Pergamon

002%3932(94)E0002-6

RIGHT-HANDERS AND LEFT-HANDERS SHOW DIFFERENCES AND IMPORTANT SIMILARITIES IN TASK INTEGRATION

WHEN PERFORMING MANUAL AND VOCAL TASKS CONCURRENTLY

KELLY MURPHY and MICHAEL PETERS*

Department of Psychology, University of Guelph, Guelph. Ontario, Canada

Abstract-For a sample of 73 consistent left-handers, 46 inconsistent left-handers and 65 right-banders, reciprocal interactions between unimanual or ixmanual tasks and concurrent reading tasks were shown to bc a complex function of task characteristics and sample composition. The suggesllon that subgrouping of left-handcrs [HELLIGE and KEY, Cerehrul Control of Speech and Limb ,Mowmentv, North Holland, Amsterdam, 19901 influences task interactions in left-handers was borne out. Be/h manual and vocal performance were measured. The effects of the vocal task on the manual task were variable for groups and conditions. but an important common trend emerged: all handedness groups showed the same robust effect: vocal task performance was better when the ri@t hand performed the manual task. Finally, speaking rate increased and tapping rate decreased durmg concurrent single hand tapping [cf. HISWCK, Brain Coqnif. 1, 119 131, 19821; and there was an opposite effect with concurrent bimanual tapping. The results favour a task integration model rather than an interference in cerebral functions space model.

INTRODUCTION

WHEN right-handed individuals perform a verbal and a skilled manual activity concurrently, right-hand performance is more seriously affected than left-hand performance [ 16, 17, 203. At the outset, it has to be emphasized that there usually is a performance decrement for both hands 120, 381 but this fact is rarely discussed in the literature (cf. the major reviews by HISCWK et al. [ 153 and HELLICE and KEE [ 111. The decrement in the left hand is presumably due to a general dual task interference effect [6, 371. The relatively greater right hand decrement, according to the “interference in functional cerebral space” model [21] is due to the specific asymmetrical interference which is thought to arise because the left hemisphere is controlling both speech and right-hand movements, and a general dual task interference effect. Interest in the vocal/manual task interference paradigm finds its origins in the hope that a simple behavioral task might allow interferences on speech/language lateralization in right-handers and subgroups of left-handers.

If the functional interference model holds, the performance of left-handers should be of some interest because the majority of left-handers have speech control in the left hemisphere,

*Address for correspondence: Department of Psychology, University of Guelph, Guelph, Ontario, Canada NIG 2Wl.

663

664 K. MUKPHY and M. PFTEKS

just like right-handers [S, 19, 23, 331. What left-handers will do in a dual task situation depends on the level at which interference takes place. There is some reason to believe that at least in some left-handers, praxis for the hands is managed in the right hemisphere, because apraxia is observed in left-handers after right hemisphere lesions [5, 191. If this were so, and if left-handers were uniform in their cortical arrangements, no assymetrical interference would be expected for left-handers. That is, the right and the left hand would only show the general dual task interference effects because there would not be any intrahemispheric functional interference. The only condition under which there would be interference between speech and left-hand movement would be if left-hand control were to act through ipsilateral descending pathways, a possibility already considered and rejected by HELLIGE and KEE

Cl 11. Current evidence [24, 36,421 suggests that in left-handers concurrent speaking interferes

more with the left hand than with right hand, but ASHTON and MCFARLAND [l] and L~MAS and KIMURA [22] failed to observe asymmetrical interference effects. The lack of documented asymmetries has several possible causes. First, as was pointed out by HIXOCK et al. [15],

