amnesic
.510*
.490*
Wernicke's
.500*
.500*
Total
.511 **
.489**
Aphasic groups
'' Significantly different from Normal group (p ** Significantly different from Normal group (p
<
.05), suggesting that no one grammatical class was experienced as especially difficult to process. When the aphasics made errors in comprehension, these errors did not load more frequently than expected on a particular grammatical class. The normal group did not mirror the expected error distribution. A greater than expected error proportion occurred on the adverb class (p < .01) and error proportions for the grammatical classes of verb and pronoun approached statistical significance at the .05 level of confidence. When the rank orderings for error proportions by grammatical class were compared for the four subject groups, similar error patterns emerged. Kendall's coefficient of concordance of .936 (p < .01) indicated a very high degree of consistency across the subject groups. These rank orders, in addition, were very similar to the expected rank order which was from greatest to least error proportion: noun, verb, adverb, adjective, pronoun, and preposition. Since the aphasic groups' patterns were similar, their performances were combined. Performance of this total group and the normal group are shown with respect to the expected rank ordering for error proportion by grammatical class in Figure 1. With minor variations, the groups showed similar error patt~rns and, furthermore, these patterns were similar to the
C. M. Shewan
330
expected error distribution. The tendencies toward difference on some grammatical classes mentioned above for the normal group can be seen. How the error patterns related to surface structure linguistic constituents was examined. Because the sentences included only simple active affirmative declarative, negative, passive, and negative passive sentences, constituents such as embedded relative clauses were not present. The surface structure of each sentence thus could be analyzed using the linguistic constituents: noun phrase (NP), verb phrase (VP), and prepositional phrase (PP). Noun
• 60
. 50
"' "'"' z
0
. 40
-EXPECTED
.\
. '. \
"' "'a.
\
'
NORMAL
,_ • - ·.;(
APHASIC
\
\
w
0 i= 0 a. 0
\
o- - _ _.,·
\
\
\
• 30
\ \ \ \
. 20
\
\\ \
,~; /
\ \
• 10
/
' v
,/
/
/
~
..... ....
' ....
..........
.............
"'-,
....
',
........
......... NOUN
VER,B
ADVERB
ADJECTIVE
'
...........
·............
----
PRONOUN
PREPOSITION
GRAMM ATICAL CLASS
Fig. 1 - Error proportions for six grammatical classes for normal and total aphasic groups and the expected error proportions.
phrases took the grammatical form of (Determiner) + (Possessive) + (Adjective) + Noun. (Optional elements are included in parentheses). Prepositional phrases took the form of Preposition + NP, while verb phrases included Auxiliary + Verb. 1 These linguistic constituents were obviously related tc grammatical form class, but not synonymous with them, since an error or the grammatical class of noun could occur within either an NP or a PP 1 Examples of the linguistic constituents are: NP - the strong boy; VP - is fighting PP - to the road.
Error patterns in auditory comprehension of aphasics
331
The rank ordering for expected error proportions for linguistic constituents, shown in Figure 2, from greatest to least was NP, VP, and PP. When the errors proportion values for the normal group were compared with the
....___...,
---
• 50
X... • 40
~
0
~ ~
....
• 30
....
......
EXPECTED NORMAL
)lo,o·-·-K APHASIC
-----
z
0 i= ~ 0 a. 0
• 20
~
a.
• 10
NOUN PHRASE
VERB PHRASE
PREPOSITIONAL PHRASE
LINGUISTIC CONSTITUENT
Fig. 2 - Error proportions for linguistic constituents for normal and total aphasic groups and the expected error proportions.
expected error proportions none of the comparisons reached statistical significance (p > .05) although the normals tended to make a smaller proportion of errors on NP and a larger proportion on PP. Figure 2 indicates that error proportions for the total aphasic group were similar to the expected error proportions and this was confirmed statistically (p > .05). Further, the total aphasic group and the normal group were similar for all three linguistic constituents (p > .05}. Whether each aphasic group would parallel this pattern was determined by comparisons between each of the three aphasic groups and the expected pattern. The amnesic and Broca's aphasics evidenced patterns similar to that expected. However, the Wernicke's aphasics differed from expected by making fewer errors on the NP (p < .01) and more errors on the VP (p < .05) constituents.
