afasia en zurdos.pdf

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Ncuropsycho) c >gy1994, Vo l . 8, No. 2, 148-159 C o p y r i g h t 1994 by t h e A m e r i c a n P s y c h o l o g i c a l A s s o c i a t i o n . I n c .0894-41O r > / 9 4 / $ 3 . ( X )

Distributions of Hemispheric Asymmetry in Left-Handers andRight-Handers: Data From Perceptual Asymmetry StudiesHo ngk e u n Ki m

A l th o u g h he m i s phe r ic a s y m m e t r y a m o n g i n d i v i d u a l s is often treated as a categor ical var iable w i t h3 values ( i .e ., left he m i s phe re do m i n a n c e , r i g h t he m i s phe r e do m i n a n c e , an d bilateral dominance) ,i t is best viewed as a c o n t i n uo us l y dist r ibuted var iable ranging from s t ro n g a s y m m e t r y in f a v o r ofth e left he m i s phe r e t hr o ug h n e a r l y e qua l a s y m m e t r y to s t r o n g a s y m m e t r y in favor of the r ighthemisphere. T he present study compared dis tr ibutions of he m i s phe r i c a s y m m e t r y in lef t -handersan d r ight-hande rs, based on behavioral indexes of he m i s phe r i c a s y m m e t r y , s uc h as v i s ua l fielda s y m m etr y o n di v i de d visual field tasks and ear a s y m m e t r y on dichot ic l i stening tasks. Meta-analyses of prior studies using these indexes indicate that dist r ibut ions of he m i s phe r i c a s y m m e t r yin lef t -handers a nd r ight-handers differ both in the m e a n and in the v a r i a n c e . R i g ht - ha n de r s ha v eg r e a te r m e a n he m i s phe ri c a s y m m e t r y than lef t -handers, whereas lef t -handers hav e greater var iancein he m i s phe r i c a s y m m e t r y than r ight-handers. W i t h i n lef t -hande rs, those w i t h o u t s inis tral relat ivesh a v e greater var iance in he m i s phe r i c a s y m m e t r y than those w i t h sin ist ra l relat ives, suggest ing thatsinistral patterns of he m i s phe r i c a s y m m e t r y de t e r m i n e d by e n v i r o n m e n t a l f a c to r s m a y b e morevariable than those determined by genet ic factors.

Evidence indicates that in a majority of the populat ion,th e left hemisphere is superior to the right hemisphere forlanguage functions, whereas th e right hemisphere is supe-rior to the left hemisphere fo r visuospatial funct ions ( f or ar e v i e w , se e Beaton, 1985). However, in a substantia lminor i ty of the populat ion, hemispher ic asymmetry doesnot follow this pattern (Annett, 1964; Bishop, 1990a;Ge s c h w i n d & Galaburda, 1987; McManus & B r y d e n ,1991). The minority w i t h anomalous types of hemispher icasymmetry m ay show a reversed direction of hemispher icasymmetry or various degrees of bilateralization of lan-g u a g e a nd visuospatia l functions . Individual var ia t ions inhemispher ic asymmetry have been ascr ibed to a m u l t i t u d eof factors, such as heredity (e.g. , Annett, 1964; Levy &Nagylaki , 1972) , cy toplasmic lef t-r ight gradient (Morgan &Corballis, 1978), fetal testosterone levels (e.g. , Geschwind& Galaburda, 1987), loss of callosal axons (W itelson &N o w a k o w s k i , 1991), birth stress (e.g., Bakan, 1971), earlybrain insul t (e.g., Satz, 1973), chance factors du ring thecourse of development (e.g., Annett, 1972), and so forth.Clinical studies indicate that the incidence of anomaloustypes of hemispher ic asymmetry is e levated in lef t-handersrelative to r ight-handers (e .g . , Bryden, Hecaen, & De-Agostini , 1983; Hecaen, DeAgostini , & Mo n z o n - Mo n te s ,1981). Thus, cases of aphasia fol lowing r ight hemispheredamage are more f requent in lef t-handers than in r ight-handers ( for a r e v i e w , se e Carter, Hohenegger, & Satz,1980; Segalowitz & B r y d e n , 1983). Rasmussen and Milner

I am grateful to Lucia A . French fo r i m p r o v e m e n t of the Englishtext .Correspondence concerning this ar t icle s h ou ld be addressed toH o n g k e u n K i m , Department of Psychotherapy, College of Reha-b i l i ta t io n Science, Taegu U n i v e r s i t y , Nam-Taegu, P.O. Box 2 1 ,Taegu, South Korea.

(1975) used sodium am yta l tes t ing to determine la teral iza-tion of speech in a group of right-handed epileptic patientsand a group of left- and mixed-handed epileptic patients. O f140 right-handed patients tested, 13 4 (96%) had speech inth e left hemisphere , 6 (4%) had speech in the r ight hemi-sphere, an d n on e had evidence of bilateral control of speech.Of 122 left- or mixed-handed patients tested, 86 (70%) hadspeech in the left hemisphere, 18 ( 1 5 % ) ha d speech in ther i g h t hemisphere , and another 18( 15% ) had bi la teral controlof speech.O n the bas is of the incidence of dysphasic sy mptom sfollowing unila teral e lectroconvuls ive therapy in severe ly

depressed patients , W arr ington and Pratt ( 1 9 7 3) reportedestimates of the incidence of left, right, and bilateral controlof speech in left- an d r ight-handers s imilar to those ofR a s m u s s e n and M ilner (1975) . The e levated incidence ofanomalous types of hemispher ic asymmetry in left-handersshows that handedness is associated, albeit imperfectly,w i t h hemispher ic asymmetry . This associa t ion presumablyreflects common factors (e.g., genetic factors, early braininsul t ) affecting lateralization of both handedness and cere-bral f u n c t i on s in at leas t some individuals . For e x a m p l e ,genetic factors determining la teral izat ion of hemispher icfunctions may also code for lateralization of handedness(e.g., Annett, 1964, 1972, 1982). Early brain insult, depend-ing on lesion location and size, may lead to changes in bothhandedness and h e m i s p h e r i c a s y m m e t r y ( B i s h op , 1990a;Rasmussen & Milner, 1975).Clinical studies and, to a lesser extent, normal studiescompar ing hemispher ic asymmetry in left- a n d r i g h t -h a n d -ers have of ten treated hemispher ic asymmetry among indi -viduals a s a categorical variable with three values: lefthemisphere dominance , r ight hemisphere dominance , andbila teral dominance (Bryden et al., 1983; Rasmussen &Milner , 1975; W arr ington & Prat t , 1973) . Ho w eve r , as

