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Dichotic Listening in Stuttering and Dyslalic Children
DICHOTIC LISTENING IN STUTTERING AND DYSLALIC CHILDREN Neil Slorach and Bonnie Noehr (Speech Therapy Department, University of Queensland)
Studies in dichotic listening have consistently shown that a greater accuracy in reporting verbal material presented to the right ear denotes lateralization of language function to the left cerebral hemisphere (Kimura, 1961; Bryden, 1964; Dirks, 1964). Kimura (1963) presented evidence of the right ear effect at four years of age, and Nagafuchi (1971) reported right ear preference in children as young as three years. Geffner and Hochberg ( 1971) reported that children from low socio-economic areas showed less of a right ear effect than did their counter-parts from higher socio-economic groups. Although speech pathologists have long considered that laterality effects may be related to specific speech disorders, e.g. stuttering, there is a dearth of studies of dichotic listening tasks among speech defective children. The theory that stutterers do not develop a sufficient margin of cerebral dominance for speech was originated by Steir ( 1911) and Sachs ( 1924) and received its early impetus through the writings of Orton ( 1927) and Travis (1931). Research into this theory was previously limited by the poor reliability of measures, such as handedness, available to assess lateralization. Laterality factors were also studied in relation to articulation disorders by Johnson and House (1937) who compared dyslalic children with normal speakers on tests of ocular and hand dominance. They concluded that handedness as measured tended to be related to severe articulatory defects but that the findings with regard to eyedness were not significant. In contrast to these :findings, Everhart (1953) found no significant differences between articulation defects and handedness in his subjects, children in grades one to six. The present investigation was designed to explore the hemispheric lateralization for speech among right handed stuttering and dyslalic children by administration of the dichotic digits test. Since the Wada ( 1960) test ' Supported by the grants from the Mayne Bequest Fund, University of Queensland and the Australian University Commission.
296
N. Slorach and B. Noehr
has shown that there is a greater probability of. right hemisphere dominance for speech in left handed and ambidextrous subjects, the present study was confined to overtly right handed subjects. The study was intended to show whether stutterers differed from dyslalics and normals in the establishment of cerebral lateralization of language function.
MATERIAL AND METHOD
Subjects Three experimental groups were formed, one consisting of stutterers, one of dyslalics, and a third control group of normal speakers. The subjects were selected from primary schools in the Brisbane (Australia) metropolitan area. Children in grades one, two and three were screened for defective speech, and each child in question was interviewed by a Senior Speech Therapist to confirm diagnostic classification. The criteria for selection included the following points: (a) normal hearing, (b) right handedness, (c) absence of structural deviations of the oral musculature, (d) at least average scholastic attainments based on teacher assessment, and (e) confirmed diagnostic category. The final sample consisted of forty-five children drawn from the middle to higher socio-economic areas with N = 15 in each of the diagnostic categories. The range in ages was from 6.25 years to 9.0 years, with a mean age of 7.4 years. Each category included thirteen males and two females.
Procedure The dichotic listening task consisted of twenty-four sets of three digit pairs presented at .5 second intervals with a fifteen second pause between sets for recall of digits. The stimulus material was presented through stereophonic headphones (Kenwood HS 3/8) from an Aiwa cassette stereophonic tape recorder (Model TP-1104 ). Each subject was given a series of trials to familiarize him with the procedure before testing was commenced. The first series of twentyfour digit sets were presented under a free recall procedure. The entire series was then repeated using an instructed order of report under which condition the subjects were required to report the left channel before the right on the first twelve sets and the right channel before the left on the second twelve sets. Because of the young age of the subjects, the words "Right" and "Left" were replaced by manual tapping on the earphones and the words "all the numbers from this side first" used in the instructions. The subjects' responses were given orally and recorded by the examiner directly on to the master scoring sheets. In order to control for any inequalities in the sound levels of the two channels the earphones were reversed for alternate subjects. In scoring the test, subjects received one point for each number reported from the corresponding ear of presentation.
