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Journal of Comparative an d Physiological Psychology1963, Vol. 56, No. 5, 899-902SPEECH LATERALIZATION IN YOUNG CHILDREN AS

DETERMINED BY AN AUDITORY TEST1DOREEN KIMURA2

Montreal Neurological InstituteSpoken digits were presented to children in such a way that different digitsarrived simultaneously at the 2 ears. Previous studies with adults had shownthat digits arriving at the right ear are more accurately reported than digitsarriving at the lef t ; this effect depends on the fac t that speech is representedin the left hemisphere. The present study demonstrated the right-ear effectfo r boys and girls as early as age 4. Boys achieved lower total scores than girlsat the early ages. These findings suggest that the left hemisphere is dominantfo r speech by age 4, for both sexes, but boys nevertheless lag behind girls indevelopment of speech perception .

It has been shown that when differentspoken digits are presented simultaneouslyto the two ears, those digits arriving at theear opposite th e dominant hemisphere aremore efficiently recognized (Kimura,1961a). In a group of neurological patientswith speech represented in the left hemi-sphere, the right ear was more efficient;in a smaller group of patients with speechrepresented in the right hemisphere, th eleft ear was more efficient. These resultswere independent of handedness. The find-ings were consistent with other studies sug-gesting that the crossed auditory pathwaysin man are more effective than the un-crossed (Kimura, 1961b; Sinha, 1959) andthat th e left temporal lobe plays a greaterpart in the perception of spoken materialthan does th e right temporal lobe (Kimura,1961b; Milner, 1958). For a group of nor-m al control Ss, in whom th e left hemispherew as assumed to be dominant , th e right earwas also more efficient. The greater e f f i -ciency of the right ear for spoken digits,then, has been shown to depend on the or-ganization of speech representation in theleft hemisphere. The technique of present-ing verbal material dichotically can thusbe used to investigate many aspects of cere-bral dominance which previously could bestudied only clinically.

1This paper was presented at the 1962 m eetingof th e Eastern Psychological Association, AtlanticCity. The investigation was supported by GrantB-2831 to Brenda Milner from the United StatesPublic Health Service.2 Now at the Neurochirurgische Klinik, Kan-tonsspital, Zurich, Switzerland.

There is a widely held belief, based onclinical observations of aphasia af ter dam-age to the left hemisphere, that speech rep-resentation is relatively fixed in one hemi-sphere in the adult, but the chronologicdevelopment of such hemispheric organiza-tion is not known. Handedness, oncethought to be a reliable index of speechrepresentation, is no longer so considered(Goodglass & Quadfasel , 1954; Rasmussen,Branch, & Mi lner , 1963). It was decided ,therefore, to present the auditory test de-scribed above to young children in an effortto discover when th e right-ear superiorityfirst appears, and thus to gain indirectlysome information about the age at whichspeech becom es lateralize d in the bra in.M E T H O D

SubjectsThe 5s were 145 no rm al children, aged 4-9 y r .

The 4-yr.-olds were tested at a nursery school in aresidential area and most of the other childrenwere tested in an elementary school in the samearea. The /Ss for each age group were chosen atrandom from th e total num ber of children at thatage level. Every attempt was made to keep thesample unselected, with tw o reservations: (a)Children with known hearing impairment were nottested, ( f a ) The n u m b e r of boys an d girls was keptroughly equivalent. Th e childre n at this schoolwere subsequently found to be of above averageintelligence, as determined from school records.O f the 145 Ss, 25 left-handed Ss were omittedfrom th e main analyses, since there are strongreasons for believing that th e incidence of right-sided speech representation is higher in naturalleft-handers than in natural right-handers (Good-glass & Quadfasel, 1954; Rasmussen et al., 1963).The bulk of the data reported in the presentstudy is therefore based on the 120 right-handed

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900 DOREEN K I M U R ATABLE 1

M E A N NUMBER OP DIGITS C O R R E C T L YR E P O R T E D F O R E A C H E A RGroup

Age 4:Ag e 5:Age 6:Age 7 :

Age 8:Age 9:

* v

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SPEEC H LATERALIZATION IN C H I L D R E N 901given, th e data are probably less reliablethan Ghent's (1961). Within these limita-tions, however, the results obtained in thisstudy are in agreement with Ghent's on themain pointthat th e greater sensitivity ofth e left thumb occurs earlier in girls thanin boys . Lef t - thumb sensitivity first ap-pears in 6-yr.-old girls, though the differ-ence between thumbs is not signif icant inthe present study. The only group for whicha signif icant difference between thumbs isobtained is the group of 9-yr.-old girls. The9-yr.-old boys show no tendency for thelef t t humb to be more sensitive.

