iUnHENT DflUXDONS IN i'SYCHOI.CXnCAl SC!t:\'Ct:

The Cognitive Neuroscience of HumanLaterality: Lessons From theBisected BrainPatricia A. Reuter-Lorenz and Andrea C. Miller^Department of Psychology, University of Michigan, Ann Arbor, Michigan

With the discovery that surgicalseparation of the cerebral hemi-spheres produces two independentand differently abled cognitivespheres, Gazzaniga, Bogen, andSperry (1962) launched a revolu-tion in the field of psychology: thestudy of human laterality. Morethan thirty-five years later, the sci-entific progeny of Roger Sperrycontinue to study split-brain pa-tients in order to solve some of themany mysteries confronting cogni-tive neuroscience today. Histori-cally, split-brain studies catalyzedresearch on hemispheric differ-ences in neurologically intact sub-jects. More recently, the bisectedbrain has served as the provingground for various models of com-plex cognitive processes. What les-sons have been learned from split-brain patients? How does researchon split-brain patients convergewith other methodologies, includ-ing the advances in neuroimaginggaining prominence in the field?

^Recommended Reading

'iGazzaniga, M.S, (1995). Conscious-ness and the cerebral hemi-

^ spheres. In M.S, Gazzaniga (Ed ),The cognitive neurosciences (pp.

'- 1391-1400). Boston: MIT Press{Ogden, J.A. (1996). SpUt brain, split! mind? Case L.B, In Fractured

^ minds: A case-study approaLh tot clinical neuropsychology (pp. 232-; 251). New York: Oxford Uni"ver-^ sitv Press.

l, E., & lacoboni, M. (Eds,) (mpiess). Tbe role of the corpus uiUo-'iiun in sensory-motoi intc'^ratiouAnatomy, physiology and heJiaowtNew York: Plenum P

Our review offers some answers tothese questions.

Consisting of more than 200 mil-lion nerve fibers, the corpus callo-sum is the major white-matterpathway connecting the left andright cortical hemispheres. In someindividuals with intractable epi-lepsy, severing this pathway is themedical treatment of choice. Scien-tists who study the cognitive con-sequences of the split-brain opera-tion have focused on two majorissues. One is to determine whattypes of mental representationsand processes can be shared be-tween the brain's hemispheres inthe absence of the corpus callosum.The other is to characterize andcompare the competencies of theleft and right hemispheres in orderto develop and test hypothesesabout normal cognitive architec-ture.

Six patients figure prominentlyin the corpus of split-brain studies.L.B., N.G., and A.A. are part of theoriginal West Coast series of pa-tients who underwent surgical sec-tion of both the corpus callosumand a minor interhemisphericpathway known as the anteriorcommissure (Bogen & Vogel, 1975).J.W., V.P., and D.R. are part of themore recent East Coast series forwhom only the corpus callosumwas sectioned (callosotomy; e.g.,Baynes, Tramo, & Gazzaniga,1992). Each individual has a uniquedevelopmental and medical his-tory. Nevertheless, important dis-coveries resulting from split-brainresearch converge beautifully withfindings from patients with local-

ized 'brain damage (due to strokeor other neurological disorders)and research on laterality effects inthe intact brain.

Because the brain is organizedcontralaterally, information fromthe right visual field (i.e., stimulion the right side of a visual fixationpoint; see Fig. 1) travels initially tothe left hemisphere of the brain,and information from tbe left vi-sual field (i.e., stimuli on the leftside of a central point) travels ini-tially to the right hemisphere. Bothvisual laterality studies with neu-rologically intact participants andsplit-brain studies require display-ing experimental stimuli to the leftor right visual field at brief expo-sure durations that prohibit eyemovements (so that the informa-tion is truly lateralized to one orthe other hemisphere). Perfor-mance differences between thehemispheres are typically n:\ore ro-bust for split-brain patients thanfor normal subjects—thus under-scoring the importance of the cor-pus callosum for integrating theoperations performed by the corti-cal hemispheres.

TWO INDEPENDENTCOGNITIVE REALMS

To what extent can the sepa-rated hemispheres operate inde-pendently? Answers to this ques-tion provide insight into thefunctions of the corpus callosumand the capacities of extracaliosalpathways. If the hemispheres stillcommunicate after the corpus cal-losum has been severed, then pat-terns of hemispheric asymmetry insplit-brain patients should reflectthis preserved cross talk.

