B ~ I N AND LANGUAGE 2, 226-236 (1975)
The Reliability of Ear Advantage in Dichotic Listening SHEILA BLUMSTEIN, HAROLD
GOODGLASS, AND V I V I E N TARTTER
Brown University, Boston VA Hospital, and Department of Neurology, Boston University
Test-re-test reliability of dichotic listening performance on consonants, vowels, and music was investigated in a population of 42 right-handed subjects. Pearson product moment correlations between (R -- L)/(R ÷ L) ear scores on first and second tests were .74 for consonants, .21 for vowels, and .46 for music. Twenty nine percent of subjects reversed ear advantage for consonants on re-testing, 19% reversed for music, and 46% for vowels. Each type of stimulus reveals a significant subgroup (15% for consonants) who retain a deviant ear advantage on retest. In any sample, subjects whose ear advantage scores are on the deviant side are more likely to reverse ear advantages on re-test than subjects who score in the modal direction. These findings are interpreted and discussed in relation to the validity of dichotic listening as an index of cerebral functional asymmetry. There is an important disparity among the various indices of cerebral dominance in man. This disparity lies in the differing estimates of the prevalence of left-brain dominance in right-handers. Dichotic listening, the simplest and most widely used experimental technique, gives results that are the most divergent from the original criterion: the occurrence of aphasia following unilateral brain damage. The present study was undertaken to evaluate the test-re-test reliability of dichotic listening and to apply the results to a consideration of whether one criterion of cerebral dominance is more valid than others. Until the introduction of the W a d a intracarotid amytal technique (Wada & Rasmussen, 1960) the determination of language dominance in the brain relied on observing whether or not an individual developed aphasia after a unilateral lesion of the right or left hemisphere. By this criterion, the incidence of left-brain dominance for language approaches 99% for right-handers. In fact, after reviewing the evidence, Zangwill (1962) states, " F o r practical purposes, the probability of right cerebral dominance in a fully right-handed individual is so low that it may be disregarded." Estimates of left-brain dominance in non-right-handers range from 53% (Goodglass & Quadfasel, 1954)to 63% (Roberts, 1969). Results from the use of the Wada technique show a somewhat greater deviation from 100% correlation between right-handedness and leftcerebral language dominance. Milner, Branch, and Rasmussen report that 92% of 95 right-handed patients reacted with aphasia to left carotid amytal injection. Results reported by Rossi and Rosadini (1967) with this 226 Copyright © 1975 by Academic Press, Inc. AII rights of reproductionin any form reserved.
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technique come closer to experience with natural lesions. Of their 74 right-handed subjects, 73 (98.5%) became asphasic only after left carotid amytal injection and one (1.4%) after injection of either side. The problem in using these figures for estimates of the natural distribution of cerebral dominance is that patients subjected to the Wada technique include a number with probable early brain damage. These result in an indeterminate number of pathologically determined right-brain dominant subjects. It is clear that ear laterality effects in dichotic listening are related to differences in the degree of participation of the two brain-halves in various classes of perceptual processing. The left-ear superiority for musical stimuli (Kimura, 1964; Spellacy & Blumstein, 1970) and the right-ear superiority for verbal stimuli (Kimura, 1961) can hardly be explained on the basis of peripheral biasing effects, such as habitual attentional patterns. The chief drawback in interpreting these results is that the incidence of right-ear superiority is commonly reported as falling in the range between 75% (Efron, personal communication) to 87% (Satz, Aehenbach & Fennel, 1967). Zurif, Caramazza, and Carbone have unpublished data giving a figure of 81% in a sample of 30. Moreover, unilateral natural lesions and pharmacological effects produce all-or-none reactions in right-handed subjects; that is, a dramatic response from the left hemisphere and no response from the right. In contrast, inferences concerning brain laterality based on dichotic listening may involve differences between ear scores averaging 6% of the total per ear (Krashen, 1972). These observations raise a question as to the conditions under which dichotic listening is a valid criterion of brain laterality for language. It is possible that the variability in dichotic listening is such that, in any sample, about 20% of the subjects will randomly show a left-ear superiority for verbal material. On the other hand, it is possible that there is a subgroup of right-handers who consistently fall into the left-ear dominant side of the distribution of dichotic listening scores. The latter possibility implies that natural lesions, dichotie listening, and the Wada technique give three different estimates, ranging from 1 to 20% of right-brain language dominance in right-handers. A finding that a significant subgroup of right-handed subjects is consistently left eared for language would be contrary to the overwhelming weight of data from pathology. Such a finding would undermine the validity of dichotic listening as a diagnostic tool for individual determinations of brain laterality. METHOD Subjects. Subjects were 42 right-handed males from Brown University. All were native speakers of English with no k n o w n hearing loss. T h e y were paid for their participation. Stimuli. T h r e e dichotic tapes were constructed to assess the c o n s i s t e n c y of the dichotic
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listening paradigm in the perception of different types of auditory stimuli. T h e s e included a c o n s o n a n t tape, a vowel tape, and a m u s i c tape. T h e s e correspond to stimuli which are normally considered to be right-ear dominant, indeterminate in ear dominance, and left-ear dominant. Since the reliability of a test obviously depends on the size of the sample, the n u m b e r o f test items in each tape (detailed below) was greater than those usually reported in the literature. T h e c o n s o n a n t stimuli consisted of the six stop c o n s o n a n t s / p t k b d g / paired with the v o w e l / a / . T h e C V syllables were produced by a trained linguist (SB) with an even intensity and uniform pitch contour. T h e onsets of the stimuli were then edited and synchronized by m e a n s of the pulse code modulation s y s t e m made available at H a s k i n s Laboratories. In addition, overall intensity of each C V syllable was equalized. T h e duration of the syllables varied b e t w e e n 350 and 375 msec. T h e test items were paired so that there were equal occurrences of voice and place distinctive feature values (cf. Blumstein & Cooper, 1972, for details). T h e r e was a total of 40 pairs. T h e vowel stimuli consisted of the stop c o n s o n a n t / b / p a i r e d with the vowels / iI e e 3 o u u/. T h e dichotic tape was prepared by the same procedure as that used for the c o n s o n a n t tape. T h e vowel test items were paired so that there were equal occurrences of the distinctive feature values for the vowel phonemes. T h e r e was a total of 