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Attenuation of dichotic-listening ear advantages by stimulus bias
Neuropsychologirr, Pnnred m Great
Vol. 23, No. 3, pp Bntam.
437 440,
1985. 0
002x-3932,‘xs $3 oo+o.oo 19X5 Pergamon Press Ltd
NOTE ATTENUATION
OF DICHOTIC-LISTENING STIMULUS BIAS
EAR ADVANTAGES
BY
MARGARET-ELLEN PIPE Victoria
University
of Wellington,
New Zealand
(Accepted 6 December 1984) Abstract--Analysis of data from a dichotic-listening experiment showed that biases of normal and retarded children towards reporting specific stimuli (stimulus bias) were inversely related to ear advantages and were reduced by auditory discrimination training.
INTRODUCTION The ear asymmetry effect in dichotic listening has been widely used to assess functional hemispheric asymmetries. However, ear asymmetries are frequently smaller [4, lo], less reliable [l, 121, and less frequently in favour of the right ear [12, 13, 141 than would be expectedifhemispheric asymmetries were their sole determinants. WEXLERet al. [13], for example, found that some 50% of their subjects failed to show statistically significant ear advantages to either ear, although 93 Y0of siyn$canr advantages favoured the right ear. Wexler et al.‘s data are, therefore, generally consistent with clinical estimates that more than 95% of right-handers have left-hemisphere speech, if it can be assumed that the non-significant ear advantages reflected the influence offactors other than functional asymmetries. In particular, small asymmetries may reflect constraints by task and performance related factors [2, 81. BRYSONet al. [2] found a significant ear asymmetry for young children when the interval between dichotic pairs was subject determined, but not when it was set at 10 sec. PIPE [S] found stronger ear advantages for groups of normal and retarded children when dichotic testing followed auditory discrimination training; prior to training ear advantages appeared to have been attenuated by performance factors. A likely source of attenuation of dichotic ear advantages is what REPP [9] has described as stimulus dominance. Stimulus dominance, or Stimulus bias as I shall call it, refers to a bias towards reporting one stimulus rather than the other of a dichoticaliy presented pair independently of the ears to which the stimuli are presented. Since stimuli are typically presented equally often to each ear over trials, the effect of such biases is to attenuate measured ear advantages. In the present note. data reported by PIPE [S] were reanalysed in order to quantify the relation between stimulus bias and ear asymmetry effects and, in particular, to determine whether the increase in ear advantages with training demonstrated in the previous study resulted from changes in stimulus bias.
METHOD
AND
RESULTS
Subjects and procedure were described in detail in the previous study [S]. In brief, 10 normal, five developmentally retarded and five Down’s syndrome children were tested on a dichotic test twice before (Tests 1 and 2) and three times after (Tests 3,4 and 5) monaural auditory discrimination training with words from the dichotic test. The test consisted of six presentations of each of six word pairs, namely ball-doll, cat-bat, coat-goat, gun-bun, pig-dig and tin-pin. Subjects responded non-verbally, either by pointing to pictures (Tests I, 2,3 and 5) or by moving response panels on a stimulus board (Training and Test 4). A measure of stimulus bias analogous to that for an ear advantage was obtained as follows. Table 1 summarizes the result from the usual dichotic-listening procedure in which two stimuli of a dichotic pair, S 1 and S2, are presented to the left and right ears, with entries in the table representing the probability or frequency of report of each stimulus when presented at each ear. The ear advantage was expressed as the difference between right- and left-ear reports, that is the difference between the column totals of Table 1, relative to total reports, following STUDDERT-KENNEDY and SHANKWEILER[ll]. Disregarding the direction of the difference to give a measure of absolute ear advantage (AEA), this measure is 437
438
NOTE
(A+C)-(B+D) mA+B+CrD-m
AEA=
(1)
x 100
Equation (1) provides a measure of the extent to which a subject’s response IS determmed by the ear of presentation of the stimuli, and can take a maximum value of lOO”d when no more than one stimulus IS reported on each trial (6). An analogous measure ofstimulus btas (SB), that is, the extent to which Sl and S2 differentially btased the subject’s report, 1s given by the differences between the row totals in Table 1, that is,
