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Semrud, CM., & Hynd, G.W, (1990). Right hemi-

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Suberi, M., & McKeever, W.F. (1977). Differentialright hemispheric memory storage of emo-tional and non-emotional faces. Neuropsycholo-gia, 15, 757-768.

Tucker, D.M., Watson, R.T., & Heilman, K.M.(1977). Discrimination and evocation of affec-tively intoned speech in patients with right pa-rietal disease. Neurology, 27, 947-958.

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Integration of Information Between theCerebral HemispheresMarie T. Banich^The Beckman Institute and Department of Psychology, University of Illinois atUrbana-Champaign, Urbana, Illinois

Despite 30 years of researchclearly demonstrating fhe comple-mentary functions of the cerebralhemispheres, we have little infor-

;!Recommended Reading

1, M.T. (1995). Interhemi-spheric processing: Theoretical

• and empirical considerations. InI RJ. Davidson & K. Hugdahl

(Eds.), Brain asymmetry (pp. 427-\ 450). Cambridge, MA: MIT Press.Helhge, J.B. (1993). Chapter 6: Vari-

, eties of interhemisplieric interac-, tion. In J.B. Hellige, Hemispheric

asymmetry: Wfiat's right and what'sleft (pp. 168-206). Cambridge,MA: Harvard University Press.

Lassonde, M., & Jeeves, M.A. (Eds.).(1994). Callosal agenesis: A naturalsplit brain? New York: PlenumPress.

Milner, A.D. (Ed.). (1995). Neuropsy-chological and developmentalstudies of the corpus callosum

' ISpecial lssuej Neuropsyihologia,% 921-1007

mation about how these two rela-tively distinct portions of the braininteract to provide the seamless be-havior we all exhibit in everydaylife. So striking are some of thedemonstrations of lateralization offunction in splif-brain patients (i.e.,individuals in whom the corpuscallosum, which connects the cere-bral hemispheres, has been sev-ered) that philosophers and neuro-scientists alike have paused toconsider whether humans mighthave two separate and unique con-sciousnesses, rather than a singlemind. Recent work has helped toexpand our understanding of theexquisite interplay between thehemispheres that provides us withunified thought. It has becomeclear that interhemispheric interac-tion has some unanticipated func-tions, such as playing a role in per-ceptually binding together disparateparts of an object or modulating at-tentional ability. Furthermore, in-

terhemispheric interaction appearsto have emergent properties, inthat under certain conditions, onecannot deduce how the hemispheresinteract based solely on how eachhemisphere operates in isolation.

Researchers attempting to un-derstand interhemispheric interac-tion have generally concentratedon two major lines of inquiry. Thefirst examines how information isrepresented as it is transferredfrom one hemisphere to the other.Thought of differently, this line ofinquiry attempts to understand the"language" that the hemispheresuse to communicate with one an-other. The second line of inquiryexamines how transfer between thehemispheres affects the brain's in-formation processing capacitiesand strategies. Thaf is, this line ofresearch attempts to understandwhat mental processes are modu-lated or influenced by interhemi-spheric interaction.

Most interaction between the ce-rebral hemispheres occurs via avery large neural band of fibersknown as the corpus callosum (seeFig. 1), which is composed of morethan 200 million nerve fibers. Al-though there are other neural path-ways by which information can betransferred between the hemi-spheres (see Fig. 1), the vast major-

l-'ublished by Cambridge University Press

CURRi:\'T DIRIXTIONS SN PSYCHOiXXUCAL SCiFiNCB

Hippocatnpal commissureCorpus callosum

Habenular commissure

Posteriorcommissure

Anterior commissure

Massa intermediaof the thalamus

Collicularcommissures

Fig. 1. A view of the brain cut down the middle so that the inside-most regions ofthe right hemisphere are shown. Labeled in this figure are the various pathwaysthrough which information can be communicated between the hemispheres. Noticethat the corpus callosum is by far the largest, and it is responsible for the vastmajority of information transfer between the cerebral hemispheres. From Banich(1997). Copyright 1997 by Houghton Mifflin Company. Used with permission.

ity of information is transferred viathe callosum. As Reuter-Lorenzand Miller (this issue) discuss, onlyrudimentary information can betransferred without a callosum.Such information includes coarsevisual information regarding mo-tion but not visual form, binary in-formation (yes/no, odd/even),general emotional tone (positive,negative), and information that al-lows for the automatic orienting ofattention.

HOW IS INFORMATIONREPRESENTED WHEN IT ISTRANSFERRED BETWEEN

THE HEMISPHERES?

