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Hand, Foot, Eye and Ear Preferences and Performance on a Dichotic Listening Test
HAND, FOOT, EYE AND EAR PREFERENCES AND PERFORMANCE ON A DICHOTIC LISTENING TEST Esther Strauss (Department of Psychology, University of Victoria, Canada)
The evidence from studies of clinical and normal populations suggests a weak relation between hand preference and hemispheric specialization for language. Based on investigations with the carotid anytal technique (Wada, 1949; Wada and Rasmussen, 1960) and unilateral electroconvulsive therapy, it is estimated that more than 95% of right-handers have speech located in the left hemisphere whereas in left-handers, the incidence of left hemisphere speech dominance is considerably lower, 60-70% (Branch, Milner and Rasmussen, 1964; McManus, cited in Bradshaw and Nettleton, 1983; Rasmussen and Milner, 1977; Strauss and Wada, 1983). There are reports of a higher incidence of aphasia in left than righthanders (see Satz, 1980, for a recent review), again suggesting that the two groups differ in terms of their brain organization. Finally, on dichotic listening and tachistoscopic tests, measures that bear some relation to cerebral speech dominance (Kimura, 1961; Strauss, Wada and Kosaka, 1986), left-handed normal people are sometimes identical in performance with the right-handed, but can also show smaller or reversed laterality effects (Hardyk and Petrinovitch, 1977; Bradshaw and Nettleton, 1983; for recent reviews). There also seems to be some association between foot preference and functional cerebral specialization. Searleman (1980) tested normal people with a verbal dichotic listening task and found that footedness was a better predictor than handedness of degree and direction of ear advantage. Strauss and Wada (1983) examined epileptic patients whose speech dominance had been ascertained using the carotid Amytal test. Based on data from those patients with late onset damage, they reported that footedness, like handedness, was linked to cerebral dominance for speech. Turning to eye preference, studies with neurologically intact individuals have reported little, if any, relation between eyedness and indicators of cerebral language specialization (see White, 1969; Searleman, 1980; . Cortex (1986) 22, 475-482
476
Esther Strauss
Porac and Coren, 1981, for reviews; but also see Levy and Gur, 1980). Strauss and Wada (1983) also examined eye preference in epileptic patients who had undergone Amy tal speech testing. They found no evidence of an association between eyedness and cerebral speech dominance among those patients with late onset of damage. In addition to the variables already discussed, Strauss and Wada (1983) examined the relation between ear preference and language lateralization. They found no evidence of a linkage between ear preference and cerebral speech dominance in epileptic patients with late onset of damage whose speech dominance had been ascertained using the carotid Amytal test. Thus, the weight of evidence to date suggests that when each lateral preference is viewed by itself, handedness and footedness, but perhaps not eyedness and earedness, are predictive of cerebral language dominance. There is also some additional evidence that the degree of congruency across indexes (all right, all left, mixed) has predictive value in assessing cerebral dominance. Searleman (1980) found that normal subjects who were left-sided (left-hand, -food, -eye) tended to report a greater number of words at the left ear than did subjects who were either right-sided or who lacked consistent sidedness. Consistent with this finding was Strauss and W ada's (1983) report that in neurological patients, a left-sided bias on all four indices (hand, foot, eye, ear) was likely to occur with right, but not left, hemisphere speech representation whereas congruent right-sidedness was typical of left, but not right, hemisphere speech dominance. The purpose of the current study was to test these ideas in normal people. Adult men and women were given a dichotic listening test to determine ear advantage. To facilitate comparison with earlier studies (e.g. Searleman, 1980), both degree and direction of ear advantage were assessed. On the basis of a lateral preference questionnaire, subjects were also classified as to handedness, footedness, eyedness, earedness and overall congruency across lateral preferences. The results revealed that footedness and overall congruency were predictive of ear advantage. Surprisingly, ear preference was the most successful factor in predicting both degree and direction of ear advantage.
MATERIALS AND METHOD
Subjects
Of the 197 undergraduate and graduate students who served as subjects, 127 were women (mean age = 25.7) and 70 were men (mean age = 24.6). Only subjects who reported no known hearing or neurological deficits were tested.
