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Language lateralization in bilinguals: More not less is needed: A reply to Paradis (1990)
BRAIN
AND
LANGUAGE
43, 528-533 (1992)
Language Lateralization in Bilinguals: More Not Less Is Needed: A Reply to Paradis (1990) ANNE BERQUIER AND RODERICK ASHTON University
of Queensland
Paradis (1990) took an extremely pessimistic view of research to date looking at the laterahzation of cerebral function in bilingual subjects. We, however, argue that some of his conclusions are, in fact, incorrect, while others speak more to the youth of the field. The problems in the latter body of work are now acknowledged and are being overcome. That the shortcomings are recognized seems to be more a case for satisfaction than of despair. o IWZ Academic press, IW.
Paradis (1990) quite rightly has taken neurolinguists to task for their neglect of methodological, subject selection and statistical problems in assessing lateralization of cerebral function in bilingual people. He then concludes, however, that studies of bilingual lateralization should be abandoned altogether because of the resultant inconsistent findings. In recent times methodologies for assessinglateralization have been refined, leading to greater validity. It is also possible, in retrospect, to delineate possible confounds in previous studies and exercise greater control in the future. Although it is not within the scope of this paper to review comprehensively all experimental or clinical problems associated with lateralization studies of bilinguals, indeed Paradis himself did not attempt this, several factors nevertheless deserve mention. EXPERIMENTAL STUDIES Recent studies have questioned propositions of discrete left hemisphere language mediation in monolinguals. Right hemisphere processing in this group has been suggested for concrete words (Paivio, 1971; Zaidel, 1983; Chiarello, 1985), high imagery words (Deloche, Seron, Scuis, & Segui, 1987), and high frequency words (Van Lanker & Kempler, 1987). In this context, the choice of verbal task variables to assessbilingual lateralization Address reprint requests to Roderick Ashton at the Department of Psychology, The University of Queensland, St. Lucia, Q4072, Australia. 528 0093-934)3/92$5.00
Copyright 8 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
NOTES AND DISCUSSION
529
requires experimental control to assessdifferences specific to bilinguals. Many bilingual studies have not controlled for these factors. In addition, studies that do not use a monolingual control group (for example, Walters & Zatorre, 1978; Gordon, 1980) do not provide comparative criteria as monolingual norms are lacking. Many studies are also conducted using children as subjects (Silverberg, Bentin, Gaziel, Obler, & Albert, 1979; Gordon & Zatorre, 1981; McKeever, Hunt, Wells, & Yazzie, 1989) or include children within the subject pool (Albanese, 1985). There have been suggestions that language lateralization could follow a maturational course, with children initially being less lateralized than adults (Witelson, 1976). If this is the case, studies conducted on children would, ipso facto, fail to identify possible processing differences between monolinguals and bilinguals as these would possibly become evident only when neural maturation is complete. Linguistic variations could also account for discrepant results in some studies. Some studies have used a mixed language bilingual group (Sussman, Franklin, & Simon, 1982), thus obscuring possible processing differences between languages. Orthographic differences between ideographic and alphabetical scripts, for example, could influence processing strategies in tachistoscopic tasks, while tonal differences between languages would be pertinent when using dichotic listening methodology. Inconsistent results could be due to these factors given the variety of languages that have been assessedacross studies. Furthermore, there have been no systematic studies to date on the assessmentof different linguistic processes. Verbal production, perception, and semantic comprehension have been separately assessedusing divergent procedures and yet, inconsistent conclusions across studies are compared by Paradis as if all language variables were processed by the same cortical neural structure. There is evidence, however, that subcortical structures such as the corpus striatum, internal capsule, thalamus, and subcortical white matter near the lateral ventricle are involved in some language processes (see Li, Hu, Zhu, Song, & Li, 1986 for a review). CLINICAL STUDIES Studies of clinical patients with brain lesions also suffer from a lack of lesion site and verbal task specificity. In many studies lesion site is not specified, and subtle or diffuse damage is not assessed. However, in at least one study using right hemisphere damaged patients (Cappa, Papagno, & Vallar, 1990), although no statistically significant difference in incidence of aphasia was found between monolinguals and bilinguals, a small subgroup of bilinguals with subcortical damage showed evidence of aphasia. These findings accentuate the necessity for appropriate matching of lesion site for bilinguals and monolinguals. Furthermore, studies of commissurotomized patients are of limited value in assessingnormal lan-
530
NOTES AND DISCUSSION
