- Email: [email protected]
Dual-task interference in left-handed subjects: Hemispheric specialization vs manual dominance
c
0028-3932 9, s3 oo+o 1991 Pergamon Presr
00 Qk
NOTE DUAL-TASK
INTERFERENCE IN LEFT-HANDED SUBJECTS: SPECIALIZATION VS MANUAL DOMINANCE
HEMISPHERIC
BAK~ARA CHERRY* and DANIEL W. KEE California (Rrcriwd
State University. 29 Jmuury
I99
Fullerton,
CA, U.S.A
I : accepted29
July
199 I )
Abstract-Thirty-six left-handed subjects performed a dichotic listening task and two concurrent (verbal plus finger-tapping) tasks. Baseline tapping scores revealed both left- and right-hand dominance within left-handed subjects. Moreover when subjects were categorized as consistent or inconsistent for ‘sidedness advantage’ (e.g. consistent=same side for dominant hand and ear advantage), it was shown that the dual-task interference effects found on the concurrent tapping tasks were due to both hemispheric specialization and manual dominance.
INTRODUCTION DUAL-TASK METHODShave been used for a number of years to assess hemispheric specialization in nonclinical right-handed populations 12, S]. For example, an asymmetrical decrement in tapping performance of the hands produced by various verbal concurrent tasks vs tapping alone indicates that the manual tapping task and the cognitive task are purportedly competing for resources from the same hemisphere [S]. Because the finger-tapping movement of the hand is primarily controlled by the contralateral hemisphere, greater right-hand interference produced by the verbal concurrent task indicates more left hemisphere involvement for the verbal activity [2, 81. Converging evidence for this left hemisphere dominance is provided by dichotic listening scores, which demonstrate a right ear (left hemisphere) advantage for verbal material in most right-handed subjects. Whereas right-handed subjects usually show reduced right-hand tapping while performing a concurrent verbal task [2, 81. recent results for left-handed subjects do not show consistent patterns of lateralized interference [S]. Studies using single linger-tapping and a variety of verbal tasks have shown either symmetrical patterns of interference for left-handers 1121, or asymmetrical dominant-hand interference; that is, left-handers show more decrements in tapping for the left hand while right-handers show more decrements in tapping for the right hand [lo, 1 I. 151. Yet those studies which also included dichotic listening tasks [IO, 151 still demonstrate left hemisphere dominance for speech (a right-ear advantage) for the majority of left-handers. In an attempt to clarify some of these results, HELLIGE and KEE 123 suggest that manual interference effects are due to both hemispheric specialization and manual dominance. and that left-handers can be classified as either left- or right-hand dominant based on baseline finger-tapping performances (from a reanalysis of their own data on lefthanders (n = 27 121) and an analysis of unpublished results by Bathurst (n =64; Bathurst, 1989). Hellige and Kee found (i) consistent proportions of subgroups for both sets of left-handed subjects: 78% left-hand dominant and 22% right-hand dominant and (ii) a dominant-hand Group by Hand interaction such that within left-handers, the left-hand-dominant group (L > R) showed more left-hand interference, while the right-hand-dominant group (R > L) showed more right-hand interference while performing concurrent verbal tasks. In addition, dichotic listening scores for the sample of 27 subjects also allowed for classification of subjects as consistent or inconsistent for ‘sidedness advantage’. That is, subjects who demonstrated dominance on the same side for both hand (baseline tapping) and ear (dichotic listening scores) were categorized as consistent (left hand/left ear and right hand/right ear), while those who showed opposite dominance for hand and ear were categorized as inconsistent (left hand,‘right ear and right hand/left ear). If dual-task interference effects are due strictly to manual dominance, symmetrical decrements in tapping performance on concurrent tasks should occur for the dominant hand regardless of whether
*Address all correspondence Los Angeles, CA 90089-1061.
to: Barbara U.S.A.
Cherry,
Department
1251
of Psychology,
University
of Southern
California.
1252
Nom
subjects are classified as consistent or inconsistent for hand and ear. II”,however. dual-task interference efrects arc due to both manual dominance and hemispheric specialization. asymmetric decrements in tapping performance should occur for the consistent but not for the inconsistent group. HELLGE and KEE L2] found greater finger-tapplng interference for the dominant hand only in the consistent group, suggesting that manual interference effects are due to at least two factors: hemispheric specialization and manual dominance. Since their findings arc only preliminary. this study provided a formal test of their hypothesis.
METHOD A total of 36 undergraduate students (20 males and I6 females) from the Introductory were used. All of the subjects spoke English as their first language and were left-handed writing-hand inspection and Edinburgh Handedness Inventory.