(A+B)-(CfD) ~ ~ ~~~ _~.~ A+B+C+D
SB=
X 100,
(2)
Measures of stimulus bias were calculated for each subject on each test by summing frequencies obtained according to equation (2) over the different dichotic pairs. Group mean SB measures for each test are shown in Fig. 1 and were submitted to a two factor (3 groups x 5 tests) analysis of variance for repeated measures on the factor of tests (least-squares solution, 15). Stimulus bias decreased significantly over tests [F (4, 68)=4.80, P
1 Ear of presentation
Stimulus
STIMULUS
E
pair
Right
Left
Sl
A
B
52
C
D
BIAS
L
%@
- AEA
,’ 4 _/=,’ 5 20 - Or Y - NORMAL 2 d
P- ---LX__.+
1 2 3 -b------
t I L
5
,o----o-___o ,’ I/ O--__o, OOWNS NNDRCPIE
1
3
DICHOTIC
FIG. 1. Mean stunulus
bias (SB) measures
L
5
TEST
and mean absolute
ear advantages
(AEA) for Tests l-5
Figure 1 also shows the group mean AEAs for each of the five tests reported by PIPE [S]. Whereas AEAs increased with training over the five tests, SB measures decreased m a perfectly complementary manner. Further evidence of the inverse relation comes from the significant (P
439
NOTE
-0
20
LO
60
PER CENT STIMULUS
80
xx)
BIAS
FIG. 2. The relation between measures of stimulus bias and absolute ear advantage for individual subjects. Data points for Tests 1 and 2 prior to training are shown as closed symbols, and points for Tests 3,4 and 5 following training are shown as open symbols, Circles, triangles and squares represent points for Normal, Down’s Syndrome and Developmentally Retarded children, respectively.
DISCUSSION The data reported here suggest that the effect of training in increasing the magnitude of dichotic-listening ear asymmetries reported by PIPE[S] resulted from the reductive effect of training on stimulus bias. Further, the present data indiacate that stimulus bias may be a major determinant of dichotic-listening performance. There have, however, been only a few attempts to control for the effects of biases to respond to specific stimuli [S, 9. 141. HARTLEY[S], for Instance, eliminated trials on which there was ‘excessive skewing’ in report of one word of a dichotic pair from ear advantage analyses. WEXLER and HALU.ES [14] selected the words for their dichotic rhymedwords test so as to minimize the influence of stimulus dominance and suggested that scoring procedures that deal separately with stimulus and ear effects, such as that proposed by REPP [9], might also be necessary. The present analysis indicates that auditory discrimination training is another approach effective in reducing the influence of stimulus specific factors on dichotic-listening performance, at least with retarded and young children. The finding that ear advantages and stimulus bias are inversely related provides empirical support for COLROLIRN’S [3] general argument that variations in ear asymmetries across subjects or stimuli do not necessarily reflect variations in the degree of asymmetry involved in processing the stimuli presented. In particular, the present analysis indicates that the contribution of variation in hemispheric asymmetry is confounded with stimulus-specific effects. The critical issue raised by the present analysis concerns the interpretation of small ear advantages. For example, although it may be theoretically convenient to find that groups of language disordered children fail to show a significant ear asymmetry if hypotheses concerning hemispheric specialization are to be supported, the present data suggest that failures to find reliable ear advantages are uninterpretable unless attempts are made to eliminate sources of bias in dichotic-listening measures. Such attempts may yield estimates of left-hemisphere speech based on dichotic listening that more closely approximate estimates based on clinical measures 112, 13, 141. A~knowlrdyumrr~rs~~ This research was supported by grant 156/80 from Victoria University of Wellington and by a Claude McCarthy Postdoctoral Fellowship to the author. Dr A. P. McLean and Dr K. G. White contributed helpful comments. Reprint requests should be addressed to Dr M.-E.Pipe. Department of Psychology, Victoria University of Wellington, Private Bag, Wellington, New Zealand.
REFERENCES I. BWMSTI’IN,S.,G~~~x~t.~ss,H. and TARTIER, V. The reliability of ear advantage
in dichotic listening. Bruirl Lung.
2. 226 -236. 1975.
