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Does manual asymmetry of right-handers change between six and nine years of age?
Human Movement North-Holland
Science 6 (1987) 321-332
321
DOES MANUAL ASYMMETRY OF RIGHT-HANDERS BETWEEN SIX AND NINE YEARS OF AGE? Jacqueline Lahoratoire
FAGARD de Psycho-Biologic
CHANGE
* de I’Enfant, Paris, France
Fagard, J., 1987. Does manual asymmetry of right-handers change between six and nine years of age? Human Movement Science 6, 321-332.
The development of manual asymmetry was studied in rhythmic tapping tasks and in repetitive timed tapping tasks on a sample of normal 6- and 9-year-olds. All subjects were right-handed. On all tappings tasks performance improved between 6 and 9 years of age. The right preferred hand induced better performance than the left non-preferred hand on all tasks except successive tapping on one key with the four fingers. On rhythmic tapping tasks the asymmetry in stability of rhythm favoring the right hand was more pronounced in younger than in older children. However. asymmetry in the discrepancy between the expected and the actual number of beats per trial on rhythmic tapping did not change with age. On the three repetitive timed tasks there was no change of asymmetry with age. These results are analyzed in the light of findings reported by other authors.
Over the course of the first ten years of life manual control undergoes many changes. The development of hand-use preference and manual asymmetry is one of these. Hand-use preference is known to be apparent by 3 years of age (Annett 1970; Bruml 1972; Ingram 1975) but may still exhibit modifications until about 8-11 years of age (Michel 1984). N umerous studies have been designed to pinpoint when hand preference develops during infancy (see Michel (1984) for a review). Similarly, there is a considerable body of data examining to what extent a given task or task condition influences manual asymmetry of performance such that the preferred hand is not always superior to the non-preferred hand in children (Ingram 1975; Wolff 1977) and * Mailing address: J. Fagard, Laboratoire de Psycho-Biologie 315, 41, rue Gay-Lussac, 75005 Paris, France.
0167-9457/87/$3.50
0 1987, Elsevier Science Publishers
de 1’Enfant. E.P.H.E.-C.N.R.S.-U.A.
B.V. (North-Holland)
322
J. Fagm-d / Manual asymmetry
ofright-handers
in adults (Provins 1956; Provins and Glencross 1968; Kimura and Vanderwolf 1970; Hermelin and O’Connor 1971; Flowers 1975; Wolff et al. 1977; Roy and McKenzie 1978; Annett et al. 1979; Todor and Kyprie 1980; Guiard et al. 1983; Todor and Smiley 1985). Conversely, little is known about age changes in manual asymmetry once handedness has become clearly established. Although there is a gradual increase in the use of the preferred hand (Connolly and Elliott 1972; Coren et al. 1981) and despite the fact that the preferred hand received considerable reinforcement mainly through writing as the child gets older, asymmetry of performance of the preferred over the non-preferred hand apparently does not increase with age. Annett, for example, reports that asymmetry of manual skill, assessed in a peg-moving task, remains stable or is even slightly greater in younger than in older children (Annett 1970; Kilshaw and Annett 1983). On some timed motor tasks such as repetitive movements, alternating hand flexion and extension, etc. Denckla indicates that ‘only younger children . . . tend to show slightly larger right-left differences due to slow left side performance’ (Denckla 1974: 736). On rhythmic tapping Wolff’s findings clearly show that the difference in stability of rhythm in favor of the preferred hand of right-handers is, at some rates, no longer significant in children over age 11 (Wolff 1977). Bruml also provides results which go against ‘the usual assumption that differences between the two hands become more marked with age’ (Bruml 1972: 12). Her findings indicate growth of skill in both hands rather than growth of differences between hands with age; however, out of the five tasks chosen to test for differential skills (pegboard, screw-turning, dynamometer and two tapping tasks), the two tapping tasks exhibit a trend towards increasing differences between hands with age, although Bruml provides no statistical data on this point. These two tapping tasks differ from Wolff’s rhythmic tapping tasks on which findings point towards a decrease in between hand differences with age: these consisted in simple fast tapping for one task and in using a pencil to tap each of a number of small squares (a subset of the Harris Laterality Test) for the other task. However, on a simple fast tapping task, as well as on the pegboard task, Finleyson and Reitan (1976) found no significant relationship between age and R-L differences of performance. It is thus still unclear whether right-left differences of performance change during the first decade of life or whether these changes are task dependent. The purpose of the present study is to examine age
J. Fagard / Manual asymmetry
of right-banders
323
changes in right-left differences of performance by comparing the same children on several different tapping tasks: (a) Wolff’s rhythmic tapping at two different rates; (b) three repetitive time tapping tasks: (1) a simple fast tapping task comparable to the task used in Bruml’s and in Finleyson and Reitan’s study for which results differ, (2) a tapping alternating between two targets which involves a certain amount of visual monitoring as in the Harris test, and (3) a sequential tapping with each finger individually, modeled on one of the tests Denckla used in her study of asymmetry in motor development. These tasks were deliberately chosen to test task situations where there has been disagreement among researchers as concerns the development of lateral asymmetry. In addition, using tasks which differ in terms of the necessary visual monitoring or in terms of the difficulty of motor sequencing, allows for comparison of the development of lateral asymmetry in different conditions of asymmetry.
