Neuropsychologia, 1977, Vol. 15, pp. 461 to 465. Pergamon Press. Printed in England.

NOTE

S I M U L T A N E O U S P E R F O R M A N C E O F T W O M O T O R A C T I V I T I E S :

T H E F A C T O R O F T I M I N G *

MICHAEL PETERS

Dept. of Psychology, University of Guelph, Guelph/Ontario, Canada

(Received 25 August 1976)

Abstraet--Three experiments are presented in which human subjects had to perform two motor tasks simultaneously. In Experiment 1, subjects were required to tap rapidly with a single index finger while reciting a nursery rhyme. In Experiment 2, subjects were required to tap rapidly with the index finger of one hand while beating a I-3-123 rhythm with the other hand. In Experiment 3, subjects were required to recite a nursery rhyme while beating the 1-3- 123 rhythm with one hand. All tested subjects were able to perform the task in Experiment 1, 10 per cent of the tested subjects were able to perform the task in Experiment 2 and none of the tested subjects were able to perform the task in Experiment 3. The results are discussed with reference to the compatibility of rhythms of tasks which are performed concurrently.

I N T R O D U C T I O N

SEVERAL studies have shown that speaking depresses the performance of dowel balancing [1-3], single finger and sequential finger and arm tapping. The right hand of righthanders is more affected by speaking during the performance of dowel balancing and sequential tasks than the left hand. In contrast, speaking seems to affect single finger tapping equally in both hands [3]. The above findings relate to questions concerning inter- hemispheric interactions, and, more generally, to the question of what factors lead to interference when two motor acts are carried out concurrently. This note is concerned with a factor which may be of considerable importance in the concurrent performance of two motor acts : rhythm.

One of the most simple rhythms is created by repetitive motor acts which are separated by equal or near equal time intervals, as in single index finger tapping. Investigation of the interference between the rhythms produced by recitation of a nursery rhyme and single index finger tapping is the objective of Experiment 1. This experiment is essentially an effort to replicate the findings of LOMAS and KaMURA [3]. An additional con- dition was added to their design. Subjects were required to slowly describe a wide clockwise circle on the floor with either foot while tapping. This type of movement cannot be considered rhythmical in nature as it does not involve any rigid temporal structure.

E X P E R I M E N T 1

Subject Twenty-four male and 24 female unpaid volunteers were used. All subjects were righthanders. Subjects

were considered to be righthanders if they performed all of the eight following activities [3] with the right hand: throw a ball, eat soup with a spoon, brush teeth, comb hair, cut bread, swing a tennis racket, hammer a nail, write own name.

Apparatus Subjects tapped the lever of a microswitch which was mounted on a wooden board. The first tap initiated a

10 sec interval during which all further taps were counted by an electronic counter. At the end of the interval the counter was automatically disconnected from the circuit.

Procedure Subjects were given the instruction to tap as rapidly as possible with the index finger without lifting the

*Supported by University of Guelph RAB Grant No. 80806.

461

462 Nm I:

Table 1. Single finger tapping rates for 24 female and 24 male subjects under various conditions. Numbers refer to the average number of taps/10 see

. N

Females

Males

Baseline Left foot Right foot rate Speaking* circling circling

right 54.3 42'5 56.4 55.7

left 50.7 47.8 49.0 48~7

right 56.0 53.8 57.0 56.1

left 51'7 49.0 50.5 50-5

*Performance during this condition significantly reduced as compared to baseline perfor- mance (dr3, 138; Frequired at P < 0.01 = 3.95, Fobtained = 6.21).

pahn of the hand or any of the other fingers from the wooden board. Baseline tapping rates were established by giving each index finger, in alternation, three 10 sec trials. The avcrage rate for each hand was used as reference value for comparisons. The subjects were presented with the following conditions in a counter- balanced design : finger tapping while reciting the rehearsed nursery rhyme "Humpty Dumpty", and finger tapping while describing a slow clockwise circle on the floor with either foot. Each of the conditions was presented with one trial only.