studies with small numbers of subjects would not be expected to find significant asymmetries for left-handers. Second, the asymmetries represent an instance of Type I error, by no means uncommon in the neuropsychological literature. Third, and this possibility was raised by HELLIGE and KEY [l 11, the inconsistency of the performance of left-handers is due to the pooling of subgroups of left-handers, in analogy to observations made in motor performance [35]. Subclassification of left-handers has been reported to influence dual task performance [ 12, 251. HICKS [12] classified left-handers according to familial handedness. However, we are swayed by BISHOP’S [2] persuasive argument that classification according to familial handedness is very unlikely to reflect underlying genotypes in a useful way, especially so in the case of left-handers. As a result, speculative attributions of underlying differences in cerebral organization on the basis of inferring genotype via familial handedness are of questionable value. PARLOW and KINSBOURNE [25] subclassified left-handers according to writing posture; the assumption being that there are differences in cerebral organization between inverted and noninverted left-handers. However, a recent study of over 600 left- handers [26] showed that writing position varied significantly with age; there is a legitimate concern whether writing posture in left-handers can meaningfully correlate with cerebral organization [3]. Subgrouping of left-handers in terms of the consistency of lateral preferences does have a possible bearing on group effects. For example, left-handers as a group fail to show hand strength asymmetries [4], in contrast to right-handers who show clear right-hand superiority. However, subgroups of left-handers with consistent and inconsistent hand preference choices have strong and opposite asymmetries in hand strength [27, 351. Our reasoning for distinguishing between consistent and inconsistent left-handers in this study was that inconsistent left-handers, who make up close to a third of the current generation of left-handers [S, 341, may differ from consistent left-handers in more fundamental ways than in hand strength asymmetry alone. The question posed was this: would subgroups of left-handers, thus defined, show different interference patterns, and could the interference test give some indication that consistent and inconsistent left-handers differ in speech localization? The purpose of this study, then, was to see whether interference effects differ for subgroups of left-handers. It should be noted that Bathurst’s work (cited by Hellige and Kee, 1990, p. 647) emphasizes the importance of subclassification of left-handers. Bathurst used a task where subjects had to solve anagrams while tapping concurrently. Left- handers who tapped faster with the right hand in the baseline condition showed a

DUAL TASK PERFORMANCL 665

disproportionately strong right-hand decrement while left-handers who tapped faster with the left hand showed no asymmetrical interference.

In the present study measures were taken ofboth manual and vocal performance. This was done because interpretations of interference effects that are based only on manual performance can lead to false conclusions, a concern addressed already by HISCOCK [ 131, with a somewhat different paradigm. For instance, would a selective interference interpretation of greater right-hand performance losses be valid if, at the same time, vocal task performance is selectively improved? An extensive literature on mechanisms ofdual task integration emphasizes how time-sharing and flexible resource allocation are used to manage two or more activities concurrently [40,41] and this approach stresses integration of activities rather than loss of performance due to interference. The stress on integration also led to the inclusion of a bimanual tapping task. In the simple tapping task, subjects tap close to their maximum rate in the control condition, and for this reason the only direction in which rates can change in the dual task condition is downwards. In contrast, the chosen bimanual task depends on integration rather than speed of movement and is not rate limited [29]. Here, dual task effects can express themselves-at least potentially-in either increases or decreases in performance. There was an additional, theoretical motivation in including the bimanual task. When performed as a single task, the bimanual task reveals strong asymmetries for right-handers but not for left-handers. If the level of task interactions for dual tasks is at the level of integration rather than at the final output level, it is possible that asymmetries for this task can be found for left-handers as well, but only in the dual task condition.

METHOD Suhjrcrs. Participants were volunteers from an undergraduate subject pool at the University of Guelph. Two

studies were conducted; an initial study with 97 subjects, and a replication study with 87 subjects.* The data from both studies were combined because there were no significant differences between the two studies. In all, there were 93 male and 91 female subjects. There were 35 male and 38 female consistent left-handers, 22 male and 24 female inconsistent left-handers, and 36 male and 29 female right-handers. Consistent left-handers were those left-handers who preferred the left hand for at least 7 out of8 preference items (write, hammer, throw, unscrew lid ofjar, use knife, use toothbrush, hold a match while striking, hold racquet), including writing. Inconsistent left-handers were those who preferred the left hand for two to six of the items, including writing. Right-handers were those who used the right hand for writing and who preferred the right hand for writing. In this population (young college students), right- handed writers who prefer the left hand for other items are very rare.