Psychology Subject Pool as assessed by self-report,
Quustionnuirus. Subjects filled out a IO-item version of the Edinburgh Handedness Inventory [9] to confirm direction and degree of handedness, and a Subject Information Questionnaire developed by E. Zaidel at the University of California, Los Angeles to assess familial sinistrality and other background information. Subject‘s writing hand posture (inverted or not) was also recorded. Information regarding familial sinistrality. other background information, and/or hand posture is available from the authors upon request. Dun/-Task Tappiny. Subjects were seated at a table in front of a small metal box (15 x 10 x 5.5 cm) on which were mounted two Micro Switch No. BA-2-V2-A2 tapping keys located 7 cm apart. The keys were attached to an Apple II Plus computer with a Mountain Hardware millisecond clock card to time 10 set trials and to record fingertapping performance. Beyond the s-itch box was a typing stand for presenting syllables (for repetition) and anagrams. In front of the switch box was a felt-covered platform (28 x 38 x 4 cm) on which subjects rested their forearms while tapping. The stimulus for the syllable repetition task was an 8 x I I in. sheet of paper on which the syllables: ,iba/, /da;. ;ga:. /pa/, /ta/ and /ka/ were printed across eight rows. For the anagram task. a set of four single-solution anagrams were presented vertically on a 13 x 20 cm card with Orator 61 10 type size. Anagrams with low imagery rating and medium-to-high familiarity ratings were used (see Ref. [I]). A different four-anagram set was used on each trial. For tapping alone trials. subjects were asked to look at a white, blank 13 x 20 cm card. For tapping alone, subjects were instructed to start tapping at the sound of a buzzer and to tap as fast as possible while staring directly ahead at the typing stand until the buzzer sounded again, marking the end of the IO set trial. Subjects were then given a practice trial for each hand to familiarize themselves with the procedure. For the syllablcrepetition task, subjects were instructed to repeat syllables across the rows out loud and continuously during the 10 set trial and to designate which syllable they were on at the end ofthe 10 sec. For tapping plus syllable repetition. subjects began tapping before the buzzer sounded. As a buzzer signaled the beginning of the 10 set trial subjects continued to tap as fast as possible while repeating the syllables. A second buzzer signaled them to stop. For the anagram solution task, subjects were asked to solve anagrams out loud working from the top of the anagram set to the bottom. If subjects solved all four anagrams before 10 set were up. they were to start over at the top. If they couldn’t solve an anagram, they were to go on to the next and come back to that particular anagram later. All other procedures were as described above. Subjects were allowed one IO set practice trial with anagram solving alone to familiarize them with the procedure. Each subject then performed 36 trials. The following conditions were represented six times in the 36 trials with the order based on a diagram-balanced Latin square [I 61: left-hand tapping alone, right-hand tapping alone, left-hand tapping plus repetition of syllables, right-hand tapping plus repetition of syllables. left-hand tapping and anagram solution, and right-hand tapping and anagram solution. nichoric listeniny. The verbal stimuli were six consonant vowel (CV) syllables: :ba:, /da;. :ga;, /pa/, .!ta/ and .‘ka:. These syllables were from natural speech samples and were timed such that the stimuli of a pair presented to each car were simultaneous within 2.5 msec. The amplitudes ofthe vowel sounds for each pair were also matched to be within 2.5 dB of each other. The 30 different sets of possible CV syllable pairs were presented in random order with the criteria that no one CV syllable be used more than three times consecutively tn one ear. Subjects were tested on four sets each of 30 syllable pairs with approx. 6 set between pair presentations and approx. 30 set between sets. Stimuli were presented via Realistic Nova 55 stereo headphones (model no. 33-2004A) connected to a Sony stereo cassette player (model no. TC-FX44). The intensity of presentation for each ear was set at 75 dB. Subjects were asked to identify syllable pairs as presented via headphones and were provided with answer sheets numbered 1 30 with all six syllables listed at each number. They were asked to line out two of the six syllables as their best guess as to what they heard. Prior to testing. subjects were exposed to 20 monaurally presented CV syllables a practice.
NOTE
RESULTS
1253
AND DISCUSSION
The dependent variable used for the analysis ofdual-task tapping was percentage reduction in tapping rate using the following formula: [(B D)/B x loo]. where B is baseline tapping rate and D is dual-task tapping rate, both measured as taps per second. The dependent variable used for analysis of dichotic listening was the number of correctly identified syllables to each ear.
Analyses did not detect significant hand differences in finger-tapping interference or ear advantages on the dichotic listening task for left-handers as a group. For dual-task tapping. analysis of variance (ANOVA) with condition (repetition vs anagrams) and hand (left vs right) as within subjects’ variables showed only a significant condition effect. [F (1, 34) = 5.97, MS = 21.97, PcO.02, (“h reduction: syllable repetition-M = 2.55; anagram solution-Mz0.35). For dichotic listening, ANOVA with ear of stimulus presentation as the within-subjects variable revealed no significant difference between ears, F< 1. The majority ofleft-handers (56”Qshowed a right-ear advantage. 44% showed a left-ear advantage.