Predictors of dichotic ear asymmetry
477
Dichotic Listening Test
The dichotic listening test (University of Victoria) consisted of 22 trials, each containing three dichotic pairs of mono-syllabic words. The subject was instructed to repeat as many of the six words as possible on each trial. The tape was played on a Sony stereo tape recorder through matched headsets. Sound amplitudes were equated (about 93 dB SPL) at each of the speakers of the set of stereo headphones by means of a sound meter. Scores were derived for each subject using the formula (R - L)/(R + L) where R represents the number of words correctly reported from right-ear presentations and L the number of words correctly reported from left-ear presentations. Questionnaire
Subjects were asked to complete a behaviorally validated self-report inventory used to assess hand, foot, eye and ear preference (Porac and Coren, 1981). This inventory contains 16 items, 4 for each index. The handedness items were: (1) With which hand do you use an eraser on paper? (2) With which hand do you remove the top card when dealing? (3) With which hand would you throw a ball to hit a target? (4) With which hand do you draw? The footedness items were: (1) With which foot would you kick a ball? (2) If you had to step onto a chair, which foot would you place on the chair first? (3) If you wanted to pick up a pebble with your toes, which foot would you use? (4) Which foot would you use to step on a bug? The eyedness items were: (1) With which eye would you peep through a keyhole? (2) If you had to look into a dark bottle to see how full it was, which eye would you use? (3) Which eye would you use to sight down a rifle? (4) Which eye would you use to sight down a telescope? The earedess items were: (I) Into which ear would you place the earphone of a transitor radio? (2) If you wanted to listen in on a conversation going on behind a closed door, which ear would you place against the door? (3) If you wanted to hear someone's heart beat, which ear would you place against their chest? (4) Imagine a small box resting on a table. The box contains a small clock. Which ear would you press against the box to find out if the clock was ticking? The items have been validated against performance measures in normal people, yielding an average question-performance concordance rate of 90% (Porac and Coren, 1981). Subjects responded to each question with an answer of left, right or both. Scores were derived for each index using the formula (R minus L)/N where R represents the number of "right" responses, L the number of "left" responses, and N the total number of items used to measure any given type of lateral preference (Porac and Coren, 1981). Scores for each index range from - 1 to + 1, where negative values represent a predominance of left responses and positive values reflect a preponderance of right responses. for example, to be classed as right-handed, the individual had to obtain a score [(R - L )/N] greater than 0 on the handedness index. If he scored 0 or less, he was classed as left-handed. In this way, the sample was subdivided into right-sided versus non-right-sided for each index of lateral preference. RESULTS
In the sample as a whole, all indices showed a predominance of right-sidedness. Handedness showed the largest dextral bias (88.2%), followed by footedness (83.2%), earedness (70.6%) and eyedness (64.5%).
478
Esther Strauss TABLE I
Mean Dichotic Laterality Scores as a Function of Sidedness Left handed Right handed Left footed Right footed Left eyed Right eyed Left eared Right eared Left sided Mixed sided Right sided
Index mean .120 .220 .079 .233 .199 .212 .050 .273 .073 .155 .279
F (1, 193) 2.17
P n.s .
7.13
<.01
.09
n.s .
23.95
<.001
5.06
<.01
With regard to overall congruency, many subjects (45.7%) were biased in favour of the right side on all four lateral preference indices and many (48.7%) lacked consistent sidedness. Few (5.6%) showed a consistent left-sided preference pattern. The effects of hand, foot, eye, ear and overall congruency on the dichotic listening scores were examined by means of five separate twoway ANOVAs (lateral preference X sex). In none of our analyses did any effect involving sex reach statistical levels of significance (p < .05). As shown in Table I, there was a tendency for the dichotic laterality scores of left-handed people to be smaller than those of the right-handed, but the difference did not reach statistical levels of significance (p = .14). There was a main effect for footedness such that dichotic laterality scores of left-footed people were significantly smaller than those of the rightfooted. There was also a significant main effect for ear preference. People who preferred the left ear had considerably lower dichotic laterality scores than did those who preferred the right ear. Finally, the effect of overall congruency was also significant such that the dichotic laterality scores progressively decreased as overall laterality went from all right-sided to mixed to all left-sided. Additional tests (Duncan's Multiple Range Test) revealed that those who preferred the right side had significantly greater laterality scores than those who preferred the left side or showed a mixed preference pattern, but the difference between the latter two groups was not significant (p > .05). A multiple regression analysis based upon the individual measures (hand, foot, eye, ear, overall laterality) revealed that a small, but significant amount of the variance (r square = .11) in dichotic listening scores could be explained by ear preference (F =23.87; dJ. = 1, 195; P <.001). No other single variable added a significant contribution (see Table II). (In this
479
Predictors of dichotic ear asymmetry
TABLE II
Correlation between Dichotic Listening Index and Lateral Preferences Dichotic Hand Foot Eye Ear
Hand
Foot
Eye
Ear
Overall
.101
.187 .575
.022 .343 .207
.330 .301 .337 .219
.107 .686 .542 .328 .377
analysis, for overall laterality, subjects were classed as being totally rightsided or not.) In order to examine the effects of the individual variables on the direction of ear advantage, subjects were classified as having a right ear advantage (REA) or a left ear advantage (LEA) on the basis of whether their dichotic laterality scores were positive or negative, respectively. If the subject had a score of 0, he was classed as having a LEA. Using this classification procedure, 72.6% of the sample showed a REA and 27.4% had a LEA. The data were analyzed using the Chi square test, with Yate's correction for continuity applied where expected frequencies were small. The findings with regard to direction of ear advantage mirrored those found for degree of asymmetry (see Table III). The relation between footedness and direction of ear advantage was significant; the incidence of LEA was highest for those who preferred the left foot. The relation between earedness and direction of ear advantage was also significant, again the left-eared having the highest incidence of LEA. Overall congruency was also related to direction of ear advantage. Subjects who showed a left-sided and mixed pattern of preferences had the highest incidence of LEA. Hand and eye preference were not related to direction of ear advantage. TABLE III
Number of Subjects Classified as Showing a REA or LEA as a Function of Sidedness Left handed Right handed Left footed Right footed Left eyed Right eyed Left eared Right eared Left sided Mixed sided Right sided
8 46 16 38 19 35 28 26 6 31 17
LEA (36%) (26%) (49%) (23%) (27%) (28%) (48%) (19%) (55%) (32%) (18%)
REA
14 129 17 126 51 92 30 113 5 65 73
(64%) (74%) (51%) (77%) (73%) (72%) (52%) (81%) (45%) (68%) (82%)
Chi square .56
n.s.