2. BRYSON,S.. MONONEN.L. J. and Yu. L. Procedural children. Neuro~~.s~,cho/~~irr 18, 243 246. 1980.
constraints
on the measurement
of laterality
in young
440
NOTE
3. COLBOURN, C. J. Can laterality be measured? Neuropsycholqia 16, 283-289, 197X. 4. GEFFEN, G., TRAUB, E. and STIERMAN, I. Language laterality assessed by unilateral ECT and dichotic monitoring. J. Neural. Neurosur-y. Psych&. 41, 354360, 197X. 5. HARTLEY, X. Y. Lateralization of speech stimuli in young Down’s syndrome children. Cortex 17. 241-248. 1981. 6. KUHN, G. M. The phi coefficient as an index of ear differences in dichotic listening. Cortex 9,447457, 1973. aspects of focal epilepsy and its neurosurgical management. In Advances in 7. MILNER, B. Psychological Neurolo,qy, D. P. PURPURA. J. K. PENRY and R. D. WALTER (Editors), Vol. 8. Raven Press, New York, 1975. 8. PIPE, M.-E. Dichotic-listening performance following auditory discrimination training in Down’s syndrome and developmentally retarded children. Cortex 19, 481-491, 1983. 9. REPP. B. H. Measuring laterality effects in dichotic listening. J. ucoust. Sot. Am. 62, 72CL-737, 1977. Psychol. Bull. 84, 503-528, 1977. IO. SEARLEMAN, A. A review of right hemisphere linguistic capabilities. 11. STUDDERT-KENNEDY, M. and SHANKWEILER, D. Hemispheric specialization for speech perception. J. UCOUSI. Sot. Am. 48, 579-594, 1970. 12. TENG, E. L. Dichotic ear difference is a poor index for the functional asymmetry between the cerebral hemispheres. Neuropsychologia 19, 235-240, 198 1. 13. WEXI.ER, B. E., H~LWES, T. and HENINGER, G. R. Use of a statistical significance criterion in drawing inferences about hemispheric dominance for language function from dichotic listening data. Brain Lang. 13, 13-18, 1981. 14. WEXI.ER, B. E. and HA~LES, T. Increasing the power of dichotic methods: the fused rhymed words test. NeuropsJ~chologia 21, 59--66, 19X3. 15. WINER, B. J. Sfatistical Principles in Experimental Design (2nd edn.). McGraw-Hill, Kogakusha, 1971.
Vol. 23, No. 3, pp Bntam.
437 440,
1985. 0
002x-3932,‘xs $3 oo+o.oo 19X5 Pergamon Press Ltd
NOTE ATTENUATION
OF DICHOTIC-LISTENING STIMULUS BIAS
EAR ADVANTAGES
BY
MARGARET-ELLEN PIPE Victoria
University
of Wellington,
New Zealand
(Accepted 6 December 1984) Abstract--Analysis of data from a dichotic-listening experiment showed that biases of normal and retarded children towards reporting specific stimuli (stimulus bias) were inversely related to ear advantages and were reduced by auditory discrimination training.
INTRODUCTION The ear asymmetry effect in dichotic listening has been widely used to assess functional hemispheric asymmetries. However, ear asymmetries are frequently smaller [4, lo], less reliable [l, 121, and less frequently in favour of the right ear [12, 13, 141 than would be expectedifhemispheric asymmetries were their sole determinants. WEXLERet al. [13], for example, found that some 50% of their subjects failed to show statistically significant ear advantages to either ear, although 93 Y0of siyn$canr advantages favoured the right ear. Wexler et al.‘s data are, therefore, generally consistent with clinical estimates that more than 95% of right-handers have left-hemisphere speech, if it can be assumed that the non-significant ear advantages reflected the influence offactors other than functional asymmetries. In particular, small asymmetries may reflect constraints by task and performance related factors [2, 81. BRYSONet al. [2] found a significant ear asymmetry for young children when the interval between dichotic pairs was subject determined, but not when it was set at 10 sec. PIPE [S] found stronger ear advantages for groups of normal and retarded children when dichotic testing followed auditory discrimination training; prior to training ear advantages appeared to have been attenuated by performance factors. A likely source of attenuation of dichotic ear advantages is what REPP [9] has described as stimulus dominance. Stimulus dominance, or Stimulus bias as I shall call it, refers to a bias towards reporting one stimulus rather than the other of a dichoticaliy presented pair independently of the ears to which the stimuli are presented. Since stimuli are typically presented equally often to each ear over trials, the effect of such biases is to attenuate measured ear advantages. In the present note. data reported by PIPE [S] were reanalysed in order to quantify the relation between stimulus bias and ear asymmetry effects and, in particular, to determine whether the increase in ear advantages with training demonstrated in the previous study resulted from changes in stimulus bias.