297
Dichotic listening in stuttering and dyslalic children
RESULTS
The results were scored for the conditions of: (a) free recall, (b) ordered recall, (c) first and second half spans for right and left ears, and (d) the numerical difference between the two ears under free recall - the interaural difference score (I.A.D.). The mean scores of the free recall condition for each diagnostic category are shown in Table I. TABLE I
Mean Scores for Each Group - Free Recall
Right ear Left ear
Stutterers
Dyslalics
Normals
43.53 35.25
40.13 34.93
52.00 33.93
A two-way analysis of variance with repeated measures showed a significant difference in favour of the right ear across the three groups (F = 16.07, p = <.001). The ordered recall condition gave mean scores which are shown in Table II. TABLE II
Mean Scores for Each Group - Ordered Recall
Stutterers Right ear Left ear
41.40 33.40
Dyslalics
Normals
37.40 38.80
48.40 36.47
A two-way analysis of variance with repeated measures on one factor showed an over-all right ear superiority, however, a significant interaction indicates that this effect was not observed across all groups. The dyslalics did not show the right ear superiority under ordered recall (Table II). Using the half-span information measure the factors analysed were the "between subjects" factor (diagnostic category) and the two "within subiects" factors - half span and ear. Mean scores for the twelve experimental conditions are shown in Table III.
N. Slorach and B. Noehr
298
TABLE III
Mean Scores for Each Group - First and Second Half Spans Dyslalics
Stutterers
Right ear Left ear
1st h/s 25.87 21.13
2nd h/s 15.80 12.27
1st h/s 22.93 22.47
2nd h/s 14.40 14.47
Normals 1st h/s 30.27 23.07
2nd h/s 18.07 13.53
A three-way analysis of variance with repeated measures on two factors showed a significant result in favour of the half span reported first (F = 57.64, p = <.001). The main effect for the "ear" factor was again significant showing an overall right ear superiority (F = 13.31, p = < .01), but significant interactions between the "diagnostic categories and ears" and "half-spans and ears" indicated that while stutterers and normals maintained the right ear superiority in both first and second half spans, the dyslalics did not show this effect in either the first or second half spans (Table III). The interaural difference scores were analysed by analysis of variance but no significant differences were observed among the three groups (F = 1.71, p = n.s.). Even when attention is restricted to the stutterers and normals, the I.A.D. scores were not significantly different when analysed by "t" test (t = 1.47, p = n.s.).
DISCUSSION
The data from the present investigation demonstrate several important points. First of all, the right ear effect was found under free recall in each of the diagnostic categories, stutterers, dyslalics and normals. This finding is in agreement with previous studies on verbal dichotic tasks (Kimura, 1961, 1963; Bryden, 1963; Dirks, 1964). Secondly, the right ear preference was maintained under ordered recall by the stutterers and normals, but not by the dyslalics. Further, the right ear effect was carried through the first and second half spans by the stutterers and normals but not by the dyslalics. The stutterers of the present investigation showed no evidence of any atypical pattern of response on the dichotic task. These findings do not support the hypothesis that stuttering is related to a lack of clearly defined cerebral dominance, as inferred from the Orton-Travis theory. Theorists who have pursued the possibility that stutterers have a constitutional predisposition based on some neurological difference will need to investigate areas other than that of cerebral dominance. The saine conclusion was reached by Quinn (1972) who found no significant differences between his
Dichotic listening in stuttering and dyslalic children
299