7.62
<.01
.00
n.s.
p
16.53
<.001
8.51
<.01
480
Esther Strauss
DISCUSSION
The goal of the present study was to determine if, in a normal population, handedness, footedness, eyedness, earedness and congruency across lateral preferences were predictive of ear asymmetry on a dichotic listening task. We did not observe an association between hand preference and ear advantage. Although a trend was apparent in our study, neither our own data nor the evidence from previously published reports (see Introduction) permit the unequivocal conclusion that a reduced or reversed laterality effect is more common in normal left-handed people. Foot preference was related to ear advantage, a finding consistent with other studies of normal (Searleman, 1980) and clinical populations (Strauss and Wada, 1983). Searleman (1980) has suggested that compared to hand preference, footedness is less susceptible to cultural bias and so may provide a more reliable indicator of cerebral lateralization. The present study found no evidence'that eye preference is linked to ear advantage. This finding is generally consistent with the current literature(White, 1969; Searleman, 1980; Porac and Coren, 1981; Strauss and Wada, 1983; but see Levy and Gur, 1980). Surprisingly, ear preference was the factor most closely related to both direction and degree of ear asymmetry on the dichotic listening task. There is an unpublished report of a similar finding. Spellacy and his colleagues (cited in Porac and Coren, 1981) presented verbal material dichotically to normal adults and found a significant relation between ear preference and direction, but not degree, of ear advantage on the dichotic listening task. The explanation for the linkage between ear preference and dichotic ear advantage is not certain although at least two possibilities can be suggested. One possibility is that the dichotic listening test is not a precise measure of functional hemispheric asymmetry (Bryden, 1982) and factors unrelated to hemispheric asymmetry may infuence performance on this behavioral measure. For example, minor differences in acuity between the ears may have contributed to the subjects ear preferences and to their dichotic listening performance. Such an explanation may well explain the relation between ear preference and dichotic ear advantage and would also be consistent with the finding that ear preference was unrelated to language laterality as determined by the carotid Amy tal test (Strauss and Wada, 1983). It should be noted, however, that Spellacy et al. measured auditory acuity in their subjects and found no relation between ear preference and auditory acuity. Moreover, there is evidence that with verbal material, a 15 dB difference in favour of the left ear is needed for it to outperform the right, on a baseline level of 80 dB (Berlin, 1977). Non-
Predictors of dichotic ear asymmetry
481
etheless, we cannot rule out the possibility that in the present study minor acuity differences affected both the ear prefen;nces and the dichotic listening performances. A second possibility is that ear preference is indeed related either directly or secondarily to language lateralization. Strauss and Wada (1983) however, did not find a relation between ear preference and language laterality, using Amytal assessment. But, their subjects were patients with medically refractory seizures, a group that may not be representative of a normal population. Moreover, the discrepancy between the results of the dichotic and carotid amytal measures of asymmetry may reflect the possibility that these tests are measuring different aspects of language that are also lateralized differently (Graves, 1980; Springer and Deutsch, 1981; Strauss, Kosaka and Wada, 1983; Witelson, 1977). Overall congruency was related to both direction and degree of ear advantage. A small proportion of subjects favoured the left side across all four lateral preference indices and this atypical group was more likely to show a LEA than subjects who were right-sided. This finding is consistent with previous reports by Searleman (1980) and Strauss and Wada (1983). However, contrary to the findings of Searleman (1980) and Strauss and Wada (1983), people who lacked consistent sidedness were also likely to show a LEA, suggesting a right hemisphere language contribution. Methodological differences may account for the discrepant results. Searleman (1980) examined hand, foot and eye preferences in normal people. Perhaps if he had also examined ear preference, his results might have mirrored the present pattern. Strauss and Wada (1983) did assess all four lateral preferences. Their conflicting observations may reflect the unique nature of their population (patients with long-standing seizure disorders) or differences in laterality measures (amytal versus dichotic). In summary, this study considered handedness, footedness, eyedness, earedness and overall congruency in an attempt to determine whether they were useful predictors of dichotic ear asymmetry in normal people. Surprisingly, ear preference was the most successful factor in predicting both degree and direction of ear advantage. However, the data suggest that knowledge of a person's ear preference does not allow for a very accurate prediction of dichotic ear asymmetry. ABSTRACT
Normal adult men and women were given a dichotic listening test to determine ear advantage. On the basis of a lateral preference questionnaire, subjects were classified as to handedness, footedness, eyedness, earedness and overall congruency across lateral preferences. Ear preference was the most successful variable in predicting ear advantage on the dichotic listening test.