guage processing as the operation is performed because of prior neural damage (cf, the problem with the Wada test noted below). Lesions incurred during childhood could cause hemispheric reorganization and, therefore, commissurotomized patients may not be an appropriate group either when comparing bilingual and monolingual language processing or in providing monolingual norms (see also Efron, 1990, for more on the problems of interpreting the data from split-brain patients.) Paradis (1990) also criticizes the methodology employed to assesslateralization, claiming that it lacks clinical validity. Recent experiments using the fused, rhymed dichotic words test (Wexler & Halwes, 1983; Wexler, 1988; Strauss, 1988), however, have yielded high correlations with previously published norms and have shown predictive validity for speech lateralization assessedby the sodium amytal tests (Strauss, 1988). There is, however, a problem with relying on the sodium amytal test (Wada, 1949; Wada & Rasmussen, 1960) as the test for hemispheric specialization for speech as, of necessity, only language production is tested, and then only in subjects with some kind of brain dysfunction. Problems are also encountered with Paradis’ (1990) interpretation of dichotic listening scores with commissurotomized subjects. If the left ear score falls to zero under dichotic stimulation conditions in these patients, this would indicate that transcollosal transfer is necessary when stimuli are in competition. In this case, left ear scores would reflect information processed by the right hemisphere rather than the left. In monaural stimulation, the task is presumably easier and can adequately be processed by ipsilateral pathways. Commissurotomized patients may also be inappropriate in this context for the reasons cited above as well as the incomplete nature of the commisurotomy: In most cases, the corpus collosum is not completely severed, and intact potential interhemispheric transmission pathways often remain (Sidtis, 1988). In addition, the 4045% correct answers reported from the left ear or left visual field in dichotic listening or tachistoscopic studies are not surprising given that chance alone would predict this level in two-choice tasks. To find such values does not invalidate the whole research thrust. STATISTICAL
CONTROVERSIES
In the concurrent task paradigm area, Paradis rather briefly supports suggestions that ANCOVA should be used to partial out differences in initial performance (Sussman, 1989; Willis & Goodwin, 1987). We strongly suggest that this notion is erroneous due to the nonequivalence of the groups on other unspecified variables. That is, subjects are not randomly assigned to a group. This leads to possible specification errors and a resultant over- or underadjustment of experimental means (Pedhazur, 1982). In addition, ANCOVA should only be used when the covariate is measured prior to the experiment, that is, when the covariate is inde-
NOTES AND DISCUSSION
531
pendent of the experimental design (Pedhazur, 1982; Tabachnik & Fidel& 1983). To quote Keppel’s (1982, p. 483) succinct statement: This method is known as the analysis of covariance and requires the measurement of subjects on some relevant ability or characteristic prior to the administration of the experimental treatments. [italics added]
In applications of ANCOVA to concurrent-task data, this prior measurement is not even attempted; supporters of the ANCOVA position use data collected during the actual experiment as the covariate. Similar problems are encountered when difference scores are used: Within-groups variance can be spuriously elevated or reduced depending on correlation levels between the original scores (Pedhazur, 1982; Albanese, 1985). We should point out here that, using our own data (see Ashton & McFarland, 1991), the correlations between no-concurrent-task scores and scores in the concurrent-language task condition are very high: r = .78 for 55 lefthanders and r = .79 for 949 right-handers when both groups are using their right hand, and I = .77 for the same sample of left-handers and r = .78 for the same sample of right-handers when both groups are using their left hands. Given such results there would seem to be no justification for the application of either ANCOVA or difference-score methodology to concurrent-task data. Furthermore, recent comparisons of statistical techniques using multivariate analyses of variance and Wilk’s Lambda tests have shown that task-specific differences between monolingual and bilingual lateralization scores are, in fact, robust (Green, Nicholson, Vaid, White, & Steiner, 1990). A simple analysis of cleaned-up raw data (i.e., data with outliers removed) using repeated-measures ANOVA would seem to be all that is required and overcomes the problems that bedevil the application of advanced statistical methods; even when used by the experienced their pitfalls are many and subtle. CONCLUSIONS
Thus, it seems extreme of Paradis (1990) to recommend the abandonment of bilingual lateralization studies in the light of improvements in methodology and the increasing specificity achieved for linguistic tasks. Although it may be premature at this stage to advocate changes in secondlanguage education or rehabilitation following neural injury based on results from flawed methodology and spurious interpretation, surely it would be more productive to exercise experimental control over all task variables and to interpret with appropriate caution than to relegate the entire topic to the “too hard” basket. Finally, care must be exercised before accepting arguments supporting the application of statistical techniques beyond the expertise of most experimenters.