Methods Subjects Sixteen 6-year-olds and sixteen 9-year-olds (eight girls and eight boys in each group) were recruited from an elementary school in Paris. They were all right-handed, as assessed by a handedness inventory (Auzias 1975). Apparatus The specially built tapping apparatus was fitted with two telegraph keys (diameter: 3 cm) mounted 25 cm apart on a plywood board and connected to two channels of an electrical recorder, which provided traces of the tapping on paper. Procedure The subjects were requested to carry out two rhythmic tapping tasks at two different paces, and three different repetitive timed tapping tasks. All the tasks were unimanual. The tasks were administered as follows:
324
J. Fugard / Manual asymmetry
of right-banders
~ Rhythmic tapping: subjects were instructed to follow a rhythm (generated by a Hewlett-Packard micro computer) by tapping the key once for each beep with their index finger and to continue tapping as steadily as possible at the same rhythm after the computer-generated rhythm was turned off and until an auditory signal indicated the end of the task. The two rhythms, based on those used by Wolff (1977) consisted of 72 beats per minute (bpm) or 152 bpm. Each task lasted 30 seconds, 15 seconds with and 15 seconds without the computer-generated rhythm. Only the last 15 seconds (without the computer-generated rhythm) were analyzed. The first 15 seconds were considered to be practice. All subjects were given the rhythmic tapping tasks first, but order of presentation was counterbalanced for rhythm and for hand. Half of the subjects started with the 72 bpm rhythm, the other half with the 152 bpm rhythm. In each subgroup half of the subjects started with the preferred hand, the other half with the non-preferred hand. Within each sex group, subjects were assigned at random into the four subgroups (4 subjects per subgroup). ~ For the three repetitive timed tasks subjects were instructed to tap as fast as possible on one key with the index finger until told to stop (maximum speed tapping), or alternatively on both keys with the same index finger (alternating tapping) or on one key with each of the four fingers successively (successive tapping). Order of presentation was counterbalanced for task and order of presentation for hand was the same as for rhythmic tapping. A brief practice period preceded the task. Outcome measures
Two measures were chosen as dependent variables for rhythmic tapping. The performance measure was the deviation score calculated separately for each hand, expressed as the difference between expected and actual number of beats. Variability was also calculated, as the standard deviation of the intertap intervals, as in Wolff’s study (Wolff and Hurwitz 1976; Wolff 1977). To correct for possible bias introduced by calculating intertap interval standard deviations on a small number of observations (Wing 1979), and to provide for valid within- and cross-group comparisons, the standard deviations were calculated for
J. Fagard / Manual asymmetry
of right-banders
325
an identical number of intervals: the first 15 intervals after the computer generated rhythm was turned off. 1 For the repetitive timed tapping tasks the performance measure was the duration required to perform 20 taps. Statistical design
Variance analyses were calculated for the two tapping tasks together, and separately, for each of the three repetitive timed tasks. Before calculating the main variance analyses, one-way ANOVAs were calculated to check that order of presentation for hand and order of presentation for rhythm had no effect on performance. None of the results were significant. A repeated measures ANOVA with age (2) and sex (2) as betweentasks and task (2) and hand (2) as within-task variables was carried out on the data for the rhythmic tapping tasks. A repeated measures ANOVA with age (2) X sex (2) as between-tasks and hand (2) as within-task variables was calculated for each of the three repetitive timed tasks.
Results Performance in rhythmic tapping was better with the right than with the left hand, and better at nine than at six years of age. At age nine the rate of tapping was very close to the expected rate. At age six the rate of tapping tended to be too fast in the slow condition and too slow in the fast condition (cf. fig. I). A 2 x 2 x 2 x 2 ANOVA for age, sex, task and hand showed a significant age effect (F(1,28) = 4.62; p < 0.05), a significant task effect (F(1,28) = 24.62; p -C O.OOl), and a significant hand effect (F&28) = 4.8; p < 0.05). There was a significant age X task interaction (F(1,28) = 18.64; p < 0.001) and a significant task X hand interaction (F(1,28) = 14.9; p < 0.001). Variability in rhythmic tapping is more pronounced at age six than at age nine, and is more marked at both ages when performing with the ’ In the initial series of analyses, standard deviations were calculated on the full 15 seconds for each condition. The intra-subject between-hands and between-tasks comparisons and intergroup comparisons presented here are highly congruent with initial data.