96 pairs on the dichotic tape. T h e music tape was a copy of K i m u r a ' s original dichotic music tape (cf. Kimura, 1964). T h e stimuli consisted of 20 dichotic pairs of melodies, each melody an excerpt from a solo passage of a classical concerto. Following each dichotic pair, a series of our melodies was presented binaurally, with instructions for the subject to identify the two which had been heard dichotically. Within each set the same instrument was used, and pitch range and t e m p o were similar. All melodies were of 4 sec duration (cf. Kimura, 1964, for further details). Procedure. Subjects were tested for consistency of ear advantages in the perception of consonants, vowels, and music. T h e order of presentation of the tapes was counterbalanced across subjects. Within each test order, earphones were reversed for half the subjects. Subjects were tested individually in a moderately quiet r o o m on a Sony T C 3 6 0 tape recorder with K o s s Pro-4 A A headphones. T h e voltage output of the two channels of the tape recorder was calibrated at approximately 70 dB sound pressure level. A test session lasted approximately one hour. E a c h subject was tested twice, the second session held at least one week after the first. Instructions, practice trials, and test order were the same for the two sessions. F o r the c o n s o n a n t s , subjects were told that they would hear two different simultaneous syllables, each taken from the s e t / p a , ta, ka, ba, da, ga/. Subjects were instructed to write the c o n s o n a n t s on an a n s w e r sheet and to g u e s s when unsure. T h e six C V syllables also appeared at the top of the a n s w e r sheet. Before the test, subjects heard the six syllables binaurally, and were given three practice dichotic trials. Forty test stimuli, with each pair separated by a 5 sec interval of silence, were then presented. A short break followed. Finally subjects were instructed to reverse their headphones, and the c o n s o n a n t tape was repeated. In all, subjects received a total of 80 dichotic c o n s o n a n t pairs. F o r the music tape, subjects were told that they would hear two different simultaneous melodies, followed 4 sec later by four binaural melodies, separated by 3 sec pauses. Subjects were instructed to m a t c h each of the dichotically presented melodies with one o f the four binaural melodies and to write on an a n s w e r sheet the n u m b e r corresponding to the position of the binaural melody in the sequence. T h e r e was a 3 sec interval b e t w e e n melodies. Again subjects were told to guess w h e n unsure. Before the test, subjects received two practice test items. After the 20 test stimuli were presented, subjects were given a rest. T h e y were then asked to reverse their h e a d p h o n e s and the music tape was replayed. T h e r e was a total of 40 dichotic pairs comprising the music tape. F o r the vowels, subjects were told that they would hear two simultaneous dichotic sylla-
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bles each taken from the s e t / b i bI be be b~ bo bo bu/. Subjects were instructed to report the two syllables orally and to g u e s s if necessary. R e s p o n s e s were simultaneously tape recorded on a U h e r 2000 Report L and transcribed by a trained phonologist (VT). Before each test there were three binaural presentations of the eight syllables. F o r the first presentation, subjects were instructed simply to listen and familiarize t h e m s e l v e s with the test items; for the other two presentations, subjects had to repeat each syllable as they heard it. This insured that they could produce all stimulus items on imitation. Subjects then received three dichotic practice items. T h e vowel tape consisted of 96 dichotic vowel pairs with a 5 sec silent interval b e t w e e n pairs. A rest was offered halfway through the test.
RESU LTS
Overall directions of ear advantages. Ear advantages were determined for both first and second presentations of the consonant tape, the vowel tape, and the music tape by computing the proportion of right- and left-ear correct responses in relation to the total correct responses (i.e., (R -- L)/(R + L) x 100). A positive index indicates an overall right ear advantage, a negative score a left ear advantage. This index accounts for lateral asymmetry in the context of overall level of accuracy (cf. StuddertKennedy & Shankweiler, 1970; Krashen, 1972). 1 The results, as summarized in Table 1, reveal a significant right ear advantage for consonants at bothtime 1 and time 2 (t = 3 . 5 3 , p < .01 andt = 4 . 4 4 , p < .001), a sig-
TABLE 1 EAR ADVANTAGES FOR CONSONANTS, VOWELS, AND MUSIC ON TWO OCCASIONS Consonants Mean n u m b e r correct time 1 time 2 time 1 time 2 time 1 time 2
L
R
49.05 55.60 52.95 60.30 Vowels 78.36 79.71 83.38 85.36 Music 30.00 26.57 31.14 27.48
(R - L)/(R + L) x 100
t-value
6.35 7.70
3.53*** 4.44****
.96 1.22
1.70" 1.98"*
-6.20 -4.28
-3.98**** -2.96***
* p--nonsignificant. ** p < .05, two-tailed. * * * p < .01, two-tailed. **** p < .02, two-tailed. 1 T h e data were analyzed not only in terms of the (R -- L)/(R + L) index but also in terms of absolute ear advantages, i.e., R -- L. T h e results of t h e s e analyses were virtually identical and thus there were no differences in the theoretical interpretations of the data based on either absolute or relative indices o f laterality.
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nificant left-ear advantage for music at both time 1 and time 2 (t = 3.98, p < .001 and t = 2.96, p < .01), a nonsignificant right-ear advantage for vowels at time 1 (t = 1.70) and time 2 (t = 1.98). These results replicate earlier findings for the lateralization of consonants, vowels, and music; namely, consistent right-ear advantages are obtained for consonants, consistent left-ear advantages for music, and a tendency for right-ear advantages for vowels. Consistency of ear advantages. T w o approaches were taken to determine to what extent performance on one occasion predicts performance on a second. T h e first was to determine Pearson product moment correlations between (R - L)/(R + L) ear differences on the two occasions. Significant correlations were obtained for consonants (r = .74) and for music ( r = .46). The correlation of .21 for vowels was not significant. The second approach was to see whether the direction (left/right) of a subject's ear preference at time 1 predicted its direction at time 2. For, while it is clear from the correlation coefficients that, at least for consonants and music, performance at time 1 is related to performance at time 2, such a relation ignores the fact that some subjects may actually have reversed ear advantages between time 1 and time 2. As research in dichotic listening has been concerned with differences in ear advantages rather than intercorrelations on dichotic tests, it is perhaps more important to consider the stability of the ear advantages across test presentations. Subjects who had equal right- and left-ear scores on either test administration (i.e., (R - L)/(R + L) = 0) were excluded from this analysis, because a change of a single point in either ear would have represented a change in ear dominance. T h e data concerning the distribution of rightand left-ear advantages on both occasions are summarized in Table 2. The data were treated by means of chi-square analyses. F o r those analyses in which cell members were < 5, Fisher's exact probability tests were conducted (cf. Ferguson, 1959). Looking first at the consonant data we find that 27 (71%) of 38 subjects retained their initial ear preferences. This breaks down to 23 of the 27 initially right-ear dominant and 4 of the 11 initially left-ear dominant subjects. In order to determine whether ear preference at time 2 is predicted by ear preference at time 1, Fisher's exact probability was computed yielding a nonsignificant p-value of .26. Consistency of ear preferences proved to be greater for music, where the overall figure for consistency was 29 (81%) of 36 S's; 24 of the 29 initially left-eared and 5 of the 7 initially right-eared retained their ear preference on re-test. T h e Fisher's exact probability for prediction from test to re-test on the music tape was .011. On vowels only 20 (64%) of 36 subjects retained their initial ear preferences, and these were in the same proportion for right- and left-ear