532
NOTES AND DISCUSSION
REFERENCES Albanese, J. F. 1985. Language lateralization in English-French bilinguals. Bruin and Language, 24, 284-296.
Ashton, R., & McFarland, K. 1991. A simple dual-task study of laterality, sex differences and handedness. Cortex, 27, 105-109. Cappa, S. F., Papagno, C., & Vallar, G. 1990. Language and verbal memory after right 28, 503-509. hemisphere stroke: A clinical-CT scan study. Neuropsychologia, Chiarello, C. 1985. Hemisphere dynamics in lexical access: Automatic and controlled priming. Brain and Language, 26, 146-172. Deloche, G., Seron, X., Scuis, G., and Segui, J. 1987. Right hemisphere language processing: Lateral difference with imageable and nonimageable ambiguous words. Bruin and Language, 30, 197-205.
Efron, R. 1990. The decline and fall of hemispheric specialization. Hillsdale, NJ: Erlbaum. Gordon, H. W. 1980. Cerebral organization in bilinguals. Brain and Language, 9, 255-268. Gordon, D. P., & Zatorre, R. J. 1981. A right-ear advantage for dichotic listening in bilingu~ children. Brain and Language, 13, 389-396. Green, A., Nicholson, N. S., Vaid, J., White, N., & Steiner, R. 1990. Hemispheric involvement in shadowing vs. interpretation: A time-sharing study of simultaneous interpreters with matched bilingual and monolingual controls. Brain and Language, 39, 107-133. Keppel, G. 1982. Design and analysis: A researcher’s handbook. Engiewood Cliffs, NJ: Prentice-Hall. Second ed. Li, X. T., Hu, C. Q., Zhu, Y. L., Song, Z. W., & Li, Y. 1986. Language disorders in patients with cerebral vascular accidents. In H. S. R. Kao & R. Hoosin (Eds.), Linguistics, psychology and the Chinese language. Hong Kong: Centre of Asian Studies, University of Hong Kong. Pp. 265-278. McKeever, W. F., Hunt, L. J., Wells, S., & Yazzie, C. 1989. Language laterality in Navajo Reservation children: Dichotic test results depend on the language context of the testing. Brain and Language, 36, 148-158. Paivio, A. 1971. Imagery and verbal processes. New York: Holt, Rinehart & Winston. Paradis, M. 1990. Language lateralization in bilinguals: Enough already! Brain and Language,
39, 576-586.
Pedhazur, E. J. 1982. Multiple regression in behavioral research. New York: Holt, Rinehart & Winston. Second ed. Sidtis, J. J. 1988. Dichotic listening after commissurotomy. In K. Hugdahl (Ed.), Handbook of dichotic listening: Theory, methodr and research. New York: Wiley. Pp. 161-184. Silverberg, R., Bentin, S., Gaziel, T., Obler, L. K., & Albert, M. L. 1979. Shift of visual field preference for English words in native Hebrew speakers. Brain and Language, 8, 184-190. Soares, C. 1984. Left hemisphere language lateraliiation in bilinguals: Use of the concurrent activities paradigm. Brain and Language, 23, 86-96. Soares, C., & Grosjean, F. 1981. Left hemisphere laterahzation in bilinguals and monolinguals. Perception and Psyc~physics, 29, 599-804. Strauss, E. 1988. Dichotic listening and sodium amytal: Functional and morphological aspects of hemispheric asymmetry. In K. Hugdahl (Ed.), Handbook of dichotic listening: Theory, methods and research. New York: Wiley. Pp. 117-138. Sussman, H. M. 1989. A reassessment of the time-sharing paradigm with ANCOVA. Bruin and Language, 37, 514-520.