Mean baseline tapping rates were slightly (but nonsignificantly) faster for the left hand (M= 5.3 I) than right hand (A4 = 5.08) for left-handers as a group; however ,27 subjects (75%) tapped faster with their left (M= 5.27) than with their right (M =4.78) hand on baseline tapping trials (L > R), while nine subjects (25”Q tapped faster with their right (A!= 5.96) than with their left (M= 5.43) hand (R > L). These percentages are similar to those observed by HELLI(;E and KCL [2] and BATHCKST (1989). When subjects were categorized into dominant hand groups, significant lateralized interference for finger-tapping was found. A group (L> R vs R > L) by condition (syllables vs anagrams) by hand (left vs right) ANOVA was performed with group as the between subjects variable and condition and hand as within subjects variables. A Group by Hand interaction was found such that the left dominant group (L> R) showed more interference with the left (M~3.25) than with the right (M=0.68) hand, while the right dominant group (R>L) showed more interference with the right (M=2.44) than with the left (M= -0.57) hand. I; (I, 34) = 8.79, MS= 23.94, PqO.006 (see Fig. 1). Within this analysis no other significant effects were found. For dichotic listening. a group (L > R vs R > L) by ear (left vs right) ANOVA revealed no Group by Ear interaction. F< I In comparison. HFLLI~X and KIX [2] reported a Group by Ear interaction approaching significance, P=O.O9. suggesting a right-ear advantage for the R > L group only. This same pattern was observed in the present study.
Subjects were next categorized into consistent vs inconsistent groups based on baseline tapping rates for hands and dichotic listening scores for ears. For example. a subject who tapped faster with his/her left than right hand on baseline tapping trials and who correctly identified more syllables presented to his;her left than right ear would be ‘consistent’. Nineteen subjects or 53”/0 were consistent (13 left hand,/left ear (LL) and 6 right hand/right ear (RR)); 17 or 47% were inconsistent (14 left hand/right ear (LR) and 3 right hand/left ear (RL)). Proportions for sidedness advantage in the present study were LL (36”/u), RR (17”/0), RL (8%) and LR (39%). In comparison, HELLICX and K~E [2] reported LL (37%), RR (1 SO/,), RL (4%) and LR (41 O/u).For the Consistent group, ANOVA with condition and dominant hand as within subjects variables showed a significant difference for the dominant (~~~3.87) vs nondominant hand (M= -0.26). F(1, l8)=20.36, MS= 15.89, P-cO.0003. There was no significant main effect for condition (repetition vs anagrams), F (I. IX)= 1.lO, MSe = 14.49, P>O.O5, nor was there a Condition by Hand interaction. Fi I. For the Inconsistent group, ANOVA with condition and dominant hand as within subjects variables showed no difference for dominant (M= 2.13) vs nondominant hand (M= 1.08). F< 1. There was a significant difference for repetition (M=3.25)vs anagrams (M= -0.04), F (1. 16)=6.75, M&=27.26. PcO.02, but no Condition by Hand interaction, F< I Thus left-handers who were consistent for sidedness advantage (same side for dommant hand and ear advantage) showed significantly greater decrements in tapping performance for their dominant vs nondominant hand while performing concurrent verbal tasks. Lefthanders who were inconsistent for sidedness advantage, however. showed equal interference for both hands.
Verbal performance (number of syllables repeated and number of anagrams solved) on dual-task trials was assessed by ANOVA. The only significant effect revealed was for condition. indicating that more syllables (!\{=36.XI I were repeated thananagramssolved (M= 1.66), F( I, 32)= 115.37. MSr=264.21, P
SUMMARY In the present study. asymmetric manual interference was observed for the dominant hand regardless of whether that hand was left or right. This finding demonstrates a manual dominance effect. This asymmetry disappeared or
Now
1254
SYLLABLE
REPETITION
6
Left =I
Hand
Right
Hand
0
3 2
I
ANAGRAM
SOLUTION
I
I
R>L
L>R BASELINE Fig.
I.
ADVANTAGE
GROUP
Percentage reduction scores for tv,o dual-tusk procedures: hyllahle repetition (upper panel) and anagram solution (lower panel) as a function of dominant hand group.
