cc-393: 3: 0?034~4403.M 0 i‘ 198: Pcrgvnon Press L:d.

NOTE

JZATION SEX DIFFERENCES IN CEREBRAL LATERAL OLD CHILDREN

IN 3- AND 4-YEAR

811.1 JO\;ES and Tt!RESh As~zn

Department of Psychology. Carleton University. Ottawa, Ontario. Canada

Abstract--Following a procedure described by Jos~s [ I 2, 131 boys and girls, 3 and -l yr of age. were

required to identify the sex of male and female faces tachistoscopically presented to the right and left visual fields. The results were in close conformity to those obtained with adults [ 13. 131. Boys showed a strong and consistent right visual held advantage in accuracy. Girls showed no strong field

advantage. It is argued that cerebral lateralization remains constant across development. Theoretical issues relating to sex differences in cerchral lateralization are discussed.

THFRC has recently hccn considcrahlc inttrcxt in the assecsmcnt ofcerebral lateralization offunction in children. e.g.

[I-9]. The thcorctical interest in developmental Studies is based upon two contrasting assumptions. One is that asymmetrical specialization of the hemispheres is purely phylogenetic rather than ontogenetic in origin and in prmciple demonstrable in the neonate and the young child, e.g. [i]. The second is that functions are bilaterally

represented at birth so that asymmetrical representation in the adult requires a gradual maturational process, e.g.

Chl. There is some behavioral evidence for both points of view dcrivcd mainly from dichotic listening experiments

[I. 73. In adults this procedure typically gives rise to a right ear advantage (REA) for speech whether subjects are required to recall [ 101 or to recognize [I I] the material. The usual assumption has been that an ear advantage reHects contralatcral hemispheric specialization in perception. in this case left hemisphere specialization for speech.

BKYDEN and ALLARD [I] argued on this basis that dichotic listening experiments show a “gradual development of cerebral Iateralization approaching the adult state by the eighth grade (p. 39X)“. though other studies have found

that the REA is roughly constant across age groups, e.g. [7].

The present work is an examination of possible sex differcnccs in hemispheric representation of function in young children. Experiments with children that have specifically examined sex differences in lateralization by means of visual and tactile procedures have in general found similar patterns of hemispheric specialization in boys and girls [3.4, I l].,However, for these tasks there was no evidence ofsix differences in adults. To answer more fully questions

about sex-related differences in the development of hemispheric specialization we may use a procedure which is

known to result in such laterality dilfercnces in adults. Recently JOWS [I?. 133 has described a task which appears in right-handed adults to result in a strong and

consistent left hemisphere advantage in males but in no lateral advantage in fcmalcs. Photographic slides of male and female faces were tachistoscopically presented to the right or left visual hemilields (RVF and LVF respectively)

and subjects were required to identify the sex of the person in the photograph. JOSES [I?. 131 argued that the task requires a familiar dichotomous classification of information which is reflected in language and parasitic upon the functional specialization of the language hemisphcrc. Male right-handers showed a consistent RVF advantage in accuracy (I I to I) while females were almost evenly split between right and left (7 to 5).

The sexual categorization task may have some advantage for use with young children since visual tasks which depend upon the acquisition of reading are impractical in this age range. It is known that by about 28 weeks of age

infants respond dilferently to male and female faces tending to smile and vocalize more for female faces [IS] and it is a matter of common observation that an infant’s first words often denote sexual distinction (“Mama” etc.). Differences between the sexes are thus identified early in ontogeny and are early fixed in the language.

If cerebral specialization is constant throughout development we should expect to find the same pattern in

children as in adults; boys should show left hemisphere specialization and girls should show hemispheric equipotentiality.

347

318 NOTE

METHOD

Subjects

Twenty-seven children from the Carleton University Pre-School uere given parental permission and agreed to take part m the expertment. The subjects fell into two age groups. 3 yr (6 boys and 7 girls) and 4 yr (6 boys and 8 girls). The mean age of the 3-yr old was 41 months and the mean age of I-yr olds was 52 months. Ail the children were right-handed as assessed by the hand they used spontaneously to pick up a pencil to write their name or initials.