332
C. M. Shewan
How closely the four subject groups paralleled one another for rank ordering for error proportion on the linguistic constituents was examined by computing Kendall's coefficient of concordance. The value of .813 revealed a high degree of similarity, suggesting similar error patterns across groups. Some of the findings in the normal group, when compared with expected error proportions, tended toward statistical significance but did not reach the .05 level of confidence. In an attempt to determine if, in fact, there were differences, a group of ten additional normal subjects was tested. The suggested position of error effect reported above was not confirmed with this group. When grammatical class was considered only the class of verb was significantly different (p < .05) from the expected error proportion. The pronoun class which had approached significance in the original normal group did not differ from the expected error proportion. The adverb class which was significant above did not show a greater than expected error proportion.
DISCUSSION
The aphasics and normal subjects demonstrated similar patterns of performance when the type of error was examined. This was true for both the total aphasic group versus the normals and for the three aphasic groups and the normals. The rank orderings of error proportions for grammatical classes were similar across the four subject groups (W = .936) and between the total aphasic group and the normals (p = .83 ). Although not an entirely independent index from grammatical class, the patterns from considering linguistic constituents were also similar. The combined aphasic and the normal groups demonstrated identical rank orderings for error proportions. This rank ordering from most to least error proportion was NP, VP, and PP. The Broca's and amnesic aphasic groups' patterns were identical to the normal, but a reversal was found in the Wernicke's group for NP and VP. With regard to position of error, the original normal group demonstrated a tendency to make a smaller error proportion in the first half of the sentence. When additional subjects were tested, however, this tendency was not substantiated and a comparison between the aphasic and the latter normal group did not reach statistical significance. Each aphasic subgroup also showed no significant difference from this normal group (p > .05). This would suggest that aphasics and normals perform similarly with regard to position of error when responding to a sentence comprehension task. Further, neither half of a sentence appears more difficult than the other. This might be explained by the method of presentation which permitted the subject to view a response plate while the sentence was being presented auditorily. The memory load might thereby have been reduced; the subject did not have to retain the entire sentence, process it, and then search for the correct
Error patterns in auditory comprehension of aphasics
333
corresponding picture. For all sentences, except the passive and negative passive, he could eliminate possibilities while the sentence was being presented. This would have the effect of not requiring that he retain the first half of the sentence while the second half was being presented. The above findings suggest that aphasics and normals behave similarly when type and position error patterns are examined. Such data support some previous reports (Shewan and Canter, 1971) in the literature that aphasics and normals demonstrate qualitative similarity even though they differ on a quantitative basis. This qualitative similarity is evident whether accuracy data are examined or whether error patterns are investigated. Of course, this evidence was gathered using a task involving comprehension of spoken sentences. That this pattern would generalize for all tasks related to comprehension is too ecompassing an interpretation. For example, Goodglass et al. (1970) found different patterns between five aphasic groups and a normal control group when using tasks such as the Peabody Picture Vocabulary Test, pointing span, a directional preposition test, and a preposition preference test. As mentioned above, these tasks may contribute to auditory comprehension but are not isomorphic with a sentence comprehension task. Therefore, it appears that the nature of the task considerably affects the pattern of results and, consequently, whether the results support the notion of qualitative similarity between aphasic and normal speakers. Related to the issue of similar processing for normals and aphasics is the pattern of performance among various types of aphasics. For error type the similar rank orderings for grammatical classes suggested that the type of aphasia was not an influential factor in the processing of grammatical classes in the sentence. When linguistic constituents were considered the Broca's and amnesic aphasic groups showed the same rank ordering for error proportion. However, the Wernicke's group showed a reversal in the order of the NP and VP, having made a greater error proportion on the VP. One possible explanation for this result relates to syntactic complexity of the sentences. The Wernicke's aphasics had most difficulty understanding the syntactically complex sentences. Since syntactic complexity for the sentences was increased by applying the negative, passive, and negative passive transformations this had the effect of making the VP surface structure more complex. Therefore, the greater error proportion on the VP constituent was probably related to the increased transformational complexity. Proportions for position of errors for all three groups of aphasics were approximately equal for both sentence halves. Therefore, for error position, aphasics perform similarly regardless of type of aphasia. These data support the generalization that various types of aphasics perform similarly in comprehending oral sentences. Quantitatively, comprehension varies among types of aphasia but the types of errors are similar. The one contrast found in the Wernicke's group should be investigated