One of the guiding principles ofinterhemispheric interaction is thatdifferent types of information aretransferred across different sec-tions of the callosum. The callosumis organized topographically, witheach section connecting nearby re-gions (i.e., the anterior callosumconnects anterior brain regions; theposterior callosum connects poste-rior brain regions). Because each of

the major types of sensory informa-tion (e.g., visual, auditory, tactile)is processed by a distinct brain re-gion, and because different higherorder representations of informa-tion (e.g., abstract visual form vs.meaning) are processed by differ-ent brain regions as well, onemight expect that the callosum con-sists of channels, each of w^hich isresponsible for transferring a dis-tinct type of information.

For the most part, this expecta-tion seems to hold. For example,when a simple flash of light is di-rected to one hemisphere and themotor response is controlled by theother hemisphere, hoth sensory(i.e., visual) and motoric informa-tion are transferred. Electrophysi-ological recordings suggest that themotor signal is sent across middleportions of the callosum that con-nect motor regions of the brain,whereas the visual signal is sentacross posterior portions of the cal-losum that connect visual regionsof the brain (Rugg, Lines, & Milner,1984). Furthermore, studies of pa-tients with cailosal tumors or par-tial section of the callosum provideevidence that, for the most part.

visual, auditory, and somatosen-sory information are transferredthrough different sections of thecallosum (e.g., Risse, Gates, Lund,Maxwell, & Rubens, 1989). Intri-guingly, there is evidence for anasymmetry in the speed of transferof sensory information between thehemispheres: Such transfer is fasterfrom the right hemisphere to theleft than from the left hemisphereto the right (Marzi, Bisiacchi, &Nicoletti, 1991).

Because not all informationtransferred between the hemi-spheres is sensory in nature, re-searchers have also attempted todetermine how higher order (e.g.,spatial, semantic) information iscommunicated. This task can besomewhat difficult because higherorder information could be trans-ferred between the hemispheres ina variety of ways. A word, for ex-ample, could be represented as avisual pattern, as a series of letters,as a series of sounds, or as a mean-ing. Hence, much of this researchhas focused not on the exact natureof information transferred, butrather on whether the representa-tion of the information involved ininterhemispheric communication issimilar to or distinct from the rep-resentation employed by eachhemisphere.

A commonly utilized method insuch endeavors is to com^pare theprocessing that occurs when infor-mation is directed to only onehemisphere with the processingthat occurs when both hemispheresreceive identical information. Inthe case of visual information,stimuli are presented to only onevisual field (i.e., only to the left orright of fixation) on some trials andto both visual fields (i.e., to bothsides of fixation) on other trials. In-formation presented to a given vi-sual field is processed initially bythe opposite hemisphere. Thus, in-formation presented to the right vi-sual field (RVF; i.e., to the right offixation) is processed initially by

Copyright © 1998 American Psychological Society

VOLUME?, \'i;VIi3i.-R !, FEfiRLiARY Vm

the left hemisphere, and informa-tion presented to the left visualfield (LVF; i.e., to the left of fixa-tion) is processed initially by theright hemisphere.

The major finding of this line ofinquiry is that the representationinvoked when both hemispheresare involved in processing can varyacross situations (see Hellige, 1993,chap. 6). In some cases, the repre-sentation employed when bothhemispheres receive information issimilar to the representation usedby one hemisphere, but not theother. For example, Hellige, Jons-son, and Michimata (1988) askedindividuals to differentiate twofaces that varied by a single feature(hair, eyes, mouth, jaw), and exam-ined performance as a function ofwhich feature distinguished thetwo faces. One might presupposethat the representation employedwhen both hemispheres saw thefaces (bilateral-visual-field, or BVF,trials) would be similar to the rep-resentation used by the hemi-sphere generally considered spe-cialized for the task (in this case,the right hemisphere, because it isusually superior at face process-ing). However, that was not thecase. The pattern of performanceon BVF trials was identical to thatobserved on RVF (left-hemisphere)trials, but different from that forLVF (right-hemisphere) trials. Infact, the representation employedon BVF trials is frequently similarto that of the hemisphere less adeptat the task. Perhaps when bothhemispheres are involved, themore adept hemisphere has to"dumb down" to meet the otherhemisphere's ability.