Sussman, H. M., Franklin, P., & Simon, T. 1982. Bilingual speech: Bilateral control. Brain and Language, l5, 125-142.
NOTES AND DISCUSSION
533
Tabachnick, B. G., & Fidel& L. S. 1983. Using multivariate statistics. New York: Harper & Row. Van Lanker, D. R., & Kempler, D. 1987. Comprehension of familiar phrases by left- but not right-hemisphere damaged patients. Brain and Language, 32, 265-277. Wada, J. 1949. A new method for the determination of the side of cerebral speech dominance: A preliminary report on the intracarotid injection of sodium amytal in man. Medicine and Biology, 14, 221-222. Wada, J., & Rasmussen, T. 1960. Intercarotoid injection of sodium amytal for the lateralization of cerebral speech dominance. Journal of Neurosurgery, 17, 266-282. Walters, J., & Zatorre, R. 1978. Laterality differences for word identification in bilinguals. Brain and Language, 6, 158-166. Wexler, B. E. 1988. Dichotic presentation as a method for single hemisphere stimulation studies. In K. Hugdahl (Ed.), Handbook of dichotic listening: Theory, methods and research. New York: Wiley. Pp. 85-116. Wexler, B. E., & Halwes, T. 1983. Increasing the power of dichotic methods: The fused rhymed words test. Neuropsychologia, 21, 59-66. Willis, W. G., & Goodwin, L. D. 1987. An alternative to interference indexes in neuropsychological time-sharing research. Neuropsychologia, 25, 719-724. Witelson, S. F. 1976. Sex and the single hemisphere: Specialization of the right hemisphere for spatial processing. Science, 193, 425-427. Zaidel, E. 1983. Disconnection syndrome as a model for laterality effects in the normal brain. In J. B. Hellige (Ed.), Cerebral hemisphere asymmetry? Method, theory and application New York: Praeger Press. Pp. 95-151.
AND
LANGUAGE
43, 528-533 (1992)
Language Lateralization in Bilinguals: More Not Less Is Needed: A Reply to Paradis (1990) ANNE BERQUIER AND RODERICK ASHTON University
of Queensland
Paradis (1990) took an extremely pessimistic view of research to date looking at the laterahzation of cerebral function in bilingual subjects. We, however, argue that some of his conclusions are, in fact, incorrect, while others speak more to the youth of the field. The problems in the latter body of work are now acknowledged and are being overcome. That the shortcomings are recognized seems to be more a case for satisfaction than of despair. o IWZ Academic press, IW.
Paradis (1990) quite rightly has taken neurolinguists to task for their neglect of methodological, subject selection and statistical problems in assessing lateralization of cerebral function in bilingual people. He then concludes, however, that studies of bilingual lateralization should be abandoned altogether because of the resultant inconsistent findings. In recent times methodologies for assessinglateralization have been refined, leading to greater validity. It is also possible, in retrospect, to delineate possible confounds in previous studies and exercise greater control in the future. Although it is not within the scope of this paper to review comprehensively all experimental or clinical problems associated with lateralization studies of bilinguals, indeed Paradis himself did not attempt this, several factors nevertheless deserve mention. EXPERIMENTAL STUDIES Recent studies have questioned propositions of discrete left hemisphere language mediation in monolinguals. Right hemisphere processing in this group has been suggested for concrete words (Paivio, 1971; Zaidel, 1983; Chiarello, 1985), high imagery words (Deloche, Seron, Scuis, & Segui, 1987), and high frequency words (Van Lanker & Kempler, 1987). In this context, the choice of verbal task variables to assessbilingual lateralization Address reprint requests to Roderick Ashton at the Department of Psychology, The University of Queensland, St. Lucia, Q4072, Australia. 528 0093-934)3/92$5.00
Copyright 8 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
NOTES AND DISCUSSION
529