right handed stuttering adults and normal controls. These findings are at variance with those of Curry and Gregory ( 1969) who found significant differences, with 55% of their stuttering adults having higher left ear scores. It may well be that certain sub-groups of stutterers do show atypical patterns of lateralization but it cannot be inferred that all stutterers have this factor as an aetiological basis. Further studies are required using overtly left handed and ambidextrous stuttering subjects to clarify this point. A point of particular interest regarding the stutterers was the fact that none of them had any impediment or difficulty in reporting the digits, although they all were stuttering in general conversation prior to the test. This was probably due to the fact that the test responses were of a non-propositional nature and there was no communicative stress in the reporting of the digits. Alternatively, the anticipation of the next series, and the attentional aspect of binaural stimulation could have acted as a distraction, leading to increased fluency. The dyslalic children who failed to maintain the right ear effect under ordered recall need further consideration. Kimura (1961, 1963) proposed a perceptual model of dichot1c ;istening and Bryden ( 1963) elaborated on a perceptual threshold model. Within this framework, stimuli from the right ear activate the appropriate trace systems within the dominant hemisphere, leading to a right ear preference for reporting verbal material. The same mechanism is thought to account for the maintenance of the right ear effect under the ordered recall condition. Since the dyslalics were not able to maintain this effect under ordered recall, it is possible that these subjects have difficulty in developing sufficiently strong neural trace patterns within the dominant hemisphere. On this basis, dyslalic children may have a true perceptual deficit related to the maturation of the auditory system. Such a proposal is congruent with the clinical observation that functional articulatory disorders are, in the majority of cases, known to improve with maturation. Speech pathologists might well consider that treatment for dyslalia could be programmed to include amplification of the auditory signals which would result in stronger neural trace patterns being activated within the dominant hemisphere. SuMMARY
In a dichotic listening experiment, three groups of right handed subjects, stutterers, dyslalics and normal children were found to have right ear preference for the reporting of verbal material. The stutterers showed no significant differences from the other two groups, indicating that cerebral dominance is not a significant factor in the aetiology of the disorder in right handed subjects. The dyslalic children were not able to maintain the right ear preference under ordered recall. This important finding has implications for the understanding of the aetiology of functional articulatory disorders.
300
N. Slorach and B. Noehr
Acknowledgments. The authors wish to thank Mrs. Madge Horan and Roger and Catherine Brown, Psychology Department, and Mrs. E.C. Usher, Head, Department of Speech Therapy, for their assistance with various aspects of the research. REFERENCES BRYDEN, M. P. (1964) Manipulation of strategies of report in dichotic listening, "Canad. J. Psycho!.," 18, 126-138. CuRRY, F. W., and GREGORY, H. H. (1969) The performance of stutterers on dichotic listening task thought to reflect cerebral dominance, "J. Sp. Hear. Res.," 12, 73-82. DIRKS, D. (1964} Perception of dichotic and monaural verbal material and cerebral dominance for speech, "Acta Oto-Laryn.," 58, 73-80. EvERHART, R. (1953) The relationship between articulation and other developmental factors in childhood, "J. Sp. Hear. Dis.," 18, 332-338. GEFFNER, D. S., and HoCHBERG, I. (1971) Ear laterality performance of children from low and middle socio-economic levels on a verbal dichotic listening task, "Cortex," 7, 193-203. JoHNSON, W., and HousE, E. (1973) Certain laterality characteristics of children with articulatory disorders, "Elem. Sch. J.," 38, 52. KIMURA, D. (1961) Cerebral dominance and the perception of verbal stimuli, "Canad. J. Psychol," 3, 166-171. (1963) Speech lateralization in young children as determined by an auditory test, "J. Comp. Physiol. Psycho!.," 56, 899-902. NAGAFUCHI, M. (1970) Development of dichotic and monaural hearing abilities in young children, "Acta Oto-Laryn.," 69, 409-415. ORTON, S. T. (1927} Studies in stuttering, "Arch. Neurol. Psychiat.," 18, 671-672. QUINN, P. (1972) Stuttering, cerebral dominance and the dichotic word test, "Med. J. Aust.," 2, 639-643. SACHS, J. (1924) In The Nature of Stuttering, C. Van Riper. Englewood Cliffs, N.J.: PrenticeHall, 1971. STEIR, E. (1911) In The Nature of Stuttering, C. Van Riper. Englewood Cliffs, N.J.: PrenticeHall, 1971. TRAVIS, L. E. (1931) Speech Pathology, Appleton-Century-Crofts, New York. WADA, J. (1960) Intracarotid injection of sodium amytal for the lateralization of cerebral speech dominance, "J. Neurosurg.," 17, 266-282.