In other cases, the nature of pro-cessing when both hemispheres arestimulated is a blend, or average, ofthe processing that takes placewithin each hemisphere. For ex-ample, when the task is namingconsonant-vowel-consonant se-quences, the pattern of errors var-ies by visual field. On LVF trials.

errors are much more frequent onthe third letter of the sequence thanthe first, whereas this difference ismuch reduced on RVF trials. Thepattern of errors on BVF trials isintermediate between that of RVFand LVF trials (Luh & Levy, 1995).These findings suggest that bothhemispheres contribute to perfor-mance so that the representationemployed in the interchange of in-formation is a blend of the differentrepresentations utilized by the twohemispheres.

One of the m ost interesting find-ings of such research is that therepresentation employed whenboth hemispheres receive informa-tion can be completely distinctfrom the representation employedby either hemisphere in isolation.In a series of studies, Karol and Iinstructed individuals to decidewhether either of two probe words(which were positioned in the samevisual field on some trials and indifferent visual fields on others)rhymed with a previously pre-sented target word. We found thatwhen a rhyme was present, perfor-mance on RVF trials, and to a lesserdegree on BVF trials, was influ-enced by whether the meaning ofthe two probe words was identical(e.g., bee and bee) or different (e.g.,bee and sea), an effect not observedfor LVF trials. We then changed thetask slightly, presenting the twowords in different cases and fonts(e.g., BEE and bee, BEE and sea) sothat they would no longer lookidentical. This manipulation didnot affect performance on RVF andLVF trials, but changed the patternfor BVF trials (Banich & Karol,1992, Experiments 4 and 5). Thus,the words' meaning and form in-teracted to affect performance onBVF trials, suggesting that the rep-resentation used when both hemi-spheres are involved in processingis one in which physical form andmeaning are linked. In contrast, thefact that font and case did not affecteither LVF or RVF performance

suggests that the individual hemi-spheres employ a representation inwhich meaning and form are sepa-rable.

HOW DOESINTERHEMISPHERIC

INTERACTIONMODULATE

COGNITIVE PROCESSES?

Researchers have also madesome progress in understandinghow interaction between the hemi-spheres influences the ways inwhich the brain processes informa-tion. Research with animals sug-gests that at least for the primarysensory areas of the brain (i.e., theregion of the brain where informa-tion from sensor receptors, such asthe eye, is first received), integra-tion of information across the cal-losum allows for a unitary sensoryworld. Because the neural systemis organized so that information onthe left side of space goes to theright half of the brain, and viceversa, there is a need to fuse thesetwo perceptual half-worlds. Thecallosum seems to play a criticalrole in this regard. Callosal connec-tions in primary sensory areas areoften limited to those regions of thesensory world that fall along themidline, such as where the RVFand LVF meet.

Corroboration of the role of thecallosum in sensory midline fusioncomes from studies of individualswho were born without a callosum,a syndrome known as callosal

'agenesis. These individuals haveinteresting sensory difficulties, in-cluding a poor ability to discernwhich of two objects placed in cen-tral vision is closer or further away(an ability that relies on the braincomputing slight differences in theretinal images received by the twoeyes), a poor ability to determinewhether two points touched on ei-ther side of the trunk are different

I'liblished by Cambridge University Press

or the same, and difficulties in lo-calizing sounds in space (a skillthat relies on the brain computingslight differences in the timing orintensity of sounds received by thetwo ears). All these difficulties relycritically on the comparison of in-formation received separately bythe two hemispheres, and hence re-quire integration across the callo-sum (Lassonde, Sauerwein, & Lep-ore, 1995).

Recently, it has been proposedthat the pattern of activation acrossthe callosum may help bind to-gether different parts of the visualworld into unitary objects. Whenstimuli on different sides of the vi-sual midline move in tandem, cellsin the two hemispheres fire syn-chronously. This synchrony reliesspecifically on the callosum, as itdoes not occur when the callosumis severed. Because parts of an ob-ject generally move together in thesame direction and at the samespeed, such temporal aspects ofcallosal firing may provide amechanism for binding togetherparts of an object that appear oneither side of the midline (Engel,Konig, Kreiter, & Singer, 1991). Be-cause people typically move theireyes so that an object of interestfalls in the center of gaze (andhence spans the midline), such cal-losal connections may be importantfor object recognition.