334
C. M. Shewan
further since syntactic complexity may have been confounded with complexity of the VP. One could develop sentences which increase the complexity of the surface structure VP consistent without requiring greater transformational complexity. Such a task would serve to differentiate whether the difficulty with the VP was related to a transformational component or to the surface structure. The general conformity of aphasic error patterns to expected pattern suggested that, at least for the forms studied, one linguistic form was no more difficult to process than another. When an aphasic fails to comprehend a sentence, then, the error does not appear to be occasioned by difficulty processing any one grammatical class or any one surface structure linguistic constituent. Moreover, a fatigue or attention factor does not seem to explain the errors. It is well known that frequency of occurrence of constructs in the English language is relevant to aphasics' verbal performance. This parameter has been found to influence aphasic behavior in many aspects of the language system, for example, phonological production (Blumstein, 1973; Trost and Canter, 1974), lexical recognition (Schuell, Jenkins and Landis, 1961), and syntactic comprehension (Shewan and Canter, 1971). Therefore, it seemed important to determine if a frequency effect was operative ±or the normal and aphasic groups. The frequency effect under consideration here dealt with the frequency of items in the tested sentence corpus rather than the overall frequency in the English language. The expected error proportions, calculated for both position and type of error, reflected the error distribution exclusively in terms of possibility of occurrence. Therefore, when subjects' error proportions were similar to expected error proportions, this could be interpreted to reflect a frequency of occurrence variable. Therefore, the fact that both the normals and the aphasic groups did not differ from expected proportions for position of errors suggests that they did not perceive one half of the sentence as more difficult than another. No primacy or recency effects emerged. Deviation from expected error proportion distribution for grammatical classes was confirmed only in the normal group for the verb class. Here, the normals made a smaller proportion of errors than expected. One interpretation is that the verb is a more salient class for them when comprehending a sentence. While a sentence may contain several nouns, pronouns, adjectives, etc., the sentences used for testing contained only one verb. Therefore, the normal speakers may focus on the verb and rarely make an error in this class. The aphasic subjects, however, either did not adopt this strategy or did not recall the meaning of the verb since their error proportion was not disproportionately small. One might inquire then why the same trend of verbal saliency for the normals did not emerge for the VP linguistic constituent. Examining the error
Error patterns in auditory comprehension of aphasics
335
proportion data for individual grammatical classes suggests one possibility. The classess which contributed to the VP constituent were verb and adverb. A smaller error proportion was made on verbs but a larger error proportion was made on adverbs (recall this class contained the grammatical morpheme "not"). Apparently the two balanced each other, with the result that no overall difference was observed for the VP. That the error proportions for NP and PP were consistent with expected proportions for both normals and aphasics lends support to the idea that the errors on the grammatical class of noun did not occur on one linguistic constituent only. This finding is consistent with what linguistic theory would predict. If a noun is not recognized, an error will occur regardless of whether the noun occurs within an NP or a PP. If an NP or PP is not comprehended, despite recognition of the noun, this is probably due to failure to comprehend other grammatical classes or the semantic relationship expressed within the linguistic constituent. The error patterns observed for normals and aphasics were quite similar and the error patterns among three aphasic groups were similar in a sentence comprehension task. These findings suggest that similar mechanisms are being used for comprehension and that the mechanism for the aphasics, while less efficient, is qualitatively similar. Of course, more data and a larger sample of subjects are needed to substantiate these findings. Furthermore, various types of stimulus materials need to be used to assess the generality of the phenomenon. Stimulus materials might investigate a wider variety of sentence types, paragraph material, and so forth. Further insight into error patterns might also be obtained by examining subjects' performance with regard to the semantic relationships expressed within sentences.
SUMMARY
The error patterns of normal and aphasic adults on a sentence comprehension test were studied. For position of error, both normals and the three aphasic groups performed consistently with the expected error distribution. Neither sentence half was experienced as more difficult to process. All groups, both aphasic and normal, demonstrated error patterns for grammatical class and for surface structure linguistic constituent which were consistent with the expected error distributions. Exceptions were noted for the normals on the verb class and for the Wernicke's aphasics on the NP and VP linguistic constituents. Rank orderings of error proportions were similar across groups for both grammatical form class and linguistic constituent. The general findings lend support to the notion that, for auditory comprehension of sentences, aphasic patients differ quantitatively from normal adult speakers, but behave qualitatively similar to them.