~2~s attenuated for subjecta who showed hand and ear dominance on oppoaitca sides. This latter tin&g ~nd~catcs that hemispheric specialization influences the cxtcnt of asymmetric manual tnterference a\ well. Our findings arc consistent with the speculation that manual dominance (measured by asymmetries III baseline tapping rates) contributes to the lateralired finger-tapping interference produccd by bcrhal concurrent tasks [ I I. 13. 171. Some have suggested that analysis of covariancc should he used with baseline tapping rata as the covariatc to adjust for manual dominance clTects [I 3. 171. This IS not unrewmable asumlng that the important assumptions of ANC‘OVA can he met L6, 141. (In the prcscnt study. no correlation was found hctwcen bascl~nc tapping rata and perccntagc reduction in finger-tappmg.) It is important to keep in mind. houcvcr. that hasclinc asymmctrlcs (l.c. manual dominance) arc neither a ncceaswy nor sullicwnt condition for producing asymmetric dual task intcrfcrencc 13, 4. 71. In summary, the present results takw together with earlier evidence indicate clearly that /JCU/Imanual dominnncc and hcmisphcric specialzation are ~nvol\ed in manual Interference. Thus, it NilI be easenhal in future dunl-task studw to minlmve the ccrntrlhutlon of hand dominance :md.or adopt procedure\ that separate the clrcct\ of hcmisphcric asymmetry from the e!Tccts of hand dominance LZJ. A[,kr~o~~/fdyrnle,~rs We arc gratcfut to Joseph Hclligc for his awstancc with thia study and for his comments on an carlicr draft of this article. We also wish to thank Krls lima1 for her invaluable ashibtance.
NATE
1255
REFERENCES 1. GILHOOLY, K. J. and HAY, D. Imagery, concreteness, age-of-acquisition, familiarity, meaningfulness values for 205 five-letter words having single-solution anagrams, B&au. Res. Method Instrum. 9, 12-17, 1977. 2. HELLIGE, J. B. and KEE, D. W. Asymmetric manual interference as an indicator of lateralizaed brain function. In Advances in Psychology: Cerebral Control of Speech and Limb Movements, G. R. HAMMOND (Editor), NorthHolland, Amsterdam, 1990. 3. HISCOCK, M., CHEESMAN,J., INCH, R., CHIPKJER,H. M. and GRAFF, L. A. Rate and variability of finger tapping as measures of lateralized concurrent task effects. Brain Cognit. 10, 87-104, 1989. 4. HISCOCK, M. and CHIPUER, H. M. Concurrent performance of rhythmically compatible or incompatible vocal and manual tasks: evidence for two sources of interference in verbal-manual timesharing. Neuropsychologia 24, 691-698, 1986. 5. HISCOCK, M., KINSBOURNE, M. and GREEN, A. Is time sharing aysmmetry a valid indicator of speech lateralization? Evidence for left handers. In Adoances in Psychology: Cerebral Control 01 Speech and Limb Movements, G. R. HAMMOND (Editor). North-Holland, Amsterdam, 1990. 6. HOWELL, D. C. Statistical Methods for Psychology, 2nd edn. Doxbury Press, Boston, 1987. 7. KEE, D. W. and CHERRY, B. Lateralized interference in finger tapping: initial value differences do not affect the outcome. Neuropsychologia 28, 3 13.-3 16, 1990. 8. KINSBOURNE,M. and HIXOCK, M. Asymmetries ofdual-task performance. In Cerebral Hemisphere Asymmetry, J. B. HELLIGE (Editor), pp. 255-334. Praeger, New York, 1983. 9. OLDFIELD, R. C. The assessment and analysis of handedness: the Edinburgh Inventory. Neuropsychologia 9, 977113, 1971. 10. ORSINI, D. L., SATZ, P., SOPER, H. V. and LIGHT, R. K. The role of familial sinistrality in cerebral organization. Neuropsychologia 23, 223-232, 1985. 11. SIMON, T. J. and SUSSMAN, H. M. The dual task paradigm: speech dominance or manual dominance? Neuropsychologia 25, 559%569, 1987. 12. SUSSMAN,H. M. Contrastive patterns of intrahemispheric interference to verbal and spatial concurrent task in right-handed, left-handed and stuttering populations. Neuropsychologia 20, 675-684, 1982. 13. SUSSMAN,H. M. A reassessment of the time-sharing paradigm with ANCOVA. Brain Lang. 37,514-520, 1989. 14. TABATHNICK, B. G. and FIDELL, L. S. Using Multivariate Statistics, 2nd edn. Harper&Row, New York, 1989. 15. VAN STRIEN, J. W. and BOUMA, A. Cerebral organization ofverbal and motorfunctions in left-handed and righthanded adults: effects of concurrent verbal tasks on unimanual tapping performance. J. clin. exp. Neuropsychol. 10, 139-156, 1988. 16. WAGENAAR. W. A. Note on the construction of diagram-balanced Latin squares. Psychol. Bull. 72, 384 386, 1969. 17. WILLIS, W. G. and GOODWIN, L. D. An alternative to interference indexes in neuropsychological time-sharing research. Neuropsychologia 25, 719-724, 1987.