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H E M I S P H E R I C A S Y M M E T R Y I N LEFT- A N D R I G H T - H A N D E R S 149individuals m a y v a r y i n degree as well as direction ofhemispher ic asymmetry , hemispher ic asymmetry is bes tviewed as a continuum ranging f rom s trong asymmetry infavor of the left hemisphere through nearly equal asymme-tr y to s trong asymmetry in f a v or of the right hemisphere(El ing, 1981; H a r s hm a n & Lundy, 1988; McManus, 1983;Segalowitz, 1987; Shankweiler & Studdert-Kennedy,1975). There ar e several sources of evidence for the notionof hemispher ic asymmetry as a continuously distributedvariable. First, handedness, a var iable known to be associ-ated with hemispher ic asymmetry , is continuously distrib-uted (e.g. , Shankweiler & Studdert-Kennedy, 1975). Sec-ond, in commissurotomized patients, th e level of skills inright hemisphere language varies considerably (e.g., Gaz-zaniga , 1983). Third, a proportion of variations in degrees ofaphasic disturbance following unilateral brain damage hasbeen ascribed "to the degree of d om i n a n c e of one h e m i -sphere in relation to lateralized processes such as speechproduction" (Luria, 1966, cited in Eling, 1981, p. 3 2 1 ) .F i n a l l y , impl ic i t in the construct of bilateral speech domi-nance is the notion that both hemispheres contribute inv a r y i n g degrees to speech production (Snyder, Novelly, &Harris, 1990). Thus, despite th e common practice oft reat ing hemispher ic asymmetry as a ca tegor ical var iable ,it is bes t v iewe d as a contin uously dis tributed var iableamong individuals.If it is accepted that hem ispheric asy m me try is a continuously distributed variable, the most informative approachto compar ing hemispher ic asymmetry in lef t-handers andr ight-handers would be to compare their distribution char-acteristics, such as m e a n s and var iances . Al though there ar ea num ber of a pr iori poss ibi l it ies in w hich dis tr ibutions ofh e m i s p h e r i c a s y m m e t r y in lef t-handers an d r ight-handersm i g h t differ, four poss ibil ities are considered here. They areillustrated in the four panels of Figure 1. Note that normaldistributions are assumed in all cases and that illustrationsw ere constructed to depict distribu tions of hemisph erica s y m m e t r y for a left-hemisphere specialized language task.First, distributions of hemispher ic asymmetry in left- andr i g ht - ha n d e r s may have the same mean and the same var i-ance (see Fig ure 1 a). This possibil ity can be regarded as an u l l hypothesis to be tested against other alternative hypoth-eses. Second, the two dis tr ibutions m ay h a v e th e same meanbut different variances, w i t h th e variance fo r left-handersgreater than th e variance fo r r ight-handers (see Figure Ib) .Third, the tw o distributions may have the same var iance butdifferent means , w i t h th e m e a n fo r r ight-handers greaterthan the mean for left-han ders (see Fig ure I c ) . Finally, thetw o dis tr ibutions m a y h a v e different m e a n s a nd differentvariances, w i t h the mean for right-handers greater than themean for left-handers and the variance for left-handersgreater than the variance for right-handers (see Figure I d ) .The first of these four a priori possibil ities, namely, thatthe two dis tr ibutions have both the same m ean and the samevar iance , is ruled out by cl inical data indic at ing an elevatedincidence of anomalous types of hemispher ic asymmetry inleft -handers relative to right-handers (e.g., Rasmussen &Milner, 1975). However, on the basis of the clinical dataalone, it is not possible, or is at best difficult, to dis t inguish

LH

Figure I. Fou r h y p oth e t ica l d is t r ibu t ions of h e misp h e r ic asy m-metry for a left hemisphere specialized task in left- and right-handers. F or s implici ty of presentation, th e difference in popula-tion p r op ort ions be tw e e n left- an d r igh t-h and e r s is ignored. R =r ig ht -ha n de rs ; L = left-handers; L H = left h e misp h e r e su p e r ior i ty ;RH = r ight hemisphere superior ity; and f = f r e q u e ncy .

a m on g th e remain ing three alternatives. W hat is needed todist inguish a m on g th e three alternatives is a continuouslydistributed index of hemispher ic asymmetry , that is, anindex ref lect ing individual var ia t ions in degree as w e l l asdirection of hemispheric asymmetry. A candidate for suchan index is behavioral indexes of hemispher ic asymmetry ,such as visual field asymmetry on divided visual field tasksand ear asymmetry on dichotic l is tening. These indexes,computed as the difference between right- and left-sensory-field scores or the transformations of such a difference (fora r e v i e w , se e Sprott & B r y d e n , 1983), are naturally contin-uously distributed and shown to be related, albeit not per-fect ly, to patterns of hemispher ic asymmetry as measuredby sodium amytal testing or other neurological procedures(e.g., Geffen & Caudrey, 1981; Hugdahl & W ester, 1992;Kimura, 1961; Strauss, 1988; Zatorre, 1989).However , there m a y b e some objections to the use ofbehavioral indexes of hemispheric asy mm etry in compar ingdis tr ibutions of hemispher ic asymmetry in left- and r ight-handers because these indexes are not perfect indicators ofhemispher ic asymmetry (Colbourn, 1978; Efron, 1990;Richardson, 1976; Satz, 1977; Schwartz & Kirsner, 1984).Evidence indicates that perceptual asymmetry reflects notonly hemispher ic asym metry but a lso other extraneous var i-ables, such as attentional biases (e.g., K im & Levine, 1992;Kim, Levine , & Kertesz , 1990; Levine, Banich, & Koch-Weser, 1984; Levy, Heller, Banich, & Burton, 1983a),sensory pathway dominance (e.g. , Efron, 1990; Lauter,1983; Sidtis, 1982), random errors in measurements (e.g.,Chiarello, Dronkers, & Hardyck, 1984; Kim, in press; Teng,1981), and so for th. However , i t may be assumed, in the


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150 H O N G K E U N K IMabsence of e v i d e n c e to the contrary , that these ext raneousv a r i a b l e s d o n o t d i f f e r e n t i a l l y i n v o l v e l e f t - h a n d e r s a n dr i g h t - h a n d e r s ( H a r s h m a n & L u n d y , 1988; Kim, 1992; Kim& L e v i n e , 199la, 1991b; S e g a l o w i t z , 1987). W i t h th i s as-s u m p t i o n , i n f l u e n c e s of ext raneous factors , ev en if t h e y a rerobust , do not inv al idate an a t tempt to compare dis t r ibut ionso f h e m i s p h e r i c a s y m m e t r y i n left- and r ight -handers on thebasis of perceptual asymmetry . Ins tead, they indicate onlyth a t high s ta t i s t ical power ( e .g . , a large sample s iz e ) i snecessary to show a statistical difference between left- andr i g h t - h a n d e r s ( B e r e n b a u m & Harshman, 1980; Harshman &L u n d y , 1988; S e g a l o w i t z & Bryden, 1983) .A n u m b e r o f s t u d i e s h a v e compared p e r c e p t u a l a s y m m e -tr y in lef t -handers and r ight -handers ( for rev iews, see An-nett , 1982; Bryden, 1987, 1988b). These s tudies hav e typ-ical ly reported that lef t -handers y i e l d a reduced m eanperceptual asymmetry re la t iv e to r ight -handers . That i s ,lef t- h a nders t y p i c a l l y s h ow a s m a l l e r m e a n r i g h t -s e n s o r y -f ield a d v a n t a g e o n v e r b a l t a s k s a n d a s m a l l e r m e a n left-sensory-f ie ld adv antage on spat ia l tasks . These resul t s areconsistent w i t h the hypothesis that left-handers have aw e a k e r m e a n h e m i s p h e r i c a s y m m e t r y t h a n r i g h t - h a n d e r s .O f t h e four possibi l i t ies depicted in Figure 1, these resultsrule out the second al ternat iv e as w el l as the f irst o n e, w h i c hsuggest that d is t r ibut ions of h e m i s p h e r i c a s y m m e t r y in left-h a n d e r s a n d r i g h t - h a n d e r s h a v e t h e s a m e m e a n ( s e e F i g u r el a & I b ) .U n f o r t u n a t e l y , nearly a l l pr ior s tudies comparing percep-t u a l a s y m m e t r y in lef t -handers a n d r i g h t - h a n d e r s h a v e o n l ycompared means, fa i l ing to tes t v ar iance di fferences ( for ane x c e p t i o n , see Bryden, 1965). Thus, despite a l a r g e n u m b e rof potentially relevant existing studies, it is not yet k n o w nw h e t h e r l e f t - h a n d e r s h a v e a greater v ar iance in perceptuala s y m m e t r y t h a n r i g h t - h a n d e r s , w h i c h w o u l d b e consis tentw i t h the hypothesis that lef t -handers hav e a greater v ar iabi l -it y in h e m i s p h e r i c a s y m m e t r y t h a n r ig h t- h a nders . O f th efour possibili t ies depicted in F i g u r e 1, it is not yet possibleto decide between the thi rd a l ternat iv e ( that the two dis t r i -b u t i o n s fo r left- a nd r ig h t- h a nders h a v e dif f erent m e a n s b u tthe same v ar iance; see Figure I c ) and the fourth a l ternat iv e( t h a t the two dis t r ibut ions hav e both di fferent means andd iffe re nt v ar iances; see Figure I d ) .

