Distributions of hand preference and hand skill asymmetry in preschool children: Theoretical implications

Neuropsychologia, Vol. 30, No. 1, pp. 27-34, 1992 Prmted m Great Britam.

0028-3932192 $5 00 + 0.00 :(> 1992 Pergamon Press plc

DISTRIBUTIONS OF HAND PREFERENCE AND HAND SKILL ASYMMETRY IN PRESCHOOL CHILDREN: THEORETICAL IMPLICATIONS FLORENCE CURT, JEAN MACCARIO and GEORGES DELLATOLAS INSERM U169, 16 avenue PV Couturier,

94807 Villejuif Cedex, France

(Received 22 February 1991; accepted 5 September 1991) Abstract-We describe the distributions of hand skill asymmetry for the Peg-moving task [I] and the Graphic test [17] among 765 preschool children. A single normal component was observed for the Peg-moving task, but two normal components for the Graphic test, which corresponded primarily to the right- and left-handed children. Hand Preference and Graphic test hand skill asymmetry were agedependent, but there was no age effect on the PMT hand skill asymmetry. Results are discussed in relation to the Annett’s Right Shift theory.

INTRODUCTION ANNETT’SRight Shift theory [3,4] is partly based on the population distribution of hand skill asymmetry. Asymmetry is defined as the difference in skill (assessed by the Peg-moving task) between the two hands, for example L-R, where L and R are the left-hand and right-hand performances. Annett built her theory on hand skill asymmetry instead of related hand preference because asymmetry, which is continuous and roughly normally distributed, is suitable for quantitative studies, whereas the J-shaped distribution of hand preference scores is more suitable for qualitative studies. Hand preference and hand asymmetry in performance are considered as two facets of a same phenomenon. BISHOP [6], has recently presented a model where the probability that one hand would be preferred for a given activity is a function of its relative skill. Annett assumes that a single biallelic gene (rs + , rs - ) induces localization of speech in the left hemisphere of the brain and favours the right hand for skilled actions. The salient features of the Annett’s model might be termed as: (i) the rs- allele has no effect on hand skill lateralization, hence, among rs - - subjects, this lateralization depends on “chance” and the L-R distribution is normal with mean at 0 (L = R); (ii) the rs + allele shifts skill to the righthand, the importance of the shift may be the same for the rs + + and the rs + - subjects for a dominant genetic model, or may be different for an additive genetic model, being m for the rs + - subjects and 2m for the rs + + subjects. For this last model, the total L-R distribution is the sum of three normal components: one, corresponding to the rs - - genotype, with a mean 0, one corresponding to the rs + - genotype with a mean m, and one corresponding to the rs + + genotype with a mean 2m. The variances of the three components are supposed to be the same. Annett’s frequency estimations of the rs + + , rs + - and rs - - genotypes are 0.3242, 0.4904 and 0.1854, respectively. These estimations are based on the frequency of aphasia induced by right cerebral lesions [2]. Annett’s theory has been criticized. MCMANUS [ 151 and TAPLEY and BRYDEN [ 171 claimed 27

2x

F. CIJKT. J. MAKAKIO and G. DTLLATOLAS

that the distribution of hand skill asymmetry is better fitted by two normal curves, one with a right shift and one with a left shift, than by the Annett’s model which postulates one or two normal curves with a right shift and one curve with no shift. Nevertheless, the Tapley and Bryden’s and the McManus’ tasks were of paper-and-pencil type and not the Peg-moving task. The J-shaped distribution of hand preference and the almost normal distribution of the L-R hand skill, might be two distinct phenomena depending on different factors. For instance, developmental factors and age and cohort effects have been described about handedness 19, 11, 161 but no age effect on L R was evident in ANNE&S PMT data 1131. The aim of this paper, based on hand preference and hand skill data collected among French preschool children, is to contribute to the above discussion: Is the population distribution of the hand skill asymmetry a single normal component or a mixture of normal components‘! Is it compatible with Annett’s models? Is it nature-of-task dependent and age or training dependent? How are handedness and hand skill asymmetry related in preschool children? Finally, using theoretical data, we address the question of the statistical power to reject the hypothesis of a single normal component in the population distribution of hand skill asymmetry, when this distribution is determined by Ann&t’s models.