Neil Slorach, Department of Speech Therapy, University of Queensland, 6 Rock Street, St. Lucia, Brisbane 4067, Australia.
Studies in dichotic listening have consistently shown that a greater accuracy in reporting verbal material presented to the right ear denotes lateralization of language function to the left cerebral hemisphere (Kimura, 1961; Bryden, 1964; Dirks, 1964). Kimura (1963) presented evidence of the right ear effect at four years of age, and Nagafuchi (1971) reported right ear preference in children as young as three years. Geffner and Hochberg ( 1971) reported that children from low socio-economic areas showed less of a right ear effect than did their counter-parts from higher socio-economic groups. Although speech pathologists have long considered that laterality effects may be related to specific speech disorders, e.g. stuttering, there is a dearth of studies of dichotic listening tasks among speech defective children. The theory that stutterers do not develop a sufficient margin of cerebral dominance for speech was originated by Steir ( 1911) and Sachs ( 1924) and received its early impetus through the writings of Orton ( 1927) and Travis (1931). Research into this theory was previously limited by the poor reliability of measures, such as handedness, available to assess lateralization. Laterality factors were also studied in relation to articulation disorders by Johnson and House (1937) who compared dyslalic children with normal speakers on tests of ocular and hand dominance. They concluded that handedness as measured tended to be related to severe articulatory defects but that the findings with regard to eyedness were not significant. In contrast to these :findings, Everhart (1953) found no significant differences between articulation defects and handedness in his subjects, children in grades one to six. The present investigation was designed to explore the hemispheric lateralization for speech among right handed stuttering and dyslalic children by administration of the dichotic digits test. Since the Wada ( 1960) test ' Supported by the grants from the Mayne Bequest Fund, University of Queensland and the Australian University Commission.
296
N. Slorach and B. Noehr
has shown that there is a greater probability of. right hemisphere dominance for speech in left handed and ambidextrous subjects, the present study was confined to overtly right handed subjects. The study was intended to show whether stutterers differed from dyslalics and normals in the establishment of cerebral lateralization of language function.
MATERIAL AND METHOD
Subjects Three experimental groups were formed, one consisting of stutterers, one of dyslalics, and a third control group of normal speakers. The subjects were selected from primary schools in the Brisbane (Australia) metropolitan area. Children in grades one, two and three were screened for defective speech, and each child in question was interviewed by a Senior Speech Therapist to confirm diagnostic classification. The criteria for selection included the following points: (a) normal hearing, (b) right handedness, (c) absence of structural deviations of the oral musculature, (d) at least average scholastic attainments based on teacher assessment, and (e) confirmed diagnostic category. The final sample consisted of forty-five children drawn from the middle to higher socio-economic areas with N = 15 in each of the diagnostic categories. The range in ages was from 6.25 years to 9.0 years, with a mean age of 7.4 years. Each category included thirteen males and two females.
Procedure The dichotic listening task consisted of twenty-four sets of three digit pairs presented at .5 second intervals with a fifteen second pause between sets for recall of digits. The stimulus material was presented through stereophonic headphones (Kenwood HS 3/8) from an Aiwa cassette stereophonic tape recorder (Model TP-1104 ). Each subject was given a series of trials to familiarize him with the procedure before testing was commenced. The first series of twentyfour digit sets were presented under a free recall procedure. The entire series was then repeated using an instructed order of report under which condition the subjects were required to report the left channel before the right on the first twelve sets and the right channel before the left on the second twelve sets. Because of the young age of the subjects, the words "Right" and "Left" were replaced by manual tapping on the earphones and the words "all the numbers from this side first" used in the instructions. The subjects' responses were given orally and recorded by the examiner directly on to the master scoring sheets. In order to control for any inequalities in the sound levels of the two channels the earphones were reversed for alternate subjects. In scoring the test, subjects received one point for each number reported from the corresponding ear of presentation.