In fact, the separated hemi-spheres are virtually isolated fromone another, so that one hemi-sphere is rarely conscious of stimu-lation presented to the other (Reu-ter-Lorenz, Nozawa, Gazzaniga, &

Copyright © 1998 American Psychological Society

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90 •

60 -

o 70 ^

50

HLeft Hemisphere• Rigbt Hemisphere

Verbal Matcbing Spztial Matchhg

Task

Verbal Matching Task

response - 'YES'

probe200 msec

delau /500 maec /

target set250 msec

+ k

D+ B

Spatial Matching Task

probe200 msec

delflu /SOO msec /

target200 msec

response - 'NO'

* n

Fig. 1. Accuracy performance of patient V.P. on two simple matching tasks, with anexample of a right-visual-field/left-hemisphere trial for each task. During the tasks,V.P. kept her eyes focused on the plus sign at all times.

Hughes, 1995). Visual and tactileinformation presented to one hemi-sphere cannot be compared withinformation presented to the otherhemisphere. The separated hemi-spheres are also unable to share ab-stract codes or semantic informa-tion (Seymour, Reuter-Lorenz, &Gazzaniga, 1994), despite previousclaims to the contrary (Sergent,1990). Coarsely coded informationabout spatial location may beshared, but this capacity may belimited to central portions of the vi-sual field (Fendrich, Wessinger, &Gazzaniga, 1996). Moreover, theseparated hemispheres can imple-ment different visuomotor pro-grams concurrently and with miru-mal interference. For example.

callosotomy patients can copy twodifferent shapes, one with the lefthand and one with the right, fasterand more accurately than normalsubjects (Franz, Eliassen, Ivry, &Gazzaniga, 1996). Thus, the au-tonomy of the heniispheres pro-duced by callosal section extendsbeyond perceptual processinginto the stages of decision makingand response preparation (Reuter-Lorenz, in press).

With two separate nrinds in thesame cranium, the split-brain pa-tient might seem ripe for inter-hemispheric conflict. Indeed, suchconflict is evident in the early post-operative period. For example,there are reports of one patientwho, while grocery shopping.

placed items in a cart with onehand but then returned the items tothe shelf with the other hand.However, these frankly conflictingresponses decrease over time, sug-gesting the emergence of mecha-nisms that' resolve interhemi-spheric competition.

In particular, it has been shownthat each hemisphere is able to in-hibit, or gate, the responses of theother (Reuter-Lorenz et al., 1995;see also Ivry, Franz, Kingstone, &Johnston, 1998). Because eitherhemisphere can gate the other'soutput, the more competent hemi-sphere can take control of perfor-mance on a task (however, seeLevy, 1977). Given the vast differ-ences in the processing abilities ofthe two hemispheres, such flexibil-ity allows split-brain patients tomeet the diverse demands of dailylife more readily.

COGNITIVEDICHOTOMIES

Researchers in the field of hu-man laterality have proposed vari-ous dichotomies to interpret differ-ences between the two cerebralhemispheres. Of course, no singledichotomy is likely to capture allthe important functional asymme-tries, especially because many dif-ferences between the hemispheresare a matter of degree. Neverthe-less, the verbal-spatial and ana-lytic-holistic dichotomies haveboth figured prominently in theo-ries of laterality. Recent split-brainstudies illustrate the value of thesedistinctions and reveal fundamen-tal asymmetries that any compre-hensive theory of human lateralitymust ultimately explain.

Left-hemisphere specializationfor language in right-handed indi-viduals has been recognized sincePaul Broca's seminal cases in themid-19th century. Right-hemi-sphere specialization for such non-

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CURRi:\'T niRfCriOl^S iN PSYCHOLCXJiCAt SCIl-NCE

verbal functions as spatial process-ing and face recognition waswidely recognized nearly a centurylater. But it was the pioneeringsplit-brain studies of Sperry, Gaz-zaniga, and their colleagues thatcatapulted the verbal-nonverbal di-chotomy into mainstream psychol-ogy. This dichotomy has been vig-orously pursued for decades invisual laterality research, and morerecently, in neuroimaging studiesof the neurologically intact. The al-ternative anaJytic-holistic di-chotomy, which w as proposed inpart to subsume the distinction be-tween verbal and nonverbal pro-cesses, also has drawn much inspi-ration from split-brain research:The left hemisphere perceives thedetails and features but not theoverall configuration, whereas theright hemisphere gets the big pic-ture while missing the details. Wenext consider each of these di-chotomies in turn.