Apparatus und stimuli

A two-channel tachistoscope (Polymetric V-l-159-B’) was used to back-project a fixation cross and slide photographs of male and female faces. Each slide was constructed so that its center would be at approximately 5’ of visual angle to the right or left of the central fixation point. Each slide subtended 2’ square in area. The persons

photographed had been asked to adopt a neutral expression. All photographs were free ofsuch obv-ious sexual cues as facial hair, baldness or long hair.

Procedwe

The subjects were shown six slides (3 male and 3 female) in the central visual field. All were able to identifv correctly the sex of the individual in the slide though words varied from child to child (“man” and “woman”. “boy” and “girl” and occasionally a “mummy” and “daddy”). In the expcrimcnt a trial was initiated by a spoken “ready” from the experimenter followed by presentation of the central held cross. The subject was instructed to maintain tixation on the cross throughout a trial and eye-movements were monitored by the experimenter. After 2 set a slide was presented for 200 msec in the RVF of LVF fields at random. Twenty different slides (IO male and 10 female) were presented in each visual field. The sex of the person in the slide was randomized over trials. The child was required to identify the sex of the individual in the slide as a “man” or as a “woman”.

RESULTS

The dependent variable was the proportion ofcorrect responses. P(C), for each visual field. Figure I shows mean values of P(C) for the two sexes and in the tuo age groups. It is apparent that boys in both age groups show a larger difference between the visual fields than do the girls. Averaged across age groups mean P(C) for the boys was 0.904 for the right visual field and 0.825 for the left while for the girls mean P(C) was identified as 0.913 for the two fields.

P(C) values were analyzed by means of a 2(Sex) x Z(Age) x Z(Visual Field) analysis of variance using the unweighted means solution for the cffcct of unequal cell sizes and an arcsine transform since proportions tend to be positively skewed. The analysis indicated signiticant main effects of Sex, F (1. 23)= 8.92, P <0.007, and Visual Field, F (I, 23)= 14.29, P<O.OOl, and a significant Sex x Visual Field interaction. F (I. 23) x 13.36. P<O.OOl. Theeffect of Age was marginally significant, F (I, 23)=3.00, P<O.lO,as was the Sex x Age interaction. F (I, 23)=4.00, P~0.07. They key effect is the Sex x Visual Field interaction. A Newman-Keuls analysis using the harmonic mean of the cell totals [I61 indicated a significant right visual field advantage (PiO.01) for boys in both agegroups and the absence ofa significant difference between the visual fields for thegirls in either agegroup. In general thereforegirls tended to be more accurate than boys since LVF accuracy for boys tends to be lower than for girls.

I 00

/

RVF- loo L”t=*---. Male Famole

085 F 085 1

080 &40.80 ; I

4 1 ‘-__

--_

FIG. I. Mean P(C) as a function for males and females for the right and left visual fields

NOTE 349

Table 1. Numbers of children showing a right visual field (RVF) advantage, a kit visual-field advantage ILVF)

or no lateral advantage (NLA)

3-yr Boys 4-yr All 3-yr

Girls J-yr All

RVF 5 6 11 3 2 5 LVF 1 0 1 2 2 4

NLA 0 0 0 1 5 6

Table 1 gives the numbers of male and female subjects who showed either visual field advantage or equal values of P(C) for the two visual fields. The distributions are clearly different for the two sexes. Only one boy did not show a right field advantage and 10 girls were split roughly evenly between the three categories. A 1’ test showed that the two distributions were significantly different, ~~(2) = 8.55. P <0.02. These distributions correspond remarkably well to those observed in adults for the sexual categorization task [Zl]. Moreover the girls showed weaker lateralization regardless of direction. The mean absolute difference between the visual fields was 0.0875 for boys and 0.04tXl for girls.