J. Fagard / Manual asymmetry
326
of right-banders
152 bDm
72 bpm
right hand
4r left hand
._--__
/’ / , -a 1
6
// / /
/I
6
9
9
AGE Fig. 1. Right-left difference of performance (difference between the actual of beats (18 for 72 bpm and 38 for 152 bpm)) on rhythmic tapping tasks.
and expected
number
left rather than with the right hand. Morever, the 72 bpm induces greater tapping variability than the 152 bpm. The difference in variability between the right and the left hand is greater at age six than at age nine (cf. fig. 2). A 2~2~2x2 ANOVA for sex, age, hand and task showed a significant age effect (F(1,28) = 17.68; p < O.OOl),a significant task
right hand ___
72 bpm
3 .r
.r -
left hand
152 bpm
.lO_
Q .r k > ,” z
.05
_
2 k-
6
I
I
4
9
6
9
AGE
Fig. 2. Right-left tapping tasks.
difference
of variability
(standard
deviation
of intertap
intervals)
on rhythmic
J. Faprd
328
/ Manual asymmetry
ofright--handers
~ On alternating tapping there was a significant age effect (F(1,28) = 34.5; p < O.OOl), a slight but significant sex effect (F(1,28) = 4.4; p < O.OS),and a significant hand effect (F(1,28) = 17.6; p < 0.001). _ On successive tapping there was a significant age effect (F(1,28) = 38.5; p < 0.001). There were no significant
interactions
between
variables.
Discussion On the five tasks reported in this research, motor performance improves between 6 and 9 years of age: 9-year-olds are better in keeping a tapping rhythm after external pacing is removed than 6year-olds. They exhibit less intertap variability than 6-year-olds in following the rhythm. They are faster than 6-year-olds in tapping twenty times with one finger on the same key, with one finger alternating between two keys or with four fingers successively on one key. The age-related difference in performance is more notable on more difficult tasks than on simple tasks. For instance, the difference in performance between 6- and 9-year-olds is significantly greater in the 152 bpm than in the 72 bpm rhythmic tapping task, the latter apparently being more difficult for the 6-year-olds than the former. On all tasks except the successive tapping the right hand induces better performance than the left hand. On rhythmic tapping the task X hand interaction on performance indicates that the poorer performance of 6-year-olds in tapping at 152 bpm as compared to 72 bpm concerns the left more than the right hand. It was expected that the degree, or even the direction, of lateral asymmetry would be task dependent. Asymmetry of performance in favor of the preferred hand of righthanders is known to be greater when visual control is needed or when precise sequencing of movements is required than in the opposite conditions (Todor and Smiley 1985). In the present research the asymmetry of performance in favor of the right hand on rhythmic tapping is greater in the fast than in the slow condition, due to the 6-year-olds’ results. In different task conditions other findings have also shown that task complexity is an important variable contributing to hand performance asymmetries (Todor and Doane 1977; Peters 1980). The absence
J. Fagard / Manual asymmetry
of right-banders
329
of R-L asymmetry on successive tapping may be related to left hand superiority on hand posturing tasks, and on tasks requiring differential finger movement, as opposed to tasks with greater speed or accuracy demand (Ingram 1975). The asymmetry of performance does not decrease significantly with age on any of the tasks. However, the significant age X hand interaction on variability measures of rhythmic tapping indicates that the difference in stability of rhythm decreases significantly between six and nine years of age. These results are congruent with Wolff’s findings that asymmetry favoring the right hand of right-handers is more pronounced in younger than in older children: in Wolff’s study, where the intertap variability was the only dependent variable reported, the cross-group comparison dealt with 9- to 12-year-olds for unimanual and bimanual alternating rhythmic tapping and with 5- to 15-year-olds only for bimanual alternating tapping (Wolff 1977). It is noteworthy that this decrease of asymmetry in tapping variability with age was also found here on unimanual tapping between 6 and 9 years of age. As regards the three repetitive timed tapping tasks, there is no significant variation with age of the R-L difference of performance. It is interesting to compare these findings to others obtained from comparable situations (Bruml 1972; Denckla 1974; Finleyson and Reitan 1976): on finger repetitive and successive movements (thumb to fingers), Denckla notes no variation with age in the R-L difference. Concerning the two tapping tasks, Bruml speaks of a trend towards an increase of the degree of asymmetry. In Finleyson and Reitan’s study lateral difference of performance did not change with age on simple fast tapping. The present results confirm indications apparent in other findings and suggest that it would be more realistic to refer to a stability of asymmetry for fast repetitive finger tapping tasks rather than the term ‘trends’, since these are either absent or non-significant. Girls are significantly faster than boys on alternating tapping but there is no significant variation in R-L asymmetry with sex. Some research on R-L asymmetry indicates greater asymmetry in girls than in boys: Annett (1970), for instance, found a higher degree of asymmetry to the right in girls than in boys in some of the samples. In Denckla’s study only younger girls (5 and 6-years-olds) show greater R-L differences than boys on some of the tasks (Denckla 1974). Wolff found that the R-L asymmetry in bimanual tapping was greater in girls under 11 than in boys of the same age (Wolff and Hurwitz 1976; Wolff