Results Table I shows the means obtained for the different conditions. An analysis of variance for repeated

measures, handedness × sex × conditions, revealed a significant effect for "hand"; the right hand tapped more often than the left (df 1'46, Fobtained 75'9, Frequired at P < 0-01 = 7.31). The conditions effect, due entirely to the depressed performance for both sexes and both hands during the speaking condition, also proved significant (dr3,138, F obtained 6.21, F required at P < 0.01-3.95). There were no significant inter- actions. Males tended to tap somewhat faster than females but the difference was not significant. Describing a circle on the floor with either foot had no significant effect on tapping performance.

Discussion Even though the procedure and design differed slightly from that used by LOMAS and KIMURA [3], their

basic finding was replicated. Speaking depresses the tapping performance of both hands. The effect was relatively small and did not apply to all subjects. Of the forty subjects, 5 male and 7 female subjects actually tapped faster during the speaking condition. With one exception, the effect was seen in both hands.

While observing the subjects, the author had the impression that the finger tapping was integrated with the speech rhythm to some extent; taps and stresses during speech seemed to coincide quite often. It may be speculated that subjects had relatively little trouble in performing the two tasks simultaneously because both rhythms allow some flexibility in terms of temporal spacing between individual motor acts. The question arises as to what would happen if one of the two motor tasks has a temporal structure which has very little flexibility. This question is explored in Experiment 2.

[:.XPE R I M E N T 2

One of the two tasks chosen for this experiment was the single linger tapping used in the previous experi- ment. The other task required subjects to beat, with the hand, the rhythm 1-3-123. North American readers will be familiar with this rhythm as the "go, go go go go" chant of cheerleaders.

Subjects Some 150 university students, 120 males and 30 females, were used. Of these, 12 students were lefthanders.

There were 6 male and 6 female lefthanders in the sample.

Procedure Subjects were told to tap continuously and as rapidly as possible with the index finger of one hand while

beating the 1-3-123 rhythm with the other hand. Subjects were asked not to let the beat of the 1-2-123 rhythm influence the single finger tapping. Subjects were required to perform the task in two combinations: left hand beating rhythm and right hand tapping and vice versa. The order of combinations was counter- balanced from subject to subject.

Initially, it had been expected that the evaluation of the simultaneous performance of the two tasks would be very difficult and plans had been made to design electronic apparatus which would assist in the evaluation. However, it turned out that evaluation was no problem. The criterion used to judge the success of the

NOTE 463

subject's performance was the experimenter's and the subject's own evaluation of indepencence of movement between the two hands. Subjects failed the task if the tapping was irregular, or if the tapping was in phase with parts of the rhythm or the entire rhythm. Subjects also failed if they did not maintain the rhythm. The actual rate of single finger tapping proved to be of little value; some subjects could achieve reasonably fast rates simply by following the I-2-123 rhythm.

Results and discussion Surprisingly enough, very few subjects were able to perform the required task. Among the subjects were

several piano players and some of these were not able to perform the task; to their own surprise. Of the 150 subjects, 15 subjects were found who could perform the task. Fourteen were righthanders and one was a lefthander. Of the righthanders who could perform the task, all were able to do the right tap/left rhythm combination. Three of the righthanders were also able to perform the right thythm/left tap combination (all three played musical instruments) but not as well as the right tap/left rhythm combination. Thus, all the righthanders who could perform the task were superior on the left rhythm/right tap combination. The probability of a result of this kind occurring due to chance is small (P <0-001, by sign test). The sole left- hander who could perform the task could only do the right rhythm/left tap combination. There were no females in the sample who could do the task.

The task proved to be too difficult for most of the subjects. Even some of the subjects who proved success- ful performers would tap in phase with the three fast beats of the 1-3-123 rhythm during the initial trials. Of the two motor activities the single finger tapping still permitted some flexibility. It is conceivable that this flexibility permitted the 15 successful subjects to perform the two activities simultaneously. What would happen, if both activities had a rigid temporal structure? This question was explored in Experiment 3.