METHOD
AND
RESULTS
Subjects and procedure were described in detail in the previous study [S]. In brief, 10 normal, five developmentally retarded and five Down’s syndrome children were tested on a dichotic test twice before (Tests 1 and 2) and three times after (Tests 3,4 and 5) monaural auditory discrimination training with words from the dichotic test. The test consisted of six presentations of each of six word pairs, namely ball-doll, cat-bat, coat-goat, gun-bun, pig-dig and tin-pin. Subjects responded non-verbally, either by pointing to pictures (Tests I, 2,3 and 5) or by moving response panels on a stimulus board (Training and Test 4). A measure of stimulus bias analogous to that for an ear advantage was obtained as follows. Table 1 summarizes the result from the usual dichotic-listening procedure in which two stimuli of a dichotic pair, S 1 and S2, are presented to the left and right ears, with entries in the table representing the probability or frequency of report of each stimulus when presented at each ear. The ear advantage was expressed as the difference between right- and left-ear reports, that is the difference between the column totals of Table 1, relative to total reports, following STUDDERT-KENNEDY and SHANKWEILER[ll]. Disregarding the direction of the difference to give a measure of absolute ear advantage (AEA), this measure is 437
438
NOTE
(A+C)-(B+D) mA+B+CrD-m
AEA=
(1)
x 100
Equation (1) provides a measure of the extent to which a subject’s response IS determmed by the ear of presentation of the stimuli, and can take a maximum value of lOO”d when no more than one stimulus IS reported on each trial (6). An analogous measure ofstimulus btas (SB), that is, the extent to which Sl and S2 differentially btased the subject’s report, 1s given by the differences between the row totals in Table 1, that is,
(A+B)-(CfD) ~ ~ ~~~ _~.~ A+B+C+D
SB=
X 100,
(2)
Measures of stimulus bias were calculated for each subject on each test by summing frequencies obtained according to equation (2) over the different dichotic pairs. Group mean SB measures for each test are shown in Fig. 1 and were submitted to a two factor (3 groups x 5 tests) analysis of variance for repeated measures on the factor of tests (least-squares solution, 15). Stimulus bias decreased significantly over tests [F (4, 68)=4.80, P
1 Ear of presentation
Stimulus
STIMULUS
E
pair
Right
Left
Sl
A
B
52
C
D
BIAS
L
%@
- AEA
,’ 4 _/=,’ 5 20 - Or Y - NORMAL 2 d
P- ---LX__.+
1 2 3 -b------
t I L
5
,o----o-___o ,’ I/ O--__o, OOWNS NNDRCPIE
1
3
DICHOTIC
FIG. 1. Mean stunulus
bias (SB) measures
L
5
TEST
and mean absolute
ear advantages
(AEA) for Tests l-5
Figure 1 also shows the group mean AEAs for each of the five tests reported by PIPE [S]. Whereas AEAs increased with training over the five tests, SB measures decreased m a perfectly complementary manner. Further evidence of the inverse relation comes from the significant (P
439
NOTE
-0
20
LO
60
PER CENT STIMULUS
80
xx)
BIAS
FIG. 2. The relation between measures of stimulus bias and absolute ear advantage for individual subjects. Data points for Tests 1 and 2 prior to training are shown as closed symbols, and points for Tests 3,4 and 5 following training are shown as open symbols, Circles, triangles and squares represent points for Normal, Down’s Syndrome and Developmentally Retarded children, respectively.