330
J. Fagard / Manual asymmetv
ofrighr-handers
1977). These different studies suggest that if there is a sex factor for lateral asymmetry, it is restricted to certain tasks at certain ages. The fact that there is a decrease in manual asymmetry on variability of rhythmic tapping tasks between 6 and 9 years of age, but no change in asymmetry of performance on any of the tapping tasks raises a number of questions. The absence of an age X hand interaction on performance measures of rhythmic tapping indicates that the greater capacity for establishing a rhythm parameter with the right than with the left hand does not change significantly with age. It has been suggested that tapping variability may be a more sensitive measure of temporal tracking than the discrepancy between the expected and the actual number of beats (Wolff 1977). This may explain the absence of interaction on performance measures observed here. The greater motor proficiency of the right hand when speed or precision are involved in the tapping tasks does not change between 6 and 9 years of age, and a comparable difference can still be observed in adults (see Todor and Smiley (1985) for a review). Concerning variability, several studies have demonstrated even in adults greater motor variability in the left than in the right hand (Annett et al. 1979; Todor and Smiley 1985). Other studies have shown that in adults the left hand is equally capable of regular tapping if external pacing is provided (Wolff et al. 1977; Brodie, cited in Todor and Smiley (1985)). The decrease of the R-L difference in stability of rhythm between 6 and 9 years of age may then reflect the fact that children progressively develop the ability for the non-preferred hand to approach the stability of the right preferred hand when an external pacing is provided. In other words, it may be the case that children become more capable of using the training rhythm to set up regular tapping for the left non-preferred hand as they mature. Age changes in lateral asymmetry may also reflect changes in the control of the execution of tapping. According to Wing, ‘departure from periodic responding may arise from imprecision in a hypothetical timekeeper and from temporal “noise” in the execution of responses triggered by the timekeeper’ (Wing 1980: 470). The significant age x hand interaction on variability of rhythmic tapping might then indicate that the R-L difference of ‘noise’ in the execution of response decreases with age. In summary, it is possible in some conditions to demonstrate a significant decrease of lateral asymmetry with age: such is the case for variability on rhythmic tapping. However, this research and other
J. Fagard / Manual asymmetry
of right-handers
331
findings referred to here indicate that on rhythmic as well as on repetitive timed tapping tasks the R-L asymmetry of performance of right-handers remains relatively stable after manual preference has been firmly established.
References Annett, M., 1970. The growth of manual preference and speed. British Journal of Psychology 61, 545-558. Annett, J., M. Annett, P.T.W. Hudson and A. Turner, 1979. The control of movement in the preferred and non-preferred hands. Quarterly Journal of Experimental Psychology 31, 641-652. Auzias, M., 1975. Enfants gauchers, enfants droitiers: une Cpreuve de lateralit& usuelle. Neuchatel: Delachaux & NiestlC. Bruml, H., 1972. Age changes in preference and skill measures of handedness. Perceptual and Motor Skills 34, 3-14. Connolly, K. and J. Elliott, 1972. ‘The evolution and ontogeny of hand function’. In: N. Blurton Jones (ed.), Ethological studies of child behavior. Cambridge: Cambridge University Press. Coren, S., C. Porac and P. Ducun, 1981. Lateral preference behavior in preschool children and young adults. Child Development 52, 443-450. Denckla, M.G., 1974. Development of motor co-ordination in normal children. Developmental Medicine and Child Neurology 16. 729-741. Finleyson, M.A.J. and R.M. Reitan, 1976. Handedness in relation to measures of motor and tactile-perceptual functions in normal children. Perceptual and Motor Skills 43. 475-481. Flowers, K., 1975. Handedness and controlled movement. British Journal of Psychology 66, 39952. Guiard, Y., G. Diaz and D. Beaubaton, 1983. Left-hand advantage in right-handers for spatial constant error: preliminary evidence in a unimanual ballistic aimed movement. Neuropsychologia 21, 111-115. Hermelin, B. and N. O’Connor. 1971. Functional asymmetry in the reading of braille. Neuropsychologia 9, 431-435. Ingram, D., 1975. Motor asymmetries in young children. Neuropsychologia 13, 95-102. Kilshaw, D. and M. Annett, 1983. Right- and left-hand skill I: effects of age, sex and hand preference showing superior skill in left-handers. British Journal of Psychology 74, 2533268. Kimura. D. and C.H. Vanderwolf, 1970. The relation between hand preference and the performance of individual finger movements by left and right hands. Brain 93, 769-774. Michel, G., 1984. Development of hand-use preference during infancy’. In: G. Young. S,. Segalowitz, C. Corted and S. Trehub (eds.), Manual specialization and the developing brain: longitudinal studies. New York: Academic Press. Peters, M., 1980. Why the preferred hand taps more quickly than the non-preferred hand: three experiments on handedness. Canadian Journal of Psychology/Revue canadienne de Psychologie 34, 62-71. Provins, K.A., 1956. Handedness and skill. Quarterly Journal of Experimental Psychology 10, 29-39. Provins, K.A. and D.J. Glencross, 1968. Handwriting. typewriting and handedness. Quarterly Journal of Experimental Psychology 20, 282-289. Roy, E.A. and C. McKenzie,, 1978. Handedness effects in kinesthetic spatial location judgements. Cortex 14, 250-258.