requires experimental control to assessdifferences specific to bilinguals. Many bilingual studies have not controlled for these factors. In addition, studies that do not use a monolingual control group (for example, Walters & Zatorre, 1978; Gordon, 1980) do not provide comparative criteria as monolingual norms are lacking. Many studies are also conducted using children as subjects (Silverberg, Bentin, Gaziel, Obler, & Albert, 1979; Gordon & Zatorre, 1981; McKeever, Hunt, Wells, & Yazzie, 1989) or include children within the subject pool (Albanese, 1985). There have been suggestions that language lateralization could follow a maturational course, with children initially being less lateralized than adults (Witelson, 1976). If this is the case, studies conducted on children would, ipso facto, fail to identify possible processing differences between monolinguals and bilinguals as these would possibly become evident only when neural maturation is complete. Linguistic variations could also account for discrepant results in some studies. Some studies have used a mixed language bilingual group (Sussman, Franklin, & Simon, 1982), thus obscuring possible processing differences between languages. Orthographic differences between ideographic and alphabetical scripts, for example, could influence processing strategies in tachistoscopic tasks, while tonal differences between languages would be pertinent when using dichotic listening methodology. Inconsistent results could be due to these factors given the variety of languages that have been assessedacross studies. Furthermore, there have been no systematic studies to date on the assessmentof different linguistic processes. Verbal production, perception, and semantic comprehension have been separately assessedusing divergent procedures and yet, inconsistent conclusions across studies are compared by Paradis as if all language variables were processed by the same cortical neural structure. There is evidence, however, that subcortical structures such as the corpus striatum, internal capsule, thalamus, and subcortical white matter near the lateral ventricle are involved in some language processes (see Li, Hu, Zhu, Song, & Li, 1986 for a review). CLINICAL STUDIES Studies of clinical patients with brain lesions also suffer from a lack of lesion site and verbal task specificity. In many studies lesion site is not specified, and subtle or diffuse damage is not assessed. However, in at least one study using right hemisphere damaged patients (Cappa, Papagno, & Vallar, 1990), although no statistically significant difference in incidence of aphasia was found between monolinguals and bilinguals, a small subgroup of bilinguals with subcortical damage showed evidence of aphasia. These findings accentuate the necessity for appropriate matching of lesion site for bilinguals and monolinguals. Furthermore, studies of commissurotomized patients are of limited value in assessingnormal lan-
530
NOTES AND DISCUSSION
guage processing as the operation is performed because of prior neural damage (cf, the problem with the Wada test noted below). Lesions incurred during childhood could cause hemispheric reorganization and, therefore, commissurotomized patients may not be an appropriate group either when comparing bilingual and monolingual language processing or in providing monolingual norms (see also Efron, 1990, for more on the problems of interpreting the data from split-brain patients.) Paradis (1990) also criticizes the methodology employed to assesslateralization, claiming that it lacks clinical validity. Recent experiments using the fused, rhymed dichotic words test (Wexler & Halwes, 1983; Wexler, 1988; Strauss, 1988), however, have yielded high correlations with previously published norms and have shown predictive validity for speech lateralization assessedby the sodium amytal tests (Strauss, 1988). There is, however, a problem with relying on the sodium amytal test (Wada, 1949; Wada & Rasmussen, 1960) as the test for hemispheric specialization for speech as, of necessity, only language production is tested, and then only in subjects with some kind of brain dysfunction. Problems are also encountered with Paradis’ (1990) interpretation of dichotic listening scores with commissurotomized subjects. If the left ear score falls to zero under dichotic stimulation conditions in these patients, this would indicate that transcollosal transfer is necessary when stimuli are in competition. In this case, left ear scores would reflect information processed by the right hemisphere rather than the left. In monaural stimulation, the task is presumably easier and can adequately be processed by ipsilateral pathways. Commissurotomized patients may also be inappropriate in this context for the reasons cited above as well as the incomplete nature of the commisurotomy: In most cases, the corpus collosum is not completely severed, and intact potential interhemispheric transmission pathways often remain (Sidtis, 1988). In addition, the 4045% correct answers reported from the left ear or left visual field in dichotic listening or tachistoscopic studies are not surprising given that chance alone would predict this level in two-choice tasks. To find such values does not invalidate the whole research thrust. STATISTICAL