To my knowledge, there has been only one s t u d y , re-ported by B r y d e n (1965), that compared v ar iances of per-c e p t u a l a s y m m e t r y in left- a n d r ight -handers . This s tudy,u s i n g a v erbal d ichot ic l i s tenin g task, found a s ig ni f ica nt lygreater v ar iance for lef t -handers than for r ight -handers , con-s is tent w i t h the hypothesis that lef t -handers hav e greaterv a r i a b i l i t y in h e m i s p h e r i c a s y m m e t r y t h a n r i g h t - h a n d e r s .Thus, pre l iminary ev idence indicates that d is t r ibut ions ofh e m i s p h e r i c a s y m m e t r y i n left- and r ight -handers have n otonly different means but a lso di fferent v ar iances . This i salso suggested by the fo l low ing theoret ical i l lus t ra t ion of-fered by Segalowitz and Bryden (1983). If we arbitrarilya s s i g n v a l u e s of 1 to left h e m i s p h e r e , -1 to r ight hemi-sphere, and 0 to bilateral control of speech to, for e x a m p l e ,R a s m u s s e n a n d M i l n e r ' s (1975) f i n d i n g s , r i g h t - h a n d e r s w i l ly i e l d a m e a n v a l u e of .92, w i t h a s tandard dev iat ion of .39,a n d l e f t - h a n d e r s w i l l yie ld a mean v alue of .55, w i t h a

standard dev iat ion of .74 ( c f . S e g a l o w i t z & B r y d e n , 1 9 8 3 ) .T h u s , r i g h t - h a n d e r s m a y h a v e greater m e a n h e m i s p h e r i ca s y m m e t r y t h a n l e f t - h a n d e r s , w h e r e a s l e f t - h a n d e r s m a yh a v e g r e a t e r v a r i a b i l i t y in h e m i s p h e r ic a s y m m e t r y t h a nright -handers .The p r i m a r y goal of the present s tudy was to i n v e s t i g a t e

whether lef t -handers hav e a greater v ar iance in perceptuala s y m m e t r y t h a n r i g h t - h a n d e r s , w h i c h w o u l d b e c o n s i s t e n tw i t h th e hypothesis that lef t -handers hav e a greater v ar iancein hemispheric asymmetry than right-handers. A prel imi-na ry rev iew of potent ia l ly re lev ant pr ior l i tera ture rev ealedth a t in a subs tant ia l num ber of these s tudies , v ar iance datawere e i ther d i rect ly reported in the text ( though not s ta t i s -t ic a l ly tes ted) or could be calculated from other resul t sl i s ted. Thus, i t was determined that the quest ion of v ar iancedifferences between lef t -handers and r ight -handers could beaddressed by reanalyz ing av ai lable data from prior s tudiesrather than b y p e r f o r m i n g a n o t h e r n o t - s o - n e w e x p e r i m e n t .Resul ts obtained from reanalyz ing pr ior s tudies were thenintegrated across s t u d i e s t h r o u g h a q u a n t i t a t i v e m e t a - a n a -lytic procedure. In addition, mean differences reported inthe l i tera ture were a lso tabulated and then integrated acrosss t u d ie s , u s i n g q u a n t i t a t i v e m e t a - a n a l y s i s . A l t h o u g h it isk n o w n t h a t l e f t -h a n d e r s t y p i c a l l y y i e l d a w e a k e r m e a n p e r -c e p t u a l a s y m m e t r y t h a n r i g h t - h a n d e r s ( f o r a r e v i e w , s e eA n n e t t , 1982), a m e t a - a n a l y s i s o f th is f i n d i n g h a s n o t y e tbeen carried out.F i n a l l y , th e present s tudy addressed th e possible rele-v a n c e o f fam il ia l s inis t ra l i ty (FS) , defined as the incidenc eof one or more lef t -handers among f i rs t -order (and some-t imes second-order ) re la tiv es , in determ ining hem isphericasymmetry, especially in left-handers. On the premise thatF S prov ides a n i n d e x of the likelihood that a l e f t - h a n d e r ha sa genet ic predisposi t ion to lef t -handedness and related pat-terns of s i n i s t r a l h e m i s p h e r i c a s y m m e t r y , l e f t- h a n d e r s w i t hlef t- h a nded re la t iv es (FS + ) may include more cases w h o s eh e m i s p h e r i c a s y m m e t r y w a s d e t e r m i n e d b y g e n e t i c f a c t o r sth a n w i l l l e f t- h a n d e r s w i t h o u t l e f t - h a n d e d r e l a t iv e s (FS-).N u m e r o u s s tudies hav e been conducted comparing th em e a n p e r c e p t u a l a s y m m e t r y of FS+ and FS- lef t -handers ,w i t h li t t le a g r e e m e n t a m o n g s t u d i e s ( f o r a r e v i e w , s e eA n n e t t , 1982; Bryden, 1988b; McKeev er & V a n D e v e n t e r ,1977). Thus, there appear to be at best weak differencesb e t w e e n FS+ and FS- l e f t - h a n d e r s in m e a n h e m i s p h e r i ca s y m m e t r y . H o w e v e r , i t h a s n o t b e e n d e t e r m i n e d w h e t h e rthe two subgroups differ in the v ar iance of hemispherica s y m m e t r y . S i n i s t r a l p a t te r n s o f h e m i s p h e r ic a s y m m e t r yd e t er m i n e d b y e n v i r o n m e n t a l f a c to r s m a y b e m o r e v a r i a b l ethan those determined by genet ic factors , ref lect ing a mul-t i tude of env i ronmental factors that could potent ia l ly affectlateralization of cerebral funct ions , fo r e x a m p l e , h i g h f e t a ltestosterone lev els , bi r th stress, ear ly brain insul t , chancefactors during the course of dev elopment , and so for th. I fthis is the case, FS- left-handers may have greater variancein h e m i sp h e ri c a s y m m e t r y t h a n FS+ lef t -handers .T h e p r e s e n t s t u d y m a d e u s e o f q u a n t i t a t i v e m e t a - a n a l y t i cprocedures. In a quant i ta t iv e meta-analys is , s ta t i s t ical pro-cedures are applied to a collection of empir ical f indingsf r o m i n d i v i d u a l s t u d i e s for the purpose of i n t e g r a t i n g t h e m