0028-3932 9, s3 oo+o 1991 Pergamon Presr
00 Qk
NOTE DUAL-TASK
INTERFERENCE IN LEFT-HANDED SUBJECTS: SPECIALIZATION VS MANUAL DOMINANCE
HEMISPHERIC
BAK~ARA CHERRY* and DANIEL W. KEE California (Rrcriwd
State University. 29 Jmuury
I99
Fullerton,
CA, U.S.A
I : accepted29
July
199 I )
Abstract-Thirty-six left-handed subjects performed a dichotic listening task and two concurrent (verbal plus finger-tapping) tasks. Baseline tapping scores revealed both left- and right-hand dominance within left-handed subjects. Moreover when subjects were categorized as consistent or inconsistent for ‘sidedness advantage’ (e.g. consistent=same side for dominant hand and ear advantage), it was shown that the dual-task interference effects found on the concurrent tapping tasks were due to both hemispheric specialization and manual dominance.
INTRODUCTION DUAL-TASK METHODShave been used for a number of years to assess hemispheric specialization in nonclinical right-handed populations 12, S]. For example, an asymmetrical decrement in tapping performance of the hands produced by various verbal concurrent tasks vs tapping alone indicates that the manual tapping task and the cognitive task are purportedly competing for resources from the same hemisphere [S]. Because the finger-tapping movement of the hand is primarily controlled by the contralateral hemisphere, greater right-hand interference produced by the verbal concurrent task indicates more left hemisphere involvement for the verbal activity [2, 81. Converging evidence for this left hemisphere dominance is provided by dichotic listening scores, which demonstrate a right ear (left hemisphere) advantage for verbal material in most right-handed subjects. Whereas right-handed subjects usually show reduced right-hand tapping while performing a concurrent verbal task [2, 81. recent results for left-handed subjects do not show consistent patterns of lateralized interference [S]. Studies using single linger-tapping and a variety of verbal tasks have shown either symmetrical patterns of interference for left-handers 1121, or asymmetrical dominant-hand interference; that is, left-handers show more decrements in tapping for the left hand while right-handers show more decrements in tapping for the right hand [lo, 1 I. 151. Yet those studies which also included dichotic listening tasks [IO, 151 still demonstrate left hemisphere dominance for speech (a right-ear advantage) for the majority of left-handers. In an attempt to clarify some of these results, HELLIGE and KEE 123 suggest that manual interference effects are due to both hemispheric specialization and manual dominance. and that left-handers can be classified as either left- or right-hand dominant based on baseline finger-tapping performances (from a reanalysis of their own data on lefthanders (n = 27 121) and an analysis of unpublished results by Bathurst (n =64; Bathurst, 1989). Hellige and Kee found (i) consistent proportions of subgroups for both sets of left-handed subjects: 78% left-hand dominant and 22% right-hand dominant and (ii) a dominant-hand Group by Hand interaction such that within left-handers, the left-hand-dominant group (L > R) showed more left-hand interference, while the right-hand-dominant group (R > L) showed more right-hand interference while performing concurrent verbal tasks. In addition, dichotic listening scores for the sample of 27 subjects also allowed for classification of subjects as consistent or inconsistent for ‘sidedness advantage’. That is, subjects who demonstrated dominance on the same side for both hand (baseline tapping) and ear (dichotic listening scores) were categorized as consistent (left hand/left ear and right hand/right ear), while those who showed opposite dominance for hand and ear were categorized as inconsistent (left hand,‘right ear and right hand/left ear). If dual-task interference effects are due strictly to manual dominance, symmetrical decrements in tapping performance on concurrent tasks should occur for the dominant hand regardless of whether
*Address all correspondence Los Angeles, CA 90089-1061.
to: Barbara U.S.A.
Cherry,
Department
1251
of Psychology,
University
of Southern
California.
1252
Nom
subjects are classified as consistent or inconsistent for hand and ear. II”,however. dual-task interference efrects arc due to both manual dominance and hemispheric specialization. asymmetric decrements in tapping performance should occur for the consistent but not for the inconsistent group. HELLGE and KEE L2] found greater finger-tapplng interference for the dominant hand only in the consistent group, suggesting that manual interference effects are due to at least two factors: hemispheric specialization and manual dominance. Since their findings arc only preliminary. this study provided a formal test of their hypothesis.
METHOD A total of 36 undergraduate students (20 males and I6 females) from the Introductory were used. All of the subjects spoke English as their first language and were left-handed writing-hand inspection and Edinburgh Handedness Inventory.