CONTROVERSIES
In the concurrent task paradigm area, Paradis rather briefly supports suggestions that ANCOVA should be used to partial out differences in initial performance (Sussman, 1989; Willis & Goodwin, 1987). We strongly suggest that this notion is erroneous due to the nonequivalence of the groups on other unspecified variables. That is, subjects are not randomly assigned to a group. This leads to possible specification errors and a resultant over- or underadjustment of experimental means (Pedhazur, 1982). In addition, ANCOVA should only be used when the covariate is measured prior to the experiment, that is, when the covariate is inde-
NOTES AND DISCUSSION
531
pendent of the experimental design (Pedhazur, 1982; Tabachnik & Fidel& 1983). To quote Keppel’s (1982, p. 483) succinct statement: This method is known as the analysis of covariance and requires the measurement of subjects on some relevant ability or characteristic prior to the administration of the experimental treatments. [italics added]
In applications of ANCOVA to concurrent-task data, this prior measurement is not even attempted; supporters of the ANCOVA position use data collected during the actual experiment as the covariate. Similar problems are encountered when difference scores are used: Within-groups variance can be spuriously elevated or reduced depending on correlation levels between the original scores (Pedhazur, 1982; Albanese, 1985). We should point out here that, using our own data (see Ashton & McFarland, 1991), the correlations between no-concurrent-task scores and scores in the concurrent-language task condition are very high: r = .78 for 55 lefthanders and r = .79 for 949 right-handers when both groups are using their right hand, and I = .77 for the same sample of left-handers and r = .78 for the same sample of right-handers when both groups are using their left hands. Given such results there would seem to be no justification for the application of either ANCOVA or difference-score methodology to concurrent-task data. Furthermore, recent comparisons of statistical techniques using multivariate analyses of variance and Wilk’s Lambda tests have shown that task-specific differences between monolingual and bilingual lateralization scores are, in fact, robust (Green, Nicholson, Vaid, White, & Steiner, 1990). A simple analysis of cleaned-up raw data (i.e., data with outliers removed) using repeated-measures ANOVA would seem to be all that is required and overcomes the problems that bedevil the application of advanced statistical methods; even when used by the experienced their pitfalls are many and subtle. CONCLUSIONS
Thus, it seems extreme of Paradis (1990) to recommend the abandonment of bilingual lateralization studies in the light of improvements in methodology and the increasing specificity achieved for linguistic tasks. Although it may be premature at this stage to advocate changes in secondlanguage education or rehabilitation following neural injury based on results from flawed methodology and spurious interpretation, surely it would be more productive to exercise experimental control over all task variables and to interpret with appropriate caution than to relegate the entire topic to the “too hard” basket. Finally, care must be exercised before accepting arguments supporting the application of statistical techniques beyond the expertise of most experimenters.
532
NOTES AND DISCUSSION
REFERENCES Albanese, J. F. 1985. Language lateralization in English-French bilinguals. Bruin and Language, 24, 284-296.
Ashton, R., & McFarland, K. 1991. A simple dual-task study of laterality, sex differences and handedness. Cortex, 27, 105-109. Cappa, S. F., Papagno, C., & Vallar, G. 1990. Language and verbal memory after right 28, 503-509. hemisphere stroke: A clinical-CT scan study. Neuropsychologia, Chiarello, C. 1985. Hemisphere dynamics in lexical access: Automatic and controlled priming. Brain and Language, 26, 146-172. Deloche, G., Seron, X., Scuis, G., and Segui, J. 1987. Right hemisphere language processing: Lateral difference with imageable and nonimageable ambiguous words. Bruin and Language, 30, 197-205.