Apparurus. Subjects tapped on microswitches which were connected to a microprocessor. There were separate switches for each hand and the subject could place the switches as desired to achieve a comfortable tapping position. The first tap initiated a IO-set period during which all taps and intertap intervals were recorded by a microcomputer.

.Sin~l/r,finylcr rcrppiny task. All tapping trials lasted IO sec. Each subject was instructed to produce complete taps rather than small amplitude oscillations, and to tap as quickly as possible. One practice trial was given to each hand, followed by four IO set trials with each hand. The beginning hand was alternated from subject to subject within a handedness group. The performance measure was the rate of tapping.

Birnarnrd tapping tusk. The tappers could be placed as desired so that each hand and arm was in a comfortable position at either side of the body. Subjects were required to produce two taps with one hand against one in the other (2: I). They were to tap at a pace that allowed them to maintain this rhythm without mistakes. In this task, there are two combinations. Subjects can perform the task “left two right one” or “right two-left one”. In the first case, the left hand is called the leading hand while in the second, the right hand is leading. Each subject was given three practice sessions with each hand leading. Subsequently, each subject performed four trials leading with the left hand and four

*The first study formed the body of the MA thesis of Kelly Murphy, and the second study was conducted by Michael Peters. in order to establish reliability of the observed effects through replication.

666 K. M~JKPHY and M. PFTEKS

trials leading with the right hand. The leading hand was alternated from subject to subject within a handedness group. The performance measure was the rate of tapping of the leading hand (the one that performed the two taps while the other performed one tap). In the analyses of the bimanual task, “left hand” refers to the performance of the left hand when issuing two taps against the single tap of the right hand, “right hand” similarly refers to the performance of the right hand issuing two taps while the left dots single taps.

Reading rash. Subjects were presented with a I IO word passage, taken from the “Velveteen Rabbit” by Margery Williams. The book had been rated at a 6th.grade level according to the Fry readability scale. The passage was printed on a standard sheet of paper, 57 cm from the subject. Lower case letters were 3 mm in height and 3 mm in width, and upper case letters were 4 mm x 4 mm in height and width. Subjects were required to read loudly and clearly and as quickly as possible without loss of quality of reading. Reading performance was recorded on tape. The performance measure was the number of words read as determined by the last word read during the IO set trial.

Dual tasks. The term dual task was used when subjects performed the finger tapping tasks and the reading task at the same time.

All subjects performed the single finger tapping task first. then the bimanual tapping task, and then the reading task. Subsequently, they performed the single finger tapping task for both hands while reading and then the bimanual finger tapping task for both hands while reading. Within each condition the order of beginning hand was counterbalanced for the tapping tasks.

RESULTS

First, there will be a description of the single and dual task performances for the single hand and the bimanual conditions, based on the means of tapping performances and words read. In discussing the results, measures of effect size and power (a =0.05) are provided in addition to F-ratios and probabilities. The effect size measure is the ETA squared value given by the SPSS program and it can be read in terms of the proportion of the variance accounted for. This measure of effect size is always smaller than the other common measure Ii’, which is computed by dividing the difference between the means in question by the common standard deviation. For example, the effect size (variance accounted for) for “condition” in Table 1 is 0.15, and effect size defined as d’ is 0.33. Degrees offreedom vary because of missing values in some cells.

In addition, and because the focus of this study is on the mutual interactions between speaking and tapping, the effects of the dual task are presented in terms of the percent change from baseline performance. This crystalizes the robust dual task effects in a clear and intelligible way.