SUBJECTS The 765 children, 350 girls and 415 boys, between 2: and 6 years, were from six different schools in the suburb of Paris in 1987. Mean age was 4.35 years (SD = 1.O) for girls and 4.29 (SD = 1.O) for boys. The examination lasted about 15 min for each child and included a test of manual preference and two hand-skill tasks: a Peg-moving task (PMT), adapted from ANNET~ [ 11, and a Graphic task (GT), adapted from TAPLEY and BRYDEN [ 171. All children participated in the manual preference assessment. 544 in the PMT which was performed in only four schools, and 566 children in the GT. For the GT, good cooperation was obtained in 93.4% of the children over 3.5 years, but only in 28% of the children less than 3.5 years (for more details see Ref. 18)).

HAND

PREFERENCE

ASSESSMENT

ProWdur~ Each child was sitting at a table of appropriate height. The hand used was observed as subjects performed tight actions, using tools placed at equal distance of both hands. The child was asked to: draw a line on a sheet of paper and erase it. cut a strip of paper with scissors, cut clay with a knife, hammer with a toy hammer. pretend to use a child’s toothbrush. a comb, and a spoon. These eight items were selected among 15 according to the results of a principal component analysis in a previous study. conducted on a sample of 80 children whose ages ranged from 3 to 6 L7]. The examiner noted which hand was used for each action. the right one, the left one, or both. The answer .&both hands” corresponds to either a change during the action or to the simultaneous use of both hands. “Right hand”, “both hands” and “left hand” answers were coded I, 0. and - I, respectively. For each child. a Handedness Score (HS) was assessed by computing the sum of the eight answers. This xore ranged between 8 (right consistent) and - 8 (left consistent). Children with HS >O were consldered as right-handers and those with HS
Figure 1 shows the HS distribution and were considered as left-handers handers. In order to test the reliability

by age group. 12.9% of the children obtained and 87.1 O/u a HS >O and were considered of the above

right-

(left-)handedness

a HS 10 as right-

definition,

a

HAliD

PREFERENCE

AND SKILL ASYMMETRY

IN PKESCHOOL

CHILDREN

29

n c3.5 (n=202) 03.5-4.5

(n=219)

q j44.5-5.5 (n=227) q >5.5 (n=116)

-8 or-7

-6 or-l

0

lto6

7to 6

HS Fig. I. Percentages of strong right-handers (HS = 7 or 8) weak right-handers (HS= 1 to 6). subjects with HS=O, weak left-handers (HS= -6 to -1) and strong left-handers (HS= -7 to -8), by age group: c3.S years of age (n=202). 3.54.5 years (n=219), 4.5 5.5 years (n=227), >S.5 years (n=l16).

reassessment was performed 1 month after the first assessment in 56 children. The reassessment showed a good reliability (kappa = 0.9 I ) for the right- (HS > 0) and left-handers (HS 10) classification. Among the 56 children, only a boy, aged less than 3.5 years, changed handedness group, obtaining a HS of -6 at the first assessment and of 6 at the second. Reliability was lower for cut-offs nearer to the boundaries, e.g. HS = 8 vs HS < 8, or HS = - 8 vs HS> -8. “Both hands” answers were rare (0% for three items, less than 1% for two items, 1.6% for “comb”, 4.2% for “hammer”, and 6.8% for “scissors”), and decreased with age. The percentage of left-handers (HS 50) tended to be higher in boys (14.8%) than girls (10.5%) and higher in the younger age-group (17.3% among those less than 3.5 years), but these differences did not reach statistical significance. Nevertheless HS and age were significantly correlated (r =O.lO, P
HAND Pry-nm:ing

rusk

SKILL

ASSESSMENT

(PMT)

Procedure. This task consisted in measuring the time (in set) each hand needed to move pegs from one row of holes #.oanother. Children were asked to shift pegs one by one to the corresponding opposite hole as fast as possible. The Irequired gesture was from left to right (left row full at start) for the right hand, and from right to left (right row full at start) for the left hand, beginning with the more distant peg. After demonstration by the examiner. 1 min was left to the child to adapt himself to the task. Then each hand was tested thrice alternatively, one child out of two started with the right hand and one child out of two with the left. Time was measured by a stopwatch. A right hand score (R) and a left hand score (L) were calculated by adding the time of the three tests. Stutisticalmrthod.s. A relative difference, calculated as the left- minus right-hand scores divided by the sum of leftand right-hand scores: (L-R)/(Li-R) was computed. This difference denoted a right-hand advantage when positive at PMT (the measure is a time). The choice of a relative score (L - R),/(L + R) instead of an absolute score (L-R) was made because overall performance L+R was a possible confounding factor in the study of the relationship between age and L-R. In fact, L + R was highly correlated with age as well as with L-R.