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231
TABLE 2 NUMBER OF SUBJECTS WHO SHOWED LEFT, RIGHT, OR NO EAR ADVANTAGES FOR TEST 1 AND TEST 2, AND NUMBER WHO MAINTAINED THESE ADVANTAGES FROM TEST 1 TO TEST 2 Consonants Test one Test two
Right-ear advantage
Left-ear advantage
Total
Right-ear advantage Left-ear advantage Total
23 4 27
7 4 11
30 8 38
No-ear advantages
Time 1 4
Time 2 0
Vowels Test one Test two
Right-ear advantage
Left-ear advantage
Total
Right-ear advantage Left-ear advantage Total
15 8 23
8 5 13
23 13 36
No-ear advantages
Time 1 3
Time 2 3
6
Music Test one Test two
Right-ear advantage
Left-ear advantage
Total
Right-ear advantage Left-ear advantage Total
5 5 10
2 24 26
7 29 36
No-ear advantages
Time 1 1
Time 2 5
6
d o m i n a n t s u b j e c t s . T h e chi s q u a r e f o r p r e d i c t i o n f r o m t e s t to r e - t e s t w a s o n l y .02, w e l l w i t h i n t h e r e a l m o f c h a n c e . G i v e n t h e s m a l l a b s o l u t e n u m b e r o f s u b j e c t s (11 f o r c o n s o n a n t s a n d 7 for m u s i c ) w h o fell o n t h e d e v i a n t side o f t h e d i s t r i b u t i o n , w e m a y q u e s tion whether the music tape really produces more stable ear preferences t h a n t h e c o n s o n a n t t a p e . C o n s i d e r i n g b o t h a n a l y s e s t o g e t h e r , h o w e v e r , it is h a r d to e s c a p e t h e c o n c l u s i o n t h a t e a r p r e f e r e n c e at t i m e 2 is n o n r a n d o m w i t h r e s p e c t to t h a t at t i m e 1, w i t h t h e p r o b a b l e e x c e p t i o n o f
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performance on vowels in a CV nonsense-syllable frame. At the same time the rate of reversals is very high. Does reversal of ear advantage occur randomly? Although the foregoing results tend to attest to the overall stability of ear preference in dichotic listening, we were concerned with those subjects who in fact reversed ear preferences between time 1 and time 2. Specifically, we wanted to determine if the proportion of people who shifted varied as a function of their initial ear advantage. That is, were those subjects who were left-eared on initial testing for consonants more likely to become right-eared than the reverse? Similarly, for music, was there more instability among subjects showing initial right-ear than initial left-ear advantages? The results are summarized in Table 3. Where appropriate, chisquare analyses or Fisher's test of exact probability were conducted. Analysis of the results indicated a significant effect for consonants (p = .001) and a nonsignificant effect for music (p = .88) and vowels (X 2 = 1.45, p < .30). Thus, for consonants, the probability for a subject who is right-eared on test 1 to remain right-eared is about .85, whereas the probability for a subject who is left-eared at time 1 to remain lefteared at time 2 is only about .3 6. Conversely for music, the probability of remaining left-eared on re-test is about .83, whereas the probability of remaining right-eared is .72. Again the vowel data are relatively unstable.
TABLE 3 NUMBER OF SUBJECTS WHO SWITCHED OR MAINTAINED EAR ADVANTAGE BETWEEN TIME 1 AND TIME 2 Consonants
Right ear (time 1) Left ear (time 1) Total
Switched advantage
Maintained advantage
Total
4 7 I1
23 4 27
27 11 38
Switched advantage
Maintained advantage
Total
8 8 16
15 5 20
23 13 36
Switched advantage
Maintained advantage
Total
5 24 29
7 29 36
Vowels
Right ear (time 1) Left ear (time 1) Total Music
Right ear (time 1) Left ear (time 1) Total
2 5 7
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Stability of ear laterality as a function of magnitude of ear differences. It is quite clear that there is a relatively high percentage of subjects who shift ear advantages on re-test. The question arises whether these subjects can be distinguished in any way from those subjects who maintain ear advantages across test sessions. One distinct possibility is that subjects who are highly lateralized, i.e., who show relatively high absolute ear difference scores, are less likely to shift ear preference than those who are not highly lateralized. In order to determine this, subjects were divided into two groups, high lateralizers and low lateralizers. High lateralizers were defined as subjects whose absolute ( R - L)/(R + L) index was at the median or higher; low lateralizers had an absolute (R - L)/(R + L) index below the median. T h e median score for consonants was 7.4, for music 8.5, for vowels 2.4. T h o s e subjects who showed no ear differences, i.e., an (R - L)/(R + L) score of 0, for time 1 were not included in the analysis. Chi-square tests revealed significant differences between high and low lateralizers for consonants (X 2 = 3.2, p < .10 > .05) and music (X ~ = 8.1, p < .01) and a nonsignificant difference for vowels (X 2 = .002). Thus, it would seem that, for the perception of consonants and music, high lateralizers are less likely to shift ear advantages than low lateralizers. Nevertheless, although high lateralizers tend to maintain their original ear advantage, the question still remains whether the maintained ear advantage reflects a smaller variability in score for these subjects between time 1 and time 2. In order to determine this, we first computed the absolute difference in ( R - L)/(R + L ) × 100 scores between time 1 and time 2 for each subject. F o r example, if a subject had an ( R - L)/(R + L) × 100 index o f - 2 on time 1 and an index o f + 1 9 on time 2, he would have an absolute shift of 21. T h e mean absolute difference between time 1 and time 2 for consonants was 6.51 with a standard deviation of 5.28, for music it was 7.65 with a standard deviation of 6.78 and for vowels it was 3.47 with a S D of 3.51. Pearson product moment correlations were computed between the absolute magnitude of ( R - L)/(R ÷ L) scores at time 1 and the absolute difference between time 1 and time 2 indices to determine if the high lateralizers had less variability than low lateralizers. T h e r e was a significant correlation for both consonants ( r = .3 l, p < .05) and music (r = .44 p < .01). Thus, the performance scores of time 1 and time 2 seem to be more stable for high lateralizers than for low lateralizers. Another way of looking at this question is to ask whether high lateralizers on consonants are the same individuals who are high lateralizers in music. A chi-square test revealed a nonsignificant relation (X 2 = .10) between high lateralizers for consonants and music. Thus, there seems to be little relation between a subject's performance on consonants and music.
