Neuropsychologia.
1976,
Vol.
14. pp.
363-370.
Pergamon
Press.
Printed
in England.
CEREBRAL LATERALIZATION OF SPEECH : THE EFFECTS OF AGE, SEX, RACE, AND SOCIOECONOMIC CLASS* THOM BOROWY~
and RONALD GOEBEL$
Department of Clinical Psychology,
and Neuropsychology Laboratory. University of Florida, Gainesville, Florida, 32601, U.S.A. (Received 16 Jltne 1975)
Ah&act--This study attempted to discover what effects the variables of age, sex, race, and socioeconomic class have on ear asymmetry. A significant right ear superiority was found at all ages studied; 5, 7, 9, and 11 yr of age. Males were found to develop essentially the same as females and white children develop the same as black children on dichotic listening tasks, which were utilized as the criteria measures of ear asymmetry. There were significant social class differences found, the middle class s’s showing a significantly greater degree of asymmetry than their lower class counterparts. All ages in both social classes, however, showed a significant right-left difference in favor of the right ear. Precise methodological controls were utilized and their relevance discussed. Possible explanations for discrepant findings between the present study and other research are also discussed.
INTRODUCTION FUNCTIONAL asymmetry
of the human brain in relation to speech has been studied by utilization of the dichotic listening technique [l-l 11. This technique involves disparate pairs of auditory stimuli (spoken words, digits, nonsense syllables, musical rhythms) which are presented simultaneously to each ear. These stimulus pairs have been presented in clusters of varying lengths and rates, and various methods of subject reporting have been utilized [l, 2, 7, 121. KIMURA [7, 81 posits that auditory stimuli presented to the ears are transmitted with greater efficiency to the contralateral hemisphere. Therefore, if speech is lateralized in the left hemisphere, verbal stimuli presented to the right ear should be more efficiently assimilated [6-81. Conversely, when non-verbal auditory stimuli are utilized, stimuli presented to the left ear (right hemisphere) are reported more often [5, 131. These “preferences” theoretically reflect a cerebral dominance for the perception and/or integration of auditory stimuli. The present study utilizes the dichotic listening technique to discern the relative importance of the variables of Age. Sex, Socioeconomic Class, and Race in relation to the ear asymmetry phenomenon. KIMURA [7] found evidence of an ear asymmetry in children as early as age 4, with the greatest ear asymmetry effects occurring in the youngest subjects. NAGAFUCHI [9] reported an even earlier age of onset of ear asymmetry based on the observation that his 3-yr-old subjects showed a significant difference between their right and left ears. Likewise, he also *Supported by NIH (NS-08209). TPresent address: Department of Psychology, University of North Florida, Jacksonville, U.S.A. fveterans Administration Hospital. Shreveport, La., U.S.A. 363
Florida 32216,
3h 1
THOM BOROIVY and RONALD GOFBEI
found that the greatest asymmetries were present in his youngest subjects. Two recent studies [I, 171 found that a significant ear asymmetry did not develop until much later. at around 9 or 10 yr of age. Further, the amount of asymmetry seemed to increase as the age increased. A possible explanation for these discrepancies can be made in relation to stimulus dificulty; the initial studies using a simple stimulus task and the later stud& employing relatively more difficult tasks. There has also been conflicting evidence as to whether malts and females develop difrerently with regard to ear asymmetry. Various btudies [3, 4, 8, 91 have found evidence supporting an earlier development of ear asymmetry in females than in males. Still other studies [I. 2, 7, 131 have found no significant sex differences in ear asymmetry. in view of the highly contradictory results in the literature and the evidence of great disparities between males and females in a number of behavioral and learning disorders [18], this variable was included in the present study. GEFFNER and HOCHRERG [22] investigated the question of social class in relation to ear asymmetry across different ages [4-71, and found right ear superiority “for all age groups from the middle socioeconomic level and for the 7-yr-old subjects from the low socioeconomic levels”. They concluded that, “the present findings agree with those reported by KIMURA (1967) that although children from low socioeconomic areas demonstrate I-ight ear superiority for verbal stimuli, the degree of such superiority is less than that observed from middle socioeconomic areas” [22]. The selection of subjects in GefTner and Hochberg’s study ~11ay have had a significant effect 011 their findings. They state, “the large majority of children from the low socioeconomic group were of black origin. whereas the children from the middle socioeconomic group were predominantly white. What, if any, influence these factors may have contributed to differences in performance cannot be ascertained from the present study. It is more than likely that various factors were operating to influence the performance of these children. The nature of these factors and their consequent influences are not clear” [22]. Due to the high degree of segregation in the make up of their social classes, it cannot bc determined from their study whether the results they found were due to socioeconomic differences or to racial differences.