336
C. M. Shewan REFERENCES
BLUMSTEIN, S.E. (1973) A Phonological Investigation of Aphasic Speech, Mouton, The Hague. FERGUSON, G.A. (1959) Statistical Analysis in Psychology and Education, McGraw-Hill, New York. GooDGLASS, H., GLEASON, ].B., and HYDE, M.R. (1970) Some dimensions of auditory language comprehension in aphasia, "J. Speech Hear. Res.," 13, 595-606. and KAPLAN, E. (1972) The Assessment of Aphasia and Related Disorders, Lea and Febiger, Philadelphia. - , KLEIN, H., CAREY, P., and JoNES, K.J. (1966) Specific semantic word categories in aphasia, "Cortex," 2, 74-89. PARISI, D., and PIZZAMIGLIO, L. (1970) Syntactic comprehension in aphasia, "Cortex," 6, 204-215. ScHOELL, H., }ENKINS, J.J., and }IMENEZ-PAB6N, E. (1964), Aphasia in Adults: Diagnosis. Prognosis and Treatment, Roeber Medical Division, Harper and Row, New York. - , - , and LANDIS, L. (1961) Relationship between auditory comprehension and word frequency in aphasia, "J. Speech Hear. Res.," 4, 30-36. SHEWAN, C.M., and CANTER, G.J. (1971) Effects of vocabulary, syntax, and sentence length on auditory comprehension in aphasic patients, "Cortex," 7, 209-226. SIEGEL, S. (1956) Nonparametric Statistics for the Behavioral Sciences, McGraw-Hill, New York. TROST, J.E., and CANTER, G.J. (1974) Apraxia of speech in patients with Broca's aphasia: a study of phoneme production accuracy and error patterns, "Brain and Language," 1, 63-79.
Cynthia M. Shewan, Ph. D., Associate Professor, Program in Communicative Disorders, University of Western Ontario, London, Ontario, Canada N6A 5C2.
Research studies and data reported from clinical tests for aphasia have demonstrated that adult aphasics have difficulty with auditory comprehension of sentence materials (Schuell, Jenkins and Jimenez-Pab6n, 1964; Goodglass and Kaplan, 1972). The severity of this comprehension deficit varies with the type of aphasia (Shewan and Canter, 1971); and aphasics, in general, show impaired performance when compared with normal speakers. These consistent differences have been found using quantitative measures. However, reports concerning qualitative differences in auditory comprehension between normals and aphasics and among types of aphasics present conflicting data. Several studies have examined subjects' correct response patterns. Parisi and Pizzamiglio (1970), studying the syntactic comprehension of four groups of aphasics, found quantitative differences among the groups; but when the Broca's and Wernicke's groups were compared, similar qualitative patterns emerged. Qualitative similarity among three aphasics groups was corroborated by Shewan and Canter (1971) who measured accuracy of comprehension for sentences varying in length, vocabulary difficulty, and syntactic complexity. By contrast, Goodglass, Gleason and Hyde (1970) found different patterns of performance among types of aphasics on comprehension tests which measured breadth of vocabulary, auditory sequential pointing-span, directional prepositions, and recognition of correct usage of prepositions. Perhaps this contrast among results should not be surprising, since the nature of the tasks was very different. Comprehending sentences is not a unitary task and is the result certainly of processing at many levels of the linguistic system, not to mention the contribution of contextual situational variables. The tasks used by Goodglass et al. (1970) did not tap comprehension of grammatical sentences, but seemed to be directed at some factors which might contribute to such an auditory comprehension task. These factors, in addition to others, are no doubt involved in comprehending sentence materials, but the nature of the interaction of these and other factors is not yet understood. Cortex (1976) 12, 325·336.
C. M. Shewan
326
That these studies examined only correct responses and that the tests measured different aspects of comprehension might have influenced the findings regarding qualitative similarity. To examine the qualitative aspects of sentence comprehension more closely, the present study examined the error patterns of aphasics and normals using a test for which quantitative (accuracy) data were known. The test used was the auditory comprehension test for sentences reported by Shewan and Canter (1971). Two specific questions were posed: ( 1) Are the error patterns of normal speakers and adult aphasics consistent with the expected error pattern, computed by statistical probability? (2) Do normal speakers and adult aphasics show qualitatively similar patterns of error? That is, when a normal speaker and an aphasic make errors, are they the same kind of errors?