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HEMISPHERIC ASYMMETRY IN LEFT- AND R I G H T - H A N D E R S 151(Wolf, 1986). Glass, McGaw, and Smith (1981) noted thatthe findings of multiple studies should be regarded as acomplex data set, no more comprehensible without statisti-cal analysis than hundreds of data points in one study.Quantitative meta-analysis is becoming increasingly popu-lar as a technique for evaluating studies within a given areaof research, including laterality studies (e.g., Boles, 1984;Bryden, 1987, 1988b; Kim & Levine, 199Ib; Marzi, Bisi-acchi, & Nicoletti, 1991). An important feature of meta-analysis is its high statistical power relative to individualtests of significance, which is, roughly speaking, due to theincreased sample size. This feature of a meta-analytic pro-cedure is particularly relevant for the present purpose. Asdiscussed, evidence indicates that perceptual asymmetryreflects not only hemispheric asymmetry but also otherextraneous variables, such as attentional biases, sensorypathway dominance, random errors, and so forth (for re-views, see Kim & Levine, 1991b; Segalowitz, 1987; Tzeng& Hung, 1985). Thus, a proportion of between-subjectsvariance in perceptual asymmetry stems from factors otherthan individual variations in hemispheric asymmetry. More-over, patterns of hemispheric asymmetry in the majority ofleft-handers do not differ from those of a typical right-hander (e.g., Rasmussen & Milner, 1975), suggesting anextensive overlap between distributions of hemisphericasymmetry in the two groups. Thus, to f ind a statisticaldifference between left- and right-handers, especially on thebasis of perceptual asymmetry, high statistical power isnecessary (Berenbaum & Harshman, 1980; Harshman &Lundy, 1988; McManus, 1984; Segalowitz & Bryden,1983). Insofar as n o n s i g n i f i c a n t differences between left-handers and right-handers in individual studies may beattributable to Type II errors (i.e., errors of incorrectlyr e t a i n i n g a false n u l l hypothesis) because of low statisticalpower, a meta-analysis may provide an opportunity to ex-amine the d i f f e r e n c e s w ith su f f ic ie n t statistical power.

F ratio:(1 )

w h e r e S\ is the var iance for lef t-handers and 5r is the var iancefor r ight-handers. T he F ratio also w as used as an effect sizeestimator for the variance differences. Thus, an F ratio value of ,say , 2, w ou l d ind ica te that sf is two times greater than sf The mean differences between left-handers an d right-handers inperceptual asy mm etry were tested w i t h th e f ol l ow ing / ratio:

Mr-M, (2)

where M r is the mean fo r right-handers, M, is the mean fo rleft-handers, nr is the num ber of r ight-handers, n, is the number ofleft-handers , and the other terms are as defined fo r Equation 1 .N o te that the standard error of differences is estimated differentlyfrom th e usual / ratio. This adjustment w as made because th epresent review indicated that S | and Sf are unequal and the usualstandard error of difference is biased with unequal var iances. Thedegrees of freedom used to test the t ratio also were adjusted forunequal variances (for specifics of this procedure, se e Hinkle,W i e r s m a , & Jurs, 1988, p. 2 5 1 ) .The effect size estimator for the mean differences, d, w asdefined as f ol l ow s:

d = M r - M,SD r (3)where SD r is the standard deviation fo r right-handers and the otherte r ms are as previously defined. Thus, a d value of , say, 1, w o u l dindicate that th e m e a n s for right-handers and lef t-handers are onestandard deviation of the r ight-handers ' distr ibutio n apart in thedirection of the expected difference (i.e., a greater mean for right-h and e r s t ha n fo r lef t-handers).

MethodData Set

Included in the present review were those prior studies thatcompared th e p e r ce p tu al asymme tr y of lef t-handers a n d r i g h t -handers under identical conditions and in w h i c h th e var iance d a tafo r each han dedn es s group w ere either directly reported in the textor could be calculated from other results listed. A total of 22studies meeting these cr iter ia were located from the issues of thef ol low i ng jou r nal s p u bl ish e d from 1965 to 1992: Brain and Cog-nition, Brain and Language, Cortex, and Neuropsvchologia. Thesestudies provided 28 experiments, which constituted the data set forth e p r e se nt r e vie w . A total of 2,192 subjects ( 1 , 1 1 2 right-handersa nd 1,080 left-handers) participated in the e x p e r ime nts . B e cau sedata for male and female subjects could not be calculated sepa-rately for most experiments, the data were collapsed over sex.

Analyses of Individual StudiesT he variance differences between lef t-handers an d r igh t-handers in p e r cep tu al asym me tr y w e r e te sted w i th th e f ol l ow ing

Meta-AnalysesT he me ta-ana lytic procedure used in the present study w a sStouffer 's Z method. This method requires adding the standardn o rm a l deviates (zs) associated w i t h the one-tailed ps obtained foreach s tudy a nd d i v i d i n g that s um b y the square root of the n u m b e rof studies (N ) be ing combine d :

z = (4)The resulti ng Z is referred to the standard norm al table for one-tailed significance testing (Rosenthal, 1984).Meta-analyses were run separately for the following categoriesof studies: verbal divided v i s ua l field studies, verbal dichoticl is tening studies, and free-vision face studies. There were too fewn o n v e rb a l divided visual studies or nonverbal dichotic listeningstudies to apply a meta-analytic procedure. F or these studies,i n di v i dua l r e su l ts onl y ar e considered.I n c l udi n g m u lt i p le results from th e same subjects in a me ta-analysis could inflate th e sample size of statistical tests an d effectsbe yond th e n u m b e r of independent studies (Rosenthal , 1984). Inthe present m eta-analyse s, six studies reported more than one set of


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152 H O N G K E U N K I Mresul ts re levant to the p re se nt me ta-anal yse s by a dm in is te r in gm ul t ipl e laterality tasks to the same subjects. These studies we rehandled by f irst a v e r a g i n g th e m ul t ipl e results an d e nte r ing th eave rage d value into th e me ta-anal ys is . H o w e v e r , s om e m u lt i p leresults from th e same subjects are included in two or m ore differ-en t m e t a - a n a l y s e s . For e x a m p l e , tw o sets of results from th e s a m esubjects, one for a ve rbal div ide d visual field task and the oth e r fo ra ve rbal dichotic l is tening task, are included in those tw o separatem et a- an alys es .

ResultsD ivided V isual Field Studies

Nine s tudies included in the meta-analyses of v erbaldiv ided v isual f ie ld s tudies a re listed in chronological orderin Table 1. These studies used letters, nonsense syllables, orw o r d s as s t i m u l i . F or each s tudy, Table 1 l i s t s numbers ,standard deviations, a nd m e a n s fo r r ight -handers a nd left-handers , respect iv ely , wi th one-tailed ps for the v ar iancedifference a nd one-tailed ps for the mean di fference.L eft -handers showed a greater v ar iance in v isual f ie lda s y m m e t r y t h a n r i g h t - h a n d e r s in e i g h t of nine s tudies(89%). Reflect ing this d i fference , th e Stouffer 's Z for thev ar iance di fference was s i g n i f i c a n t l y p o s i ti v e (Z = 3 .815,p < .0001). T he m e d i a n v a l u e of F w as 1.634; t h a t is, onaverage, the variance of left-handers was about 1.6 timesgreater than th e v ar iance of r ight -handers .R i g h t - h a n d e r s ha d a greater mean v isual f ie ld asymmetry( i .e . , a greater mean r ight v isual f ie ld adv antage) than left-handers in sev en of n i n e s t u d i e s (78%). Reflect ing thisdifferenc e, th e S t o u f f e r ' s Z for the mean di fference w ass ig nif ic a ntly positive (Z = 2.140, p < .05). The medianv a l u e of d w as .225; that is, on av erage , th e m e a n s fo rlef t -handers a nd r ight -handers were about one-fourth of as tandard dev iat ion of the r ight-handers ' dis t r ibut ion apar t .In summary, meta-analyses of v erbal d iv ided v isual f ie lds tudies indicate that d is t r ibut ions of v isual f ie ld asy mm etryfo r v erbal tasks in lef t -handers and r ight -handers hav e bothdifferent m e a n s a n d different v ar iances . Rig ht -handersh a v e a greater mean r ight v isual field a d v a n t a g e t h a n left-

handers , whereas lef t -handers hav e a g r e a te r v a r i a n c e th a nright -handers . This pat tern i s i l lus t ra ted in Figure I d .O n l y t w o n o n v e r b a l d i v i d e d v i s u a l field s t u d i e s ( H e l l e r &L e v y , 1981; Kim et al . , 1990) were av ai lable . Both s tudiesused photographs of faces as s t i m u l i . In H e l l e r an d L e v y ' s( 1 9 8 1 ) s tudy, lef t -handers had a greater v ar iance than r ight -handers , whereas in Kim et al.'s (1990) s t u d y , th e reversewas t rue . In nei ther case, howev er , was the v ar iance differ-ence s ta t i s t ical ly s ig ni f ican t . In both s tudies , r ight -han dershad a greater mean than lef t -handers ( i .e . , a greater leftv i s u a l f ie ld adv antage for r ight -handers ) . The mean differ-ence was s ta t i s t ical ly s igni f icant only in Kirn et a l . ' s ( 1990 )s t u d y (p < .05).Dichotic Listening Studies