METHOD Subjects Gs) m the present study were 120 children (60 males and 60 females) enrolled in the pubhc school system of Alachua County, Florida. Handedness was determined by verbal report and performance on manual tasks involving throwing, catching, kicking and sighting. Socioeconomic status was determined from school records concerning the amount of family income, occupation of the parents, and tea&et evaluation. The experimental design contained 24 individual cells comprised of 5 Ss each (Table 1). Dichotic listerlittg tasks
The dichotic listening task consisted of 30 sets of 3 digit pairs which were presented in such a way that the 2 digits in each pair were presented simultaneously, one to the right and one to the left ear. Each trial consisted of 3 pairs of digits which were presented at the rate of 2 pairs/second. The digits heard on the tape were l-5, 8, 9, 10, 12-15, 18. The tape had been created so that the digits in each pair had the identical onset time and the same loudness. Since there are auditory differences in spoken digits which cannot be controlled for acoustically, other control methods were necessary. Therefore, the tape was constructed so that the first 15 trials were identical to the second 15 trials. However, after trial 15 the headphones were revcrscd on each S so that half the Ss heard channel A in the right car first and half heard channel B in the right eal first. This constitutes both a within subject and between subject reversal procedure. These methodological controls ensured that each S heard exactly the same stimuli in each ear, which eliminates the possible bias that more readily perceived digits are presented more often lo one ear than the other
C‘EREBRAL
LATERALIZATION
OF SPEECH:
THF
EFFECTS
Table 1. Schematic representation
Race : Class Age
h~is__:
:
Sex:
Mi.ddlc
Class
OF AGE,
SEX,
RACE,
I.ow
SOCIOECONOMIC
CLASS
365
of the experimental design
_____.__
__-
AND
BM_~__
_-___1.m
Clans
5 -_
-7
9 --
11
L
_J- Y
u_
.5
J --
N-F
M-F
PI-F
M-F
H-F
II-F
H-F
M-F
WF
M-F
Claqs
9 H-F
& M-F
Procedure Each S was tested individually by the senior author in a quiet room within the S’s own school. Testing for handedness preceded the administration of the Digit Span Scale of the Wechslcr Intelligence Scale for Children [23]. The first 5 trials on the tape were used as practice trials and not counted in the scoring; thus, the total possible score for each ear was 90. The Ss were instructed to listen to the stimuli presented on the tapes and verbally report back to the experimenter what they heard. There was no requirement that the Ss report in any specific order. There was a 10 set inter-trial interval in which the verbal reporting took place. Data analysis An inspection of Table 1 reveals that the experimental design is orthogonal with respect to all variables except Race and Social Class. This situation prompted the splitting of the data analyses into 2 stages where, in Stage 1, the data were fractionated so as to produce two 4-way orthogonal designs: Ear Y Age x Sex x Race, and Ear x Age x Sex x Social Class. The price paid for obtaining orthogonality was the sacrificing of one-third of the subject data pool for each 4-way design. On the other hand, the advantage gained was that Race and Social Class could be assessed independently. Stage 2 concerned the investigation of the Sex, Age, and Ear variables on the entire sample of Ss using the technique of multiple linear regression (see WARD and JFNNIKGS [24]. The computer program u
1976,
Vol.