482
Esther Strauss
Acknowledgement. This research was supported by grant ~A7933 from the Natural Sciences and Engineering Research Council, Canada. I thank Dr. Joan Borod, Dr. Roger Graves and especially, Dr. Josef Zaide for their helpful comments. REFERENCES BERLIN, C.I. Hemispheric asymmetry in auditory tasks. In S. Hamad, R.W. Doty, L. Goldstein, J. Jaynes and G. Krauthamer (Eds.), Lateralization in the Nervous System. New York: Academic Press, 1977. BRADSHAW, J., and NETILETON, N. Human Cerebral Asimmetry. New Jersey: Prentice Hall, 1983. BRANCH, c., MILNER, B., and RASMUSSEN, T. Intracarotid sodium Amytal for the lateralization of cerebral speech dominance. Journal of Neurosurgery, 21: 399-405, 1964. BRYDEN, M.P. Laterality: Functional Asymmetry in the Intact Brain. New York: Academic Press, 1982. GRAYES, R. Mouth asymmetry, dichotic ear advantage and tachistoscopic visual field advantage as measures of language lateralization. Neuropsychologia, 21: 641-650, 1980. HARDYCK, c., and PETRINOVITCH, L.F. Left-handedness. Psychological Bulletin, 84: 385404, 1977. KIMURA, D. Cerebral dominance and the perception of verbal stimuli. Canadian Journal of Psychology, 15: 166-177, 1961. KIMURA, D. Functional hemispheric asymmetry of the brain in dichotic listening. Cortex, 3: 163-178, 1967. LEVY, J., and GUR, R.c. Individual differences in psychoneurological organization. In J. Herron (Ed.), Neuropsychology of Left-handedness. New York: Academic Press, 1980. PORAC, c., and COREN, S. Lateral Preferences and Human Behavior. New York: SpringerVerlag, 1981. RASMUSSEN, T., and MILNER, B. The role of early left-brain injury in determining lateralization of cerebral speech functions. Annals of the New York Academy of Sciences, 299: 355-369, 1977. SATZ, P. Incidence of aphasia in left-handers: A test of some hypothetical models of cerebral speech organization. In J. Herron (Ed.), Neuropsychology of Left-handedness. New York: Academic Press, 1980. SEARLEMAN, A. Subject variables and cerebral organization for language. Cortex, 16: 239-254, 1980. SPRINGER, S., and DEUTSCH, G. Left Brain, Right Brain. San Francisco: W.H. Freeman, 1981. STRAUSS, E., KOSAKA, B., and WADA, J. The neurobiological basis of lateralized cerebral function: A review. Human Neurobiology, 2: 115-127, 1983. STRAUSS, E., and WADA, J. Lateral preferences and cerebral speech dominance. Cortex, 19: 165-177, 1983. STRAUSS, E., WADA, J., and KOSAKA, B. Visual laterality effects and cerebral speech dominance determined by the carotid amytal test. Neuropsychologia,23: 567-570, 1985. W ADA, J. A new method for the determination of the side of cerebral speech dominance. A preliminary report on the intracarotid injection of sodium amy tal in man. Medical Biology, 14: 221-222, 1949. W ADA, J., and RASMUSSEN, T. Intracarotid injection of sodium amytal for the lateralization of cerebral speech dominance. Experimental and clinical observations. Journal of Neurosurgery, 17: 266-282, 1960. WHITE, M.J. Laterality differences in perception. Psychological Bull., 72: 387-405, 1969. WITELSON, S.F. Anatomic asymmetry in the temporal lobes: Its documentation, phylogenesis, and relationship to functional asymmetry. Annals of the New York Academy of Sciences, 299: 328-354, 1977. Esther Strauss, Ph.D., Dpt. of Psychology, University of Victoria, Victoria, British Columbia, P.O.Box 1700, Canada.