Thir teen s tudies included in the meta-analyses of v e r b a ldichot ic l i s tening s tudies are listed in chronological order inTable 2, w i t h associated statist ics for each s t u d y . Theses tudies used dig i t s , nonsense syl lables , or w o r d s a s s t i m u l i .L eft -handers showed a greater v ar iance in ear a s y m m e t r ythan r igh t -hande rs in 10 of 13 s tudies ( 77% ). Reflect ing thisdi fference , th e S t o u f f e r ' s Z for the v ar iance di fference w a ss i g n i f i c a n t l y posi t iv e (Z = 3.900, p < .0001). The m e d i a nv a l u e of F w as 1.243.

Right-handers had a greater mean ear a s y m m e t r y (i.e., agreater r ight ear a d v a n t a g e ) t h a n l e f t - h a n d e r s in 13 of 13studies (100%). Reflect ing this d i fference , th e Stouffer 's Zfo r th e m e a n d i f f e r e n c e w a s s i g n i f i c a n t l y p o s i t i v e ( Z =9 . 1 1 4 , p < .0001). T he m e d i a n v a l u e of d w a s .503.In s u m m a r y , c on s i s te n t w i t h r e s u l ts o f m e t a - a n a l y s e s o fv e r b a l d i v i d e d v i s u a l f ield s tudies , meta-analyses of v erbaldichot ic l i s tening s tudies indicate that r ight -handers hav e agreater mean r ight ea r a d v a n t a g e t h a n l e f t -h a n d e r s , w h e r e a slef t -handers hav e greater v ar iance than r ight -handers ( seeF i g u r e I d ) .O n l y on e nonv erbal d ichot ic l i s tening s tudy (Curry , 1967)w a s a v a i l a b l e . In th is s t u d y , i n w h i c h e n v i r o n m e n t a l s o u n d swere used a s s t i m u l i , r i g h t - h a n d e r s ha d both greater v ar i -ance and a greater mean (i .e., a greater left e ar a d v a n t a g e )

Table 1Verbal Divided Visual Field Studies: Mean Perceptual Asymmetries and StandardDeviations for Right-Handers (RH) and Left-Handers (LH)

nStudyB r yden (1965)O rbach (1967)M cKe e ve r , V an De ve nte r ,& Suberi (1973)M cKe e ve r & Van De ve nte r(1977)B r ads haw, Gates,& Nettleton (1977)H a u n (1978)Bradshaw & Taylor (1979)Dagenbach (1986)K i m , L e v i n e , & Ke rte sz (1990)

RH20254880241224503 1

LH202124712412481243 2

RH2.5003.0707.6037.059

19.2026.20723.3281.2904.530

SDLH2.9503.7005.300

7.94146.84710.2383 3 . 3 2 11.6505.790

P.239.190.968.155.000.056.033.026.091

RH0.7253.1603.0002.613

13.0009.80030.0002.4502.350

MLH-0.2001.0951.290

2.1278.0008.93015.5002.5403.060

P.146.024.136.347.316.402.018.649.705


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H EMI SP H ERI C A SY MMETRY I N LEF T- A ND RI G H T- H A ND ERS 153Table 2Verbal Dichotic Listening Studies: Mean Perceptual Asymmetries an d StandardDeviations for Right-Handers (RH) and Left-Handers (LH)

nStudy

B r yden (1965)Curry (1967)Curry & Rutherford (1967)Satz, Achenbach, Patt ishall ,& F e n n e l l (1967)K nox & Boone (1970)B r i g g s & N e b e s ( 1 9 7 6 )M c K e e v e r & V an D ev en t er(1977)Fennell , Satz, V an D en Abell ,Bowers, & Thomas (1978)H u gd ah l & Andersson (1984)O r s in i , Satz, Soper,& Light (1985)D agen b ach ( 1986)B r yd e n ( I 9 8 8 a )O brzut , Conrad, Bryden,& Boliek (1988)

RH202512521 14080262021 550

21 915

LH202512411 14071422025 7

1247815

RH0.2105.6452.21022.2798.3007.0029.770

10.20020.7530.217

1.3900.7890.240

SDLH0.4505.6002.50022.2968.1006.99410.216

12.15023.0520.244

1.8600.8800.684

P.001.515.345.493.530.503.349.178.326.036.011.113.000

RH0.1105.24010.28023.480

54.2283.2757.08016.30020.0000.187

1.2000.5540.363

MLH

-0.0902.4004.53012.64047.2570.7503.64514.400

-10.0000.1250.5800.134-0.452

P.041.040.000.011.030.055.019.245.000.002.009.000.000

than left-handers. However, neither of these differences wasstatistically significant.Free-Vision Laterality Studies

The three studies included in the meta-analyses of free-vision laterality studies ar e listed in chronological order inTable 3, with associated statistics for each study. Thesestudies used the free-vision chimeric face task developed byLevy, Hel ler , Banich, and Burton (1983b) or a s imilarface-processing task. This task involves judgin g w hich ofth e tw o mirror-imaged chimeric faces, on e w i t h a smile tothe subject's left or one w i t h a smile to the subject's right,looks happier. Subjects as a group show a strong bias tochoose th e face with th e smile to their left as lookinghappier (for details of this task, se e L e v y et al., 1983b).Left-handers showed a greater variance in perceptuala s y m m e t r y for this task than right-handers in all studies.Reflecting this difference, th e Stouffer ' s Z for the variancedifference w as signif icantly posit ive ( Z = 1.809, p < .05).The median value of F w as 1.360.Right-handers had a greater mean perceptual asymmetry(i.e., a greater leftward bias) fo r this task than left-handers

in all three studies. Reflecting this difference, the Stouffer'sZ for the mean difference was s ig nif icantly positive ( Z =3.494, p < .0001). The median value of d w as .384.In summary, consistent with the results of the meta-analyses of verbal divided visual field studies as w e l l as ofverbal dichotic listening studies, the meta-analyses of non-verbal free-vision face studies indicate that right-handershave a greater mean leftward bias than left-handers, whereasleft-handers have greater variance than right-handers (seeFigure I d ) .Familial Sinistrality

In 6 of 22 studies, it was possible to compare means andvariances in v i s u a l field or ear asymmetries for FS+ right-handers and FS- right-handers, and in 8 of 22 studies, it waspossible to compare means and variances for FS+ left-handers and FS- left-handers. These studies are listed inTable 4, w i t h associated statistics for each s t u d y . Half ofthese studies were verbal divided visual field studies and theother half were verbal dichotic l i s tening studies. Because ofthe re l a t iv el y small number of studies available, dividedvisual field and dichotic listening studies were analyzed