Verbal-Nonverbal Asymmetries

Hemispheric differences in ver-bal and spatial abilities are clear.For example, consider the perfor-mance of patient V.P. on twosimple matching tasks. We haveobtained these data as part of alarge-scale investigation on work-ing memory in the separated hemi-spheres {unpublished observa-tions). In the verbal task, a set oftwo, three, or four target letters isflashed briefly to the left or rightvisual field, followed by a probeletter in the same visual field. V.P.then indicates whether or not theprobe letter is the same as one ofthe targets. In the spatial task, a dotis presented briefly in the left orright visual field, followed by aprobe circle. V.P. then indicateswhether or not the probe is in thesame location as the target dot. Fig-ure 1 illustrates these tasks andpresents results showing that witha 500-ms delay between the target

and probe, the left hemisphere isclearly superior in the verbal task,whereas the right hemisphere ex-cels at the spatial task. Moreover,we find that the magnitude ofasymmetry increases as the delaybetween the target and probe in-creases to 3 s. These results indicatethat the hemispheres are differen-tially specialized for verbal andspatial representations. The in-creased asymmetry with delaymay stem partly from hemisphericdifferences in the quality of theseinitial representations. However,the processes that maintain verbaland spatial representations inworking memory may also be dif-ferentially lateralized in the leftand right hemispheres (e.g.. Smith& Jonides, 1997).

Working memory asymmetriesunderscore a crucial dimension ofthe verbal-nonverbal dichotomy:Its explanatory power lies in anemphasis on the relative compe-tence of each hemisphere for en-gaging verbal versus nonverbal-spatial processing operations. Thispoint is further evident from tasksthat use letters as stimuli but reveala right-hemisphere superiority. Forexample, the right hemisphere ofpatient L.B. is better than the leftwhen matching letter shapes ormentally rotating letters, eventhough the left hemisphere is supe-rior at discriminating the letters'identities (Corballis & Sergent,1989; Eviatar & Zaidel, 1994).

Although verbal and nonverbalasymmetries do exist, not all hemi-spheric differences are accuratelydescribed by this dichotomy. Theleft hemisphere is superior to theright in some abilities that are notobviously linguistic, such as mak-ing skilled hand movements, cod-ing temporal sequences, and ana-lyzing certain types of spatialrelations (e.g., Kosslyn & Koenig,1992). Furthermore, the right hemi-sphere excels with certain figura-tive and connotative aspects of lan-guage (see Beeman & Chiarello,

this issue). Because one goal of lat-erality research is to specify funda-mental processing differences be-tween the two hemispheres,researchers have attempted to gobeyond the verbal-nonverbal di-chotomy. '

Analytic VersusHolistic Processing

The most prominent alternativeto the verbal-nonverbal dichotomyinvolves the distinction betweenanalytic and holistic processing.According to this distinction, theleft hemisphere is specialized forsequential analysis of features anddetailed elements that make upcomplex stimuli, whereas the righthemisphere is specialized for si-multaneous, gestalt-type process-ing that encodes elements in paral-lel as a synthesized whole. Thisdistinction was prompted by earlyevidence from split-brain patientsindicating right-hemisphere domi-nance for recognizing faces andprocessing configurations versusleft-hemisphere superiority foranalyzing features and details (e.g..Levy, 1977).

New and striking results sup-porting this distinction continue toemerge from split-brain research.In a recent study, Gazzaniga (inter-viewed in Singer, Ghedd, & An-gier, 1997) tested J.W.'s perceptionof paintings of faces made fromconfigurations of objects, such asfruits. When a painting was pre-sented to his right hemisphere, J.W.perceived it as a face, but when thesame stimulus was presented to hisleft hemisphere, he perceived itonly as fruits. Numerous otherstudies on face perception in split-brain patients confirm a right-hemisphere superiority for facesand other complex, hard-to-nameforms (see, e.g.. Stone, Nisenson,Eliassen, & Gazzaniga, 1996). Theright hemisphere's disadvantage atanalyzing elementary features is

Copyright © 19'>8 American Psycho logical Society

VOr.UML- 7, NUMBHR i, I-T:BRUARY

evident in a different line of re-search on visual imagery. Kosslynand his colleagues showed tbatJ.W.'s right hemisphere bas a spe-cific deficit in imagining the partsof complex forms, whereas his lefthemisphere's ability to imaginesuch parts produces an advantageon a variety of imagery tasks (e.g.,Kosslyn & Koenig, 1992).