332
J. Fagard / Manual asymmetry of right-handers
Todor, J.I. and T. Deane, 1977. Handedness classification: preference versus proficiency. Perceptual and Motor Skills 45, 1041-1042. Todor, J.I. and P.M. Kyprie, 1980. Hand differences in the rate and variability of rapid tapping. Journal of Motor Behavior 12, 57-62. Todor, J.I. and L. Smiley, 1985. ‘Performance differences between the hands: implications for studying disruption to limb praxis’. In: E.A. Roy (ed.), Neuropsychological studies of apraxia and related disorders. Amsterdam: North-Holland. Wing, A.M., 1979. A note on estimation of the autocovariance function in the analysis of timing of repetitive responses. British Journal of Mathematical and Statistical Psychology 32, 143-145. Wing, A., 1980. ‘The long and short of timing in response sequences’. In: G.E. Stelmach and J. Requin (eds.), Tutorials in motor behavior. Amsterdam: North-Holland. Wolff, P.H., 1977. The development of manual asymmetries in motor sequencing skills. Annals of the New York Academy of Sciences 299, 319-327. Wolff, P.H. and I. Hurwitz, 1976. Sex differences in finger tapping: a developmental study. Neuropsychologia 14, 35-41. Wolff, P.H., I. Hurwitz and H. Moss, 1977. Serial organization of motor skills in left- and right-handed adults. Neuropsychologia 15, 539-546.
Science 6 (1987) 321-332
321
DOES MANUAL ASYMMETRY OF RIGHT-HANDERS BETWEEN SIX AND NINE YEARS OF AGE? Jacqueline Lahoratoire
FAGARD de Psycho-Biologic
CHANGE
* de I’Enfant, Paris, France
Fagard, J., 1987. Does manual asymmetry of right-handers change between six and nine years of age? Human Movement Science 6, 321-332.
The development of manual asymmetry was studied in rhythmic tapping tasks and in repetitive timed tapping tasks on a sample of normal 6- and 9-year-olds. All subjects were right-handed. On all tappings tasks performance improved between 6 and 9 years of age. The right preferred hand induced better performance than the left non-preferred hand on all tasks except successive tapping on one key with the four fingers. On rhythmic tapping tasks the asymmetry in stability of rhythm favoring the right hand was more pronounced in younger than in older children. However. asymmetry in the discrepancy between the expected and the actual number of beats per trial on rhythmic tapping did not change with age. On the three repetitive timed tasks there was no change of asymmetry with age. These results are analyzed in the light of findings reported by other authors.
Over the course of the first ten years of life manual control undergoes many changes. The development of hand-use preference and manual asymmetry is one of these. Hand-use preference is known to be apparent by 3 years of age (Annett 1970; Bruml 1972; Ingram 1975) but may still exhibit modifications until about 8-11 years of age (Michel 1984). N umerous studies have been designed to pinpoint when hand preference develops during infancy (see Michel (1984) for a review). Similarly, there is a considerable body of data examining to what extent a given task or task condition influences manual asymmetry of performance such that the preferred hand is not always superior to the non-preferred hand in children (Ingram 1975; Wolff 1977) and * Mailing address: J. Fagard, Laboratoire de Psycho-Biologie 315, 41, rue Gay-Lussac, 75005 Paris, France.
0167-9457/87/$3.50
0 1987, Elsevier Science Publishers
de 1’Enfant. E.P.H.E.-C.N.R.S.-U.A.
