NOTE A SIMPLE DUAL-TASK STUDY OF LATERALITY, SEX-DIFFERENCES AND HANDEDNESS Roderick Ashton and Ken McFarland (Department of Psychology, University of Queensland)
INTRODUCTION
The dual-task technique for studying lateralization of brain function was introduced by Kinsbourne and Cook (1971). The essence of the technique is that if subjects have to carry out two tasks at once then performance declines. Thus, in the Kinsbourne and Cook study they timed how long subjects could balance pieces of wooden dowel on their left- and right index fingers. Then the subjects repeated the task whilst at the same time reciting a rhyme. The outcome was that right-hand performance declined when both tasks were performed concurrently. Left-hand performance, in this initial study, actually showed a slight impro vement over performing the balancing task alone. The explanation for these results is always couched in terms of brain lateralization of function: the left cerebral hemisphere (in right-banders) is the one controlling language and the right-hand. Thus, when performed concurrently, the motor and language task interfere with each other, and motor performance declines. In the case of left-hand balancing, as this hand is controlled by the right-hemisphere (which does not partake in the control of the pro duction of the rhyme) its performance does not decline; the language and motor tasks are being controlled by different hemispheres. The balancing task is a difficult one to control and, more importantly for the present study, is hard to administer to large numbers of subjects. Consequently, the motor task component of the dual- or concurrent-task paradigm was changed to a tapping task (see e.g., Hicks, 1975; Lomas and Kimura, 1976; McFarland and Ashton, 1975). Here the dependent variable for the motor task became the number of taps a subject could produce in some time period (e.g., 10 sees) whilst also carrying out either no other concurrent task or whilst, again, performing a verbal, usually a vocalisation, task. Lomas and Kimura (1976) had subjects un der different conditions recite not only a ryhme, but also a nonsense word (Ia-la). This mo dification allowed the experimenter to dissect out the effect of the cognitive component of speech from the pure motor effects (moving the vocal apparatus; see McFarland, Ashton and Jeffrey, 1989). While the "verbal" task is relatively easy to administer and requires no ela borate equipment, nearly all tapping task studies have used complex equipment and data re cording methods. This need necessitates; logistically, the use of small numbers of subjects. One purpose of the present paper is to demonstrate a technique that can be used to collect data from a large number of subjects at once. The use of the method thus enables the pro duction of a normative data base that, perhaps, can be employed to look at anomalous la teralization of brain function and ultimately lead to a standardized test procedure for clini cal populations. A second purpose of the present work was to look at putative sex differences in lateraCortex, (1991) 27, 105-109
106
R. Ashton and K. McFarland
lization of function. One current suspicion in this area of research is that females may be less lateralized for language than males (see e.g., McGlone, 1980, and Sundet, 1988, for a review and discussion of problems of interpretation). Because the technique employed herein ena bled the assessment of a large number of subjects this suggested sex diference became a se cond major focus of the investigation. Finally, left-banders constitute a further group where atypical brain lateralization has been found (Bradshaw and Nettleton, 1983). Given that we could test so many subjects a preliminary data base was collected for them. The data are still, however, sparse as we only found a less than 90Jo incidence of left-hand preference for wri ting (the criterion used). This is a slightly lower figure than that presented by Steenhuis and Bryden (1989) for a similar sample using a similar preference test. Left-banders were also di vided into familial and non-familial samples. This was done to investigate possible differen ces between "genetic left-banders" (another in the family) and "non-genetic" (the subject being the only left-bander in the family; for further see discussion by Bradshaw and Nettle ton, 1983). The reader is, however, referred to Bishop (1990) for a negative theoretical ana lysis of the impact of the familialarity variable on differences between left-banders in their cerebral specialization for language, and to Orsini et al. (1985) for an absence of experimen tal support for this concept.