Psychology Subject Pool as assessed by self-report,
Quustionnuirus. Subjects filled out a IO-item version of the Edinburgh Handedness Inventory [9] to confirm direction and degree of handedness, and a Subject Information Questionnaire developed by E. Zaidel at the University of California, Los Angeles to assess familial sinistrality and other background information. Subject‘s writing hand posture (inverted or not) was also recorded. Information regarding familial sinistrality. other background information, and/or hand posture is available from the authors upon request. Dun/-Task Tappiny. Subjects were seated at a table in front of a small metal box (15 x 10 x 5.5 cm) on which were mounted two Micro Switch No. BA-2-V2-A2 tapping keys located 7 cm apart. The keys were attached to an Apple II Plus computer with a Mountain Hardware millisecond clock card to time 10 set trials and to record fingertapping performance. Beyond the s-itch box was a typing stand for presenting syllables (for repetition) and anagrams. In front of the switch box was a felt-covered platform (28 x 38 x 4 cm) on which subjects rested their forearms while tapping. The stimulus for the syllable repetition task was an 8 x I I in. sheet of paper on which the syllables: ,iba/, /da;. ;ga:. /pa/, /ta/ and /ka/ were printed across eight rows. For the anagram task. a set of four single-solution anagrams were presented vertically on a 13 x 20 cm card with Orator 61 10 type size. Anagrams with low imagery rating and medium-to-high familiarity ratings were used (see Ref. [I]). A different four-anagram set was used on each trial. For tapping alone trials. subjects were asked to look at a white, blank 13 x 20 cm card. For tapping alone, subjects were instructed to start tapping at the sound of a buzzer and to tap as fast as possible while staring directly ahead at the typing stand until the buzzer sounded again, marking the end of the IO set trial. Subjects were then given a practice trial for each hand to familiarize themselves with the procedure. For the syllablcrepetition task, subjects were instructed to repeat syllables across the rows out loud and continuously during the 10 set trial and to designate which syllable they were on at the end ofthe 10 sec. For tapping plus syllable repetition. subjects began tapping before the buzzer sounded. As a buzzer signaled the beginning of the 10 set trial subjects continued to tap as fast as possible while repeating the syllables. A second buzzer signaled them to stop. For the anagram solution task, subjects were asked to solve anagrams out loud working from the top of the anagram set to the bottom. If subjects solved all four anagrams before 10 set were up. they were to start over at the top. If they couldn’t solve an anagram, they were to go on to the next and come back to that particular anagram later. All other procedures were as described above. Subjects were allowed one IO set practice trial with anagram solving alone to familiarize them with the procedure. Each subject then performed 36 trials. The following conditions were represented six times in the 36 trials with the order based on a diagram-balanced Latin square [I 61: left-hand tapping alone, right-hand tapping alone, left-hand tapping plus repetition of syllables, right-hand tapping plus repetition of syllables. left-hand tapping and anagram solution, and right-hand tapping and anagram solution. nichoric listeniny. The verbal stimuli were six consonant vowel (CV) syllables: :ba:, /da;. :ga;, /pa/, .!ta/ and .‘ka:. These syllables were from natural speech samples and were timed such that the stimuli of a pair presented to each car were simultaneous within 2.5 msec. The amplitudes ofthe vowel sounds for each pair were also matched to be within 2.5 dB of each other. The 30 different sets of possible CV syllable pairs were presented in random order with the criteria that no one CV syllable be used more than three times consecutively tn one ear. Subjects were tested on four sets each of 30 syllable pairs with approx. 6 set between pair presentations and approx. 30 set between sets. Stimuli were presented via Realistic Nova 55 stereo headphones (model no. 33-2004A) connected to a Sony stereo cassette player (model no. TC-FX44). The intensity of presentation for each ear was set at 75 dB. Subjects were asked to identify syllable pairs as presented via headphones and were provided with answer sheets numbered 1 30 with all six syllables listed at each number. They were asked to line out two of the six syllables as their best guess as to what they heard. Prior to testing. subjects were exposed to 20 monaurally presented CV syllables a practice.
NOTE
RESULTS
1253
AND DISCUSSION
The dependent variable used for the analysis ofdual-task tapping was percentage reduction in tapping rate using the following formula: [(B D)/B x loo]. where B is baseline tapping rate and D is dual-task tapping rate, both measured as taps per second. The dependent variable used for analysis of dichotic listening was the number of correctly identified syllables to each ear.
Analyses did not detect significant hand differences in finger-tapping interference or ear advantages on the dichotic listening task for left-handers as a group. For dual-task tapping. analysis of variance (ANOVA) with condition (repetition vs anagrams) and hand (left vs right) as within subjects’ variables showed only a significant condition effect. [F (1, 34) = 5.97, MS = 21.97, PcO.02, (“h reduction: syllable repetition-M = 2.55; anagram solution-Mz0.35). For dichotic listening, ANOVA with ear of stimulus presentation as the within-subjects variable revealed no significant difference between ears, F< 1. The majority ofleft-handers (56”Qshowed a right-ear advantage. 44% showed a left-ear advantage.