B.V. (North-Holland)
322
J. Fagm-d / Manual asymmetry
ofright-handers
in adults (Provins 1956; Provins and Glencross 1968; Kimura and Vanderwolf 1970; Hermelin and O’Connor 1971; Flowers 1975; Wolff et al. 1977; Roy and McKenzie 1978; Annett et al. 1979; Todor and Kyprie 1980; Guiard et al. 1983; Todor and Smiley 1985). Conversely, little is known about age changes in manual asymmetry once handedness has become clearly established. Although there is a gradual increase in the use of the preferred hand (Connolly and Elliott 1972; Coren et al. 1981) and despite the fact that the preferred hand received considerable reinforcement mainly through writing as the child gets older, asymmetry of performance of the preferred over the non-preferred hand apparently does not increase with age. Annett, for example, reports that asymmetry of manual skill, assessed in a peg-moving task, remains stable or is even slightly greater in younger than in older children (Annett 1970; Kilshaw and Annett 1983). On some timed motor tasks such as repetitive movements, alternating hand flexion and extension, etc. Denckla indicates that ‘only younger children . . . tend to show slightly larger right-left differences due to slow left side performance’ (Denckla 1974: 736). On rhythmic tapping Wolff’s findings clearly show that the difference in stability of rhythm in favor of the preferred hand of right-handers is, at some rates, no longer significant in children over age 11 (Wolff 1977). Bruml also provides results which go against ‘the usual assumption that differences between the two hands become more marked with age’ (Bruml 1972: 12). Her findings indicate growth of skill in both hands rather than growth of differences between hands with age; however, out of the five tasks chosen to test for differential skills (pegboard, screw-turning, dynamometer and two tapping tasks), the two tapping tasks exhibit a trend towards increasing differences between hands with age, although Bruml provides no statistical data on this point. These two tapping tasks differ from Wolff’s rhythmic tapping tasks on which findings point towards a decrease in between hand differences with age: these consisted in simple fast tapping for one task and in using a pencil to tap each of a number of small squares (a subset of the Harris Laterality Test) for the other task. However, on a simple fast tapping task, as well as on the pegboard task, Finleyson and Reitan (1976) found no significant relationship between age and R-L differences of performance. It is thus still unclear whether right-left differences of performance change during the first decade of life or whether these changes are task dependent. The purpose of the present study is to examine age
J. Fagard / Manual asymmetry
of right-banders
323
changes in right-left differences of performance by comparing the same children on several different tapping tasks: (a) Wolff’s rhythmic tapping at two different rates; (b) three repetitive time tapping tasks: (1) a simple fast tapping task comparable to the task used in Bruml’s and in Finleyson and Reitan’s study for which results differ, (2) a tapping alternating between two targets which involves a certain amount of visual monitoring as in the Harris test, and (3) a sequential tapping with each finger individually, modeled on one of the tests Denckla used in her study of asymmetry in motor development. These tasks were deliberately chosen to test task situations where there has been disagreement among researchers as concerns the development of lateral asymmetry. In addition, using tasks which differ in terms of the necessary visual monitoring or in terms of the difficulty of motor sequencing, allows for comparison of the development of lateral asymmetry in different conditions of asymmetry.
Methods Subjects Sixteen 6-year-olds and sixteen 9-year-olds (eight girls and eight boys in each group) were recruited from an elementary school in Paris. They were all right-handed, as assessed by a handedness inventory (Auzias 1975). Apparatus The specially built tapping apparatus was fitted with two telegraph keys (diameter: 3 cm) mounted 25 cm apart on a plywood board and connected to two channels of an electrical recorder, which provided traces of the tapping on paper. Procedure The subjects were requested to carry out two rhythmic tapping tasks at two different paces, and three different repetitive timed tapping tasks. All the tasks were unimanual. The tasks were administered as follows:
324
J. Fugard / Manual asymmetry
of right-banders
~ Rhythmic tapping: subjects were instructed to follow a rhythm (generated by a Hewlett-Packard micro computer) by tapping the key once for each beep with their index finger and to continue tapping as steadily as possible at the same rhythm after the computer-generated rhythm was turned off and until an auditory signal indicated the end of the task. The two rhythms, based on those used by Wolff (1977) consisted of 72 beats per minute (bpm) or 152 bpm. Each task lasted 30 seconds, 15 seconds with and 15 seconds without the computer-generated rhythm. Only the last 15 seconds (without the computer-generated rhythm) were analyzed. The first 15 seconds were considered to be practice. All subjects were given the rhythmic tapping tasks first, but order of presentation was counterbalanced for rhythm and for hand. Half of the subjects started with the 72 bpm rhythm, the other half with the 152 bpm rhythm. In each subgroup half of the subjects started with the preferred hand, the other half with the non-preferred hand. Within each sex group, subjects were assigned at random into the four subgroups (4 subjects per subgroup). ~ For the three repetitive timed tasks subjects were instructed to tap as fast as possible on one key with the index finger until told to stop (maximum speed tapping), or alternatively on both keys with the same index finger (alternating tapping) or on one key with each of the four fingers successively (successive tapping). Order of presentation was counterbalanced for task and order of presentation for hand was the same as for rhythmic tapping. A brief practice period preceded the task. Outcome measures
Two measures were chosen as dependent variables for rhythmic tapping. The performance measure was the deviation score calculated separately for each hand, expressed as the difference between expected and actual number of beats. Variability was also calculated, as the standard deviation of the intertap intervals, as in Wolff’s study (Wolff and Hurwitz 1976; Wolff 1977). To correct for possible bias introduced by calculating intertap interval standard deviations on a small number of observations (Wing 1979), and to provide for valid within- and cross-group comparisons, the standard deviations were calculated for
J. Fagard / Manual asymmetry
of right-banders
325
an identical number of intervals: the first 15 intervals after the computer generated rhythm was turned off. 1 For the repetitive timed tapping tasks the performance measure was the duration required to perform 20 taps. Statistical design
Variance analyses were calculated for the two tapping tasks together, and separately, for each of the three repetitive timed tasks. Before calculating the main variance analyses, one-way ANOVAs were calculated to check that order of presentation for hand and order of presentation for rhythm had no effect on performance. None of the results were significant. A repeated measures ANOVA with age (2) and sex (2) as betweentasks and task (2) and hand (2) as within-task variables was carried out on the data for the rhythmic tapping tasks. A repeated measures ANOVA with age (2) X sex (2) as between-tasks and hand (2) as within-task variables was calculated for each of the three repetitive timed tasks.