MATERIALS AND METHOD
Subjects Over the past years around 1000 first and second year university students have been te sted. Their ages ranged from 17 to 58 years. Of these 949 were self-declared right-banders (for writing) with 648 being female and 301 males. In the case of left-banders the sample size was 55 only, but the numbers can be compared to a sample of 653 right-banders collected at the same time. This comparison gives an incidence of 8.4%. Of these 55, 31 were female and 24 male with 19 female and 10 males reporting another left-bander in their families. Procedure Subjects were tested in groups which varied in size from 50 to 260. The participants were divided into pairs and each alternated in their roles as "subject" or "experimenter". Each subject was required to tap with a pen between two crosses printed 6 em apart in the side-to-side plane on sheets of paper. Fresh sheets of paper were supplied for each con dition. Subjects were instructed to perform the tapping task for 10 sees and this was timed by the "experimenter" of the pair. The participants were told as a group that they were to attempt to hit a cross with each tap, but to alternate between the two crosses as fast and as accurately as possible. Each subject used either the left- or right-hand in one of three con ditions: saying nothing, concurrently saying a tongue-twister ("she sells sea shells on the sea shore"), or concurrently saying "Ia-la" repeatedly. The six combinations thus resulting were repeated three times each giving 18 sets of taps per 10 sees per subject. The 18 trials were ran domized and different for each group. The groups were instructed as to the next condition and then given a fresh "target" sheet. Following performance of the tapping task, the students counted the number of pen marks made on the paper for each trial. The number of marks ranged from about 30 to 60 taps per 10 sees (see below, Table 1). These numbers were quite consistent over groups and not out of the range of data reported previously (e.g., McFarland and Ashton, 1978) where much more precise targets of two electronic switches were used. Accuracy of the pen marks in relation to the target crosses in the present study was not assessed. The data for analysis were the mean number of taps across the three repetitions for each of the six possible com binations of hand x condition.
107
Dual- Task study of laterality TABLE I
Mean Number of Taps Produced in 10 Seconds by Each Hand in Each Condition for Left- and Righthanders (standard deviations in parentheses). Hand used Hand preference
Right
Sex
Left
Nothing
T. Twister
La-La
Nothing
T. Twister
La-La
Right
Female (648) Male (301)
48 (8.44) 50.38 (8.38)
42.87 (7.9) 45 (8.15)
46.02 (8.15) 48.42 (8.08)
40.5 (7.34) 42.52 (6.87)
39.03 (7.09) 40.79 (7.11)
39.73 (6.75) 41.58 (7.21)
Left
Female (31) Male (24)
40.77 (6.02) 45.17 (7.17)
37.45 (5.42) 41.42 (8.05)
40.48 (6.30) 44.04 (6.91)
46.26 (8.12) 48.04 (7.01)
41.39 (6.46) 44.79 (8.03)
44.45 (8.11) 46.25 (8.03)
RESULTS
Right-handers
The data showed remarkable stability: deviating by, on average, only two taps per tense conds between a group of 85 students or 260 students. The overall mean numbers of taps and standard deviation per combination of hand and conditions and over sex are shown in Table I. A 3-way ANOV A (sex X hand x condition) run on the data showed all three main ef fects as being highly significant: For sex (F=18.95; d.f.=1, 947; p .05). The familiarity factor was non-significant, and neither of these factors occurred in any significant interactions. There were, as expected, significant main effects for hand and condition (F= 59.70; d. f.= 1, 51; p< .001 and F= 31.521; d. f.= 1, 102; p < .001 respectively). However, no significant interactions were found. The only inte resting finding for the left-banders was a reversal from the pattern seen in the right-banders; for left-banders the left hand was faster than the right. Overall though, for left-banders both hands showed impairment when the subjects were reciting the tongue-twister. The small number of left-banders does, however, militate against any strong conclusions being made about them. The relevant data are shown in Table I. Accuracy of the Technique
One way of checking if the tapping technique employed here can accurately tap an in dividual's hemispheric specialization for language is to look at reversals. That is, for right banders, those who tap faster when concurrently speaking rather than showing a decrement. The data were that 25 of the 301 male right-banders (8.30Jo) actually showed such an incre ment, as did 56 of the 648 females (8.6%). Thus 8.5% of total sample showed a reversal of the expected result. In a similar vein, 7.0% of the male right-banders showed neither a de crement nor an enhancement in right-hand performance, as did 6.3% of the females; a total of 6.5% of the entire sample showed no effect on motor performance as a result of concur rently carrying out the speech task. Overall then, 15.1% (n = 143) of the 949 right banders
108
R. Ashton and K. McFarland
sampled did not show the expected results. There are probably too few left-banders in the present sample for reversals, and no change effects, to be meaningful. However, looking at their overall results, 41.13% of the 55 left-banders tested showed either an enhancement or no effect on their left-hand performance when concurrently reciting the tongue-twister.