297
Dichotic listening in stuttering and dyslalic children
RESULTS
The results were scored for the conditions of: (a) free recall, (b) ordered recall, (c) first and second half spans for right and left ears, and (d) the numerical difference between the two ears under free recall - the interaural difference score (I.A.D.). The mean scores of the free recall condition for each diagnostic category are shown in Table I. TABLE I
Mean Scores for Each Group - Free Recall
Right ear Left ear
Stutterers
Dyslalics
Normals
43.53 35.25
40.13 34.93
52.00 33.93
A two-way analysis of variance with repeated measures showed a significant difference in favour of the right ear across the three groups (F = 16.07, p = <.001). The ordered recall condition gave mean scores which are shown in Table II. TABLE II
Mean Scores for Each Group - Ordered Recall
Stutterers Right ear Left ear
41.40 33.40
Dyslalics
Normals
37.40 38.80
48.40 36.47
A two-way analysis of variance with repeated measures on one factor showed an over-all right ear superiority, however, a significant interaction indicates that this effect was not observed across all groups. The dyslalics did not show the right ear superiority under ordered recall (Table II). Using the half-span information measure the factors analysed were the "between subjects" factor (diagnostic category) and the two "within subiects" factors - half span and ear. Mean scores for the twelve experimental conditions are shown in Table III.
N. Slorach and B. Noehr
298
TABLE III
Mean Scores for Each Group - First and Second Half Spans Dyslalics
Stutterers
Right ear Left ear
1st h/s 25.87 21.13
2nd h/s 15.80 12.27
1st h/s 22.93 22.47
2nd h/s 14.40 14.47
Normals 1st h/s 30.27 23.07
2nd h/s 18.07 13.53
A three-way analysis of variance with repeated measures on two factors showed a significant result in favour of the half span reported first (F = 57.64, p = <.001). The main effect for the "ear" factor was again significant showing an overall right ear superiority (F = 13.31, p = < .01), but significant interactions between the "diagnostic categories and ears" and "half-spans and ears" indicated that while stutterers and normals maintained the right ear superiority in both first and second half spans, the dyslalics did not show this effect in either the first or second half spans (Table III). The interaural difference scores were analysed by analysis of variance but no significant differences were observed among the three groups (F = 1.71, p = n.s.). Even when attention is restricted to the stutterers and normals, the I.A.D. scores were not significantly different when analysed by "t" test (t = 1.47, p = n.s.).
DISCUSSION
The data from the present investigation demonstrate several important points. First of all, the right ear effect was found under free recall in each of the diagnostic categories, stutterers, dyslalics and normals. This finding is in agreement with previous studies on verbal dichotic tasks (Kimura, 1961, 1963; Bryden, 1963; Dirks, 1964). Secondly, the right ear preference was maintained under ordered recall by the stutterers and normals, but not by the dyslalics. Further, the right ear effect was carried through the first and second half spans by the stutterers and normals but not by the dyslalics. The stutterers of the present investigation showed no evidence of any atypical pattern of response on the dichotic task. These findings do not support the hypothesis that stuttering is related to a lack of clearly defined cerebral dominance, as inferred from the Orton-Travis theory. Theorists who have pursued the possibility that stutterers have a constitutional predisposition based on some neurological difference will need to investigate areas other than that of cerebral dominance. The saine conclusion was reached by Quinn (1972) who found no significant differences between his
Dichotic listening in stuttering and dyslalic children
299