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DISCUSSION The results of this study indicate that the distribution of subjects as either left- or right-ear dominant in dichotic tasks is not random: subjects tend to remain in the same category of ear superiority on re-test. This stability was most striking in the case of music, but short of significance in the case of consonants. In spite of the weakness of the latter finding, the underlying consistency in performance is reflected in the high rank order correlations for size and direction of ear differences between test 1 and test 2. The ear directionality for vowels is very weak and totally unstable on re-test. Despite the apparent stability of both consonant and music perception, the fact that as many as 29% of the subjects switch ear advantages for consonants, and 19% of the subjects switch ear advantages for music indicates the low reliability of the dichotic listening procedure for the investigation of ear dominance in any individual. In addition, the absolute variability of the subjects' performance on re-test is so large as to bring into question any assessment of a normal subject's auditory processing on the basis of a single testing. This is especially the case for those subjects who show very small ear advantages, as the average variability is often greater than individual ear difference scores. Because of this variability, the proportion of "true" right-eared to "true" left-eared subjects is difficult to determine. However, a good estimate can be obtained from that portion of the population which has the same ear preferences on both tests. For consonants, this amounts to 4/27 (15%) left-ear dominant and 23/27 (85%) right-ear dominant. This correction brings the proportion of left-ear dominant subjects for consonants down to little more than half of what it appeared to be from the first test administration. This correction is understandable if one assumes that approximately the same variability is imposed on the 15% of truly left-ear dominant and 85% of truly right-ear dominant subjects. A n y single sample is bound to show a greater spurious increase of leftear dominant than of right-ear dominant subjects. This explains our finding that there is a greater proportion of reversals on re-test for subjects in the deviant side of the ear preference distribution than for those on the modal side. The average percentage of subjects who are likely to appear in the wrong ear dominance column in any sampling can easily be estimated by solving a simple algebraic formula2 if we assume that: 1) the same per2 1. Let Rt and L t represent the true proportion of R and L ear advantages in the population. 2. L e t R0 and L0 represent the observed proportions of R and L ear advantages. 3. Let x be the percentage of individuals in each ear-advantage group who are misclassifted in any sample.
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centage applies to both left- and right-ear dominant subjects; 2) the subjects who reversed ears on re-test should be allocated to the left- and right-eared groups in the same proportion as the subjects who remained unchanged; 3) practice effects can be ignored and that tests 1 and 2 can be averaged as independent samples of the population, as determined by Step (2). Using the data of the present experiment the percent of false eardominance allocations is 14.58% for consonants, 9.75% for music, and 22.2% for vowels. These figures turn out to be half of the total percentage of individuals who reverse ear preference between test and retest. While there is no basis, from pathology, for estimating the proportion of individuals with left vs. right cerebral dominance for music, the evidence with respect to language is very strong, as we have noted. For right-handers, this proportion approaches 100% left-cerebral dominance. The estimated incidence of "true" left-ear superiority for dichotically presented consonants is 15%. The disparity between this finding and the near-zero incidence of aphasia from right-cerebral lesions needs to be reconciled. It might be suggested that auditory language processing, as measured by dichotic listening, may have a different distribution of brain laterality from speech production. If this were so, however, we should expect 15% of receptive aphasias in right-brain-injured right-handers. No such experience has been reported. Consequently, it appears that one or more factors, in addition to cerebral functional asymmetry of language processing, are contributing to the side of ear preference in dichotic listening. These factors are possibly peripheral to the cortical level-perhaps relating to the distribution of contralaterally to ipsilateraUy sensitive neurones in the lower auditory centers. It would enormously increase the utility of dichotic listening as an index of brain laterality, if these factors could be measured.
ACKNOWLEDGMENTS This work was supported in part by U S P H S Grants No. N S 07615 to Clark University, N S 10776 to Brown University, and N S 06209 to Boston University. T h e authors gratefully acknowledge Franklin S. Cooper for making available the facilities of H a s kins Laboratories, Michael S t u d d e r t - K e n n e d y for helping to construct the stimuli, and Errol Baker and Bill H u g g i n s for their critical c o m m e n t s and assistance.
REFERENCES Blumstein, S., & Cooper, W. 1972. Identification versus discrimination of distinctive features in speech perception. Quarterly Journal of Experimental Psychology, 24, 207-214.
4. T h e n R0 = Rt -- Rtx + Ltx and x = (R 0 -- Rt)/(L t -- Rt). T h e s a m e value of x is obtained by setting Lo in place of R0 and making the appropriate substitutions.
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Cooper, F. S., & Mattingly, 1. G. 1969. Computer-controlled PCM system for the investigation of dichotic speech perception. Paper presented at the 79th Meeting of the Acoustical Society of America, Philadelphia. Ferguson, G. 1966. Statistical analysis in psychology and education. New York: McGraw-Hill. Goodglass, H., & Quadfasel, F. A. 1954. Language laterality in left-handed aphasics. Brain, 77, 521-548. Kimura, D. 1964. Left-right differences in the presentation of melodies. Quarterly Journal of Experimental Psychology, 16, 355-358. Kimura, D. 1961. Some effects of temporal-lobe damage on auditory perception. Canadian Journal of Psychology, 16, 355-358. Krashen, S. 1972. An unbiased procedure for comparing degree of lateralization of dichotically presented stimuli. In Krashen, S. Language and the Left Hemisphere. Working Papers in Phonetics 24. Los Angeles: UCLA. Milner, B., Branch, C., & Rasmussen, T. 1966. Evidence for bilateral speech representation in some non-right-handers. Transactions of the American Neurological A ssociation. New York: Springer. Pp. 305-306. Roberts, L. 1966. Aphasia, apraxia and agnosia in abnormal states of cerebral dominance. In P. J. Vinken and G. W. Brnyn (Eds.) Handbook of Clinical Neurology, Vol. 4. Amsterdam: North-Holland Publishing Co. Pp. 312-326. Rossi, G. F., & Rosadini, G. 1967. Experimental analysis of cerebral dominance in man. In C. M. Millkan and F. C. Darley, (Eds.) Brain Mechanisms Underlying Speech and Language. New York: Grune and Stratton. Pp. 167-184. Satz, P., Achenbach, I., & Fennell, E. 1967. Relations between assessed manual laterality and predicted speech laterality. Neuropsychologia, 5, 295-310. Spellacy, F., & Blumstein, S. 1970. The influence of language set on ear preference in phoneme recognition. Cortex, 6, 430-439. Wada, J., & Rasmussen, T. 1960. Intracarotid injection of sodium amytal for the lateralization of cerebral speech dominance: Experimental and clinical observations. Journal of Neurosurgery, 17, 266-282. Zangwill, O. L. 1962. Dyslexia in relation to cerebral dominance. In J. Money (Ed.) Reading Disability. Baltimore: The Johns Hopkins Press.