Mean baseline tapping rates were slightly (but nonsignificantly) faster for the left hand (M= 5.3 I) than right hand (A4 = 5.08) for left-handers as a group; however ,27 subjects (75%) tapped faster with their left (M= 5.27) than with their right (M =4.78) hand on baseline tapping trials (L > R), while nine subjects (25”Q tapped faster with their right (A!= 5.96) than with their left (M= 5.43) hand (R > L). These percentages are similar to those observed by HELLI(;E and KCL [2] and BATHCKST (1989). When subjects were categorized into dominant hand groups, significant lateralized interference for finger-tapping was found. A group (L> R vs R > L) by condition (syllables vs anagrams) by hand (left vs right) ANOVA was performed with group as the between subjects variable and condition and hand as within subjects variables. A Group by Hand interaction was found such that the left dominant group (L> R) showed more interference with the left (M~3.25) than with the right (M=0.68) hand, while the right dominant group (R>L) showed more interference with the right (M=2.44) than with the left (M= -0.57) hand. I; (I, 34) = 8.79, MS= 23.94, PqO.006 (see Fig. 1). Within this analysis no other significant effects were found. For dichotic listening. a group (L > R vs R > L) by ear (left vs right) ANOVA revealed no Group by Ear interaction. F< I In comparison. HFLLI~X and KIX [2] reported a Group by Ear interaction approaching significance, P=O.O9. suggesting a right-ear advantage for the R > L group only. This same pattern was observed in the present study.
Subjects were next categorized into consistent vs inconsistent groups based on baseline tapping rates for hands and dichotic listening scores for ears. For example. a subject who tapped faster with his/her left than right hand on baseline tapping trials and who correctly identified more syllables presented to his;her left than right ear would be ‘consistent’. Nineteen subjects or 53”/0 were consistent (13 left hand,/left ear (LL) and 6 right hand/right ear (RR)); 17 or 47% were inconsistent (14 left hand/right ear (LR) and 3 right hand/left ear (RL)). Proportions for sidedness advantage in the present study were LL (36”/u), RR (17”/0), RL (8%) and LR (39%). In comparison, HELLICX and K~E [2] reported LL (37%), RR (1 SO/,), RL (4%) and LR (41 O/u).For the Consistent group, ANOVA with condition and dominant hand as within subjects variables showed a significant difference for the dominant (~~~3.87) vs nondominant hand (M= -0.26). F(1, l8)=20.36, MS= 15.89, P-cO.0003. There was no significant main effect for condition (repetition vs anagrams), F (I. IX)= 1.lO, MSe = 14.49, P>O.O5, nor was there a Condition by Hand interaction. Fi I. For the Inconsistent group, ANOVA with condition and dominant hand as within subjects variables showed no difference for dominant (M= 2.13) vs nondominant hand (M= 1.08). F< 1. There was a significant difference for repetition (M=3.25)vs anagrams (M= -0.04), F (1. 16)=6.75, M&=27.26. PcO.02, but no Condition by Hand interaction, F< I Thus left-handers who were consistent for sidedness advantage (same side for dommant hand and ear advantage) showed significantly greater decrements in tapping performance for their dominant vs nondominant hand while performing concurrent verbal tasks. Lefthanders who were inconsistent for sidedness advantage, however. showed equal interference for both hands.
Verbal performance (number of syllables repeated and number of anagrams solved) on dual-task trials was assessed by ANOVA. The only significant effect revealed was for condition. indicating that more syllables (!\{=36.XI I were repeated thananagramssolved (M= 1.66), F( I, 32)= 115.37. MSr=264.21, P
SUMMARY In the present study. asymmetric manual interference was observed for the dominant hand regardless of whether that hand was left or right. This finding demonstrates a manual dominance effect. This asymmetry disappeared or
Now
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SYLLABLE
REPETITION
6
Left =I
Hand
Right
Hand
0
3 2
I
ANAGRAM
SOLUTION
I
I
R>L
L>R BASELINE Fig.
I.
ADVANTAGE
GROUP
Percentage reduction scores for tv,o dual-tusk procedures: hyllahle repetition (upper panel) and anagram solution (lower panel) as a function of dominant hand group.