The evidence from studies of clinical and normal populations suggests a weak relation between hand preference and hemispheric specialization for language. Based on investigations with the carotid anytal technique (Wada, 1949; Wada and Rasmussen, 1960) and unilateral electroconvulsive therapy, it is estimated that more than 95% of right-handers have speech located in the left hemisphere whereas in left-handers, the incidence of left hemisphere speech dominance is considerably lower, 60-70% (Branch, Milner and Rasmussen, 1964; McManus, cited in Bradshaw and Nettleton, 1983; Rasmussen and Milner, 1977; Strauss and Wada, 1983). There are reports of a higher incidence of aphasia in left than righthanders (see Satz, 1980, for a recent review), again suggesting that the two groups differ in terms of their brain organization. Finally, on dichotic listening and tachistoscopic tests, measures that bear some relation to cerebral speech dominance (Kimura, 1961; Strauss, Wada and Kosaka, 1986), left-handed normal people are sometimes identical in performance with the right-handed, but can also show smaller or reversed laterality effects (Hardyk and Petrinovitch, 1977; Bradshaw and Nettleton, 1983; for recent reviews). There also seems to be some association between foot preference and functional cerebral specialization. Searleman (1980) tested normal people with a verbal dichotic listening task and found that footedness was a better predictor than handedness of degree and direction of ear advantage. Strauss and Wada (1983) examined epileptic patients whose speech dominance had been ascertained using the carotid Amytal test. Based on data from those patients with late onset damage, they reported that footedness, like handedness, was linked to cerebral dominance for speech. Turning to eye preference, studies with neurologically intact individuals have reported little, if any, relation between eyedness and indicators of cerebral language specialization (see White, 1969; Searleman, 1980; . Cortex (1986) 22, 475-482
476
Esther Strauss
Porac and Coren, 1981, for reviews; but also see Levy and Gur, 1980). Strauss and Wada (1983) also examined eye preference in epileptic patients who had undergone Amy tal speech testing. They found no evidence of an association between eyedness and cerebral speech dominance among those patients with late onset of damage. In addition to the variables already discussed, Strauss and Wada (1983) examined the relation between ear preference and language lateralization. They found no evidence of a linkage between ear preference and cerebral speech dominance in epileptic patients with late onset of damage whose speech dominance had been ascertained using the carotid Amytal test. Thus, the weight of evidence to date suggests that when each lateral preference is viewed by itself, handedness and footedness, but perhaps not eyedness and earedness, are predictive of cerebral language dominance. There is also some additional evidence that the degree of congruency across indexes (all right, all left, mixed) has predictive value in assessing cerebral dominance. Searleman (1980) found that normal subjects who were left-sided (left-hand, -food, -eye) tended to report a greater number of words at the left ear than did subjects who were either right-sided or who lacked consistent sidedness. Consistent with this finding was Strauss and W ada's (1983) report that in neurological patients, a left-sided bias on all four indices (hand, foot, eye, ear) was likely to occur with right, but not left, hemisphere speech representation whereas congruent right-sidedness was typical of left, but not right, hemisphere speech dominance. The purpose of the current study was to test these ideas in normal people. Adult men and women were given a dichotic listening test to determine ear advantage. To facilitate comparison with earlier studies (e.g. Searleman, 1980), both degree and direction of ear advantage were assessed. On the basis of a lateral preference questionnaire, subjects were also classified as to handedness, footedness, eyedness, earedness and overall congruency across lateral preferences. The results revealed that footedness and overall congruency were predictive of ear advantage. Surprisingly, ear preference was the most successful factor in predicting both degree and direction of ear advantage.
MATERIALS AND METHOD
Subjects
Of the 197 undergraduate and graduate students who served as subjects, 127 were women (mean age = 25.7) and 70 were men (mean age = 24.6). Only subjects who reported no known hearing or neurological deficits were tested.