_!$ixt.~ of’ reuding on tapping

Single lzund tapping, siryle and dual tusk. Table 1 shows the performance during the single tapping task and concurrent reading. Significance levels and associated information are given in Table 1. There was a main effect for Sex; males tapped faster than females. The main effect for hand (preferred hand faster) was qualified by a significant Group x Hand interaction and this was due to the fact that inconsistent left-handers showed smaller between hand differences than right-handers and consistent left-handers. This point is clarified by the simple effects for preferred vs non-preferred hand for the three groups, shown at the bottom of Table 1. A significant main effect for Condition indicated that tapping rates declined while reading for all groups. The significant interaction between Condition x Hand was due to the fact that the performance of the preferred hand was more depressed than that of the nonpreferred hand during concurrent reading.

Bimanual tapping, single ad duul task. Table 2 shows the tapping performances during the bimanual task. For the purposes of this analysis, it is sufficient to discuss only the performance of the hand that performs the “2” beat; the performance of the hand that performs the “1” beat is complementary and adds nothing to the discussion. Previous work has shown that right-handers perform better when the right hand performs the two beats and

DUAL TASK PERFORMANCE 667

Table 1. Overall single hand tapping performances by the different sex and handedness groups; reading and nonreading conditions, taps/l0 set and S.D.‘s

Males Females Single hand tapping Single hand tapping

No reading Reading No reading Reading

PRH

CL 54. I 5.8

PRH

IL 52.2 5.1

PRH

RH 54.4 4.9

NPH PRH N=35

51.3 50.6 6.2 6.5

NPH PRH N=21

51.8 49.9 5.3 5.5

NPH PRH N=36

50.6 51.1 5.4 6.9

NPH PRH

48.6 49.0 6.1 5.3

NPH PRH

49.6 48.1 5.0 3.9

NPH PRH

48.4 49.7 6.3 5.0

Sex [F(l, 177)=41.6, P<O.OOOl] Condition [F(l, 177)=73.4, P<0.0001] Hand [F(I, 177)=108.0, P<0.0001] Group x Hand [F(2, 177)=11.4, P<O.OOOl] Condition x Hand [F(l, 177)=6.5, P<O.O120] Simple effect for preferred vs nonpreferred hand CL [F(l, 72)=65.9, PiO.00001 IL [F(l, 45)=4.0, P<O.O530] RH [F(l,64)=74.9, P<O.OOOO]

NPH PRH N=3X

46.2 44.3 5.7 7.8

NPH PRH N=24

47.1 44.6 4.1 5.1

NPH PRH N=29

47.1 44.6 4.1 5.7

NPH

42.2 7.8

NPH

43.9 4.5

NPH

43.9 4.5

Effect size Power 0.19 1 .oOO 0.29 1.000 0.38 1.000 0.11 0.992 0.04 0.712

0.48 1.000 0.08 0.493 0.54 1.000

the left hand performs the single beat [29, 391. For undifferentiated samples of left-handers [29, 391, no significant asymmetries are observed and this was the case in both replications here.

There was a significant main effect for Sex, males tapping faster. There was a significant main effect for Condition, all groups tapping slightly faster during the concurrent reading that during the tapping only condition. Because the bimanual tapping+ reading condition was always given after the single task bimanual tapping condition, the increase in tapping might be attributed to a practice effect. Single task tapping rates can increase slightly after prolonged practice [31]. However, performance of the 2:l tapping is not rate limited or rate dependent because subjects establish their own personal rate [29] and it is unlikely that a rather minimal practice would result in subjects performing faster in the difficult dual task condition. Finally, there was a significant interaction between Group x Hand, due to better performance by the inconsistent left-handers with the right (nonpreferred) hand. As the simple effects of preferred vs non-preferred hand at the botton of Table 2 indicate, consistent left-handers showed no significant asymmetry between the preferred and non-preferred hands.