30

F. CUKT, J. MACCAKIO and G. DELLATOLAS

Inspection of the scores led to the exclusion of one child with aberrant left-hander with a particularly unskilful right hand. Fit of the distributions

values (more than 3 SD from the mean): a

of hand skill relative

diflerence

Inspection of the values showed an excess of scores equal to zero, which can be explained by the rounding of values, time being manually measured with a stopwatch. This excess did not allow proper fits on individual values, hence we grouped the values in tailored intervals with observed frequencies nearly equal and not too low (grossly between 5 and 10). A single normal distribution was first fitted, by maximization ofthe Log Likelihood [lo]. Tests offit were performed by comparison of the observed and estimated frequencies, using the Likelihood Ratio Chi-square. When the test of fit was found significant, mixtures of two populations were fitted and tests of fit performed again. The distribution of the total sample and the distributions by age-group were studied. Results. The mean of the relative hand skill difference was 0.037 (SD = 0.053) in the total sample. The mean (WI=0.048; SD = 0.048) among right-handers (HS > 0) differed significantly (P
The observed

distribution

was not significantly

Fits ofthe

For each age group, a single normal difference (x2 test gave a P consistantly Graphic

in the total sample

different

distributions

from a single normal

by age group

distribution

(P=O.l).

(Fig. 2)

distribution fitted well the observed distributions of the relative hand skill greater than 0.1). Figure 2 shows the observed and the fitted distributions.

test (CT)

On a sheet of paper, small circles were drawn according to a certain pattern. The child was asked to in each circle, following the pattern, with a felt-tip pen and to mark as many circles as he could in 20 sec. was tested twice. The order of four tests was R,L,L,R among the children who used the pencil with the in the manual preference assessment, and L,R,R,L among the children who used the pencil with the left answer “both hands” had never been observed for the pencil during the manual preference assessment. and left hand scores were calculated by adding the number of circles properly marked off during both

Procedure.

make Each right hand. Right tests.

a dot hand hand The hand

Statistical methods. A relative difference, calculated as the right- minus left-hand scores divided by the sum of left and right hand scores: (R - L)/(L + R) were computed. This difference denote a right-hand advantage when positive (the measure is a number of circles filled). Inspection of the scores led to the exclusion of two children with aberrant values (more than 3 SD from the mean): two right-handers with a particularly unskilful left hand.

Fit of the distributions

We used for CT the method

described

of hand skill rvlatioe

diference

above (see PMT).

Results. The mean of the relative hand skill difference was 0.138 (SD=0.136) in the total sample. The mean (WI= 0.173; SD = 0.095) among right-handers (HS > 0) differed significantly (P
in the total sample

The observed distribution was very different from a single normal distribution tit was obtained with a two normal component distribution. Fits

of the distributions

(PcO.001).

But a quitesatisfactory

by aye group (Fig. 3)

For the youngest age class (< 3.5 years, n = 43,36 right-handers and 7 left-handers) the observed distribution was not significantly different from a unique normal distribution (P=O.55). For the other three age groups, the hypothesis of one normal distribution was rejected (P < 0.001) and a good fit obtained with two normal components (P=O.39,0.53,0.40, respectively for the three age groups). The second normal component corresponded mainly to the left-handers (HSIO) (Fig. 3).

HAND PREFERENCE

AND SKILL

ASYMMETRY

IN PRESCHOOL

CHILDREN

31

R

s

(a)

10,

b)

ULeft-handers 0

: 1 I Ai/ L&i (L - R)/ (L + R)

10

Left-handers

(L - RI / (L + RI

(d)

(c)

I

8

I

6

a

’ . \

.

.

4

2

.

L.

PO.2

5

.

-0.1

0.0

-

3

0.1

0.2

I

0.3

G

L 8.2

OLeft-handers

.I ..*. -0.1

(L - Fl)/ (L + R)

0.3

0.0

(L R) / (L + R)

Fig. 2. Peg-moving task: For each age group, the observed distribution of relative hand skill difference fits well with a single normal component. The fitted distribution is represented by the continuous line and the observed distribution by the histogram.