DISCUSSION
It is obvious that even such a simple procedure as described herein revealed a lateralized effect of language causing a decline in right-hand performance in right-banders. The la-la condition, which has the motor components of speech without a large verbal/linguistic aspect, has only a marginal effect on motor performance. Left-hand performance is hardly affected under either concurrent task condition. Perhaps more important than the mere confirmation of previous findings in this area was the total absence of an interaction involving sex. The only sex difference found was a main effect, with males tapping faster than females. The data for the left-banders, whilst mainly non-significant are, nevertheless, interesting in that first, no effect of familialness was apparent. This result is in line with the findings of Orsini eta!. (1985). Second, the reversal of the main effect for hand in left- versus right-ban ders (with their left-hand being faster than their right) confirms that the task used is sensitive with respect to handedness. Finally, the lack of any interaction between hand and condition in the case of left-banders is explained by inspection of the means: both hands are interfered with by concurrent speech. For an individualleft-hander this says nothing about the latera lization of speech, but does confirm that as a group left-banders have less clear lateralization than do right-banders, with some being left-hemisphere specialized, some right and some with bilateral representation of language (Springer and Deutsch, 1989). One final point has to be made, and that is that although the technique is, as usual with most in the field, sensitive for detecting lateralization of brain function for speech in groups of subjects (particularly right-banders), it is not sensitive in individual cases. The fact that 15.1 o/o of right-banders, on a simple test of the accuracy of the technique, do not show the expected decline in performance of their right hand when reciting the tongue-twister, argues against such a simple procedure being used alone as a test for cerebrallateralization of func tion.
ABSTRACT
Nine-hundred and eight-one right-handed and 55 left-handed subjects were required to tap with a pen for 10 sees between targets 6 em apart whilst either saying nothing, reciting a tongue-twister or saying la-la. The numbers of dots produced in 10 sees in each condition were analysed. The right-handed group demonstrated the usually found degradation in their right-hand performance whilst reciting the meaningful words, but showed a lesser effect when saying Ia-la. Their left-handed performance was essentially unaffected. Absolutely no evi dence for the presence of a sex difference in lateralization of language was found. The much smaller sample of left-handed subjects presented a complex picture of no significant effects, whether they were the sole left-banders in their family, or had other left-handed close rela tions.
REFERENCES
BISHOP, D.V.M. On the futility of using familial sinistrality to subclassify handedness groups. Cortex, 26: 153-155, 1990. BRADSHAW, J.L., and NETTLETON, N.C. Human Cerebral Asymmetry. Englewood Cliffs: Prentice Hall, 1983.
Dual-Task study of laterality
109
HICKS, R.E. lntrahemispheric response competition between vocal and unimanual performance in nor mal adult human males. Journal of Comparative and Physiological Psychology, 89: 50-60, 1975. KINSBOURNE, M., and CooK, K. Generalized and lateralized effects of concurrent verbalization on a uni manual skill. Quarterly Journal ofExperimental Psychology, 23: 341-346, 1971. LOMAS, J., and KIMURA, D. Intrahemispheric interaction between speaking and sequential manual acti vity. Neuropsychologia, 14: 23-33, 1976. McFARLAND, K., and ASHTON, R. The Iateralized effects of cognitive activity on a unimanual skill. Cor tex, 11: 283-290, 1975. McFARLAND, K., and AsHTON, R. The Iateralized effects of concurrent cognitive and motor performan ce. Perception and Psychophysics, 23: 344-349, 1978. McFARLAND, K., ASHTON, R., and JEFFREY, C.K. Lateralized dual-task performance: The effects of spa tial and muscular repositioning. Neuropsycho/ogia, 27: 1267-1276, 1989. McGLONE. J. Sex differences in human brain asymmetry: A critical survey. Behavioral and Brain Scien ces, 3: 215-264, 1980. ORSINI, D.L., SATZ, P., SOPER, H.V., and LIGHT, R.K. The role of familial sinistrality in cerebral orga nization. Neuropsycho/ogia, 23: 223-232, 1985. SPRINGER, S.P., and DEUTSCH, G. Left Brain, Right Brain. New York: Freeman, 1989. STEENHUIS, R.E., and BRYDEN, M.P. Different dimensions of hand preference that relate to skilled and unskilled activities. Cortex, 25: 289-304, 1989. SuNDET, K. Sex differences in severity and type of aphasia. Scandinavian Journal ofPsychology, 29: 168 179, 1988. Roderick Ashton, Department of Psychology, University of Queensland, St. Lucia, QLD, 4072, Australia. The data base is available for other analyses on the Macintosh computer as an annotated Microsoft EXCEL file. Send a blank disk if required.