The Reliability of Ear Advantage in Dichotic Listening SHEILA BLUMSTEIN, HAROLD
GOODGLASS, AND V I V I E N TARTTER
Brown University, Boston VA Hospital, and Department of Neurology, Boston University
Test-re-test reliability of dichotic listening performance on consonants, vowels, and music was investigated in a population of 42 right-handed subjects. Pearson product moment correlations between (R -- L)/(R ÷ L) ear scores on first and second tests were .74 for consonants, .21 for vowels, and .46 for music. Twenty nine percent of subjects reversed ear advantage for consonants on re-testing, 19% reversed for music, and 46% for vowels. Each type of stimulus reveals a significant subgroup (15% for consonants) who retain a deviant ear advantage on retest. In any sample, subjects whose ear advantage scores are on the deviant side are more likely to reverse ear advantages on re-test than subjects who score in the modal direction. These findings are interpreted and discussed in relation to the validity of dichotic listening as an index of cerebral functional asymmetry. There is an important disparity among the various indices of cerebral dominance in man. This disparity lies in the differing estimates of the prevalence of left-brain dominance in right-handers. Dichotic listening, the simplest and most widely used experimental technique, gives results that are the most divergent from the original criterion: the occurrence of aphasia following unilateral brain damage. The present study was undertaken to evaluate the test-re-test reliability of dichotic listening and to apply the results to a consideration of whether one criterion of cerebral dominance is more valid than others. Until the introduction of the W a d a intracarotid amytal technique (Wada & Rasmussen, 1960) the determination of language dominance in the brain relied on observing whether or not an individual developed aphasia after a unilateral lesion of the right or left hemisphere. By this criterion, the incidence of left-brain dominance for language approaches 99% for right-handers. In fact, after reviewing the evidence, Zangwill (1962) states, " F o r practical purposes, the probability of right cerebral dominance in a fully right-handed individual is so low that it may be disregarded." Estimates of left-brain dominance in non-right-handers range from 53% (Goodglass & Quadfasel, 1954)to 63% (Roberts, 1969). Results from the use of the Wada technique show a somewhat greater deviation from 100% correlation between right-handedness and leftcerebral language dominance. Milner, Branch, and Rasmussen report that 92% of 95 right-handed patients reacted with aphasia to left carotid amytal injection. Results reported by Rossi and Rosadini (1967) with this 226 Copyright © 1975 by Academic Press, Inc. AII rights of reproductionin any form reserved.
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technique come closer to experience with natural lesions. Of their 74 right-handed subjects, 73 (98.5%) became asphasic only after left carotid amytal injection and one (1.4%) after injection of either side. The problem in using these figures for estimates of the natural distribution of cerebral dominance is that patients subjected to the Wada technique include a number with probable early brain damage. These result in an indeterminate number of pathologically determined right-brain dominant subjects. It is clear that ear laterality effects in dichotic listening are related to differences in the degree of participation of the two brain-halves in various classes of perceptual processing. The left-ear superiority for musical stimuli (Kimura, 1964; Spellacy & Blumstein, 1970) and the right-ear superiority for verbal stimuli (Kimura, 1961) can hardly be explained on the basis of peripheral biasing effects, such as habitual attentional patterns. The chief drawback in interpreting these results is that the incidence of right-ear superiority is commonly reported as falling in the range between 75% (Efron, personal communication) to 87% (Satz, Aehenbach & Fennel, 1967). Zurif, Caramazza, and Carbone have unpublished data giving a figure of 81% in a sample of 30. Moreover, unilateral natural lesions and pharmacological effects produce all-or-none reactions in right-handed subjects; that is, a dramatic response from the left hemisphere and no response from the right. In contrast, inferences concerning brain laterality based on dichotic listening may involve differences between ear scores averaging 6% of the total per ear (Krashen, 1972). These observations raise a question as to the conditions under which dichotic listening is a valid criterion of brain laterality for language. It is possible that the variability in dichotic listening is such that, in any sample, about 20% of the subjects will randomly show a left-ear superiority for verbal material. On the other hand, it is possible that there is a subgroup of right-handers who consistently fall into the left-ear dominant side of the distribution of dichotic listening scores. The latter possibility implies that natural lesions, dichotie listening, and the Wada technique give three different estimates, ranging from 1 to 20% of right-brain language dominance in right-handers. A finding that a significant subgroup of right-handed subjects is consistently left eared for language would be contrary to the overwhelming weight of data from pathology. Such a finding would undermine the validity of dichotic listening as a diagnostic tool for individual determinations of brain laterality. METHOD Subjects. Subjects were 42 right-handed males from Brown University. All were native speakers of English with no k n o w n hearing loss. T h e y were paid for their participation. Stimuli. T h r e e dichotic tapes were constructed to assess the c o n s i s t e n c y of the dichotic
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listening paradigm in the perception of different types of auditory stimuli. T h e s e included a c o n s o n a n t tape, a vowel tape, and a m u s i c tape. T h e s e correspond to stimuli which are normally considered to be right-ear dominant, indeterminate in ear dominance, and left-ear dominant. Since the reliability of a test obviously depends on the size of the sample, the n u m b e r o f test items in each tape (detailed below) was greater than those usually reported in the literature. T h e c o n s o n a n t stimuli consisted of the six stop c o n s o n a n t s / p t k b d g / paired with the v o w e l / a / . T h e C V syllables were produced by a trained linguist (SB) with an even intensity and uniform pitch contour. T h e onsets of the stimuli were then edited and synchronized by m e a n s of the pulse code modulation s y s t e m made available at H a s k i n s Laboratories. In addition, overall intensity of each C V syllable was equalized. T h e duration of the syllables varied b e t w e e n 350 and 375 msec. T h e test items were paired so that there were equal occurrences of voice and place distinctive feature values (cf. Blumstein & Cooper, 1972, for details). T h e r e was a total of 40 pairs. T h e vowel stimuli consisted of the stop c o n s o n a n t / b / p a i r e d with the vowels / iI e e 3 o u u/. T h e dichotic tape was prepared by the same procedure as that used for the c o n s o n a n t tape. T h e vowel test items were paired so that there were equal occurrences of the distinctive feature values for the vowel phonemes. T h e r e was a total of 96 pairs on the dichotic