E X P E R I M E N T 3

In this experiment, subjects were required to recite the rehearsed nursery rhyme "Humpty Dumpty" while beating the 1-3-123 rhythm with either the right or the left hand. Subjects were instructed to recite the nursery rhyme at normal speed, with proper rhythmic intonation. They were asked not to use one rhythm to pace the other. None of the 100 subjects sampled were able to perform the task. Among these subjects were the 15 subjects who were able to perform the task in Experiment 2. It is interesting to note that several subjects realized that they would not be able to do the task while preparing for its performance.

C O N C L U S I O N

The three tasks presented in Experiments 1, 2 and 3 represent a gradient with regard to rigidity of temporal structure. The difficulty in performing the tasks increases accordingly. The factors of task complexity and attention (KINsBOURNE and Coo~ [2]) are very likely involved in the increasing difficulty of the tasks. Nevertheless, the independent role of the rigidity of temporal structure is emphasized by considering the effects of speaking on dowel balancing performance. Dowel balancing requires a high degree of attention and can be considered a complex visuo-motor task. The effects of speaking on dowel balancing, however, are small [1-3] or even insignificant [4]. Dowel balancing may not be a very good example, since, unlike the tasks presented in this paper, dowel balancing requires visual feedback for adequate performance. Good examples involving complex concurrent motor activities which do not depend on visual feedback are found in the field of music. Musicians routinely perform very complex movements simultaneously with both hands while playing an instrument; and singing is compatible with this activity. In comparison, the tasks in Experi- ments 2 and 3 are quite simple. How, then, could one account for the severe difficulty which performance of these tasks presents? In the following discussion, only those motor activities which do not depend on visual feedback shall be considered--independence of visual feedback seems a characteristic of motor movements involved in the production of rhythms.

As working hypothesis it is suggested here that the central nervous system, in the voluntary guidance of movement, can produce only one basic rhythm at a time. The successful concurrent activity of two motor systems is possible if the following conditions apply: 1. One motor system produces a rhythm but the other produces movements which are not subject to rigid

temporal restrictions. 2. One motor system produces a rhythm and the other produces rhythms which are based on preformed

species specific motor patterns, such as walking or running. 3. One motor system produces a rhythm and the other simply follows this rhythm. There are severe limits to

the ability of the central nervous systems to perform voluntarily guided bilaterally symmetrical move- ments. This can be ascertained by tapping simultaneously with both index fingers on a surface. Even at relatively low tapping speeds the two movements will get out of phase.

4. One motor system produces a rhythm and the other functions on an opportunistic basis, i.e. it is active

464 Nol~

only during those points in time when pauses in the dominant rhythm permit activity. This may be termed a time sharing mechanism.

5. One motor system dominates in generating rhytlun and the rhythm produced by the other system is integrated with the dominant rhythm in such a manner that the two rhythms produce predictable and interlocking patterns of stresses and pauses.

The above propositions point out a number of ways in which two concurrent voluntarily guided motor patterns, of which one or both involve a rhythm, can be made compatible. If these propositions exhaust the list of options for compatibility, it can be seen that the task required of subjects in Experiment 3 would be impossible to perform. The requirement of independent production of rhythms by systems which guide speech and hand movement allows none of the mechanisms for compatibility.

The task demanded of subjects in Experiment 2 is interesting as it seems to allow some degree of successful concurrent activity for a small number of subjects. Previously, it was pointed out that there seemed to be some accommodation of the single finger tapping to the 1-3-123 beat. It is possible that the subjects who were able to perform the task were able to make sure of options 3 and 4 for compatibility of movements. This can be investigated experimentally.

The asymmetry observed in the ability to perform the task in Experiment 2 deserves some comment. It may be speculated that the 1-3-123 rhythm, requiring greater attention because of its more severe temporal constraints, was produced as the dominant rhythm. If so, this asymmetry could be accounted for in the following way. When, in righthanders, the left hemisphere produces the basic rhythm, the right hemisphere is overruled and tends to follow the rhythm produced by the left hemisphere. If the dominant rhythm is produced by the right hemisphere, however, the left is capable of guiding single finger movement at least to some extent. One possible interpretation of this finding, to be substantiated by further research, is that in righthanders the left hemisphere dominates in generating motor rhythms. Further research will have to find motor tasks which are somewhat easier to perform than the task required in Experiment 2 so that the con- clusions will not have to depend on a small number of subjects.