INTRODUCTION
The dual-task technique for studying lateralization of brain function was introduced by Kinsbourne and Cook (1971). The essence of the technique is that if subjects have to carry out two tasks at once then performance declines. Thus, in the Kinsbourne and Cook study they timed how long subjects could balance pieces of wooden dowel on their left- and right index fingers. Then the subjects repeated the task whilst at the same time reciting a rhyme. The outcome was that right-hand performance declined when both tasks were performed concurrently. Left-hand performance, in this initial study, actually showed a slight impro vement over performing the balancing task alone. The explanation for these results is always couched in terms of brain lateralization of function: the left cerebral hemisphere (in right-banders) is the one controlling language and the right-hand. Thus, when performed concurrently, the motor and language task interfere with each other, and motor performance declines. In the case of left-hand balancing, as this hand is controlled by the right-hemisphere (which does not partake in the control of the pro duction of the rhyme) its performance does not decline; the language and motor tasks are being controlled by different hemispheres. The balancing task is a difficult one to control and, more importantly for the present study, is hard to administer to large numbers of subjects. Consequently, the motor task component of the dual- or concurrent-task paradigm was changed to a tapping task (see e.g., Hicks, 1975; Lomas and Kimura, 1976; McFarland and Ashton, 1975). Here the dependent variable for the motor task became the number of taps a subject could produce in some time period (e.g., 10 sees) whilst also carrying out either no other concurrent task or whilst, again, performing a verbal, usually a vocalisation, task. Lomas and Kimura (1976) had subjects un der different conditions recite not only a ryhme, but also a nonsense word (Ia-la). This mo dification allowed the experimenter to dissect out the effect of the cognitive component of speech from the pure motor effects (moving the vocal apparatus; see McFarland, Ashton and Jeffrey, 1989). While the "verbal" task is relatively easy to administer and requires no ela borate equipment, nearly all tapping task studies have used complex equipment and data re cording methods. This need necessitates; logistically, the use of small numbers of subjects. One purpose of the present paper is to demonstrate a technique that can be used to collect data from a large number of subjects at once. The use of the method thus enables the pro duction of a normative data base that, perhaps, can be employed to look at anomalous la teralization of brain function and ultimately lead to a standardized test procedure for clini cal populations. A second purpose of the present work was to look at putative sex differences in lateraCortex, (1991) 27, 105-109
106
R. Ashton and K. McFarland
lization of function. One current suspicion in this area of research is that females may be less lateralized for language than males (see e.g., McGlone, 1980, and Sundet, 1988, for a review and discussion of problems of interpretation). Because the technique employed herein ena bled the assessment of a large number of subjects this suggested sex diference became a se cond major focus of the investigation. Finally, left-banders constitute a further group where atypical brain lateralization has been found (Bradshaw and Nettleton, 1983). Given that we could test so many subjects a preliminary data base was collected for them. The data are still, however, sparse as we only found a less than 90Jo incidence of left-hand preference for wri ting (the criterion used). This is a slightly lower figure than that presented by Steenhuis and Bryden (1989) for a similar sample using a similar preference test. Left-banders were also di vided into familial and non-familial samples. This was done to investigate possible differen ces between "genetic left-banders" (another in the family) and "non-genetic" (the subject being the only left-bander in the family; for further see discussion by Bradshaw and Nettle ton, 1983). The reader is, however, referred to Bishop (1990) for a negative theoretical ana lysis of the impact of the familialarity variable on differences between left-banders in their cerebral specialization for language, and to Orsini et al. (1985) for an absence of experimen tal support for this concept.