tape. T h e music tape was a copy of K i m u r a ' s original dichotic music tape (cf. Kimura, 1964). T h e stimuli consisted of 20 dichotic pairs of melodies, each melody an excerpt from a solo passage of a classical concerto. Following each dichotic pair, a series of our melodies was presented binaurally, with instructions for the subject to identify the two which had been heard dichotically. Within each set the same instrument was used, and pitch range and t e m p o were similar. All melodies were of 4 sec duration (cf. Kimura, 1964, for further details). Procedure. Subjects were tested for consistency of ear advantages in the perception of consonants, vowels, and music. T h e order of presentation of the tapes was counterbalanced across subjects. Within each test order, earphones were reversed for half the subjects. Subjects were tested individually in a moderately quiet r o o m on a Sony T C 3 6 0 tape recorder with K o s s Pro-4 A A headphones. T h e voltage output of the two channels of the tape recorder was calibrated at approximately 70 dB sound pressure level. A test session lasted approximately one hour. E a c h subject was tested twice, the second session held at least one week after the first. Instructions, practice trials, and test order were the same for the two sessions. F o r the c o n s o n a n t s , subjects were told that they would hear two different simultaneous syllables, each taken from the s e t / p a , ta, ka, ba, da, ga/. Subjects were instructed to write the c o n s o n a n t s on an a n s w e r sheet and to g u e s s when unsure. T h e six C V syllables also appeared at the top of the a n s w e r sheet. Before the test, subjects heard the six syllables binaurally, and were given three practice dichotic trials. Forty test stimuli, with each pair separated by a 5 sec interval of silence, were then presented. A short break followed. Finally subjects were instructed to reverse their headphones, and the c o n s o n a n t tape was repeated. In all, subjects received a total of 80 dichotic c o n s o n a n t pairs. F o r the music tape, subjects were told that they would hear two different simultaneous melodies, followed 4 sec later by four binaural melodies, separated by 3 sec pauses. Subjects were instructed to m a t c h each of the dichotically presented melodies with one o f the four binaural melodies and to write on an a n s w e r sheet the n u m b e r corresponding to the position of the binaural melody in the sequence. T h e r e was a 3 sec interval b e t w e e n melodies. Again subjects were told to guess w h e n unsure. Before the test, subjects received two practice test items. After the 20 test stimuli were presented, subjects were given a rest. T h e y were then asked to reverse their h e a d p h o n e s and the music tape was replayed. T h e r e was a total of 40 dichotic pairs comprising the music tape. F o r the vowels, subjects were told that they would hear two simultaneous dichotic sylla-
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bles each taken from the s e t / b i bI be be b~ bo bo bu/. Subjects were instructed to report the two syllables orally and to g u e s s if necessary. R e s p o n s e s were simultaneously tape recorded on a U h e r 2000 Report L and transcribed by a trained phonologist (VT). Before each test there were three binaural presentations of the eight syllables. F o r the first presentation, subjects were instructed simply to listen and familiarize t h e m s e l v e s with the test items; for the other two presentations, subjects had to repeat each syllable as they heard it. This insured that they could produce all stimulus items on imitation. Subjects then received three dichotic practice items. T h e vowel tape consisted of 96 dichotic vowel pairs with a 5 sec silent interval b e t w e e n pairs. A rest was offered halfway through the test.
RESU LTS
Overall directions of ear advantages. Ear advantages were determined for both first and second presentations of the consonant tape, the vowel tape, and the music tape by computing the proportion of right- and left-ear correct responses in relation to the total correct responses (i.e., (R -- L)/(R + L) x 100). A positive index indicates an overall right ear advantage, a negative score a left ear advantage. This index accounts for lateral asymmetry in the context of overall level of accuracy (cf. StuddertKennedy & Shankweiler, 1970; Krashen, 1972). 1 The results, as summarized in Table 1, reveal a significant right ear advantage for consonants at bothtime 1 and time 2 (t = 3 . 5 3 , p < .01 andt = 4 . 4 4 , p < .001), a sig-
TABLE 1 EAR ADVANTAGES FOR CONSONANTS, VOWELS, AND MUSIC ON TWO OCCASIONS Consonants Mean n u m b e r correct time 1 time 2 time 1 time 2 time 1 time 2
L
R
49.05 55.60 52.95 60.30 Vowels 78.36 79.71 83.38 85.36 Music 30.00 26.57 31.14 27.48
(R - L)/(R + L) x 100
t-value
6.35 7.70
3.53*** 4.44****
.96 1.22
1.70" 1.98"*
-6.20 -4.28
-3.98**** -2.96***
* p--nonsignificant. ** p < .05, two-tailed. * * * p < .01, two-tailed. **** p < .02, two-tailed. 1 T h e data were analyzed not only in terms of the (R -- L)/(R + L) index but also in terms of absolute ear advantages, i.e., R -- L. T h e results of t h e s e analyses were virtually identical and thus there were no differences in the theoretical interpretations of the data based on either absolute or relative indices o f laterality.
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nificant left-ear advantage for music at both time 1 and time 2 (t = 3.98, p < .001 and t = 2.96, p < .01), a nonsignificant right-ear advantage for vowels at time 1 (t = 1.70) and time 2 (t = 1.98). These results replicate earlier findings for the lateralization of consonants, vowels, and music; namely, consistent right-ear advantages are obtained for consonants, consistent left-ear advantages for music, and a tendency for right-ear advantages for vowels. Consistency of ear advantages. T w o approaches were taken to determine to what extent performance on one occasion predicts performance on a second. T h e first was to determine Pearson product moment correlations between (R - L)/(R + L) ear differences on the two occasions. Significant correlations were obtained for consonants (r = .74) and for music ( r = .46). The correlation of .21 for vowels was not significant. The second approach was to see whether the direction (left/right) of a subject's ear preference at time 1 predicted its direction at time 2. For, while it is clear from the correlation coefficients that, at least for consonants and music, performance at time 1 is related to performance at time 2, such a relation ignores the fact that some subjects may actually have reversed ear advantages between time 1 and time 2. As research in dichotic listening has been concerned with differences in ear advantages rather than intercorrelations on dichotic tests, it is perhaps more important to consider the stability of the ear advantages across test presentations. Subjects who had equal right- and left-ear scores on either test administration (i.e., (R - L)/(R + L) = 0) were excluded from this analysis, because a change of a single point in either ear would have represented a change in ear dominance. T h e data concerning the distribution of rightand left-ear advantages on both occasions are summarized in Table 2. The data were treated by means of chi-square analyses. F o r those analyses in which cell members were < 5, Fisher's exact probability tests were conducted (cf. Ferguson, 1959). Looking first at the consonant data we find that 27 (71%) of 38 subjects retained their initial ear preferences. This breaks down to 23 of the 27 initially right-ear dominant and 4 of the 11 initially left-ear dominant subjects. In order to determine whether ear preference at time 2 is predicted by ear preference at time 1, Fisher's exact probability was computed yielding a nonsignificant p-value of .26. Consistency of ear preferences proved to be greater for music, where the overall figure for consistency was 29 (81%) of 36 S's; 24 of the 29 initially left-eared and 5 of the 7 initially right-eared retained their ear preference on re-test. T h e Fisher's exact probability for prediction from test to re-test on the music tape was .011. On vowels only 20 (64%) of 36 subjects retained their initial ear preferences, and these were in the same proportion for right- and left-ear