Results Performance in rhythmic tapping was better with the right than with the left hand, and better at nine than at six years of age. At age nine the rate of tapping was very close to the expected rate. At age six the rate of tapping tended to be too fast in the slow condition and too slow in the fast condition (cf. fig. I). A 2 x 2 x 2 x 2 ANOVA for age, sex, task and hand showed a significant age effect (F(1,28) = 4.62; p < 0.05), a significant task effect (F(1,28) = 24.62; p -C O.OOl), and a significant hand effect (F&28) = 4.8; p < 0.05). There was a significant age X task interaction (F(1,28) = 18.64; p < 0.001) and a significant task X hand interaction (F(1,28) = 14.9; p < 0.001). Variability in rhythmic tapping is more pronounced at age six than at age nine, and is more marked at both ages when performing with the ’ In the initial series of analyses, standard deviations were calculated on the full 15 seconds for each condition. The intra-subject between-hands and between-tasks comparisons and intergroup comparisons presented here are highly congruent with initial data.
J. Fagard / Manual asymmetry
326
of right-banders
152 bDm
72 bpm
right hand
4r left hand
._--__
/’ / , -a 1
6
// / /
/I
6
9
9
AGE Fig. 1. Right-left difference of performance (difference between the actual of beats (18 for 72 bpm and 38 for 152 bpm)) on rhythmic tapping tasks.
and expected
number
left rather than with the right hand. Morever, the 72 bpm induces greater tapping variability than the 152 bpm. The difference in variability between the right and the left hand is greater at age six than at age nine (cf. fig. 2). A 2~2~2x2 ANOVA for sex, age, hand and task showed a significant age effect (F(1,28) = 17.68; p < O.OOl),a significant task
right hand ___
72 bpm
3 .r
.r -
left hand
152 bpm
.lO_
Q .r k > ,” z
.05
_
2 k-
6
I
I
4
9
6
9
AGE
Fig. 2. Right-left tapping tasks.
difference
of variability
(standard
deviation
of intertap
intervals)
on rhythmic
J. Faprd
328
/ Manual asymmetry
ofright--handers
~ On alternating tapping there was a significant age effect (F(1,28) = 34.5; p < O.OOl), a slight but significant sex effect (F(1,28) = 4.4; p < O.OS),and a significant hand effect (F(1,28) = 17.6; p < 0.001). _ On successive tapping there was a significant age effect (F(1,28) = 38.5; p < 0.001). There were no significant
interactions
between
variables.
Discussion On the five tasks reported in this research, motor performance improves between 6 and 9 years of age: 9-year-olds are better in keeping a tapping rhythm after external pacing is removed than 6year-olds. They exhibit less intertap variability than 6-year-olds in following the rhythm. They are faster than 6-year-olds in tapping twenty times with one finger on the same key, with one finger alternating between two keys or with four fingers successively on one key. The age-related difference in performance is more notable on more difficult tasks than on simple tasks. For instance, the difference in performance between 6- and 9-year-olds is significantly greater in the 152 bpm than in the 72 bpm rhythmic tapping task, the latter apparently being more difficult for the 6-year-olds than the former. On all tasks except the successive tapping the right hand induces better performance than the left hand. On rhythmic tapping the task X hand interaction on performance indicates that the poorer performance of 6-year-olds in tapping at 152 bpm as compared to 72 bpm concerns the left more than the right hand. It was expected that the degree, or even the direction, of lateral asymmetry would be task dependent. Asymmetry of performance in favor of the preferred hand of righthanders is known to be greater when visual control is needed or when precise sequencing of movements is required than in the opposite conditions (Todor and Smiley 1985). In the present research the asymmetry of performance in favor of the right hand on rhythmic tapping is greater in the fast than in the slow condition, due to the 6-year-olds’ results. In different task conditions other findings have also shown that task complexity is an important variable contributing to hand performance asymmetries (Todor and Doane 1977; Peters 1980). The absence
J. Fagard / Manual asymmetry
of right-banders
329
of R-L asymmetry on successive tapping may be related to left hand superiority on hand posturing tasks, and on tasks requiring differential finger movement, as opposed to tasks with greater speed or accuracy demand (Ingram 1975). The asymmetry of performance does not decrease significantly with age on any of the tasks. However, the significant age X hand interaction on variability measures of rhythmic tapping indicates that the difference in stability of rhythm decreases significantly between six and nine years of age. These results are congruent with Wolff’s findings that asymmetry favoring the right hand of right-handers is more pronounced in younger than in older children: in Wolff’s study, where the intertap variability was the only dependent variable reported, the cross-group comparison dealt with 9- to 12-year-olds for unimanual and bimanual alternating rhythmic tapping and with 5- to 15-year-olds only for bimanual alternating tapping (Wolff 1977). It is noteworthy that this decrease of asymmetry in tapping variability with age was also found here on unimanual tapping between 6 and 9 years of age. As regards the three repetitive timed tapping tasks, there is no significant variation with age of the R-L difference of performance. It is interesting to compare these findings to others obtained from comparable