14. pp.
363-370.
Pergamon
Press.
Printed
in England.
CEREBRAL LATERALIZATION OF SPEECH : THE EFFECTS OF AGE, SEX, RACE, AND SOCIOECONOMIC CLASS* THOM BOROWY~
and RONALD GOEBEL$
Department of Clinical Psychology,
and Neuropsychology Laboratory. University of Florida, Gainesville, Florida, 32601, U.S.A. (Received 16 Jltne 1975)
Ah&act--This study attempted to discover what effects the variables of age, sex, race, and socioeconomic class have on ear asymmetry. A significant right ear superiority was found at all ages studied; 5, 7, 9, and 11 yr of age. Males were found to develop essentially the same as females and white children develop the same as black children on dichotic listening tasks, which were utilized as the criteria measures of ear asymmetry. There were significant social class differences found, the middle class s’s showing a significantly greater degree of asymmetry than their lower class counterparts. All ages in both social classes, however, showed a significant right-left difference in favor of the right ear. Precise methodological controls were utilized and their relevance discussed. Possible explanations for discrepant findings between the present study and other research are also discussed.
INTRODUCTION FUNCTIONAL asymmetry
of the human brain in relation to speech has been studied by utilization of the dichotic listening technique [l-l 11. This technique involves disparate pairs of auditory stimuli (spoken words, digits, nonsense syllables, musical rhythms) which are presented simultaneously to each ear. These stimulus pairs have been presented in clusters of varying lengths and rates, and various methods of subject reporting have been utilized [l, 2, 7, 121. KIMURA [7, 81 posits that auditory stimuli presented to the ears are transmitted with greater efficiency to the contralateral hemisphere. Therefore, if speech is lateralized in the left hemisphere, verbal stimuli presented to the right ear should be more efficiently assimilated [6-81. Conversely, when non-verbal auditory stimuli are utilized, stimuli presented to the left ear (right hemisphere) are reported more often [5, 131. These “preferences” theoretically reflect a cerebral dominance for the perception and/or integration of auditory stimuli. The present study utilizes the dichotic listening technique to discern the relative importance of the variables of Age. Sex, Socioeconomic Class, and Race in relation to the ear asymmetry phenomenon. KIMURA [7] found evidence of an ear asymmetry in children as early as age 4, with the greatest ear asymmetry effects occurring in the youngest subjects. NAGAFUCHI [9] reported an even earlier age of onset of ear asymmetry based on the observation that his 3-yr-old subjects showed a significant difference between their right and left ears. Likewise, he also *Supported by NIH (NS-08209). TPresent address: Department of Psychology, University of North Florida, Jacksonville, U.S.A. fveterans Administration Hospital. Shreveport, La., U.S.A. 363
Florida 32216,
3h 1
THOM BOROIVY and RONALD GOFBEI
found that the greatest asymmetries were present in his youngest subjects. Two recent studies [I, 171 found that a significant ear asymmetry did not develop until much later. at around 9 or 10 yr of age. Further, the amount of asymmetry seemed to increase as the age increased. A possible explanation for these discrepancies can be made in relation to stimulus dificulty; the initial studies using a simple stimulus task and the later stud& employing relatively more difficult tasks. There has also been conflicting evidence as to whether malts and females develop difrerently with regard to ear asymmetry. Various btudies [3, 4, 8, 91 have found evidence supporting an earlier development of ear asymmetry in females than in males. Still other studies [I. 2, 7, 131 have found no significant sex differences in ear asymmetry. in view of the highly contradictory results in the literature and the evidence of great disparities between males and females in a number of behavioral and learning disorders [18], this variable was included in the present study. GEFFNER and HOCHRERG [22] investigated the question of social class in relation to ear asymmetry across different ages [4-71, and found right ear superiority “for all age groups from the middle socioeconomic level and for the 7-yr-old subjects from the low socioeconomic levels”. They concluded that, “the present findings agree with those reported by KIMURA (1967) that although children from low socioeconomic areas demonstrate I-ight ear superiority for verbal stimuli, the degree of such superiority is less than that observed from middle socioeconomic areas” [22]. The selection of subjects in GefTner and Hochberg’s study ~11ay have had a significant effect 011 their findings. They state, “the large majority of children from the low socioeconomic group were of black origin. whereas the children from the middle socioeconomic group were predominantly white. What, if any, influence these factors may have contributed to differences in performance cannot be ascertained from the present study. It is more than likely that various factors were operating to influence the performance of these children. The nature of these factors and their consequent influences are not clear” [22]. Due to the high degree of segregation in the make up of their social classes, it cannot bc determined from their study whether the results they found were due to socioeconomic differences or to racial differences.