MATERIAL AND METHOD
Subjects
The clinical group consisted of twenty-seven aphasics evenly distributed among the types of Broca's, Wernicke's, and amnesic aphasia. Each group contained patients who varied in severity of language impairment, being classified as mild, moderate, or severe. The aphasics were neurologically stable at the time of testing and were native speakers of English. The aphasics ranged in age from 20 to 83 years, with a mean age of 53.5 years. Their educational level ranged from 8 to 19 years of formal education, with a mean of 13.0 years. Etiology in the majority of patients was cerebrovascular disorder. The remaining patients were divided between traumatic and surgicalfor-tumor etiologies, with one brain abscess. Nine normal adults served as the control group. They were comparable with the aphasic patients for chronological age and educational level. They ranged in age from 22 to 71 years, with a mean age of 53.7 years. Education ranged from 6 to 20 years, with a mean of 12.6 years. Test Procedures The auditory comprehension test
A detailed description of the auditory comprehension test is available elsewhere (Shewan and Canter, 1971). Briefly, the test contained forty-two sentences which varied systematically in the parameters of length, vocabulary difficulty, and syntactic complexity. There were seven types of sentences (six sentences of each type) designed by altering each of the three parameters independently to a moderate or a hard degree of difficulty. The subject's task was to demonstrate comprehension of the orally presented sentences by pointing to its corresponding visual picture on a response plate. Each plate contained the correct pictorial representation of the sentence presented and three "dummy" pictures (error responses). Each of the dummy pictures contrasted with the presented sentence in only one critical item. For example, in the sentence The girl is reading a book, the contrasted critical items would be
Error patterns in auditory comprehension of aphasics
327
girl, read, and book. This was not possible for the longer sentences because it necessitated including more than four alternatives. For these, the selected contrasting items were noted. Procedures
Each subject was screened for adequate auditory sensitivity, auditory discrimination, visual perception, and minimal language comprehension. After completion of these screening tests, the auditory comprehension test was presented. The subject's responses were recorded and each error was classified according to both position and type as described below. Each dummy picture was examined and coded for position and type of error. The position of the error referred to whether the error occurred in the first half or the second half of the sentence. The type of error was coded for grammatical form class (noun, verb, etc.) and for surface structure linguistic constituent (noun phrase, verb phrase, and prepositional phrase). For example, for the presented sentence The girl is reading a book, one dummy picture represented The boy is reading a book. When a subject selected the latter picture the error would be coded as occurring in the first half of the sentence and an error on the grammatical class of noun and within a noun phrase linguistic constituent. Because each error position and type had different possibilities of occurrence, error frequencies were converted to error proportions for analysis of the results. For example, 58 errors were possible in the first sentence half position while 68 errors were possible in the second sentence half. Therefore, the proportions of possible errors for these frequencies were .460 and .540, respectively. The frequency data for grammatical form class and linguistic constituent were converted to proportions in the same manner.
RESULTS
To determine whether the aphasic group and the normal group differed from the expected error proportion for position of error, the data were analysed by two-tailed z-tests for differences between proportions. As shown in Table I the total aphasic group did not differ from the expected error proportion and the z-test confirmed this (p < .05). The normal group tended to make a smaller proportion of errors than expected in the first sentence half and the difference approached statistical significance (p = .06 ). When the total aphasic group was compared with the normals, a significant difference was found (p < .01), due to the smaller error proportion in the first sentence half made by the normal group. Whether the three aphasic groups demonstrated patterns similar to the expected error pattern was examined. As Table I shows none of the comparisons was significant (p > .05), suggesting that the aphasics, regardless of type, performed similarly with respect to error position on the sentence comprehension task. The grammatical form classes of critical items on which errors could occur were noun, verb, adjective, adverb, pronoun, and preposition. Here,
C. M. Shewan
328
TABLE I
Error Proportions for Error Position in Aphasic and Normal Groups and Expected Error Proportions
Sentence half First
Second
Expected error proportion
.460
.540
Normal group
.309
.691
Broca's
.527**
.473**
Amnesic
.510*
.490*
Wernicke's
.500*
.500*
Total
.511 **
.489**
Aphasic groups
'' Significantly different from Normal group (p ** Significantly different from Normal group (p
<
.05), suggesting that no one grammatical class was experienced as especially difficult to process. When the aphasics made errors in comprehension, these errors did not load more frequently than expected on a particular grammatical class. The normal group did not mirror the expected error distribution. A greater than expected error proportion occurred on the adverb class (p < .01) and error proportions for the grammatical classes of verb and pronoun approached statistical significance at the .05 level of confidence. When the rank orderings for error proportions by grammatical class were compared for the four subject groups, similar error patterns emerged. Kendall's coefficient of concordance of .936 (p < .01) indicated a very high degree of consistency across the subject groups. These rank orders, in addition, were very similar to the expected rank order which was from greatest to least error proportion: noun, verb, adverb, adjective, pronoun, and preposition. Since the aphasic groups' patterns were similar, their performances were combined. Performance of this total group and the normal group are shown with respect to the expected rank ordering for error proportion by grammatical class in Figure 1. With minor variations, the groups showed similar error patt~rns and, furthermore, these patterns were similar to the
C. M. Shewan
330
expected error distribution. The tendencies toward difference on some grammatical classes mentioned above for the normal group can be seen. How the error patterns related to surface structure linguistic constituents was examined. Because the sentences included only simple active affirmative declarative, negative, passive, and negative passive sentences, constituents such as embedded relative clauses were not present. The surface structure of each sentence thus could be analyzed using the linguistic constituents: noun phrase (NP), verb phrase (VP), and prepositional phrase (PP). Noun
• 60
. 50
"' "'"' z
0
. 40
-EXPECTED
.\
. '. \
"' "'a.