situations (Bruml 1972; Denckla 1974; Finleyson and Reitan 1976): on finger repetitive and successive movements (thumb to fingers), Denckla notes no variation with age in the R-L difference. Concerning the two tapping tasks, Bruml speaks of a trend towards an increase of the degree of asymmetry. In Finleyson and Reitan’s study lateral difference of performance did not change with age on simple fast tapping. The present results confirm indications apparent in other findings and suggest that it would be more realistic to refer to a stability of asymmetry for fast repetitive finger tapping tasks rather than the term ‘trends’, since these are either absent or non-significant. Girls are significantly faster than boys on alternating tapping but there is no significant variation in R-L asymmetry with sex. Some research on R-L asymmetry indicates greater asymmetry in girls than in boys: Annett (1970), for instance, found a higher degree of asymmetry to the right in girls than in boys in some of the samples. In Denckla’s study only younger girls (5 and 6-years-olds) show greater R-L differences than boys on some of the tasks (Denckla 1974). Wolff found that the R-L asymmetry in bimanual tapping was greater in girls under 11 than in boys of the same age (Wolff and Hurwitz 1976; Wolff
330
J. Fagard / Manual asymmetv
ofrighr-handers
1977). These different studies suggest that if there is a sex factor for lateral asymmetry, it is restricted to certain tasks at certain ages. The fact that there is a decrease in manual asymmetry on variability of rhythmic tapping tasks between 6 and 9 years of age, but no change in asymmetry of performance on any of the tapping tasks raises a number of questions. The absence of an age X hand interaction on performance measures of rhythmic tapping indicates that the greater capacity for establishing a rhythm parameter with the right than with the left hand does not change significantly with age. It has been suggested that tapping variability may be a more sensitive measure of temporal tracking than the discrepancy between the expected and the actual number of beats (Wolff 1977). This may explain the absence of interaction on performance measures observed here. The greater motor proficiency of the right hand when speed or precision are involved in the tapping tasks does not change between 6 and 9 years of age, and a comparable difference can still be observed in adults (see Todor and Smiley (1985) for a review). Concerning variability, several studies have demonstrated even in adults greater motor variability in the left than in the right hand (Annett et al. 1979; Todor and Smiley 1985). Other studies have shown that in adults the left hand is equally capable of regular tapping if external pacing is provided (Wolff et al. 1977; Brodie, cited in Todor and Smiley (1985)). The decrease of the R-L difference in stability of rhythm between 6 and 9 years of age may then reflect the fact that children progressively develop the ability for the non-preferred hand to approach the stability of the right preferred hand when an external pacing is provided. In other words, it may be the case that children become more capable of using the training rhythm to set up regular tapping for the left non-preferred hand as they mature. Age changes in lateral asymmetry may also reflect changes in the control of the execution of tapping. According to Wing, ‘departure from periodic responding may arise from imprecision in a hypothetical timekeeper and from temporal “noise” in the execution of responses triggered by the timekeeper’ (Wing 1980: 470). The significant age x hand interaction on variability of rhythmic tapping might then indicate that the R-L difference of ‘noise’ in the execution of response decreases with age. In summary, it is possible in some conditions to demonstrate a significant decrease of lateral asymmetry with age: such is the case for variability on rhythmic tapping. However, this research and other
J. Fagard / Manual asymmetry
of right-handers
331
findings referred to here indicate that on rhythmic as well as on repetitive timed tapping tasks the R-L asymmetry of performance of right-handers remains relatively stable after manual preference has been firmly established.
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Todor, J.I. and T. Deane, 1977. Handedness classification: preference versus proficiency. Perceptual and Motor Skills 45, 1041-1042. Todor, J.I. and P.M. Kyprie, 1980. Hand differences in the rate and variability of rapid tapping. Journal of Motor Behavior 12, 57-62. Todor, J.I. and L. Smiley, 1985. ‘Performance differences between the hands: implications for studying disruption to limb praxis’. In: E.A. Roy (ed.), Neuropsychological studies of apraxia and related disorders. Amsterdam: North-Holland. Wing, A.M., 1979. A note on estimation of the autocovariance function in the analysis of timing of repetitive responses. British Journal of Mathematical and Statistical Psychology 32, 143-145. Wing, A., 1980. ‘The long and short of timing in response sequences’. In: G.E. Stelmach and J. Requin (eds.), Tutorials in motor behavior. Amsterdam: North-Holland. Wolff, P.H., 1977. The development of manual asymmetries in motor sequencing skills. Annals of the New York Academy of Sciences 299, 319-327. Wolff, P.H. and I. Hurwitz, 1976. Sex differences in finger tapping: a developmental study. Neuropsychologia 14, 35-41. Wolff, P.H., I. Hurwitz and H. Moss, 1977. Serial organization of motor skills in left- and right-handed adults. Neuropsychologia 15, 539-546.