Table 3Free-Vision Face Studies: Mean Perceptual Asymmetries and Standard Deviationsfor Right-Handers (RH) and Left-Handers (LH)

StudyL e v y , Heller , Banich,& B ur t o n (1 9 8 3 b)H op tman & L e v y (1988)K i m , Levine, & Kertesz (1990)

nRH1 1 1403 1

LH1 1 1403 2

RH.440.558.548

SDLH

.499

.654.639

P.094.163.201

RH.303.354.259

MLH.134

-.027.157

P.004.003.249


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154 H O N G K E U N K IMTable 4Mean Perceptual Asymmetries and Standard Deviations for Subjects With No FamilvHistory of Sinistratify (FS-) and Subjects With Such History (FS+)

nStudy FS- FS + FS-

SDFS +

MP FS- FS+ P

R igh t-h and e r sVerbal divided visual field studiesM cKe e ve r , V an Deventer,& Suberi ( 1973)M c K e e v e r & V an D ev en t er(1977)K i m , Lev i n e, & Kertesz (1990)Verbal dichotic l is tening studiesM c K e e v e r & V an D e ve nte r(1 9 7 7 )O r s i n i , Satz, Soper,& Li ght (1985)B r yd e n (!988a)

24442244

145143

24369

367076

7.9517.3884.0079.3600.2400.800

7.2406.7115.069

10.3790.1600.770

.328

.281

.816

.742

.000

.361

4.0903.1301.3646.9000.1800.520

11470

.910

.980

.778

.300

.2000.619

.837

.766

.048

.429

.235

.186Left-handersVerbal divided visual field studies

B r yden (1965)M c K e e v e r & V an D e ve nte r(1 9 7 7 )Bradshaw & Taylo r ( 1979)K i m , L e vine , & Kertesz (1990)Verbal dichotic l istening studiesBry de n (1965)McK e e ve r & V an D e ve nte r(1977)O r s i n i , Satz, Soper,& Li ght (1985)B r y d e n ( 1988a)

163 424191634

141143

43 7241343 7

11 676

2.5007.02739.1926.780

0.3701 1 .6620.2700.800

0.9008.50726.9544 . 1 3 0

0.7208.5870.2100.770

.059.865

.035.042

.967.037

.003

.361

0.4400.4701 2.0003.4200.0105.0000.1200.520

-13192 ,-0

.750.650

.000.530

.3602.40000

.130

.619

.994.045

.237.676

.808.854

.369

.186

together in the first meta-analyses. When significant effectsemerged in the firs t analyses, the effects were then sepa-rately analyzed fo r divided v i s u a l f i e ld studies a n d dichoticl i s t e n i n g studies.

Meta-analyses of right-handers showed that FS+ and FS-right-handers did not significantly differ either in meanperceptual asymmetry (Z = .714, ns) or in the variance ofperceptual asymmetry (Z = 1.427, ns). Meta-analyses ofl ef t -handers showed that FS+ and FS- left-handers did notsignificantly differ in mean perceptual asymmetry (Z =1.236, ns). However, FS- left-handers had a s i g n i f i c a n t l ygreater variance of perceptual asymmetry than FS+ left-handers (Z = 2.504, p < .01). In six of eight studies (75%),the variance for FS- left-handers was greater than thevariance for FS+ left-handers. When divided visual fieldstudies and dichotic listening studies were analyzed sepa-rately, the effect was significant for divided visual studies(Z = 2.002, p < .05) and approached statistical significancefo r dichotic listening studies (Z = 1.539, p < .10).

The greater variance for FS- than FS + left-handers sug-gests that the greater variance found for left- than forright-handers may involve FS- left-handers more than FS +left-handers. Consistent with this hypothesis, FS- left-handers had a significantly greater variance than right-handers (Z = 3.904, p < .0001), whereas no significantd i f f e r e n c e in variance emerged between FS+ left-handersand right-handers (Z = . 8 1 1 , ns).

DiscussionThe present study addressed the question of whetherd i s t r i b u t i o n s of perceptual asymmetry in left-handers and

right-handers di f fer in the mean or in the variance or both.Meta-analyses of 28 experiments collected from the litera-ture support the hypothesis that distributions of perceptualasymmetry in lef t- a n d right-handers di f fer both in the meanand in the variance. Whereas right-handers had greatermean perceptual asymmetry than left-handers, left-handershad greater variance around the mean than right-handers.This was true for a variety of laterality tasks: divided visualfield tasks, dichotic l istening tasks, and a free-vision facetask. Of the 28 experiments considered, 26 (93%) yieldedgreater mean asymmetry for right-handers, 22 (79%)yielded greater variance for left-handers, and 20 (71%)yielded both a greater mean for right-handers and greatervariance for left-handers.

Perceptual asymmetry has been shown to be related,albeit not perfectly, to patterns of hemispheric asymmetry asmeasured by sodium amytal testing or other neurologicalprocedures (e.g., Ge ffe n & Caudrey, 1981; Hugdahl &Wester, 1992; Kimura, 1961; Strauss, 1988; Zatorre, 1989).Although evidence indicates that perceptual asymmetry re-flects not only hemispheric asymmetry but also other extra-neous variables, such as attentional biases, sensory pathwaydominance, random errors, and so forth (Hellige, Bloch, &


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H E M I S P H E R I C A S Y M M E T R Y I N LEFT- A N D R I G H T -H A N D E R S 155Taylor, 1988; K im & Levine , 1991b; Segalowitz, 1987;Tzeng & Hung, 1985), these extraneous variables m a y n o tdifferentially involve lef t-handers and r ight-handers (Harsh-m an & L u n d y , 1988; Kim et al, 1990; Segalowitz, 1987).Thus, the present pattern of results, that is, the greater meanperceptual asymmetry for right-handers and greater vari-ance fo r left-handers, is consistent with th e hypothesis thatr ight-handers have greater mean hemispher ic asymmetrythan left-handers, whereas left-handers have greater vari-ability in hemispher ic asymmetry around the mean thanr ight-handers .Another important finding in the present review is thatF S- left-handers had greater variance in perceptual asym-metry than F S + r ight-handers , sugges t ing a greater vari-ability in hemispheric asymmetry for FS- left-handers.Also, the greater v ar iance in perceptual asym metry for left-than fo r right-handers w a s d u e more to FS- left-handersthan to FS+ right-handers. Relative to FS + r ight-handers ,F S - left-handers m ay have included more cases in w h i c hhandedness and sinistral patterns of hemispheric asymmetryw ere determined by env ironmen tal factors. O n the basis ofthis premise, th e greater variability in hemispheric asym-metry for FS- lef t-handers sugges ts that , at least w i t h i nleft -handers, patterns of hemispher ic asym metry determinedby environmental factors ar e more variable than those de -t e r m i n e d by genetic factors. The greater heterogeneity inpatterns of h e m i s p h e ri c a s y m m e t ry a m on g F S - left-handersm ay reflect a m u l t i t u d e of environmental factors thatcould potentially induce sinistral patterns of hemispherica s y m m e t r y , such as high fetal testosterone levels (e.g.,Geschwind & Galaburda, 1987), birth stress (e.g. , Bakan,1971), early brain insult (e.g., Satz, 1973), chance factorsdur ing the course of development (e.g. , Annett, 1972), andso for th.Bishop (1990a, 1990b) rece ntly argued th at it may befutile to subclass ify lef t-handers into FS+ and FS typesbecause famil ia l s inis tra l i ty is an extremely weak indicatorof w h e t h e r or not a person's handedness or hemispher icasymmetry is influenced by genetic factors. However, th egreater variance for FS- t h a n F S + left-handers shows thather conclus ions may be premature . Al though famil ia l s in-istrality m a y n ot be a perfect, or even strong, indicator of aleft -hander 's genetic predisposition to left-handedness, ac-cording to the present f indings , the difference between FS 4 -an d FS- lef t-handers in the proportion of so-called geneticleft-handers is large enough to be statistically significant.Fur thermore , the current genetic models of handedness andhemispher ic asymmetry of ten postula te equal var iance fo rdistributions with and without a genetic bias (cf. Bishop,1990a). This assumption m a y b e in error g iven that F S +and FS- dis tr ibutions differ in var iance .A l t ho u g h the greater var iance in perceptual asymmetryfor left-handers is interpreted here as reflecting greatervariability in hemispheric asymmetry, alternative interpre-tations are, of course, possible. For instance, it is often thecase that subjects classified as right-handers typically in-clude only strong right-handers, whereas subjects classifiedas left-handers include not only strong left-handers but alsow e a k or mixed lef t-handers . According to this view, the