RELATIONSHIPS

BETWEEN

HANDEDNESS

AND HAND

SKILL

Statistical methods We used a multivariate logistic regression of the probability differences for PMT and CT, with age and sex as covariables.

of right-handedness

(HS >O) on relative hand skill

Results PMT and hand preference group. The logistic regression gave the equation: Logit (PRH)= 1.66 + 32.04 x (L - R)/(L + R), where PRHstands for Pr(HS > 0). The PMT coefficient was highly significant (P
PMT

GT and hand preference

group

and GT relative hand skill asymmetry were significantly correlated (r=0.45, The model with both regressors was not an improvement on the model with only

P
32

F. CCKI.

J. MAC‘CAKIO

and

G.

DMLATOLAS

M

0

E

-0 4

-0.2

00

IR

Left-handers

..I 04

0.2

0.6

.2

(R

L) / (L + RI

04

06

L) / (L + RI

m

Cc)

(d

7..

1 .

r

:

.

0

l \. (

Leftmhanders

.

.;

.

i

02

m

l.---04

0.6

q Leh-banders

,, -0 2

L) I (L + R)

00

CR

L) / (L + RI

Fig. 3. Graphic test: Only for the younger age group ( < 3.5years) the ohscrved distribution of relative hand skill diffcrencc lits with a single normal component. F\)r the other age groups, the observed distributions differ significantly (PO) and to the left-banders (HS
GT as regressor: the PMT coefficient was not significant if the GT result was taken into account, but the converse was not true. We concluded that GT conveys a significant amount of information about handedness group when added to the PMT only model (PC 0.001), but the converse is not true and all the information available in the PMT is already contained in the GT.

POWER

STUDlES

ON THEORETICAL POPULATIONS WITH ANNETT’S MODELS

IN ACCORDANCE

Sl~Ifi.sll~~tr/ r7~c~th0d.~ Power is dclincd by the rejection probability of a single component hypotheslc. Theoretic populations were formed 111accordance with Annctt’s models: (a ) we:~dopted for each component proportion the Annett’s figures. i.e. pI = Pr( rs - ~ ) = 0.1854, p1 = Pr(r + -- ) = 0.4904. pz= Pr(rs+ + )=0.3242:

(c) the standard error WIS asaumcd the same and equal to one for all components.

HAND

PREFERENCE

AND

SKILL

ASYMMETRY

IN PRESCHOOL

(‘HILDRLN

33

Such theoretic populations were produced for various values of /I [0, (0.5) 2.51 for the dominant and additive models. Histograms with constant class frequency of 5% were deduced from these theoretic populations. Then, using the maximum likelihood, a single normal population was fitted. The non-centrahty parameter was then evaluated 1141. Using a classical approximation of the non-central ;1’ distribution [IS], we computed the power of the test.

Results

Figure 4 shows the power of tests for detecting a mixture of normal components in a population, for the dominant and additive Annett’s models (2 or 3 normal components), as a function of the sample size N and of the values of the shift parameter p. For a given N and p, the power is lower for the three component model than for the two component model. For jr< 1, the power remains very close to the type I error of value 0.05. In other words, for a small shift between the components, it is quite difficult to detect these models with a reasonable N. For example, in the two component model, if !1= I, a 50% power needs 18 560 subjects. If we consider a p value close to Annett’s value (p = 1.5) the power to detect a mixture of two normal components achieves only 20% with N as large as 1000 subjects.

0

2.0

1.0

I*

Fig. 4. Probability to reject a single normal component if the real distribution is a mixture of two normalcomponents (Annett’sdominant model):O.l854N(O. 1)+0.8146 N(~L. l).oramixtureofthrec normal components (Ann&t’s additive model): 0.1854 N(0, 1 )+0.4904 N(n. I )+0.3242 N(2 I!, I). according to 11and to the number of subjects (n).

DISCUSSION

AND CONCLUSIONS

The J-shaped distribution of hand preference is age-dependant in preschool children; at 3 years of age handedness seems less lateralized than at 6 years of age. Some longitudinal children studies [l l] tend to confirm this developmental aspect of the human hand preference. In Annett’s theory, handedness among rs- - subjects, i.e. without the right-shift allele, and also among some animals, depends on chance und is U-shaped. In our sense, a true random mechanism would lead to a normal HS distribution (with the mode at the mean) which is never observed, and the U-shaped distribution implicitly contains some developmental or learning process. The age effect in the GT distribution could be explained