DISCUSSION The data reported here suggest that the effect of training in increasing the magnitude of dichotic-listening ear asymmetries reported by PIPE[S] resulted from the reductive effect of training on stimulus bias. Further, the present data indiacate that stimulus bias may be a major determinant of dichotic-listening performance. There have, however, been only a few attempts to control for the effects of biases to respond to specific stimuli [S, 9. 141. HARTLEY[S], for Instance, eliminated trials on which there was ‘excessive skewing’ in report of one word of a dichotic pair from ear advantage analyses. WEXLER and HALU.ES [14] selected the words for their dichotic rhymedwords test so as to minimize the influence of stimulus dominance and suggested that scoring procedures that deal separately with stimulus and ear effects, such as that proposed by REPP [9], might also be necessary. The present analysis indicates that auditory discrimination training is another approach effective in reducing the influence of stimulus specific factors on dichotic-listening performance, at least with retarded and young children. The finding that ear advantages and stimulus bias are inversely related provides empirical support for COLROLIRN’S [3] general argument that variations in ear asymmetries across subjects or stimuli do not necessarily reflect variations in the degree of asymmetry involved in processing the stimuli presented. In particular, the present analysis indicates that the contribution of variation in hemispheric asymmetry is confounded with stimulus-specific effects. The critical issue raised by the present analysis concerns the interpretation of small ear advantages. For example, although it may be theoretically convenient to find that groups of language disordered children fail to show a significant ear asymmetry if hypotheses concerning hemispheric specialization are to be supported, the present data suggest that failures to find reliable ear advantages are uninterpretable unless attempts are made to eliminate sources of bias in dichotic-listening measures. Such attempts may yield estimates of left-hemisphere speech based on dichotic listening that more closely approximate estimates based on clinical measures 112, 13, 141. A~knowlrdyumrr~rs~~ This research was supported by grant 156/80 from Victoria University of Wellington and by a Claude McCarthy Postdoctoral Fellowship to the author. Dr A. P. McLean and Dr K. G. White contributed helpful comments. Reprint requests should be addressed to Dr M.-E.Pipe. Department of Psychology, Victoria University of Wellington, Private Bag, Wellington, New Zealand.
REFERENCES I. BWMSTI’IN,S.,G~~~x~t.~ss,H. and TARTIER, V. The reliability of ear advantage
in dichotic listening. Bruirl Lung.
2. 226 -236. 1975.
2. BRYSON,S.. MONONEN.L. J. and Yu. L. Procedural children. Neuro~~.s~,cho/~~irr 18, 243 246. 1980.
constraints
on the measurement
of laterality
in young
440
NOTE
3. COLBOURN, C. J. Can laterality be measured? Neuropsycholqia 16, 283-289, 197X. 4. GEFFEN, G., TRAUB, E. and STIERMAN, I. Language laterality assessed by unilateral ECT and dichotic monitoring. J. Neural. Neurosur-y. Psych&. 41, 354360, 197X. 5. HARTLEY, X. Y. Lateralization of speech stimuli in young Down’s syndrome children. Cortex 17. 241-248. 1981. 6. KUHN, G. M. The phi coefficient as an index of ear differences in dichotic listening. Cortex 9,447457, 1973. aspects of focal epilepsy and its neurosurgical management. In Advances in 7. MILNER, B. Psychological Neurolo,qy, D. P. PURPURA. J. K. PENRY and R. D. WALTER (Editors), Vol. 8. Raven Press, New York, 1975. 8. PIPE, M.-E. Dichotic-listening performance following auditory discrimination training in Down’s syndrome and developmentally retarded children. Cortex 19, 481-491, 1983. 9. REPP. B. H. Measuring laterality effects in dichotic listening. J. ucoust. Sot. Am. 62, 72CL-737, 1977. Psychol. Bull. 84, 503-528, 1977. IO. SEARLEMAN, A. A review of right hemisphere linguistic capabilities. 11. STUDDERT-KENNEDY, M. and SHANKWEILER, D. Hemispheric specialization for speech perception. J. UCOUSI. Sot. Am. 48, 579-594, 1970. 12. TENG, E. L. Dichotic ear difference is a poor index for the functional asymmetry between the cerebral hemispheres. Neuropsychologia 19, 235-240, 198 1. 13. WEXI.ER, B. E., H~LWES, T. and HENINGER, G. R. Use of a statistical significance criterion in drawing inferences about hemispheric dominance for language function from dichotic listening data. Brain Lang. 13, 13-18, 1981. 14. WEXI.ER, B. E. and HA~LES, T. Increasing the power of dichotic methods: the fused rhymed words test. NeuropsJ~chologia 21, 59--66, 19X3. 15. WINER, B. J. Sfatistical Principles in Experimental Design (2nd edn.). McGraw-Hill, Kogakusha, 1971.