Interaction between the hemi-spheres not only influences sen-sory processing, but also modu-lates the processing capacity of thebrain. Belger and I (Banich & Bel-ger, 1990) found that fhe ability ofinferhemispheric interaction to fa-cilitate performance increases asthe computational complexity ofthe task, which we define as fhenumber (and nature) of the stepsinvolved, increases. For example, ifit must be decided whether twodigits add to 10 (summation task),performance is befter if one digit ispresented to each hemisphere (sothat the hemispheres musf interact

to make a decision) than if bothdigits are presented to the samehemisphere (in which case, no in-teraction is required). In contrast, ifthe task is to decide whether twodigits are identical (physical-identity task), interhemispheric in-teraction does not yield a perfor-mance advantage. The summationtask is more con:\plex because notonly is perceptual processing of thedigits required (as in the physical-identity task), but then some iden-tification and addition must be per-formed as well These results areconsistent with others demonstrat-ing thaf performance is betterwhen operations are dividedacross the hemispheres (e.g., di-recting a digit to be added to a tar-get to one hemisphere, while di-recting another digit to be subtractedfrom the target to the other hemi-sphere) than when bofh operationsmust be performed by the samehemisphere (Liederman, 1986).

We believe this effect occurs be-cause the computational power ofthe brain may be increased by di-viding processing over as muchneural space as possible (in thiscase, over the two hemispheres),much the way that computationalpower of computers is increased bydividing a task over many sysfems.Such a division is possible becausefor most tasks (with the possibleexceptions of speech output andphonetic processing), specializa-tion of the hemispheres is relativerafher than absolute. Thus, eventhough one hemisphere may do aparticular task less capably or effi-ciently than the other, it nonethe-less has the capacity to contribute(see also Beeman & Chiarello andChabris & Kosslyn, this issue). Astasks get more difficulf, any costoverhead imposed by having to co-ordinate processing between thehemispheres is more than offset bythe increased computational capac-ity provided by having both hemi-spheres involved.

Indirect support for such an idea

has been provided by the recentsurge of brain-imaging studies. Al-though these studies are designedto examine specific cognitive pro-cesses (e.g., memory), the manipu-lations employed often vary incomplexity (e.g., in how manyitems musf be retrieved frommemory). As task demands are in-creased, often there is not onlygreater activation of the brain re-gion specialized for that task, butgreater activation over both hemi-spheres as well (e.g.. Braver et al.,1997). Such findings are consistentwith the idea that as tasks get moredifficult to perform, more re-sources are recruited from bothhen:\ispheres.

Interhemispheric interactionalso seems to modulate fhe abilityto select certain information forprocessing (e.g., the words on thispage) while filtering out other in-formation (e.g., the backgroundnoise that occurs while you arereading). Using three well-knownparadigms, Alessandra Passarotti,Joel Shenker, Daniel Weissman,and I have demonstrated that inter-hemispheric interaction aids taskperformance when a high degree,but not a low degree, of selection isrequired. In one of these para-digms, the individual must pay at-tention to the overall shape (globalform) of an item and ignore thesmall shapes (local form) of whichit is composed (or vice versa); inanother, individuals must decide ifa pair of items matches, and thatdecision must be made by attend-ing to one attribute of the pair (e.g.,their shapes) but not another (e.g.,their colors); and in the third para-digm, the Stroop paradigm, an in-dividual must pay attention to(and name) the color of the ink inwhich a word is presented whileignoring the meaning of the word.When there is no interference be-tween the two types of informationand thus little need for selection,interaction between the hemi-spheres does not aid performance.

Copyright © 1998 American Psychological Society

For example, interhemispheric in-tefaction is not especially usefulwhen the global and local form ofan item are the same, when a pairof items have the same form andthe same color, or when the color aword names is concordant with itsink color (e.g., "red" is printed inred ink). However, when there isconflicting information and indi-viduals must select one type of in-formation over another, the degreeof interference engendered by theconflicting information can be re-duced by interhemispheric interac-tion. For example, when an item'sglobal and local shape lead to dif-ferent responses, when itemsmatch in form but not color, andwhen the word "blue" is printed inred ink, interaction between thehemispheres leads to superior per-formance. Furthermore, these re-sults, for the most part, are rela-tively independent of hemisphericasymmetries for the task, onceagain suggesting that interhemi-spheric interaction may affect pro-cessing in a manner independent ofhemispheric specialization.

THE IMPORTANCE OFINTERHEMISPHERIC

INTERACTION FROM ACLINICAL OR

LIFE-SPAN PERSPECTIVE

The nature of interaction be-tween the hemispheres may also beimportant in a number of neuro-psychological syndromes, and mayhave implications for developmentand aging. For example, the corpuscallosum seems especially vulner-able to damage caused by multiplesclerosis (e.g., Rao et al., 1989) andclosed head injury (Gale, Johnson,Bigler, & Blatter, 1995), and themorphology and function of thecorpus callosum are different inpeople with schizophrenia than inneurologically intact individuals(e.g., David, Minne, Jones, Harvey,

& Ron, 1995). The implications ofsuch findings remain unclear atpresent, but it is possible that someof the attentional difficulties ob-served in people with these syn-dronnes may be lir\ked to disruptedinterhemispheric interaction.