&fects qf tapping on reading performance

Reading during single hand tapping. The effects of the manual tasks on the reading task are summarized in Table 3. There were no sex differences and data were combined across sex. A significant main effect for Condition was due to the fact that subjects read more words when tapping at the same time than when only reading. A significant group effect derived from a

668 K. MURPHY and M. PETERS

Table 2. Overall bimanudl hand tapping performances by the different sex and handedness groups; reading and nonreading conditions, taps’10 set and S.D.‘s. Only the performance of the hand performing the “2” beats in

the 2:l task is shown

Males Females Single hand tapping Single hand tapping

No reading Reading No reading Reading

PRH

CL 28.5 5.4

PRH

IL 26.7 5.0

PRH

RN 26.8 5.3

NPH PRH N=35

29.4 29.4 5.5 6.1

NPH PRH N=22

21.3 27.5 4.9 5.0

NPH PRH N=36

25.6 27.4 4.8 5.0

NPH PRH

28.6 25.8 6.0 5.5

NPH PRH

28.2 23.9 5.3 3.7

NPH PRH

25.1 26.5 5.4 3.2

Sex [F(l, 178)=7.5. P<O.O07] Condition [F(l, 178)=7.8, P<O.O06] Group x Hand [F(2, 178)=21.2, P<O.OOOl] Simple effect for preferred vs nonpreferred hand CL [F(l. 72)=0.2. P<O.66] IL [F(l,45)=9.38, P<O.O03] RH [F(l, 74)=33.7, P<O.OOO]

NPH PRH N=38

25.8 25.1 5.2 5.8

NPH PRH N=24

24.8 25.3 3.9 4.5

NPH PRH N=29

24.8 26.7 4.0 4.4

Effect size 0.04 0.04 0.19

0.00 0.17 0.35

NPH

26.1 5.3

NPH

26.0 5.0

NPH

25.4 4.4

Power 0.778 0.793 1.000

0.050 0.849 1.000

slightly higher mean number of words read by the consistent left-handers. This effect did not appear in the separate replications but emerged in the pooled data. Finally, there was a significant effect of Hand; subjects in all three groups read more words when the right hand was tapping then when the left hand was tapping.

Reading during bimanual tapping. There were no sex differences and the data were pooled across sex (Table 3). There was a significant main effect of condition; subjects in all three groups read fewer words during bimanual tapping than when reading alone. In addition, there was a significant effect of Hand; in all three groups, regardless of handedness, reading rate was less severely reduced when the right hand was tapping than when the left hand was tapping.

Baseline/single task performances expressed in terms of percent change

Figures 1 and 2 were constructed by setting the single task performances for each group equal to 100%. All dual task performances were then expressed relative to this 100% standard. Use of a percentage standard eliminated sex differences and allows a clear summary statement. The following robust effects can be seen in single finger tapping (Fig. 1). All subjects had a reduced tapping rate compared to baseline, and all subjects read more words when tapping at the same time. In addition, more words were read when the right hand was tapping than when the left hand was tapping. Right-handers and consistent left- handers showed an asymmetric interference effect during concurrent reading: tapping rate of the preferred hand was proportionately more reduced than the tapping rate of the non- preferred hand. Inconsistent left-handers showed no significant asymmetry. This group also

DUAL TASK PEKFOKMANCE 669

Table 3. Numbers of word read in 10 set when reading only, and when tapping in the single hand and himanual conditions. In contrast to reading only, reading speed increases when tapping

with the single hand and decreases over control when tapping himanually

Single hand tapping Words read

Bimanual tapping

Control Left hand Right hand Control Left hand Right hand

CL 49.3 53.5 54.0 49.3 44.2 44.8 5.4 6.3 6.4 5.4 9.1 9.5

IL 46.1 50.7 51.4 46.7 41.8 43.3 4.5 5.6 5.9 4.5 9.1 8.3

RH 46.5 51.2 52.5 46.5 42.X 43.x 7.1 7.6 8.6 7.1 10.7 10.9

Condition Condition [F(2, 362)=257.2, P<O.OOOOl] [F(2, 362)=43.1, P<O.OOOl] Effect size 0.58, Power 1.000 Effect size 0.19, Power 1.000

Hand Hand [F(l, 1X3)=22.5, P<O.OOOOl] [F(l, 181)=16.9, P<O.OOOl] Effect size 0.11, Power 0.997 Effect size 0.09, Power 0.983

Group [F(2, 181)=3.2, P<O.O4] Effect size 0.03, Power 0.608

showed a minimal performance asymmetry when tapping with the single hand. Thus, the role of initial performance asymmetries in dual task asymmetries [ 11, 1 S] has to be assessed relative to handedness group.