\
'
NORMAL
,_ • - ·.;(
APHASIC
\
\
w
0 i= 0 a. 0
\
o- - _ _.,·
\
\
\
• 30
\ \ \ \
. 20
\
\\ \
,~; /
\ \
• 10
/
' v
,/
/
/
~
..... ....
' ....
..........
.............
"'-,
....
',
........
......... NOUN
VER,B
ADVERB
ADJECTIVE
'
...........
·............
----
PRONOUN
PREPOSITION
GRAMM ATICAL CLASS
Fig. 1 - Error proportions for six grammatical classes for normal and total aphasic groups and the expected error proportions.
phrases took the grammatical form of (Determiner) + (Possessive) + (Adjective) + Noun. (Optional elements are included in parentheses). Prepositional phrases took the form of Preposition + NP, while verb phrases included Auxiliary + Verb. 1 These linguistic constituents were obviously related tc grammatical form class, but not synonymous with them, since an error or the grammatical class of noun could occur within either an NP or a PP 1 Examples of the linguistic constituents are: NP - the strong boy; VP - is fighting PP - to the road.
Error patterns in auditory comprehension of aphasics
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The rank ordering for expected error proportions for linguistic constituents, shown in Figure 2, from greatest to least was NP, VP, and PP. When the errors proportion values for the normal group were compared with the
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• 50
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EXPECTED NORMAL
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• 20
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NOUN PHRASE
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Fig. 2 - Error proportions for linguistic constituents for normal and total aphasic groups and the expected error proportions.
expected error proportions none of the comparisons reached statistical significance (p > .05) although the normals tended to make a smaller proportion of errors on NP and a larger proportion on PP. Figure 2 indicates that error proportions for the total aphasic group were similar to the expected error proportions and this was confirmed statistically (p > .05). Further, the total aphasic group and the normal group were similar for all three linguistic constituents (p > .05}. Whether each aphasic group would parallel this pattern was determined by comparisons between each of the three aphasic groups and the expected pattern. The amnesic and Broca's aphasics evidenced patterns similar to that expected. However, the Wernicke's aphasics differed from expected by making fewer errors on the NP (p < .01) and more errors on the VP (p < .05) constituents.
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How closely the four subject groups paralleled one another for rank ordering for error proportion on the linguistic constituents was examined by computing Kendall's coefficient of concordance. The value of .813 revealed a high degree of similarity, suggesting similar error patterns across groups. Some of the findings in the normal group, when compared with expected error proportions, tended toward statistical significance but did not reach the .05 level of confidence. In an attempt to determine if, in fact, there were differences, a group of ten additional normal subjects was tested. The suggested position of error effect reported above was not confirmed with this group. When grammatical class was considered only the class of verb was significantly different (p < .05) from the expected error proportion. The pronoun class which had approached significance in the original normal group did not differ from the expected error proportion. The adverb class which was significant above did not show a greater than expected error proportion.