METHOD Subjects Gs) m the present study were 120 children (60 males and 60 females) enrolled in the pubhc school system of Alachua County, Florida. Handedness was determined by verbal report and performance on manual tasks involving throwing, catching, kicking and sighting. Socioeconomic status was determined from school records concerning the amount of family income, occupation of the parents, and tea&et evaluation. The experimental design contained 24 individual cells comprised of 5 Ss each (Table 1). Dichotic listerlittg tasks
The dichotic listening task consisted of 30 sets of 3 digit pairs which were presented in such a way that the 2 digits in each pair were presented simultaneously, one to the right and one to the left ear. Each trial consisted of 3 pairs of digits which were presented at the rate of 2 pairs/second. The digits heard on the tape were l-5, 8, 9, 10, 12-15, 18. The tape had been created so that the digits in each pair had the identical onset time and the same loudness. Since there are auditory differences in spoken digits which cannot be controlled for acoustically, other control methods were necessary. Therefore, the tape was constructed so that the first 15 trials were identical to the second 15 trials. However, after trial 15 the headphones were revcrscd on each S so that half the Ss heard channel A in the right car first and half heard channel B in the right eal first. This constitutes both a within subject and between subject reversal procedure. These methodological controls ensured that each S heard exactly the same stimuli in each ear, which eliminates the possible bias that more readily perceived digits are presented more often lo one ear than the other
C‘EREBRAL
LATERALIZATION
OF SPEECH:
THF
EFFECTS
Table 1. Schematic representation
Race : Class Age
h~is__:
:
Sex:
Mi.ddlc
Class
OF AGE,
SEX,
RACE,
I.ow
SOCIOECONOMIC
CLASS
365
of the experimental design
_____.__
__-
AND
BM_~__
_-___1.m
Clans
5 -_
-7
9 --
11
L
_J- Y
u_
.5
J --
N-F
M-F
PI-F
M-F
H-F
II-F
H-F
M-F
WF
M-F
Claqs
9 H-F
& M-F
Procedure Each S was tested individually by the senior author in a quiet room within the S’s own school. Testing for handedness preceded the administration of the Digit Span Scale of the Wechslcr Intelligence Scale for Children [23]. The first 5 trials on the tape were used as practice trials and not counted in the scoring; thus, the total possible score for each ear was 90. The Ss were instructed to listen to the stimuli presented on the tapes and verbally report back to the experimenter what they heard. There was no requirement that the Ss report in any specific order. There was a 10 set inter-trial interval in which the verbal reporting took place. Data analysis An inspection of Table 1 reveals that the experimental design is orthogonal with respect to all variables except Race and Social Class. This situation prompted the splitting of the data analyses into 2 stages where, in Stage 1, the data were fractionated so as to produce two 4-way orthogonal designs: Ear Y Age x Sex x Race, and Ear x Age x Sex x Social Class. The price paid for obtaining orthogonality was the sacrificing of one-third of the subject data pool for each 4-way design. On the other hand, the advantage gained was that Race and Social Class could be assessed independently. Stage 2 concerned the investigation of the Sex, Age, and Ear variables on the entire sample of Ss using the technique of multiple linear regression (see WARD and JFNNIKGS [24]. The computer program u