Efron, R. 1990. The decline and fall of hemispheric specialization. Hillsdale, NJ: Erlbaum. Gordon, H. W. 1980. Cerebral organization in bilinguals. Brain and Language, 9, 255-268. Gordon, D. P., & Zatorre, R. J. 1981. A right-ear advantage for dichotic listening in bilingu~ children. Brain and Language, 13, 389-396. Green, A., Nicholson, N. S., Vaid, J., White, N., & Steiner, R. 1990. Hemispheric involvement in shadowing vs. interpretation: A time-sharing study of simultaneous interpreters with matched bilingual and monolingual controls. Brain and Language, 39, 107-133. Keppel, G. 1982. Design and analysis: A researcher’s handbook. Engiewood Cliffs, NJ: Prentice-Hall. Second ed. Li, X. T., Hu, C. Q., Zhu, Y. L., Song, Z. W., & Li, Y. 1986. Language disorders in patients with cerebral vascular accidents. In H. S. R. Kao & R. Hoosin (Eds.), Linguistics, psychology and the Chinese language. Hong Kong: Centre of Asian Studies, University of Hong Kong. Pp. 265-278. McKeever, W. F., Hunt, L. J., Wells, S., & Yazzie, C. 1989. Language laterality in Navajo Reservation children: Dichotic test results depend on the language context of the testing. Brain and Language, 36, 148-158. Paivio, A. 1971. Imagery and verbal processes. New York: Holt, Rinehart & Winston. Paradis, M. 1990. Language lateralization in bilinguals: Enough already! Brain and Language,
39, 576-586.
Pedhazur, E. J. 1982. Multiple regression in behavioral research. New York: Holt, Rinehart & Winston. Second ed. Sidtis, J. J. 1988. Dichotic listening after commissurotomy. In K. Hugdahl (Ed.), Handbook of dichotic listening: Theory, methodr and research. New York: Wiley. Pp. 161-184. Silverberg, R., Bentin, S., Gaziel, T., Obler, L. K., & Albert, M. L. 1979. Shift of visual field preference for English words in native Hebrew speakers. Brain and Language, 8, 184-190. Soares, C. 1984. Left hemisphere language lateraliiation in bilinguals: Use of the concurrent activities paradigm. Brain and Language, 23, 86-96. Soares, C., & Grosjean, F. 1981. Left hemisphere laterahzation in bilinguals and monolinguals. Perception and Psyc~physics, 29, 599-804. Strauss, E. 1988. Dichotic listening and sodium amytal: Functional and morphological aspects of hemispheric asymmetry. In K. Hugdahl (Ed.), Handbook of dichotic listening: Theory, methods and research. New York: Wiley. Pp. 117-138. Sussman, H. M. 1989. A reassessment of the time-sharing paradigm with ANCOVA. Bruin and Language, 37, 514-520.
Sussman, H. M., Franklin, P., & Simon, T. 1982. Bilingual speech: Bilateral control. Brain and Language, l5, 125-142.
NOTES AND DISCUSSION
533
Tabachnick, B. G., & Fidel& L. S. 1983. Using multivariate statistics. New York: Harper & Row. Van Lanker, D. R., & Kempler, D. 1987. Comprehension of familiar phrases by left- but not right-hemisphere damaged patients. Brain and Language, 32, 265-277. Wada, J. 1949. A new method for the determination of the side of cerebral speech dominance: A preliminary report on the intracarotid injection of sodium amytal in man. Medicine and Biology, 14, 221-222. Wada, J., & Rasmussen, T. 1960. Intercarotoid injection of sodium amytal for the lateralization of cerebral speech dominance. Journal of Neurosurgery, 17, 266-282. Walters, J., & Zatorre, R. 1978. Laterality differences for word identification in bilinguals. Brain and Language, 6, 158-166. Wexler, B. E. 1988. Dichotic presentation as a method for single hemisphere stimulation studies. In K. Hugdahl (Ed.), Handbook of dichotic listening: Theory, methods and research. New York: Wiley. Pp. 85-116. Wexler, B. E., & Halwes, T. 1983. Increasing the power of dichotic methods: The fused rhymed words test. Neuropsychologia, 21, 59-66. Willis, W. G., & Goodwin, L. D. 1987. An alternative to interference indexes in neuropsychological time-sharing research. Neuropsychologia, 25, 719-724. Witelson, S. F. 1976. Sex and the single hemisphere: Specialization of the right hemisphere for spatial processing. Science, 193, 425-427. Zaidel, E. 1983. Disconnection syndrome as a model for laterality effects in the normal brain. In J. B. Hellige (Ed.), Cerebral hemisphere asymmetry? Method, theory and application New York: Praeger Press. Pp. 95-151.