Predictors of dichotic ear asymmetry
477
Dichotic Listening Test
The dichotic listening test (University of Victoria) consisted of 22 trials, each containing three dichotic pairs of mono-syllabic words. The subject was instructed to repeat as many of the six words as possible on each trial. The tape was played on a Sony stereo tape recorder through matched headsets. Sound amplitudes were equated (about 93 dB SPL) at each of the speakers of the set of stereo headphones by means of a sound meter. Scores were derived for each subject using the formula (R - L)/(R + L) where R represents the number of words correctly reported from right-ear presentations and L the number of words correctly reported from left-ear presentations. Questionnaire
Subjects were asked to complete a behaviorally validated self-report inventory used to assess hand, foot, eye and ear preference (Porac and Coren, 1981). This inventory contains 16 items, 4 for each index. The handedness items were: (1) With which hand do you use an eraser on paper? (2) With which hand do you remove the top card when dealing? (3) With which hand would you throw a ball to hit a target? (4) With which hand do you draw? The footedness items were: (1) With which foot would you kick a ball? (2) If you had to step onto a chair, which foot would you place on the chair first? (3) If you wanted to pick up a pebble with your toes, which foot would you use? (4) Which foot would you use to step on a bug? The eyedness items were: (1) With which eye would you peep through a keyhole? (2) If you had to look into a dark bottle to see how full it was, which eye would you use? (3) Which eye would you use to sight down a rifle? (4) Which eye would you use to sight down a telescope? The earedess items were: (I) Into which ear would you place the earphone of a transitor radio? (2) If you wanted to listen in on a conversation going on behind a closed door, which ear would you place against the door? (3) If you wanted to hear someone's heart beat, which ear would you place against their chest? (4) Imagine a small box resting on a table. The box contains a small clock. Which ear would you press against the box to find out if the clock was ticking? The items have been validated against performance measures in normal people, yielding an average question-performance concordance rate of 90% (Porac and Coren, 1981). Subjects responded to each question with an answer of left, right or both. Scores were derived for each index using the formula (R minus L)/N where R represents the number of "right" responses, L the number of "left" responses, and N the total number of items used to measure any given type of lateral preference (Porac and Coren, 1981). Scores for each index range from - 1 to + 1, where negative values represent a predominance of left responses and positive values reflect a preponderance of right responses. for example, to be classed as right-handed, the individual had to obtain a score [(R - L )/N] greater than 0 on the handedness index. If he scored 0 or less, he was classed as left-handed. In this way, the sample was subdivided into right-sided versus non-right-sided for each index of lateral preference. RESULTS
In the sample as a whole, all indices showed a predominance of right-sidedness. Handedness showed the largest dextral bias (88.2%), followed by footedness (83.2%), earedness (70.6%) and eyedness (64.5%).
478
Esther Strauss TABLE I
Mean Dichotic Laterality Scores as a Function of Sidedness Left handed Right handed Left footed Right footed Left eyed Right eyed Left eared Right eared Left sided Mixed sided Right sided
Index mean .120 .220 .079 .233 .199 .212 .050 .273 .073 .155 .279
F (1, 193) 2.17
P n.s .
7.13
<.01
.09
n.s .
23.95
<.001
5.06
<.01
With regard to overall congruency, many subjects (45.7%) were biased in favour of the right side on all four lateral preference indices and many (48.7%) lacked consistent sidedness. Few (5.6%) showed a consistent left-sided preference pattern. The effects of hand, foot, eye, ear and overall congruency on the dichotic listening scores were examined by means of five separate twoway ANOVAs (lateral preference X sex). In none of our analyses did any effect involving sex reach statistical levels of significance (p < .05). As shown in Table I, there was a tendency for the dichotic laterality scores of left-handed people to be smaller than those of the right-handed, but the difference did not reach statistical levels of significance (p = .14). There was a main effect for footedness such that dichotic laterality scores of left-footed people were significantly smaller than those of the rightfooted. There was also a significant main effect for ear preference. People who preferred the left ear had considerably lower dichotic laterality scores than did those who preferred the right ear. Finally, the effect of overall congruency was also significant such that the dichotic laterality scores progressively decreased as overall laterality went from all right-sided to mixed to all left-sided. Additional tests (Duncan's Multiple Range Test) revealed that those who preferred the right side had significantly greater laterality scores than those who preferred the left side or showed a mixed preference pattern, but the difference between the latter two groups was not significant (p > .05). A multiple regression analysis based upon the individual measures (hand, foot, eye, ear, overall laterality) revealed that a small, but significant amount of the variance (r square = .11) in dichotic listening scores could be explained by ear preference (F =23.87; dJ. = 1, 195; P <.001). No other single variable added a significant contribution (see Table II). (In this
479
Predictors of dichotic ear asymmetry
TABLE II
Correlation between Dichotic Listening Index and Lateral Preferences Dichotic Hand Foot Eye Ear
Hand
Foot
Eye
Ear
Overall
.101
.187 .575
.022 .343 .207
.330 .301 .337 .219
.107 .686 .542 .328 .377
analysis, for overall laterality, subjects were classed as being totally rightsided or not.) In order to examine the effects of the individual variables on the direction of ear advantage, subjects were classified as having a right ear advantage (REA) or a left ear advantage (LEA) on the basis of whether their dichotic laterality scores were positive or negative, respectively. If the subject had a score of 0, he was classed as having a LEA. Using this classification procedure, 72.6% of the sample showed a REA and 27.4% had a LEA. The data were analyzed using the Chi square test, with Yate's correction for continuity applied where expected frequencies were small. The findings with regard to direction of ear advantage mirrored those found for degree of asymmetry (see Table III). The relation between footedness and direction of ear advantage was significant; the incidence of LEA was highest for those who preferred the left foot. The relation between earedness and direction of ear advantage was also significant, again the left-eared having the highest incidence of LEA. Overall congruency was also related to direction of ear advantage. Subjects who showed a left-sided and mixed pattern of preferences had the highest incidence of LEA. Hand and eye preference were not related to direction of ear advantage. TABLE III
Number of Subjects Classified as Showing a REA or LEA as a Function of Sidedness Left handed Right handed Left footed Right footed Left eyed Right eyed Left eared Right eared Left sided Mixed sided Right sided
8 46 16 38 19 35 28 26 6 31 17
LEA (36%) (26%) (49%) (23%) (27%) (28%) (48%) (19%) (55%) (32%) (18%)
REA
14 129 17 126 51 92 30 113 5 65 73
(64%) (74%) (51%) (77%) (73%) (72%) (52%) (81%) (45%) (68%) (82%)
Chi square .56
n.s.