DISCUSSION

In general and in detail these results correspond closely to those obtained with adults [ 12, 131. The simplest explanation for this correspondence is that cerebral lateralization is constant across the course of development in both scxcs. The high degrees of accuracy shown even by the 3 yr olds is consistent with the previous suggestion [ 12. l3] that sexual categorization is early established in the language leading to an advantage for the hemisphere specialized for language. Language may therefore be more asymmetrically represented in males and more bilaterally represented in females.

The results are difficult to square with the theory proposed by B~IFFEHY and GRAY [2]. In order to account for the more rapid development of linguistic skills in females, they argurd that females are characterized by stronger left hemisphere lateralization ofspeech early in ontogeny. Like many theorists they assumed that a more asymmetrical representation of a function should lead to a more eflicient performance. There is evidence against this view. For example. sexual categorization offaces by left-handed females appeared to be more strongly lateralized than in right- handed females but the left-handers were the less accurate [ 131. In theory relatively more efficient use of language early in development may be consistent with a more symmetrical cerebral representation. Evidence for greater asymmetry early in development in females than in males tends to be anatomical and morphological in nature (see [Z]), This data is very variable and susceptible of a number of interpretations (see the critique in HARRIS [ 171 of the use made of such data in assessing early sex differences). Behavioral data either shows no sex differences [3,4,7.9] or as here indicates less specialization of the hemispheres of function in females.

At all events, the present data show that there is no simple relationship between degree of lateralization and accuracy. If anything the less strong lateralized females tended to be the more accurate. One might perhaps speculate that the experimental task happened to be one at which girls naturally excel so that their superiority compensates for the fact that function is not lateralized. This argument could only be answered by the use of other language-based categorization tasks for which girls are not expected to have any inherent advantage. We suspect, however, that the expectation that girls excel boys at distinguishing male from female faces may be false. YOUNG and Et.t.ts [9] found that boys made about 41, fewer errors than girls in a same-different comparisons of faces. The procedure is obviously different from the one we have used here. Nevertheless the Young and Ellis experiment does suggest that girls hav,e no genrrul superiority at processing faces.

Oneargument in favor ofthe proposition that functions are bilaterally represented in the infant brain has been the assumption that a more symmetrically organized system would have greater developmental Hexibility including some ability to reorganize effectively following trauma [ 11. In general infant brains tend to be more plastic than adult brains and bilateral representation may allow even greater Hexibility. If some functions are more likely to be bilaterally represented in the female brain in both children and adults, females. for this reason. may be in some respects developmentally less at risk than males. At the moment the clinical data on sex differences in response to brain injury in young children (e.g. [lg]) is simply too scanty to permit any firm conclusions.

Ackno~/edgeme,lts-Research supported by Grant A9946 from NSERC to the senior author. We thank the staff, parents and children of the Carleton University Pre-School.

350 NOTE

REFERENCES

1 BRYDEU. &I. P. and .ALL-\RP. F. Dichotis listening and the debelopmcnt of llngul\tii processes. In .~~~rnnwtric~~! Funcricv~ of‘fhr Broirf. \I. KIVSB:WR\E (Editor). CVP. Sew 1.ork. 1978.

2. BCFF~RI. .A. IV. H. and GR.\L. J. A. Sex differences in the dc\clopmcnt of perceptual and linguiwc skills. In &v&r D$rrmcrz: Tiwir O~royetl~ nnd Sigrrificorfcr, C. OLS~TED and D. G. T\\L!)R IEditarsr. Churchrll. London. lY72.

3. C.\RTER. G. 1. and KIXSBC)CRYE. St. The ontogeny ofcerebral I,ltcraliration ofspatial mental set. Dtir[ Ps~clwi. 15.241-245. 1979.

4. FL\SER\. R. C. and B~I.LIUC. J. D. Developmental change in hemispheric specialization for tactile spatial ability. Dtirl Ps~clwi. 15, 344-372, 19i9.

- 2. KISSDOL.RS.E. ii. Evolution of language in relation to lateral action. In .-i~~mm~~~rictri Func.rion r,l’ihc Brtrin. xi. ~ISSll~xR~E (Editort. CL’P, Xew York. 197s.