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TABLE 2 NUMBER OF SUBJECTS WHO SHOWED LEFT, RIGHT, OR NO EAR ADVANTAGES FOR TEST 1 AND TEST 2, AND NUMBER WHO MAINTAINED THESE ADVANTAGES FROM TEST 1 TO TEST 2 Consonants Test one Test two
Right-ear advantage
Left-ear advantage
Total
Right-ear advantage Left-ear advantage Total
23 4 27
7 4 11
30 8 38
No-ear advantages
Time 1 4
Time 2 0
Vowels Test one Test two
Right-ear advantage
Left-ear advantage
Total
Right-ear advantage Left-ear advantage Total
15 8 23
8 5 13
23 13 36
No-ear advantages
Time 1 3
Time 2 3
6
Music Test one Test two
Right-ear advantage
Left-ear advantage
Total
Right-ear advantage Left-ear advantage Total
5 5 10
2 24 26
7 29 36
No-ear advantages
Time 1 1
Time 2 5
6
d o m i n a n t s u b j e c t s . T h e chi s q u a r e f o r p r e d i c t i o n f r o m t e s t to r e - t e s t w a s o n l y .02, w e l l w i t h i n t h e r e a l m o f c h a n c e . G i v e n t h e s m a l l a b s o l u t e n u m b e r o f s u b j e c t s (11 f o r c o n s o n a n t s a n d 7 for m u s i c ) w h o fell o n t h e d e v i a n t side o f t h e d i s t r i b u t i o n , w e m a y q u e s tion whether the music tape really produces more stable ear preferences t h a n t h e c o n s o n a n t t a p e . C o n s i d e r i n g b o t h a n a l y s e s t o g e t h e r , h o w e v e r , it is h a r d to e s c a p e t h e c o n c l u s i o n t h a t e a r p r e f e r e n c e at t i m e 2 is n o n r a n d o m w i t h r e s p e c t to t h a t at t i m e 1, w i t h t h e p r o b a b l e e x c e p t i o n o f
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performance on vowels in a CV nonsense-syllable frame. At the same time the rate of reversals is very high. Does reversal of ear advantage occur randomly? Although the foregoing results tend to attest to the overall stability of ear preference in dichotic listening, we were concerned with those subjects who in fact reversed ear preferences between time 1 and time 2. Specifically, we wanted to determine if the proportion of people who shifted varied as a function of their initial ear advantage. That is, were those subjects who were left-eared on initial testing for consonants more likely to become right-eared than the reverse? Similarly, for music, was there more instability among subjects showing initial right-ear than initial left-ear advantages? The results are summarized in Table 3. Where appropriate, chisquare analyses or Fisher's test of exact probability were conducted. Analysis of the results indicated a significant effect for consonants (p = .001) and a nonsignificant effect for music (p = .88) and vowels (X 2 = 1.45, p < .30). Thus, for consonants, the probability for a subject who is right-eared on test 1 to remain right-eared is about .85, whereas the probability for a subject who is left-eared at time 1 to remain lefteared at time 2 is only about .3 6. Conversely for music, the probability of remaining left-eared on re-test is about .83, whereas the probability of remaining right-eared is .72. Again the vowel data are relatively unstable.
TABLE 3 NUMBER OF SUBJECTS WHO SWITCHED OR MAINTAINED EAR ADVANTAGE BETWEEN TIME 1 AND TIME 2 Consonants
Right ear (time 1) Left ear (time 1) Total
Switched advantage
Maintained advantage
Total
4 7 I1
23 4 27
27 11 38
Switched advantage
Maintained advantage
Total
8 8 16
15 5 20
23 13 36
Switched advantage
Maintained advantage
Total
5 24 29
7 29 36
Vowels
Right ear (time 1) Left ear (time 1) Total Music
Right ear (time 1) Left ear (time 1) Total
2 5 7
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Stability of ear laterality as a function of magnitude of ear differences. It is quite clear that there is a relatively high percentage of subjects who shift ear advantages on re-test. The question arises whether these subjects can be distinguished in any way from those subjects who maintain ear advantages across test sessions. One distinct possibility is that subjects who are highly lateralized, i.e., who show relatively high absolute ear difference scores, are less likely to shift ear preference than those who are not highly lateralized. In order to determine this, subjects were divided into two groups, high lateralizers and low lateralizers. High lateralizers were defined as subjects whose absolute ( R - L)/(R + L) index was at the median or higher; low lateralizers had an absolute (R - L)/(R + L) index below the median. T h e median score for consonants was 7.4, for music 8.5, for vowels 2.4. T h o s e subjects who showed no ear differences, i.e., an (R - L)/(R + L) score of 0, for time 1 were not included in the analysis. Chi-square tests revealed significant differences between high and low lateralizers for consonants (X 2 = 3.2, p < .10 > .05) and music (X ~ = 8.1, p < .01) and a nonsignificant difference for vowels (X 2 = .002). Thus, it would seem that, for the perception of consonants and music, high lateralizers are less likely to shift ear advantages than low lateralizers. Nevertheless, although high lateralizers tend to maintain their original ear advantage, the question still remains whether the maintained ear advantage reflects a smaller variability in score for these subjects between time 1 and time 2. In order to determine this, we first computed the absolute difference in ( R - L)/(R + L ) × 100 scores between time 1 and time 2 for each subject. F o r example, if a subject had an ( R - L)/(R + L) × 100 index o f - 2 on time 1 and an index o f + 1 9 on time 2, he would have an absolute shift of 21. T h e mean absolute difference between time 1 and time 2 for consonants was 6.51 with a standard deviation of 5.28, for music it was 7.65 with a standard deviation of 6.78 and for vowels it was 3.47 with a S D of 3.51. Pearson product moment correlations were computed between the absolute magnitude of ( R - L)/(R ÷ L) scores at time 1 and the absolute difference between time 1 and time 2 indices to determine if the high lateralizers had less variability than low lateralizers. T h e r e was a significant correlation for both consonants ( r = .3 l, p < .05) and music (r = .44 p < .01). Thus, the performance scores of time 1 and time 2 seem to be more stable for high lateralizers than for low lateralizers. Another way of looking at this question is to ask whether high lateralizers on consonants are the same individuals who are high lateralizers in music. A chi-square test revealed a nonsignificant relation (X 2 = .10) between high lateralizers for consonants and music. Thus, there seems to be little relation between a subject's performance on consonants and music.