7.62
<.01
.00
n.s.
p
16.53
<.001
8.51
<.01
480
Esther Strauss
DISCUSSION
The goal of the present study was to determine if, in a normal population, handedness, footedness, eyedness, earedness and congruency across lateral preferences were predictive of ear asymmetry on a dichotic listening task. We did not observe an association between hand preference and ear advantage. Although a trend was apparent in our study, neither our own data nor the evidence from previously published reports (see Introduction) permit the unequivocal conclusion that a reduced or reversed laterality effect is more common in normal left-handed people. Foot preference was related to ear advantage, a finding consistent with other studies of normal (Searleman, 1980) and clinical populations (Strauss and Wada, 1983). Searleman (1980) has suggested that compared to hand preference, footedness is less susceptible to cultural bias and so may provide a more reliable indicator of cerebral lateralization. The present study found no evidence'that eye preference is linked to ear advantage. This finding is generally consistent with the current literature(White, 1969; Searleman, 1980; Porac and Coren, 1981; Strauss and Wada, 1983; but see Levy and Gur, 1980). Surprisingly, ear preference was the factor most closely related to both direction and degree of ear asymmetry on the dichotic listening task. There is an unpublished report of a similar finding. Spellacy and his colleagues (cited in Porac and Coren, 1981) presented verbal material dichotically to normal adults and found a significant relation between ear preference and direction, but not degree, of ear advantage on the dichotic listening task. The explanation for the linkage between ear preference and dichotic ear advantage is not certain although at least two possibilities can be suggested. One possibility is that the dichotic listening test is not a precise measure of functional hemispheric asymmetry (Bryden, 1982) and factors unrelated to hemispheric asymmetry may infuence performance on this behavioral measure. For example, minor differences in acuity between the ears may have contributed to the subjects ear preferences and to their dichotic listening performance. Such an explanation may well explain the relation between ear preference and dichotic ear advantage and would also be consistent with the finding that ear preference was unrelated to language laterality as determined by the carotid Amy tal test (Strauss and Wada, 1983). It should be noted, however, that Spellacy et al. measured auditory acuity in their subjects and found no relation between ear preference and auditory acuity. Moreover, there is evidence that with verbal material, a 15 dB difference in favour of the left ear is needed for it to outperform the right, on a baseline level of 80 dB (Berlin, 1977). Non-
Predictors of dichotic ear asymmetry
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etheless, we cannot rule out the possibility that in the present study minor acuity differences affected both the ear prefen;nces and the dichotic listening performances. A second possibility is that ear preference is indeed related either directly or secondarily to language lateralization. Strauss and Wada (1983) however, did not find a relation between ear preference and language laterality, using Amytal assessment. But, their subjects were patients with medically refractory seizures, a group that may not be representative of a normal population. Moreover, the discrepancy between the results of the dichotic and carotid amytal measures of asymmetry may reflect the possibility that these tests are measuring different aspects of language that are also lateralized differently (Graves, 1980; Springer and Deutsch, 1981; Strauss, Kosaka and Wada, 1983; Witelson, 1977). Overall congruency was related to both direction and degree of ear advantage. A small proportion of subjects favoured the left side across all four lateral preference indices and this atypical group was more likely to show a LEA than subjects who were right-sided. This finding is consistent with previous reports by Searleman (1980) and Strauss and Wada (1983). However, contrary to the findings of Searleman (1980) and Strauss and Wada (1983), people who lacked consistent sidedness were also likely to show a LEA, suggesting a right hemisphere language contribution. Methodological differences may account for the discrepant results. Searleman (1980) examined hand, foot and eye preferences in normal people. Perhaps if he had also examined ear preference, his results might have mirrored the present pattern. Strauss and Wada (1983) did assess all four lateral preferences. Their conflicting observations may reflect the unique nature of their population (patients with long-standing seizure disorders) or differences in laterality measures (amytal versus dichotic). In summary, this study considered handedness, footedness, eyedness, earedness and overall congruency in an attempt to determine whether they were useful predictors of dichotic ear asymmetry in normal people. Surprisingly, ear preference was the most successful factor in predicting both degree and direction of ear advantage. However, the data suggest that knowledge of a person's ear preference does not allow for a very accurate prediction of dichotic ear asymmetry. ABSTRACT
Normal adult men and women were given a dichotic listening test to determine ear advantage. On the basis of a lateral preference questionnaire, subjects were classified as to handedness, footedness, eyedness, earedness and overall congruency across lateral preferences. Ear preference was the most successful variable in predicting ear advantage on the dichotic listening test.