The interplay between the hemi-spheres may also be linked tochanges in cognitive processingwith development. During child-hood, the speed with which infor-mation can be relayed between thehemispheres increases because thefatty insulation around neurons,called myelin, continues to increasein size around callosal neurons un-til some time during the late teenyears. In essence, the hemispheresof young children are more func-tionally disconnected than those ofadults. Children do not exhibit thesame advantages of dividing pro-cessing across the hemispheres asobserved in older individuals, butthat changes as they grow older(Liederman, Merola, & Hoffman,1986). Interhemispheric interactionmay be disrupted in dyslexia (Mar-kee, Brown, Moore, & Theberge,1996), and may be atypical in atten-tional deficit disorder (Giedd et al.,1994). Future work is likely to bedirected to more carefully explicat-ing the nature of the relationshipbetween interhemispheric interac-tion and certain developmentalsyndromes.

CONCLUSIONT

In sum, the functionally distinctcerebral hemispheres appear to co-ordinate their performance in mul-tiple ways via the corpus callosum,which allows for dynamic inter-change of information. The natureof the representations used forsuch communication is not wellelucidated at present, especially forhigher order information. How-ever, it is clear that to further un-derstand the implications of later-

alization of function, it will becritical to understand interhemi-spheric integration, as research al-ready suggests that the whole ismore than the sum of its parts.

Acknowledgments—Preparation of thisarticle was supported by National Insti-tute of Mental Health Grant ROlMH54217. I thank Mark Beeman, Chris-topher Chabris, Emanue! Donchin, andWendy Heller for helpful comments.

Note

1. Address correspondence toMarie T. Banich, Beckman Institute andDepartment of Psychology, Universityof Illinois at Urbana-Champaign, 405N. Mathews, Urbana, IL 61801; e-mail:mbanich@s .psych.uiuc.edu.

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Banich, M.T. (1997). Neiiropsychology: The neuralbases of menial function. Boston: Houghton Mif-flin.

Banich, M.T., & Belger, A. (1990). Inter hemisphericinteraction: How do the hemispheres divideand conquer a task? Cortex. 26. 77-94,

Bdnich, M.T., & Karol, D.L. (1992). The sum of theparts does not equal the whole: Evidence frombihemispheric processing, journal of Experi-mental Psycbologi/: Human Perception and Perfor-mance. 18, 763-784.

Braver, T.S., Cohen, J.D., Nystrom, L.E., Jonides, J.,Smith, E.E,, & Noll, D.C. (1997). A parametricstudy of prefrontal cortex involvement Ln hu-man working memory. Neuroimage. 5. 49-62.

David, A.5., Minne, C , Jones, P., Harvey, I., &Ron, M.A. (1995). Structure and function of thecorpus callosum in schizophrenia: What's theconnection? European Psychiatry, 1Q{\). 28-35.

Engel, A.K,, Konig, P., Kreiter, A.K,, & Singer, VV.(1991). Interhemispheric synchronization ofoscillatory neuional responses in cat visualcortex. Science. 152, 1177-1179.

Gale, S.D., Johnson, S.C., Bigler, E.D., & Bfatter,D.D. (1995). Nonspecific white matter degen-eration following traumatic brain injury, jour-nal of the International Neuropsychological Soci-ety, I, 17-28.

Giedd, J.N., Castellanos, F.X., Casey, B.J., Kozuch,P., King, A.C, Hamburger, S.D., & Rapoport,J.L. (1994). Quantitative morphology of thecorpus callosum in attention deficit hyperac-tivity disorder. American journal of Psychiatry,151, 665-669.

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The American Psychological Society was founded in 1988 as an independent,multipurpose organization to advance the discipline of psychology,, to preservethe scientific base of psychology, to promote public understanding of psycho-logical science and its applications, to enhance the quality of graduate educa-tion, and to encourage the "giving away" of psychology in the public interest.All members of the American Psychological Society receive Psychological Sci-ence and the APS Observer as part of their annual membership dues, which are$122.00 per year through 1998. APS's second journal, Current Directions inPsychological Science, consists of reader-friendly reviews of current researchareas. It is available by subscription. For membership information and applica-tions, contact the American Psychological Society, 1010 Vermont Avenue,NW, Suite 1100, Washington, DC 20005^907. Telephone: 202-783-2077; Fax:202-783-2083; Internet: aps@aps.washington.dc.us.

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