DISCUSSION
The aphasics and normal subjects demonstrated similar patterns of performance when the type of error was examined. This was true for both the total aphasic group versus the normals and for the three aphasic groups and the normals. The rank orderings of error proportions for grammatical classes were similar across the four subject groups (W = .936) and between the total aphasic group and the normals (p = .83 ). Although not an entirely independent index from grammatical class, the patterns from considering linguistic constituents were also similar. The combined aphasic and the normal groups demonstrated identical rank orderings for error proportions. This rank ordering from most to least error proportion was NP, VP, and PP. The Broca's and amnesic aphasic groups' patterns were identical to the normal, but a reversal was found in the Wernicke's group for NP and VP. With regard to position of error, the original normal group demonstrated a tendency to make a smaller error proportion in the first half of the sentence. When additional subjects were tested, however, this tendency was not substantiated and a comparison between the aphasic and the latter normal group did not reach statistical significance. Each aphasic subgroup also showed no significant difference from this normal group (p > .05). This would suggest that aphasics and normals perform similarly with regard to position of error when responding to a sentence comprehension task. Further, neither half of a sentence appears more difficult than the other. This might be explained by the method of presentation which permitted the subject to view a response plate while the sentence was being presented auditorily. The memory load might thereby have been reduced; the subject did not have to retain the entire sentence, process it, and then search for the correct
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corresponding picture. For all sentences, except the passive and negative passive, he could eliminate possibilities while the sentence was being presented. This would have the effect of not requiring that he retain the first half of the sentence while the second half was being presented. The above findings suggest that aphasics and normals behave similarly when type and position error patterns are examined. Such data support some previous reports (Shewan and Canter, 1971) in the literature that aphasics and normals demonstrate qualitative similarity even though they differ on a quantitative basis. This qualitative similarity is evident whether accuracy data are examined or whether error patterns are investigated. Of course, this evidence was gathered using a task involving comprehension of spoken sentences. That this pattern would generalize for all tasks related to comprehension is too ecompassing an interpretation. For example, Goodglass et al. (1970) found different patterns between five aphasic groups and a normal control group when using tasks such as the Peabody Picture Vocabulary Test, pointing span, a directional preposition test, and a preposition preference test. As mentioned above, these tasks may contribute to auditory comprehension but are not isomorphic with a sentence comprehension task. Therefore, it appears that the nature of the task considerably affects the pattern of results and, consequently, whether the results support the notion of qualitative similarity between aphasic and normal speakers. Related to the issue of similar processing for normals and aphasics is the pattern of performance among various types of aphasics. For error type the similar rank orderings for grammatical classes suggested that the type of aphasia was not an influential factor in the processing of grammatical classes in the sentence. When linguistic constituents were considered the Broca's and amnesic aphasic groups showed the same rank ordering for error proportion. However, the Wernicke's group showed a reversal in the order of the NP and VP, having made a greater error proportion on the VP. One possible explanation for this result relates to syntactic complexity of the sentences. The Wernicke's aphasics had most difficulty understanding the syntactically complex sentences. Since syntactic complexity for the sentences was increased by applying the negative, passive, and negative passive transformations this had the effect of making the VP surface structure more complex. Therefore, the greater error proportion on the VP constituent was probably related to the increased transformational complexity. Proportions for position of errors for all three groups of aphasics were approximately equal for both sentence halves. Therefore, for error position, aphasics perform similarly regardless of type of aphasia. These data support the generalization that various types of aphasics perform similarly in comprehending oral sentences. Quantitatively, comprehension varies among types of aphasia but the types of errors are similar. The one contrast found in the Wernicke's group should be investigated
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further since syntactic complexity may have been confounded with complexity of the VP. One could develop sentences which increase the complexity of the surface structure VP consistent without requiring greater transformational complexity. Such a task would serve to differentiate whether the difficulty with the VP was related to a transformational component or to the surface structure. The general conformity of aphasic error patterns to expected pattern suggested that, at least for the forms studied, one linguistic form was no more difficult to process than another. When an aphasic fails to comprehend a sentence, then, the error does not appear to be occasioned by difficulty processing any one grammatical class or any one surface structure linguistic constituent. Moreover, a fatigue or attention factor does not seem to explain the errors. It is well known that frequency of occurrence of constructs in the English language is relevant to aphasics' verbal performance. This parameter has been found to influence aphasic behavior in many aspects of the language system, for example, phonological production (Blumstein, 1973; Trost and Canter, 1974), lexical recognition (Schuell, Jenkins and Landis, 1961), and syntactic comprehension (Shewan and Canter, 1971). Therefore, it seemed important to determine if a frequency effect was operative ±or the normal and aphasic groups. The frequency effect under consideration here dealt with the frequency of items in the tested sentence corpus rather than the overall frequency in the English language. The expected error proportions, calculated for both position and type of error, reflected the error distribution exclusively in terms of possibility of occurrence. Therefore, when subjects' error proportions were similar to expected error proportions, this could be interpreted to reflect a frequency of occurrence variable. Therefore, the fact that both the normals and the aphasic groups did not differ from expected proportions for position of errors suggests that they did not perceive one half of the sentence as more difficult than another. No primacy or recency effects emerged. Deviation from expected error proportion distribution for grammatical classes was confirmed only in the normal group for the verb class. Here, the normals made a smaller proportion of errors than expected. One interpretation is that the verb is a more salient class for them when comprehending a sentence. While a sentence may contain several nouns, pronouns, adjectives, etc., the sentences used for testing contained only one verb. Therefore, the normal speakers may focus on the verb and rarely make an error in this class. The aphasic subjects, however, either did not adopt this strategy or did not recall the meaning of the verb since their error proportion was not disproportionately small. One might inquire then why the same trend of verbal saliency for the normals did not emerge for the VP linguistic constituent. Examining the error
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proportion data for individual grammatical classes suggests one possibility. The classess which contributed to the VP constituent were verb and adverb. A smaller error proportion was made on verbs but a larger error proportion was made on adverbs (recall this class contained the grammatical morpheme "not"). Apparently the two balanced each other, with the result that no overall difference was observed for the VP. That the error proportions for NP and PP were consistent with expected proportions for both normals and aphasics lends support to the idea that the errors on the grammatical class of noun did not occur on one linguistic constituent only. This finding is consistent with what linguistic theory would predict. If a noun is not recognized, an error will occur regardless of whether the noun occurs within an NP or a PP. If an NP or PP is not comprehended, despite recognition of the noun, this is probably due to failure to comprehend other grammatical classes or the semantic relationship expressed within the linguistic constituent. The error patterns observed for normals and aphasics were quite similar and the error patterns among three aphasic groups were similar in a sentence comprehension task. These findings suggest that similar mechanisms are being used for comprehension and that the mechanism for the aphasics, while less efficient, is qualitatively similar. Of course, more data and a larger sample of subjects are needed to substantiate these findings. Furthermore, various types of stimulus materials need to be used to assess the generality of the phenomenon. Stimulus materials might investigate a wider variety of sentence types, paragraph material, and so forth. Further insight into error patterns might also be obtained by examining subjects' performance with regard to the semantic relationships expressed within sentences.
SUMMARY
The error patterns of normal and aphasic adults on a sentence comprehension test were studied. For position of error, both normals and the three aphasic groups performed consistently with the expected error distribution. Neither sentence half was experienced as more difficult to process. All groups, both aphasic and normal, demonstrated error patterns for grammatical class and for surface structure linguistic constituent which were consistent with the expected error distributions. Exceptions were noted for the normals on the verb class and for the Wernicke's aphasics on the NP and VP linguistic constituents. Rank orderings of error proportions were similar across groups for both grammatical form class and linguistic constituent. The general findings lend support to the notion that, for auditory comprehension of sentences, aphasic patients differ quantitatively from normal adult speakers, but behave qualitatively similar to them.
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BLUMSTEIN, S.E. (1973) A Phonological Investigation of Aphasic Speech, Mouton, The Hague. FERGUSON, G.A. (1959) Statistical Analysis in Psychology and Education, McGraw-Hill, New York. GooDGLASS, H., GLEASON, ].B., and HYDE, M.R. (1970) Some dimensions of auditory language comprehension in aphasia, "J. Speech Hear. Res.," 13, 595-606. and KAPLAN, E. (1972) The Assessment of Aphasia and Related Disorders, Lea and Febiger, Philadelphia. - , KLEIN, H., CAREY, P., and JoNES, K.J. (1966) Specific semantic word categories in aphasia, "Cortex," 2, 74-89. PARISI, D., and PIZZAMIGLIO, L. (1970) Syntactic comprehension in aphasia, "Cortex," 6, 204-215. ScHOELL, H., }ENKINS, J.J., and }IMENEZ-PAB6N, E. (1964), Aphasia in Adults: Diagnosis. Prognosis and Treatment, Roeber Medical Division, Harper and Row, New York. - , - , and LANDIS, L. (1961) Relationship between auditory comprehension and word frequency in aphasia, "J. Speech Hear. Res.," 4, 30-36. SHEWAN, C.M., and CANTER, G.J. (1971) Effects of vocabulary, syntax, and sentence length on auditory comprehension in aphasic patients, "Cortex," 7, 209-226. SIEGEL, S. (1956) Nonparametric Statistics for the Behavioral Sciences, McGraw-Hill, New York. TROST, J.E., and CANTER, G.J. (1974) Apraxia of speech in patients with Broca's aphasia: a study of phoneme production accuracy and error patterns, "Brain and Language," 1, 63-79.
Cynthia M. Shewan, Ph. D., Associate Professor, Program in Communicative Disorders, University of Western Ontario, London, Ontario, Canada N6A 5C2.