~2~s attenuated for subjecta who showed hand and ear dominance on oppoaitca sides. This latter tin&g ~nd~catcs that hemispheric specialization influences the cxtcnt of asymmetric manual tnterference a\ well. Our findings arc consistent with the speculation that manual dominance (measured by asymmetries III baseline tapping rates) contributes to the lateralired finger-tapping interference produccd by bcrhal concurrent tasks [ I I. 13. 171. Some have suggested that analysis of covariancc should he used with baseline tapping rata as the covariatc to adjust for manual dominance clTects [I 3. 171. This IS not unrewmable asumlng that the important assumptions of ANC‘OVA can he met L6, 141. (In the prcscnt study. no correlation was found hctwcen bascl~nc tapping rata and perccntagc reduction in finger-tappmg.) It is important to keep in mind. houcvcr. that hasclinc asymmctrlcs (l.c. manual dominance) arc neither a ncceaswy nor sullicwnt condition for producing asymmetric dual task intcrfcrencc 13, 4. 71. In summary, the present results takw together with earlier evidence indicate clearly that /JCU/Imanual dominnncc and hcmisphcric specialzation are ~nvol\ed in manual Interference. Thus, it NilI be easenhal in future dunl-task studw to minlmve the ccrntrlhutlon of hand dominance :md.or adopt procedure\ that separate the clrcct\ of hcmisphcric asymmetry from the e!Tccts of hand dominance LZJ. A[,kr~o~~/fdyrnle,~rs We arc gratcfut to Joseph Hclligc for his awstancc with thia study and for his comments on an carlicr draft of this article. We also wish to thank Krls lima1 for her invaluable ashibtance.
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REFERENCES 1. GILHOOLY, K. J. and HAY, D. Imagery, concreteness, age-of-acquisition, familiarity, meaningfulness values for 205 five-letter words having single-solution anagrams, B&au. Res. Method Instrum. 9, 12-17, 1977. 2. HELLIGE, J. B. and KEE, D. W. Asymmetric manual interference as an indicator of lateralizaed brain function. In Advances in Psychology: Cerebral Control of Speech and Limb Movements, G. R. HAMMOND (Editor), NorthHolland, Amsterdam, 1990. 3. HISCOCK, M., CHEESMAN,J., INCH, R., CHIPKJER,H. M. and GRAFF, L. A. Rate and variability of finger tapping as measures of lateralized concurrent task effects. Brain Cognit. 10, 87-104, 1989. 4. HISCOCK, M. and CHIPUER, H. M. Concurrent performance of rhythmically compatible or incompatible vocal and manual tasks: evidence for two sources of interference in verbal-manual timesharing. Neuropsychologia 24, 691-698, 1986. 5. HISCOCK, M., KINSBOURNE, M. and GREEN, A. Is time sharing aysmmetry a valid indicator of speech lateralization? Evidence for left handers. In Adoances in Psychology: Cerebral Control 01 Speech and Limb Movements, G. R. HAMMOND (Editor). North-Holland, Amsterdam, 1990. 6. HOWELL, D. C. Statistical Methods for Psychology, 2nd edn. Doxbury Press, Boston, 1987. 7. KEE, D. W. and CHERRY, B. Lateralized interference in finger tapping: initial value differences do not affect the outcome. Neuropsychologia 28, 3 13.-3 16, 1990. 8. KINSBOURNE,M. and HIXOCK, M. Asymmetries ofdual-task performance. In Cerebral Hemisphere Asymmetry, J. B. HELLIGE (Editor), pp. 255-334. Praeger, New York, 1983. 9. OLDFIELD, R. C. The assessment and analysis of handedness: the Edinburgh Inventory. Neuropsychologia 9, 977113, 1971. 10. ORSINI, D. L., SATZ, P., SOPER, H. V. and LIGHT, R. K. The role of familial sinistrality in cerebral organization. Neuropsychologia 23, 223-232, 1985. 11. SIMON, T. J. and SUSSMAN, H. M. The dual task paradigm: speech dominance or manual dominance? Neuropsychologia 25, 559%569, 1987. 12. SUSSMAN,H. M. Contrastive patterns of intrahemispheric interference to verbal and spatial concurrent task in right-handed, left-handed and stuttering populations. Neuropsychologia 20, 675-684, 1982. 13. SUSSMAN,H. M. A reassessment of the time-sharing paradigm with ANCOVA. Brain Lang. 37,514-520, 1989. 14. TABATHNICK, B. G. and FIDELL, L. S. Using Multivariate Statistics, 2nd edn. Harper&Row, New York, 1989. 15. VAN STRIEN, J. W. and BOUMA, A. Cerebral organization ofverbal and motorfunctions in left-handed and righthanded adults: effects of concurrent verbal tasks on unimanual tapping performance. J. clin. exp. Neuropsychol. 10, 139-156, 1988. 16. WAGENAAR. W. A. Note on the construction of diagram-balanced Latin squares. Psychol. Bull. 72, 384 386, 1969. 17. WILLIS, W. G. and GOODWIN, L. D. An alternative to interference indexes in neuropsychological time-sharing research. Neuropsychologia 25, 719-724, 1987.