MATERIALS AND METHOD
Subjects Over the past years around 1000 first and second year university students have been te sted. Their ages ranged from 17 to 58 years. Of these 949 were self-declared right-banders (for writing) with 648 being female and 301 males. In the case of left-banders the sample size was 55 only, but the numbers can be compared to a sample of 653 right-banders collected at the same time. This comparison gives an incidence of 8.4%. Of these 55, 31 were female and 24 male with 19 female and 10 males reporting another left-bander in their families. Procedure Subjects were tested in groups which varied in size from 50 to 260. The participants were divided into pairs and each alternated in their roles as "subject" or "experimenter". Each subject was required to tap with a pen between two crosses printed 6 em apart in the side-to-side plane on sheets of paper. Fresh sheets of paper were supplied for each con dition. Subjects were instructed to perform the tapping task for 10 sees and this was timed by the "experimenter" of the pair. The participants were told as a group that they were to attempt to hit a cross with each tap, but to alternate between the two crosses as fast and as accurately as possible. Each subject used either the left- or right-hand in one of three con ditions: saying nothing, concurrently saying a tongue-twister ("she sells sea shells on the sea shore"), or concurrently saying "Ia-la" repeatedly. The six combinations thus resulting were repeated three times each giving 18 sets of taps per 10 sees per subject. The 18 trials were ran domized and different for each group. The groups were instructed as to the next condition and then given a fresh "target" sheet. Following performance of the tapping task, the students counted the number of pen marks made on the paper for each trial. The number of marks ranged from about 30 to 60 taps per 10 sees (see below, Table 1). These numbers were quite consistent over groups and not out of the range of data reported previously (e.g., McFarland and Ashton, 1978) where much more precise targets of two electronic switches were used. Accuracy of the pen marks in relation to the target crosses in the present study was not assessed. The data for analysis were the mean number of taps across the three repetitions for each of the six possible com binations of hand x condition.
107
Dual- Task study of laterality TABLE I
Mean Number of Taps Produced in 10 Seconds by Each Hand in Each Condition for Left- and Righthanders (standard deviations in parentheses). Hand used Hand preference
Right
Sex
Left
Nothing
T. Twister
La-La
Nothing
T. Twister
La-La
Right
Female (648) Male (301)
48 (8.44) 50.38 (8.38)
42.87 (7.9) 45 (8.15)
46.02 (8.15) 48.42 (8.08)
40.5 (7.34) 42.52 (6.87)
39.03 (7.09) 40.79 (7.11)
39.73 (6.75) 41.58 (7.21)
Left
Female (31) Male (24)
40.77 (6.02) 45.17 (7.17)
37.45 (5.42) 41.42 (8.05)
40.48 (6.30) 44.04 (6.91)
46.26 (8.12) 48.04 (7.01)
41.39 (6.46) 44.79 (8.03)
44.45 (8.11) 46.25 (8.03)
RESULTS
Right-handers
The data showed remarkable stability: deviating by, on average, only two taps per tense conds between a group of 85 students or 260 students. The overall mean numbers of taps and standard deviation per combination of hand and conditions and over sex are shown in Table I. A 3-way ANOV A (sex X hand x condition) run on the data showed all three main ef fects as being highly significant: For sex (F=18.95; d.f.=1, 947; p .05). The familiarity factor was non-significant, and neither of these factors occurred in any significant interactions. There were, as expected, significant main effects for hand and condition (F= 59.70; d. f.= 1, 51; p< .001 and F= 31.521; d. f.= 1, 102; p < .001 respectively). However, no significant interactions were found. The only inte resting finding for the left-banders was a reversal from the pattern seen in the right-banders; for left-banders the left hand was faster than the right. Overall though, for left-banders both hands showed impairment when the subjects were reciting the tongue-twister. The small number of left-banders does, however, militate against any strong conclusions being made about them. The relevant data are shown in Table I. Accuracy of the Technique
One way of checking if the tapping technique employed here can accurately tap an in dividual's hemispheric specialization for language is to look at reversals. That is, for right banders, those who tap faster when concurrently speaking rather than showing a decrement. The data were that 25 of the 301 male right-banders (8.30Jo) actually showed such an incre ment, as did 56 of the 648 females (8.6%). Thus 8.5% of total sample showed a reversal of the expected result. In a similar vein, 7.0% of the male right-banders showed neither a de crement nor an enhancement in right-hand performance, as did 6.3% of the females; a total of 6.5% of the entire sample showed no effect on motor performance as a result of concur rently carrying out the speech task. Overall then, 15.1% (n = 143) of the 949 right banders
108
R. Ashton and K. McFarland
sampled did not show the expected results. There are probably too few left-banders in the present sample for reversals, and no change effects, to be meaningful. However, looking at their overall results, 41.13% of the 55 left-banders tested showed either an enhancement or no effect on their left-hand performance when concurrently reciting the tongue-twister.