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DISCUSSION The results of this study indicate that the distribution of subjects as either left- or right-ear dominant in dichotic tasks is not random: subjects tend to remain in the same category of ear superiority on re-test. This stability was most striking in the case of music, but short of significance in the case of consonants. In spite of the weakness of the latter finding, the underlying consistency in performance is reflected in the high rank order correlations for size and direction of ear differences between test 1 and test 2. The ear directionality for vowels is very weak and totally unstable on re-test. Despite the apparent stability of both consonant and music perception, the fact that as many as 29% of the subjects switch ear advantages for consonants, and 19% of the subjects switch ear advantages for music indicates the low reliability of the dichotic listening procedure for the investigation of ear dominance in any individual. In addition, the absolute variability of the subjects' performance on re-test is so large as to bring into question any assessment of a normal subject's auditory processing on the basis of a single testing. This is especially the case for those subjects who show very small ear advantages, as the average variability is often greater than individual ear difference scores. Because of this variability, the proportion of "true" right-eared to "true" left-eared subjects is difficult to determine. However, a good estimate can be obtained from that portion of the population which has the same ear preferences on both tests. For consonants, this amounts to 4/27 (15%) left-ear dominant and 23/27 (85%) right-ear dominant. This correction brings the proportion of left-ear dominant subjects for consonants down to little more than half of what it appeared to be from the first test administration. This correction is understandable if one assumes that approximately the same variability is imposed on the 15% of truly left-ear dominant and 85% of truly right-ear dominant subjects. A n y single sample is bound to show a greater spurious increase of leftear dominant than of right-ear dominant subjects. This explains our finding that there is a greater proportion of reversals on re-test for subjects in the deviant side of the ear preference distribution than for those on the modal side. The average percentage of subjects who are likely to appear in the wrong ear dominance column in any sampling can easily be estimated by solving a simple algebraic formula2 if we assume that: 1) the same per2 1. Let Rt and L t represent the true proportion of R and L ear advantages in the population. 2. L e t R0 and L0 represent the observed proportions of R and L ear advantages. 3. Let x be the percentage of individuals in each ear-advantage group who are misclassifted in any sample.
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centage applies to both left- and right-ear dominant subjects; 2) the subjects who reversed ears on re-test should be allocated to the left- and right-eared groups in the same proportion as the subjects who remained unchanged; 3) practice effects can be ignored and that tests 1 and 2 can be averaged as independent samples of the population, as determined by Step (2). Using the data of the present experiment the percent of false eardominance allocations is 14.58% for consonants, 9.75% for music, and 22.2% for vowels. These figures turn out to be half of the total percentage of individuals who reverse ear preference between test and retest. While there is no basis, from pathology, for estimating the proportion of individuals with left vs. right cerebral dominance for music, the evidence with respect to language is very strong, as we have noted. For right-handers, this proportion approaches 100% left-cerebral dominance. The estimated incidence of "true" left-ear superiority for dichotically presented consonants is 15%. The disparity between this finding and the near-zero incidence of aphasia from right-cerebral lesions needs to be reconciled. It might be suggested that auditory language processing, as measured by dichotic listening, may have a different distribution of brain laterality from speech production. If this were so, however, we should expect 15% of receptive aphasias in right-brain-injured right-handers. No such experience has been reported. Consequently, it appears that one or more factors, in addition to cerebral functional asymmetry of language processing, are contributing to the side of ear preference in dichotic listening. These factors are possibly peripheral to the cortical level-perhaps relating to the distribution of contralaterally to ipsilateraUy sensitive neurones in the lower auditory centers. It would enormously increase the utility of dichotic listening as an index of brain laterality, if these factors could be measured.
ACKNOWLEDGMENTS This work was supported in part by U S P H S Grants No. N S 07615 to Clark University, N S 10776 to Brown University, and N S 06209 to Boston University. T h e authors gratefully acknowledge Franklin S. Cooper for making available the facilities of H a s kins Laboratories, Michael S t u d d e r t - K e n n e d y for helping to construct the stimuli, and Errol Baker and Bill H u g g i n s for their critical c o m m e n t s and assistance.
REFERENCES Blumstein, S., & Cooper, W. 1972. Identification versus discrimination of distinctive features in speech perception. Quarterly Journal of Experimental Psychology, 24, 207-214.
4. T h e n R0 = Rt -- Rtx + Ltx and x = (R 0 -- Rt)/(L t -- Rt). T h e s a m e value of x is obtained by setting Lo in place of R0 and making the appropriate substitutions.
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Cooper, F. S., & Mattingly, 1. G. 1969. Computer-controlled PCM system for the investigation of dichotic speech perception. Paper presented at the 79th Meeting of the Acoustical Society of America, Philadelphia. Ferguson, G. 1966. Statistical analysis in psychology and education. New York: McGraw-Hill. Goodglass, H., & Quadfasel, F. A. 1954. Language laterality in left-handed aphasics. Brain, 77, 521-548. Kimura, D. 1964. Left-right differences in the presentation of melodies. Quarterly Journal of Experimental Psychology, 16, 355-358. Kimura, D. 1961. Some effects of temporal-lobe damage on auditory perception. Canadian Journal of Psychology, 16, 355-358. Krashen, S. 1972. An unbiased procedure for comparing degree of lateralization of dichotically presented stimuli. In Krashen, S. Language and the Left Hemisphere. Working Papers in Phonetics 24. Los Angeles: UCLA. Milner, B., Branch, C., & Rasmussen, T. 1966. Evidence for bilateral speech representation in some non-right-handers. Transactions of the American Neurological A ssociation. New York: Springer. Pp. 305-306. Roberts, L. 1966. Aphasia, apraxia and agnosia in abnormal states of cerebral dominance. In P. J. Vinken and G. W. Brnyn (Eds.) Handbook of Clinical Neurology, Vol. 4. Amsterdam: North-Holland Publishing Co. Pp. 312-326. Rossi, G. F., & Rosadini, G. 1967. Experimental analysis of cerebral dominance in man. In C. M. Millkan and F. C. Darley, (Eds.) Brain Mechanisms Underlying Speech and Language. New York: Grune and Stratton. Pp. 167-184. Satz, P., Achenbach, I., & Fennell, E. 1967. Relations between assessed manual laterality and predicted speech laterality. Neuropsychologia, 5, 295-310. Spellacy, F., & Blumstein, S. 1970. The influence of language set on ear preference in phoneme recognition. Cortex, 6, 430-439. Wada, J., & Rasmussen, T. 1960. Intracarotid injection of sodium amytal for the lateralization of cerebral speech dominance: Experimental and clinical observations. Journal of Neurosurgery, 17, 266-282. Zangwill, O. L. 1962. Dyslexia in relation to cerebral dominance. In J. Money (Ed.) Reading Disability. Baltimore: The Johns Hopkins Press.