Another interpretation is possible as well. MILrqER [5], on the basis of sodium amytal studies, has suggested that the programming of learned sequences of hand and arm movements appears to be a function of the speak- ing hemisphere, irrespective of hand preference, If this conclusion applies to the production of rhythms as well, one could assume that the 1-3-123 rhythm was produced in the left hemisphere of the sample of right- banders used in this experiment. It is possible, then, that the integration of rhythms between left and right hand is easier, in righthanders, if the same hemisphere programs the basic rhythm and guides the single finger tapping. In Experiment 2, presumably, this meant that the left hemisphere programmed the produc- tion of the 1-3-123 rhythm for the left hand and guided the single finger tapping of the right hand.

R E F E R E N C E S

1. HICKS, R. E. lntrahemispheric response competition between vocal and unimanual performance in normal adult human males. J. comp. physiol. Psyehol. 89, 50-60, 1975

2. K1NSBOURNE, M. and CooK, J. Generalized and lateralized effects of concurrent verbalization on a unimanual skill. Q. Jl exp. P.wchol. 23, 341-345, 1971

3. LoMAs, J. and K~MURA, D. Intrahemispheric interaction between speaking and sequential manual activity. Neuropsychologia 14, 23-45, 1976

4. MAJERES, R. L. The effect of unimanual performance on speed of verbalization. J. mot. Behav. 7, 57-58, 1975

5. MILNER, B. personal communication.

NOTE 463

R@sum@ :

On pr~sente 3 exp@riences dans lesquelles des sujets on£

accomplir simultan@ment 2 t~ches motrices. Dans la premiere ex-

perience, les sujets doivent taper rapidement avec l'index tandis

qu'ils r6citent une chanson £nfantine. Dans la deuxi~me exp@rience,

les sujets doivent taper rapidement avec l'index d'une main tandis

qu'ils battent un rythme 1-3-123 avec l'autre main. Dans la troi-

si~me experience, les sujets doivent r@citer une chanson enfantine

tandis qu'ils battent le rythme 1-3-123 avec une main. Tousles su-

jets test@s ~taient capables d'acconiplir la t~che de la premiere

exp@rience, i0 % des sujets examines ~taient capables d'accomplir

la t~che de la deuxi~me experience mais aucun des sujets n'@tait

capable d'accomplir la t~che de la troisi@me experience. On disc~te

ces r6sultats en se r~f6rant ~ la compatibilit~ des rythmes des

diverses @preuves accomplies concuremmento

Deutschsp rachi~.e Zusa,~ F, e f?.ssum~::

Es wurden dI~ei Experi,~en:e ds.rgestel!t, in denen Versuehs-

personen ~leichzeitig zwei moLo~ische Aufgaben l~Ssen muSten.

In elnem ersten Experlmenr W~rde~l die Versuchspersonen atlf-

gefordert mix einem einze!nen Zeigefinger ganz rasch auf-

zutippen, w:ihrend ein Kinderreim rezitiert ',~'erden muSte.

Im zwelten Expe1~iment mu~ten clie Versuohspersonen m~3gliohst

rasoh mit dem Ze~gefinger einer Hand auftippen, w[ihrend mit

der anderen Hand ein Rhythmus I - 3 - 123 gesc~lagen werden

muSte. Im Experiment dre! muSten die Versuchspersonen einen

Kinderreim rezitieren, w~brend sie mit einer Hand de.~l ~ -

3 - 123 - Rhythmus schlagen muSten. Alle 6etesteten Perse-

nen waren in der Lage, das Experiment [~r. I erfolgreich

auszuf[~hren. 10 ~ dot getesteten Probanden konnte die Be-

dlngun~en des Experimentes 2 erfHllen rand keine Versuehs-

person war in der Lage, den Anforder~Jngen des Experi~entes

3 zu gendgen. Die Er2obnisse w~rden im }[inblick auf die

Kompatibilit~t der verschiedenen Testrhyt~u~en in lhrer

konkurrierenden Wirkung bei L0sung der Testsufgsben dis-

kutiert.