DISCUSSIONThe present results indicate that, as earlyas age four, spoken material arriving at theright ear is more accurately reported thanspoken material arriving at the left, sug-gesting that le f t cerebral dominance forspeech is established by the age of four ,and probably earlier. This conclusion ap-pears to conflict with clinical reports thatleft-sided brain damage in children, even

when it occurs as late as age six, does notpermanently affect speech, and that lesionsof th e right hemisphere result in aphasiamore f requently in children than in adults(Basser, 1962). The foregoing facts suggestsome participation of the r ight hemispherein speech functions in the child, and haveled Zangwill (1960) to propose that le f tcerebral dominance is established graduallyduring childhood. The conflict between theclinical findings and the data reported here,however, is not i r reconcilable. The presentstudy strongly indicates an early prepo-tence of the left hemisphere for speech func-tions, but it does not rule out participationof th e r ight hemisphere. The fu r the r f a c t ofrecovery from aphasia a f te r lef t -sided dam-age m ay mean only that speech representa-tion is less rigidly established in childrenthan in adults . Zangwill ' s statement con-cerning the gradual development of domi-nance may theref ore stand with referenceto the development of i r reversible asym-metry of the two hemispheres.The preceding statements have been con-cerned with the lateralization of speech inthe le f t hemisphere. The quest ion thenarises, when does the analogous lateral iza-

tion of certain nonverbal abilities to theright hemisphere take place? Ghent's(1961) study on the development of thumbsensitivity suggests that one of the func-tions which is to become more dependent onth e minor hemisphere is not so establisheduntil age six in girls and even later in boys,i.e., later than the first occurrence of theright-ear effect in children. The two studiestaken together raise the possibility thatcerebral do m inance for speech is establishedbefore cerebral dominance for nonverbalabilities. It appears, at least, that the allo-cation of certain speech functions to theleft hemisphere (as indicated by the right-ear effect) precedes th e allocation of someaspects of nonverba l somesthetic fun ctionto the right hemisphere (as indicated bylef t - thumb sensitivity).These results must be interpreted withcaution. A strong objection to the compar-ison of Ghent's study and the present oneis that they employed measures in differentsense modalitiesin somesthesis and audi-tion. Thus it cannot, on the basis of thesetw o studies alone, be concluded that allverbal functions which become to some de-gree lateralized do so before all nonverbalfunct ions which become lateralized. To es-tablish conclusively the priority of speechlateralization, the asymmetries in percep-tion resulting f rom specialization of thetw o hemispheres should be studied withinone sense modality, i.e., within th e auditorymodality one should discover whether left-right differences in perception occur forverbal material before nonverbal. Such astudy must await th e development of anauditory task more dependent on the r ight(nondominant) hemisphere, which can bepresented dichotically to young children.Sex Differences in the Development ofSpeech

It was expected, on the basis of Ghent's(1961) study, that boys might develop ther ight-ear effect later than girls. No suchsex difference w as found in the age rangetested. It is still possible that a sex differ-ence might appear if younger children couldbe studied, and an unsuccessful attempt w asm a d e to test 3-yr.-old children. However, atthe age of four , boys are significantly betteron the r ight ear than on the left , and there

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902 D O R E E N KIMURAis no trend for this effect to disappear at theearly ages, for boys or girls. In fact , th edifference between ears is accentuated inth e very young children. There is thus noevidence from these data that speech be-comes represented in the left hemisphereany later in boys than in girls, though thepossibility cannot be ruled out.Although no significant sex difference wasobtained for the right-ear effect, there wasa difference between boys and girls in over-al l efficiency on the task. This differencewas evident at age four , but it did notachieve statistical significance; it was sig-nificant at ages five and six, disa ppe aredat age seven, and did not reappear at ageseight or nine.Sex differences in cognitive abilities havebeen extensively reviewed by Terman andTyler (1954). There is evidence that girlsexcel boys in almost all speaking skills, atleast in the early years. The present studysuggests that boys lag behind girls in theperception of speech sounds as well as intheir production. That this is simply aslower development, rather than a perma-nent difference, is indicated by the fac t thatth e difference disappears at age seven. Simi-la r findings led McCarthy to suggest that,with regard to language ability, girls "gothrough the developmental cycle more rap-idly than do boys, but both arrive eventu-ally at approximately the same level" (Mc-Carthy, 1930, p. 82).Another suggestion of a sex difference inth e present study was found among th elef t-handed children. Of the 11 le f t -handedgirls, 10 were more efficient on the right ear( t h e normal effect) , and one showed nodifference between ears. Of the 14 left-handed boys, only nine were more efficienton th e right ear, and the other five weremore efficient on the lef t ear, a findingtypical of Ss with speech represented inth e right hemisphere (Kimura, 1961a). Withsuch small numbers as the above, no defi-nite conclusions can be drawn, but a morecomprehensive comparison of lef t-handedgirls and left-handed boys might repayinvestigation. Pertinent to this line ofthought is a study by Taylor (1962) inwhich digits were presented dichotically tochildren with reading disorders. The find-ings again indicated atypical speech later-

alization in the boys of this gr oup but notin the girls. Thus althoug h there is no evi-dence at present that norm al r ight-handedboys develop speech lateralization differ-ently from girls, there may be such a sexdifference in children who show other signso f irregular cerebral organization.

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