6. LEVSEBERG, E. Biological Found~irions o/Lmyutrye. John Wile>. New York. 1967. i. SESTOU. Sf. r\. and GLFFE\. C;. Dexelopmcnt ofthreestrategiss ofattentlon in Jichotic monitoring. Der,l f’.s~ci!oi.

IS, ZYYm.3 IO. 1979. S. WIIFI.SW. S. F. Hemispheric specialization for linguistic and nonlinguistic tactual gerception using a

dichotomous stimulation technique. Cortex 10, 3-17. 1974. 9. YWNG. A. W. and ELLIS, H. D. An experimental investigation of developmental differences in ability to

recognize facts presenrzd to the left and right cerebral hemisplxres. .Vtwrops~cl~ologiu 14, 495498, 1976.

IO. KIULXA. D. Functional asymmetry, of the brain in dichotic listening. Cwtrs 3. 163-175. 1967. I I. JOSFS. B. anJ SOLLSFR. R. Rsropnrtton memory for dichotically presented word pairs in right- and left-handed

males. Corfcs, in press. I?. Jou~s B. Sex and visual field cffccts on accuracy and decision making when subjects classify male and female

faces. Corirx 15. 551-560. 1979. 13. Joues, B. Sex and handedness as factors in visual-field organization for a categorization task. Joumc~l c$

E.xcperimrrrrof P.s.bd~oloy,v: ff unum Perceptim ad Per/hwncc, 6. 494500. 1980. IJ. Lcvv. J. and REIIX M. Variations in cerebral orgamzation as a function of handedness. hand posture in writing

and sex. J. t*spl. Psyrhol. GLW. 107. 119-144. 1978. 15. ~\IIREXS, R. Beitrag zur Entwicklung der Physiognomie und Mimikrrkennens. Z./h P.Y_P. anyew. P.syc/d. 2,

-l I-1 -4.54, 599433, 1951. 16. LVISER, B. J. Stcrfisficrrl Principkz in E.~perinwntnf DL’siqtt. McGraw-Hill, New York. 1962. 17. H,\RRIS, L. J. Sex differences in spatial ability: possible environmental, genetic and neurological factors. In

.-ls,tmwfriccrf Functim o/‘rhe Bruin. M. KINSHOI:RNF (Editor). CVP. New York, 1978. 18. Htc~rs, H. Acquired aphasia in children and the ontogcncsis of hemisphere functional specialization. Brain

Luw/. 3, 114-134. 1976.

En suivant une procBdure d&rite pric6demment par JONES (12, 13). on

a demand6 B des g.ar$ons et 1 des filles de 3 et 4 ans de reconnaitre les visages

d’hommes et de feonnes prEsenc&s tachistoscopiquement BUX h&mi-champs visuels droit

et gauche. Les resultat s’accordent parfaitement avec ceux obtenus avec les

adultes (12, 13). Cher les gar~ons, il existe un avantage. en exactitude, du

champ visuel droit. important et cohsrent. Len filles ne montrent pas ce

fort avantage du champ visuel, On en tire argument pour admetcre que la latzra-

lisation cerdbrale reste constante pendant le d6veloppement. On discute les

problPmes cheoriques lids aux diffGrences dues au sexe dans la lat6ralisation

cgrdbrale.

i%ch einem van Jones angegebenen Ve;fahren sollten Jungen und MSdchen im Alter

van 3 und 4 Jahren das Geschlecht van mtinlichen und weiblichen Gesichtern identifi- .

zieren, die tachistoskopisch ins rechte und linke Gesichtsfeld projiziert wurden. Die

Ergebnisse entsprachen denen bei Erwachsenen. Jungen zeigten eine stiirkere und

konsistente tjberlegenheit des rechten Gesichtsfeldes, Madchen keinen Gesichtsfetd-

vorteil. Es wird argumentiert, dafl die zerebrale Later&&rung wtihrend der Lebens-

entwicklung konstanr bleibt. Theoretische tjberlegungen in bezug auf Geschlechtsunter-

schiede bei der zerebralen Lateralisbrung werden diskutiert.