Reading and one-handed tapping % Change In ” of words and tafm / 10 aec

16

-10’ CL IL RH

= L Tam m R Taps DL Word3 m R Words

N CL - 73. IL - 46, RH - m

Fig. 1. The changes in % in reading rates and tapping rates when subjects are reading and tapping with a single hand are shown. Increases in performance compared to single task performance are seen above the line, and decreases are shown below the line. Concurrent reading decreases tapping rates, more so in the preferred hands for CLs and RHs. Concurrent tapping increasrs reading rates, more so when the right hand taps, in consistent left-handers (CLs), inconsistent left-handers (ILs) and right-

handers (RHs).

670 K. MIJKPHY and M. PETERS

In the bimanual tapping task (Fig. 2) it can be seen that all groups increased their tapping rate during dual task performance, and decreased their reading rate during dual task performance. In addition, the depression of the reading rate during the dual task was less severe when attention was focused on the right hand (i.e. when the right hand performs the “2” beat).

Reading and two-handed tapping PL Change In ” of words and ~.SPS / 10 set

-12’ CL IL RH

= L Taps mm R TOPS =L Words =A Word3

N CL - 73. IL - 4.3, RH - 16

Fig. 2. Shows the changes in u/u in reading rates and tapping rates when subjects are reading and tapping with both hands. Only the tapping performance of the hand that performs the “2” beats ofthe 2:l task is shown. Effects are opposite of those seen for single and tapping. Subjects read fewer words while tapping bimanually, but the reduction is less severe when the right hand taps the “2” beat. for

CLs. ILs and RHs.

The two figures give evidence that as performance of one task declines, performance of the other improves. The asymmetrical effect of speaking on preferred hand tapping is seen only in the single tapping task, and the effect is minor and subject to subject sample composition. Perhaps the most interesting finding concerns the faster reading performance when the right hand is tapping; in the single tapping conditions this shows up in increased reading rates and in the bimanual tapping condition it shows up in less reduced reading rates when the right hand is tapping.

DISCUSSION

In considering these results, it is tempting to speculate in great detail on all observed interactions between hand and handedness group. However, the very point made by this study is that particulars of task selection and subject sample composition are likely to yield numerous different patterns of interaction between task and subject characteristics. For this reason, the Discussion will only resolve around the major substantive points which are likely robust and valid for many different dual task conditions. First, subjects engage trade-off practices when performing a manual and vocal task concurrently. A similar tendency was observed for tapping and a memory task [7] and for a tapping and vocal task studied by HIXYXK [I 31. Because Hiscock asked his subjects to focus attention deliberately on either the manual or vocal task, the situation is not quite comparable to the present task and no clear trade-off effect was observed. For instance, when asked to focus on the manual task, his subjects produced more words, as was observed in the present task, but they also made more

DUAL TASK I’ER~ORMANCE 671

errors in the tongue twister. The specific direction of the trade-off depends on the task characteristics. This is illustrated dramatically in Figs l(a) and (b). In single tapping and reading, tapping suffers and reading rate increases. Tapping suffers relatively more in the preferred hand for consistent left-handers and right-handers, but not in inconsistent left- handers. While the effects of reading on tapping are variable, the effects of tapping on reading are uniform for all three groups: reading rate increases in both hands in all groups, and in the riyht hand for all groups. This finding qualifies the generality of HELLIGE and KEE’S [ 1 l] point concerning initial baseline asymmetries. Thus, inconsistent left-handers show no asymmetrical effect of reading on tapping in the preferred and nonpreferred hand, presumably because they do not have much of an asymmetry in the baseline condition. However, the asymmetrical effect of tapping on reading in inconsistent left-handers is just as strong in inconsistent left-handers as it is in the other two groups.