DISCUSSION
It is obvious that even such a simple procedure as described herein revealed a lateralized effect of language causing a decline in right-hand performance in right-banders. The la-la condition, which has the motor components of speech without a large verbal/linguistic aspect, has only a marginal effect on motor performance. Left-hand performance is hardly affected under either concurrent task condition. Perhaps more important than the mere confirmation of previous findings in this area was the total absence of an interaction involving sex. The only sex difference found was a main effect, with males tapping faster than females. The data for the left-banders, whilst mainly non-significant are, nevertheless, interesting in that first, no effect of familialness was apparent. This result is in line with the findings of Orsini eta!. (1985). Second, the reversal of the main effect for hand in left- versus right-ban ders (with their left-hand being faster than their right) confirms that the task used is sensitive with respect to handedness. Finally, the lack of any interaction between hand and condition in the case of left-banders is explained by inspection of the means: both hands are interfered with by concurrent speech. For an individualleft-hander this says nothing about the latera lization of speech, but does confirm that as a group left-banders have less clear lateralization than do right-banders, with some being left-hemisphere specialized, some right and some with bilateral representation of language (Springer and Deutsch, 1989). One final point has to be made, and that is that although the technique is, as usual with most in the field, sensitive for detecting lateralization of brain function for speech in groups of subjects (particularly right-banders), it is not sensitive in individual cases. The fact that 15.1 o/o of right-banders, on a simple test of the accuracy of the technique, do not show the expected decline in performance of their right hand when reciting the tongue-twister, argues against such a simple procedure being used alone as a test for cerebrallateralization of func tion.
ABSTRACT
Nine-hundred and eight-one right-handed and 55 left-handed subjects were required to tap with a pen for 10 sees between targets 6 em apart whilst either saying nothing, reciting a tongue-twister or saying la-la. The numbers of dots produced in 10 sees in each condition were analysed. The right-handed group demonstrated the usually found degradation in their right-hand performance whilst reciting the meaningful words, but showed a lesser effect when saying Ia-la. Their left-handed performance was essentially unaffected. Absolutely no evi dence for the presence of a sex difference in lateralization of language was found. The much smaller sample of left-handed subjects presented a complex picture of no significant effects, whether they were the sole left-banders in their family, or had other left-handed close rela tions.
REFERENCES
BISHOP, D.V.M. On the futility of using familial sinistrality to subclassify handedness groups. Cortex, 26: 153-155, 1990. BRADSHAW, J.L., and NETTLETON, N.C. Human Cerebral Asymmetry. Englewood Cliffs: Prentice Hall, 1983.
Dual-Task study of laterality
109
HICKS, R.E. lntrahemispheric response competition between vocal and unimanual performance in nor mal adult human males. Journal of Comparative and Physiological Psychology, 89: 50-60, 1975. KINSBOURNE, M., and CooK, K. Generalized and lateralized effects of concurrent verbalization on a uni manual skill. Quarterly Journal ofExperimental Psychology, 23: 341-346, 1971. LOMAS, J., and KIMURA, D. Intrahemispheric interaction between speaking and sequential manual acti vity. Neuropsychologia, 14: 23-33, 1976. McFARLAND, K., and ASHTON, R. The Iateralized effects of cognitive activity on a unimanual skill. Cor tex, 11: 283-290, 1975. McFARLAND, K., and AsHTON, R. The Iateralized effects of concurrent cognitive and motor performan ce. Perception and Psychophysics, 23: 344-349, 1978. McFARLAND, K., ASHTON, R., and JEFFREY, C.K. Lateralized dual-task performance: The effects of spa tial and muscular repositioning. Neuropsycho/ogia, 27: 1267-1276, 1989. McGLONE. J. Sex differences in human brain asymmetry: A critical survey. Behavioral and Brain Scien ces, 3: 215-264, 1980. ORSINI, D.L., SATZ, P., SOPER, H.V., and LIGHT, R.K. The role of familial sinistrality in cerebral orga nization. Neuropsycho/ogia, 23: 223-232, 1985. SPRINGER, S.P., and DEUTSCH, G. Left Brain, Right Brain. New York: Freeman, 1989. STEENHUIS, R.E., and BRYDEN, M.P. Different dimensions of hand preference that relate to skilled and unskilled activities. Cortex, 25: 289-304, 1989. SuNDET, K. Sex differences in severity and type of aphasia. Scandinavian Journal ofPsychology, 29: 168 179, 1988. Roderick Ashton, Department of Psychology, University of Queensland, St. Lucia, QLD, 4072, Australia. The data base is available for other analyses on the Macintosh computer as an annotated Microsoft EXCEL file. Send a blank disk if required.