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Acknowledgement. This research was supported by grant ~A7933 from the Natural Sciences and Engineering Research Council, Canada. I thank Dr. Joan Borod, Dr. Roger Graves and especially, Dr. Josef Zaide for their helpful comments. REFERENCES BERLIN, C.I. Hemispheric asymmetry in auditory tasks. In S. Hamad, R.W. Doty, L. Goldstein, J. Jaynes and G. Krauthamer (Eds.), Lateralization in the Nervous System. New York: Academic Press, 1977. BRADSHAW, J., and NETILETON, N. Human Cerebral Asimmetry. New Jersey: Prentice Hall, 1983. BRANCH, c., MILNER, B., and RASMUSSEN, T. Intracarotid sodium Amytal for the lateralization of cerebral speech dominance. Journal of Neurosurgery, 21: 399-405, 1964. BRYDEN, M.P. Laterality: Functional Asymmetry in the Intact Brain. New York: Academic Press, 1982. GRAYES, R. Mouth asymmetry, dichotic ear advantage and tachistoscopic visual field advantage as measures of language lateralization. Neuropsychologia, 21: 641-650, 1980. HARDYCK, c., and PETRINOVITCH, L.F. Left-handedness. Psychological Bulletin, 84: 385404, 1977. KIMURA, D. Cerebral dominance and the perception of verbal stimuli. Canadian Journal of Psychology, 15: 166-177, 1961. KIMURA, D. Functional hemispheric asymmetry of the brain in dichotic listening. Cortex, 3: 163-178, 1967. LEVY, J., and GUR, R.c. Individual differences in psychoneurological organization. In J. Herron (Ed.), Neuropsychology of Left-handedness. New York: Academic Press, 1980. PORAC, c., and COREN, S. Lateral Preferences and Human Behavior. New York: SpringerVerlag, 1981. RASMUSSEN, T., and MILNER, B. The role of early left-brain injury in determining lateralization of cerebral speech functions. Annals of the New York Academy of Sciences, 299: 355-369, 1977. SATZ, P. Incidence of aphasia in left-handers: A test of some hypothetical models of cerebral speech organization. In J. Herron (Ed.), Neuropsychology of Left-handedness. New York: Academic Press, 1980. SEARLEMAN, A. Subject variables and cerebral organization for language. Cortex, 16: 239-254, 1980. SPRINGER, S., and DEUTSCH, G. Left Brain, Right Brain. San Francisco: W.H. Freeman, 1981. STRAUSS, E., KOSAKA, B., and WADA, J. The neurobiological basis of lateralized cerebral function: A review. Human Neurobiology, 2: 115-127, 1983. STRAUSS, E., and WADA, J. Lateral preferences and cerebral speech dominance. Cortex, 19: 165-177, 1983. STRAUSS, E., WADA, J., and KOSAKA, B. Visual laterality effects and cerebral speech dominance determined by the carotid amytal test. Neuropsychologia,23: 567-570, 1985. W ADA, J. A new method for the determination of the side of cerebral speech dominance. A preliminary report on the intracarotid injection of sodium amy tal in man. Medical Biology, 14: 221-222, 1949. W ADA, J., and RASMUSSEN, T. Intracarotid injection of sodium amytal for the lateralization of cerebral speech dominance. Experimental and clinical observations. Journal of Neurosurgery, 17: 266-282, 1960. WHITE, M.J. Laterality differences in perception. Psychological Bull., 72: 387-405, 1969. WITELSON, S.F. Anatomic asymmetry in the temporal lobes: Its documentation, phylogenesis, and relationship to functional asymmetry. Annals of the New York Academy of Sciences, 299: 328-354, 1977. Esther Strauss, Ph.D., Dpt. of Psychology, University of Victoria, Victoria, British Columbia, P.O.Box 1700, Canada.