The most parsimonious explanation of this discrepancy is likely found in the common laterality of speech motor control in all groups; almost all right-handers and the majority of left-handers have motor speech control in the left hemisphere. PETERS [26] has recently suggested that clear left hemisphere specialization of motor speech control in left-handers is likely considerably more common than clear left hemisphere specialization for linguistic functions in general, and this argument, if valid, would account for the observation that the effects of right-handed tapping on speaking are as clearly expressed in left-handers as in right- handers.

Contrary to the single tapping task, bimanual tapping rates increase significantly during concurrent speaking. It must be noted, however, that bimanual tapping rates even for the faster hand (the one that beats the two beats) are quite slow compared to single hand repetitive tapping rates. All three groups decrease their reading while performing bimanual tapping. The reading rates in all three groups are affected in the same way: all groups read more slowly, and the effect is less severe when reading occurs during right-hand tapping.

When the effects of speaking on tapping and tapping on speaking are examined, it is easier to interpret the results in terms ofefforts to integrate the two streams of motor response than in terms of interference effects [9, 10, 141. In all cases, the change in rates of movement, be it manual tapping or speech motor production, is in the direction of a common rate. The uniform efrects of right-hand performance on speaking are interpreted in terms of the better ability to integrate speech and right-hand movement because speech motor control and the right hand motor outflow are on the same side and all groups. This interpretation is the exact opposite of what the “interference in cerebral functional space” model suggests and in full agreement with the findings of GUARD'S [9] elegant study on the integration of singing the score while playing the piano. The interference in cerebral functional space model, of course, was formulated with only halfthe information-that is, knowledge of the effects a concurrent vocal task on manual performance, but not knowledge of how manual performance effects vocal performance.

Thcsc results caution that dual task studies, rather than informing in a generally valid way about cerebral organization of right-handers and left-handers, can be expected to give quite variable results, depending on whether it is possible to integrate the two concurrent motor streams. Thus, we have come to the same conclusions reached in the thoughtful review by HISUXK et ul. [15], albeit by a different route. Depending on the nature of the tasks, anything can be expected. When both tasks require complex chains of timed events that cannot be subsumed under a common denominator, performance breaks down completely 1321. At the other extreme would be complete compatibility of vocal and manual

672 K. MURPHY and M. PETEKS

movements, as in some musical performances, where one activity stabilizes and enhances the other (cf. Ref. [21]). The differential effects of concurrent speaking on tapping in consistent and inconsistent left-handers are interpreted in terms of the way in which attention comes to bear on manual movement in these two groups, and on which of the two tasks receives focal attention. In unimanual tapping and reading, the simple repetitive nature of the tapping task allows focusing of attention on the reading task. Overall, this benefits the reading rate to the detriment of the tapping rates. The asymmetry of interference is explained in terms of the fact that consistent left-handers and right-handers focus attention consistently on the preferred hand; because the preferred hand demands focused attention [28], withdrawal of attention will affect the preferred hand more severely than the nonpreferred hand. Inconsistent left- handers have left-long practice in directing attention to either hand, depending on the activity. As a result, there is no selective effect on the nonpreferred hand.

In contrast, the bimanual task is quite difficult, and subjects are compelled to focus attention on the preferred hand rather than reading in order to maintain performance [28, 301. Because the preferred hand receives focused attention, there is no asymmetrical effect on this hand in the bimanual task. In both the dual unimanual and the bimanual tasks the changes in rates of the component tasks are an outcome of an attempt to harmonize the rates in both activities.

In summary, whether or not the vocal task has an asymmetrical effect on manual performance depends on the disposition of subjects to focus attention on the preferred hand, and the attentional requirements of the tapping task. In contrast, the asymmetries that are observed in the effects of the manual on the vocal task are interpreted in terms of structural factors, with emphasis of the proximity of control mechanisms for right-hand movement and speech motor control in the left hemisphere for both right- and left-handed individuals.

Acknowlrdgeme,lt-This work was supported by NSERCC grant No. A 7054 to the first author.

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II.

12.

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