F. CURT, J.

34

MAC~ARIO and G. DELLATOLAS

by systematic training of the preferred hand, i.e. the hand which uses the pencil, between 3 and 6 years of age. For PMT there is no such training, and as in ANNETT’Sdata [4,13], there is no obvious age effect on the PMT hand skill asymmetry. The distribution of hand skill asymmetry defined as the relative difference between hands, depends on the nature of the task used to assess hand skill. For the PMT, this distribution is compatible with a single normal distribution. What is shown here is the difficulty of detecting Annett’s genetic shift by population distribution modelling. The power study shows that the number of subjects in our sample is too small (n= 544) to confirm or to invalidate the predictions of Annett’s right shift model [3, 41. For the GT, there are two normal distributions, one with a right shift and the other with a left shift; they correspond primarily to the right-handers and to the left-handers, in acordance with TAPLEY and BRYDEN [17] and MCMANUS [15]. When the absolute difference is used as in ANNETT’Sstudies [3,4], the above conclusions remain valid. The relationship between hand preference and hand skill asymmetry can be described by a logistic model as BISHOP [6] suggested. The relationship between GT and handedness group is stronger than that between PMT and handedness group. Is the age-independant PMT hand skill asymmetry a better marker of cerebral lateralization of function, e.g. cerebral speech dominance, as Annett’s models suggest, than the age-dependant handedness or GT hand skill asymmetry? More data seem necessary to answer this question, as the relationship between handedness and cerebral speech dominance remains unclear [S, 121, and the relationship between PMT hand skill asymmetry and cerebral speech dominance has not yet been explored enough.

REFERENCES of manual preference and speed. Br. J. Psychol. 61, 545-558, 1970. 2. ANNETT. M. Hand preference and the laterality of cerebral speech. Cortex 11, 305 328. 1975. 3. ANNETT, M. and KILSHAW, D. Right and left-hand skill II: Estimating the parameters of the distribution of L -R differences in males and females. Br. J. Psychol. 74, 269-283, 1983. 4. ANNETT, M. Lxfi, Right, Hand und Brain: The Right Shij Theory. Erlbaum, London, 1985. 5. BASSO,A., FAKAROLA, M., GKASSI, M. P., LAIACONA, M. and ZANOBIO, M. E. Aphasia in left-handers. Brain and Language 38, 233-252, 1990. 6. BISHOP, D. V. M. Does hand proficiency determine hand preference? Br. J. Psychol. 80, l9l- 199, 1989. I. DE AC;OSTINI,M. and DELLATOLAS, G. Une ipreuve simple pour &valuer la prCf&ence manuelle chez I’enfant B partir de 3 ans. Enfance 41, 139%147, 1988. 8. Dr: AGOSTINI, M., PAI& C., GOUDO~, D. and DELLATOLAS, G. Manual preference and skill development in preschool children. Dee. Neuropsych., in press. 9. DELLATOLAS,G., TUBEKT, P., CASTKESANA,A., MESBAH, M., GIALLONARDO, T., LAZARATOU. H. and LELLOUVH, J. Age and cohort effects in adult handedness. Neuropsychologia 29, 255 261, 1991. IO. EVERITT, B. S. and HAND, D. J. Finite Mixture Distribution. Chapman and Hall, London, 1981. II. FENNEL. E. B., SAW, P. and MORRIS, R. The development ofhandedness and dichotic ear listening asymmetries in relation to school achievement: a longitudinal study. J. e.xp. Child Psycho!. 35, 248-262, 1983. 12. H~CAFN, H. Les Gauchers. P.U.F., Paris, 1984. 13. KILSHAW, D. and ANNETT, M. Right- and left-hand skill I: Effects of age, sex and hand preference showing superior skill in left-handers. Br. J. Psycho/. 74, 253-268, 1983. 14. L~HMANN, E. L. Testing Statisticul Hyiotheses. Wiley, New York, 1986. 15. MCMANUS, I. C. Right- and left-hand skill: Failure of the right shift model. Br. J. Pswhol. 76, l-16, 1985. 16. PORAC, C. and COR~N, S. Lateral Preferences and Human Behavior. Springer, New York, 1981. between the hands. 17. TAPLEY, S. M. and BRYDEN, M. P. A group test for the assessment of performance I. ANNETT, M. The growth

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18. ZELEN, M. and SEVEKO, N., C. In Handbook (Editors). Dover, New York, 1970.

CI/ Mathematical

Functions,

M. ABKAMOWITZ and A. STEGUN