greater variance in perceptual asymmetry fo r left- than fo rright-handers m ay also reflect this factor. However, thisview would predict equal variances in perceptual asymme-try fo r left- and r ight-handers in studies testing only strongleft-handers and strong right-handers. In the present review,there were 11 experiments that tested only strong left-handers and strong right-handers, and of these, 9 ( 8 2 % )yielded greater variance for left-handers. Thus, the greatervariance fo r left- than fo r r ight-handers is independent ofsome samples of left-handers that include both strong an dweak lef t-handers .Another explanation for the greater variance fo r left-handers may be that it is a methodological nuisance.From this point of view, the heterogeneity of var ianceshould be eliminated by data transformation (e.g., logtransformation) rather than be a subject of systematic in-ves t igat ion. However , in a great major i ty of studies re -v i e w e d , th e var iance differences were in the expected di-rection, that is , greater variance for left-handers, and inno case was the var iance for r ight-hand ers s ign if icantlygreater than the variance for left-handers. These system-atic variance differences across studies are inconsistentwith the view that they stem from methodological nui-sance. A s B r y k an d R a u d e n b u s h (1988, p. 402) cogentlyargued, "The practice of routinely searching for datat ransformation that wil l el iminate heterogeneity is mis-guided, because it could lead to a failure to recognize thesubstantive significance of heterogeneity." The substan-tive significance of the present heterogeneity of varianceis that it reflects greater variability in hemispher ic asym-metry fo r left- than for right-handers.In passing, prior s tudies have routinely violated thea s s u m p t i on of h om og e n e i t y of v a r i a n c e i n th e i r t e st i n g ofm e a n p e r c e p t u a l a s y m m e t r y d i f f e re n c e s be t w e e n left-h a n d e r s a n d r i g h t -h a n d e r s . T e c h n i c a l l y , u n l e s s t h e v a r i -a n c e d i f f e r e n c e be t w e e n tw o groups reaches s ta t is t icals i g n i f i c a n c e , i t may not be a ser ious er ror to tes t the meand i f fe r e n c e in a u s u a l m a n n e r ( f or s t a t i s t i c a l m e a n s oftesting mean differences with unequal variances, see, fore x a m p l e , H i n k l e et al. , 1988, p. 2 5 1 ) . How e v e r , th ep r e s e n t m e t a -a n a l y s i s s h ow e d t h a t t h e v a r i a n c e s f or left-a n d r i g h t -h a n d e r s a r e s i g n i f i c a n t l y d i f f e r e n t , p r e s u m a bl yr e f l e c t i n g g r e a t e r v a r i a b i l i t y in h e m i s p h e r i c a s y m m e t r yfo r left- t h a n for right-handers. T h u s , i t may be safer nott o a s s u m e e q u a l v a r i a n c e s , e v e n w h e n t h e v a r i a n c e d i f -fe r e n c e be t w e e n the tw o g r o u p s does n ot reach s ta t is t icals i g n i f i c a n c e .

A l t h o u g h the p r e s e n t r e v i e w w a s i n t e n d e d to e x a m i n edistributions of hemispheric a s y m m e t r y fo r both left-h e m i s p h e r e s p e c i a l i ze d v e r ba l t a sk s a n d r i g h t -h e m i s p h e r es p e c i a l i z e d n on v e r ba l t a s k s , t h e r e w e r e t oo f e w n on v e r -b al s tudies , except fo r f ree-vis ion s tudies , to apply am e t a -a n a l y t i c a l p r oc e d u re . A s B r y d e n (198 7 ) noted, it iseasy to assume, but not necessar i ly t rue , that " the f u n c -t i o n s of the r ight hemisphere are re la ted to handedness inm u c h th e same w a y that language functions are , but inreverse" ( p . 62). Thus , the p r e s e n t f i n d i n g t h a t d i s t r i bu -t i on s of h e m i s p h e r i c a s y m m e t r y i n left- a n d r i g h t -h a n d e r sdiffer in bot h m e a n a n d var iance is g e n e r a l i z a bl e m or e to


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156 H O N G K E U N K IMd i s t r i b u t i o n s o f h e m i s p h e r i c a s y m m e t r y f o r v e r b a l f u n c -t ions than to d i s t r i b u t i o n s o f h e m i s p h e r i c a s y m m e t r y fo rn o n v e r b a l f u n c t i o n s . N o n e t h e l e s s , i t i s n o t a b l e t h a t t h epresent m e t a - a n a l y s e s o f n o n v e r b a l f r e e - v i s i o n s t u d i e sy i e l d e d r e s u l t s c o m p a r a b l e to those of the v e r b a l l a t e r a l -it y s t u d i e s . T h a t i s , o n t h e n o n v e r b a l f a c e - p r o c e s s i n gt a s k , r i g h t - h a n d e r s s h o w e d greater m e a n p e r c e p t u a la s y m m e t r y (i.e., a greater l e f t w a r d b i a s ) t h a n l e f t - h a n d -ers, a n d l e f t - h a n d e r s showed greater v a r i a n c e t h a n r i g h t -h a n d e r s . A l t h o u g h f r e e - v i s i o n m o d a l i t y i s n o t a c o m m o nt y p e o f l a t e r a l i t y t a s k , i t h a s b e e n s h o w n t h a t p e r c e p t u a la s y m m e t r i e s o n f r e e - v i s i o n t a s k s a r e s i g n i f i c a n t l y corre-l a te d w i t h those o n d i v i d e d v i s u a l fi e l d t a s ks ( K i m &L e v i n e , 1 9 9 1 b ; K i m e t a l . , 1990; Levy et a l . , 1 9 8 3 a ) .T h u s , f u r t h e r s t u d i e s m a y i n d i c a t e t h a t d i s t r i b u t i o n s o fh e m i s p h e r i c a s y m m e t r y i n l ef t - a n d r i g h t - h a n d e r s d i f f e rin both m e a n a n d v a r i a n c e n o t o n l y f o r v e r b a l f u n c t i o n sb u t also f o r n o n v e r b a l f u n c t i o n s .A l t h o u g h t h e m a j o r i t y o f e x p e r i m e n t s i n c l u d e d in th epresent rev iew yie lded the expected pat terns of resul t s ( i .e . ,greater mean perceptual asymmetry for r ight -handers andgreater v ar iance for lef t -hande rs ) , the num be r of experi -ments in which these resul t s reached s ta t i s t ical s igni f icancew as r e l a t i v e l y s m a l l . In part icular , only 1 6 of 28 e x p e r i -m e n t s ( 5 7 % ) s h o w e d a s i g n i f i c a n t l y greater mean for r ight -handers at the p < .05 level (one-tailed), and only 8 of 28e x p e r im e n t s ( 2 9 % ) s h o w e d s i g n i f i c a n t l y g r e a te r v a r i a n c efo r lef t -handers a t the same p lev el . This i s not surpr is ingbecause high s ta t i s t ical power or a large sample m a y b enecessary to show a s ta t i s t ical d i fference between left-handers and r ight -hand ers (B eren bau m & Harsh ma n, 1980;Harshman, 1988; Segalowi tz & Bryden, 1983) . In f act ,a n a l y s e s of sample s iz e showed that , on av erage , thosee x p e r i m e n t s y i e l d i n g s i g n i f i c a n t differences h a d largersamples (M = 55.5 r ight -handers and 58.2 lef t -handers )than those y i e l d i n g n o n s i g n i f i c a n t d i f f e r e n c e s (M = 32.6r i g h t - h a n d e r s a n d 3 0 . 8 l e f t - h a n d e r s ) , s u g g e s t i n g t h a t as m a l l sample contr ibutes to a lack of s ta t i s t ical lys i g n i f i c a n t effects .It also is of some interes t that more s igni f icant meandifferences were found than s igni f icant v ar iance di ffer-ences . O ne possibi l i ty m a y b e that higher s ta t is t ical power isnecessary to demonstrate s ign i f icant v ar iance di fferencest h a n s i g n i f i c a n t m e a n d i f f e r e n c e s . A l t e r n a t i v e l y , a n d p e r-haps more pars imoniously , th is pat tern may be related to thefact that s tudies support ing the n u l l hypothesis are lessl ikel y to be p u b l i s h e d ( R o s e n t h a l , 1979). Because priors tudies hav e reported only mean di fferences between left-an d right -handers , fa i l ing to test v ar iance di fferences , thisp u b l i s h i n g bias, if i t exists, m ay also h a v e biased the presentsample of studies with respect to mean di fferences but notw i t h respect to v ar iance di fferences . As a result , the presentsample of studies m ay i n c l u d e more cases of s i g n i f i c a n tmean di fferences than s igni f icant v ar iance di fferences .In a n a l t e r n a t i v e approach to the present rev iew, Bryden(1987, 1988b) used th e fact that many s tudies report howmany lef t -handers or r ight -handers show a left or r ight earadv a ntage on dichot ic l i s ten ing tasks as the basis for re-