right handed stuttering adults and normal controls. These findings are at variance with those of Curry and Gregory ( 1969) who found significant differences, with 55% of their stuttering adults having higher left ear scores. It may well be that certain sub-groups of stutterers do show atypical patterns of lateralization but it cannot be inferred that all stutterers have this factor as an aetiological basis. Further studies are required using overtly left handed and ambidextrous stuttering subjects to clarify this point. A point of particular interest regarding the stutterers was the fact that none of them had any impediment or difficulty in reporting the digits, although they all were stuttering in general conversation prior to the test. This was probably due to the fact that the test responses were of a non-propositional nature and there was no communicative stress in the reporting of the digits. Alternatively, the anticipation of the next series, and the attentional aspect of binaural stimulation could have acted as a distraction, leading to increased fluency. The dyslalic children who failed to maintain the right ear effect under ordered recall need further consideration. Kimura (1961, 1963) proposed a perceptual model of dichot1c ;istening and Bryden ( 1963) elaborated on a perceptual threshold model. Within this framework, stimuli from the right ear activate the appropriate trace systems within the dominant hemisphere, leading to a right ear preference for reporting verbal material. The same mechanism is thought to account for the maintenance of the right ear effect under the ordered recall condition. Since the dyslalics were not able to maintain this effect under ordered recall, it is possible that these subjects have difficulty in developing sufficiently strong neural trace patterns within the dominant hemisphere. On this basis, dyslalic children may have a true perceptual deficit related to the maturation of the auditory system. Such a proposal is congruent with the clinical observation that functional articulatory disorders are, in the majority of cases, known to improve with maturation. Speech pathologists might well consider that treatment for dyslalia could be programmed to include amplification of the auditory signals which would result in stronger neural trace patterns being activated within the dominant hemisphere. SuMMARY
In a dichotic listening experiment, three groups of right handed subjects, stutterers, dyslalics and normal children were found to have right ear preference for the reporting of verbal material. The stutterers showed no significant differences from the other two groups, indicating that cerebral dominance is not a significant factor in the aetiology of the disorder in right handed subjects. The dyslalic children were not able to maintain the right ear preference under ordered recall. This important finding has implications for the understanding of the aetiology of functional articulatory disorders.
300
N. Slorach and B. Noehr
Acknowledgments. The authors wish to thank Mrs. Madge Horan and Roger and Catherine Brown, Psychology Department, and Mrs. E.C. Usher, Head, Department of Speech Therapy, for their assistance with various aspects of the research. REFERENCES BRYDEN, M. P. (1964) Manipulation of strategies of report in dichotic listening, "Canad. J. Psycho!.," 18, 126-138. CuRRY, F. W., and GREGORY, H. H. (1969) The performance of stutterers on dichotic listening task thought to reflect cerebral dominance, "J. Sp. Hear. Res.," 12, 73-82. DIRKS, D. (1964} Perception of dichotic and monaural verbal material and cerebral dominance for speech, "Acta Oto-Laryn.," 58, 73-80. EvERHART, R. (1953) The relationship between articulation and other developmental factors in childhood, "J. Sp. Hear. Dis.," 18, 332-338. GEFFNER, D. S., and HoCHBERG, I. (1971) Ear laterality performance of children from low and middle socio-economic levels on a verbal dichotic listening task, "Cortex," 7, 193-203. JoHNSON, W., and HousE, E. (1973) Certain laterality characteristics of children with articulatory disorders, "Elem. Sch. J.," 38, 52. KIMURA, D. (1961) Cerebral dominance and the perception of verbal stimuli, "Canad. J. Psychol," 3, 166-171. (1963) Speech lateralization in young children as determined by an auditory test, "J. Comp. Physiol. Psycho!.," 56, 899-902. NAGAFUCHI, M. (1970) Development of dichotic and monaural hearing abilities in young children, "Acta Oto-Laryn.," 69, 409-415. ORTON, S. T. (1927} Studies in stuttering, "Arch. Neurol. Psychiat.," 18, 671-672. QUINN, P. (1972) Stuttering, cerebral dominance and the dichotic word test, "Med. J. Aust.," 2, 639-643. SACHS, J. (1924) In The Nature of Stuttering, C. Van Riper. Englewood Cliffs, N.J.: PrenticeHall, 1971. STEIR, E. (1911) In The Nature of Stuttering, C. Van Riper. Englewood Cliffs, N.J.: PrenticeHall, 1971. TRAVIS, L. E. (1931) Speech Pathology, Appleton-Century-Crofts, New York. WADA, J. (1960) Intracarotid injection of sodium amytal for the lateralization of cerebral speech dominance, "J. Neurosurg.," 17, 266-282.
Neil Slorach, Department of Speech Therapy, University of Queensland, 6 Rock Street, St. Lucia, Brisbane 4067, Australia.