v i e w i n g that l i tera ture . For example , on av erage , about 82%of r ight -handers but only 64% of lef t -handers showed anexpected r ig ht ear adv antag e for v erbal d ichot ic l i s teningtasks (Bry den, 1987) . This f ind ing presuma bly ref lects d i f-ferences in cerebral organiz at ion between the two groups.H o w e v e r , i n essence, b y d i c h o t o m i z i n g a n e s s e n t i a l l y c o n -t i n u o u s v ar iable , th is approach does n o t take ful l a d v a n t a g eof i n f o r m a t i o n a b o u t h e m i s p h e r ic a s y m m e t r y c o n t a in e d inp e r c e p t u a l a s y m m e t r y . M o r e o v e r , a s B r y d e n ( 1 9 8 7 ) w a saware , c lass i f icat ion of subjects according to thei r d i rect ionof sensory f ield a d v a n t a g e c o u l d g i v e a false implicat ionthat subjects are b eing c lass i f ied according to thei r d i rect ionof h e m i s p h e r i c d o m i n a n c e . In r e a l i t y , a left e a r a d v a n t a g e ,fo r e x a m p l e , is not an i n d e x of r ig h t h e m i s p h e r e d o m i n a n c eb ut merely increases th e likelihood of r i g h t h e m i s p h e r i cdominance re la t iv e to a r ight ear adv antage (Satz , 1977;Speaks, 1988).M y f ina l c o m m e n t s c o n c e r n w h y nearly a ll p r e v i o u sl a teral i ty s tudies hav e fa i led to address the quest ion ofv ar iance di fferences between lef t -handers and r ight -hand-ers . This quest ion i s ra ther cur ious , par t icular ly because on eof the ear l ier s tudies , reported b y B r y d e n (1965), specifi-c a l ly addressed t h e q u e s t io n a n d f o u n d s i g n i f i c a n t l y g r e a t e rv a r i a n c e f o r l e f t - h a n d e r s . H o w e v e r , n e g l e c t of q u e s t i o n sr e g a r d i n g v a r i a n c e d i f f e r e n c e s or, more b r o a d l y , i n d i v i d u a ldifferenc es is not l i m i t e d to the la teral i ty l i tera ture b u toccurs in many o ther areas in experim ental psy chology( B r y k & R a u d e n b u s h , 1 9 8 8) . H i s t o r i c a l l y , t h e r e h a v e b e e ntw o t r a d i t i o n s in p s y c h o l o g y o n e t h a t h a s th e goal ofc h a r a c t e r i z i n g the modal human being and the o ther that hast h e g o a l o f c h a r a c t e r i z in g i n d i v i d u a l d i f fe r e n c e s ( C r o n b a c h ,1957). In the former , nomothet ic t radi t ion, i n d i v i d u a l dif-f e r e n c e s hav e often been v iewed "as a methodological nui -sance or an un w elco me obstacle in the pursu i t of inferencesa b o u t the effects of t reatments on means" ( B r y k & R a u d e n -bush, 1988, p. 396). The n e g l e c t of quest ions regardingv a r i a n c e d i f fe r e n c e s b e t w e e n left- a nd r ig h t- h a nders ma y bea n u n f o r t u n a t e l e g a c y o f t h i s t radi t ion ( see Kim & L ev ine ,in press) .In c o n c l u s i o n , in r e a d i n g th e la teral i ty l i tera ture , on e oftenencounters such s ta tements a s "L eft -handers a re less later-al iz ed than r ight -handers ." This s ta tement i s h i g h l y a m b i g -u o u s because i t could mean that each indiv idua l lef t -handedperson h as less different ia ted pat terns of h e m i s p h e r i c a s y m -metry than each i n d i v i d u a l r ig h t- h a nded person, or it couldmean that lef t -handers a re more v ar iable in thei r la teral iz a-t ion than r ight -handers , or i t could mean something e lse( A n n e t t , 1982; Colbourn, 1978; Harshman, 1988; M c -M a n u s , 1983). In fact , when hemispheric asymmetry i sconceptual iz ed as a con t inuou sly dis t ributed v a r iable , as i tshould be, it does not m a k e a great deal of sense to askwhether one group of i n d i v i d u a l s is more or less la teral iz edthan another group. Ins tead, on e should b e a s k i n g w h e t h e rdis t r ibut ions of hemispheric asymmetry in the two groupsd iffe r in the mean, in the variance, or in both. The presents tudy a l lows speci f ic answers to such quest ions regardingleft-handers an d right -handers . That is , r ight -handers hav egreater mean hemispheric asy mm etry than lef t-handers ,whereas lef t -handers hav e greater v ar iance in hemispheric


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H E M I S P H E R I C A S Y M M E T R Y I N LEFT- A N D R I G H T - H A N D E R S 157asymmetry than right-handers. FS- left-handers havegreater variance in hemispheric asymmetry t h a n FS+ left-handers, suggesting that sinistral patterns of hemisphericasymmetry determined by environmental factors may bemore variable than those determined by genetic factors. Tothe extent that meta-analyses are less subject to samplingerror, measurement error, low statistical power, and otherartifacts than are i n d i v i d u a l studies (Schmidt, 1992), thepresent f i n d i n g s represent a reliable characterization of dis-tributions of hemispheric asymmetry in left- and right-handers.

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