Other attempts to investigate theanalytic-holistic distinction haveyielded mixed results, however.Hierarchical stimulus configura-tions have been particularly popu-lar for testing the dichotomy. Thesestimuli consist of large lettersformed from appropriate arrange-ments of smaller letters (e.g., alarge H made up of small Ss), rep-resenting the global and local lev-els, respectively. Robertson and hercolleagues (see, e.g., Robertson,Lamb, & Zaidel, 1993) have shownthat localized right-bemispberedamage impairs processing at theglobal level, as the holistic charac-terization of right-hemisphere pro-cessing would predict. Likewise,localized left-hemisphere damageimpairs local-level processing, aswould be expected if the left hemi-sphere is specialized for analyzingparts of complex configurations.However, some visual lateralitystudies of neurologically intact ob-servers have not found global-localprocessing asymmetries (see VanKleeck, 1989, for a review). Simi-larly, although Robertson et al.(1993) reported a left-hemisphereadvantage for local processing anda right-bemisphere advantage forglobal processing for patients L.B.,N.G., and A.A., a foUow-up studyby Weekes, Carusi, and Zaide)(1997) found a global-processingadvantage for both hemispheres ofthese patients.

It is important to recognize thatthe logic of inferring a general, fun-damental processing asymmetryfrom hemispheric differences onspecific tasks such as face recogni-tion and letter identification may

be fundamentally flawed. Perhapsthe subsystems that process facesand visual language are domain-specific (Moscovitch, Winocur, &Behrmann, 1997; Polk & Farah,1998). That is, these processors maybe activated exclusively by particu-lar classes of stimuli. If a process-ing subsystem in the right hemi-sphere is specialized for faces, itmay indeed process facial featuresand their relations in a gestalt fash-ion. However, to the extent thatthis "module" is domain-specific,it may not be more generally appli-cable to processing any other ge-stalt-like configurations.

THE BISECTED BRAININ COGNITIVE

NEUROSCIENCE

Split-brain studies have played avital role in unearthing basic prin-ciples governing interhemisphericinteractions and human laterality.Here, we survey three importantways in which split-brain researchwill continue to play a vital role inthe field of cognitive neuroscience.

Convergence With LesionStudies andFunctional Neuroimaging

Because cognitive neuroscienceis a field of diverse methodologies,results that converge on the sameconclusion provide powerful evi-dence. Numerous examples of con-vergence exist among results fromsplit-brain patients, studies of pa-tients with localized brain damage,and functional neuroimaging. Sev-eral of these examples are particu-larly revealing about the uniquestatus of evidence from split-brainpatients.

For example, studies of single-word reading in the separatedhemispheres of J.W. (Reuter-Lorenz & Baynes, 1992) help tosbow how the isolated language-

do niinant and -nondominanthemispheres read. Not surpris-ingly, J.W.'s left hemisphere readsnormally. In contrast, his righthemisphere uses a serial, letter-by-letter strategy resembling thatfound in patients with pure alexia{a reading disorder that is acquiredin adulthood and results fromdamage to left occipital-temporalregions of the brain). Processing ofsingle letters' identities by J.W.'sright hemisphere is also impaired;this deficit is like that shown bypure alexics. In pure alexia and inthe isolated right hemisphere, aplodding letter-by-letter strategyresults because letter identities can-not be accessed efficiently and au-tomatically, as they are in normalreading.

These results have helped re-solve a prevailing debate. Previ-ously, it was unclear whether theletter-by-letter strategy seen inpure alexia reflects the limitedreading competence of the intactright hemisphere or the impairedreading of the dysfunctional lefthemisphere. By demonstrating let-ter-by-letter reading in the righthemisphere of a split-brain patient,this work suggests that letter-by-letter reading in pure alexics re-flects reliance on the right hemi-sphere rather than on the impairedleft hemisphere. These conclusionsconverge with neuroimaging evi-dence for a left-hemisphere letter-processing module in the intactbrain (Petersen, Fox, Snyder, &Raichle, 1990; Polk & Farah, 1998).In this example, a split-brain studyillustrates the cognitive conse-quences {i.e., letter-by-letter read-ing) associated with the relative ab-sence of a particular processingsubsystem, namely, the letter-identity processor.

Revealing the Modulesof Cognition

By characterizing the competen-cies of the separated hemispheres.

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the cognitive neuroscientist gainsinsight about which cognitive op-erations can be dissociated. Wesummarize two examples to illus-trate this point. Studies of thebisected brain, patients with local-ized brain damage, and neuroim-aging evidence (see Mangun et a!.,1994) reveal that the right hemi-sphere has a special role in allocat-ing attention to spatial locations. Incontrast, recent evidence indicatesthat the left hemisphere is essentialfor attending to objects. For ex-ample, one study examined J.W.'sspeed in responding to a visual tar-get following a cue. J.W.'s lefthemisphere responded morequickly when the cue and targetappeared in different locationswithin the same object than whenthey appeared in two different ob-jects. In contrast, the right hemi-sphere did not show this within-object advantage, suggesting that itorients attention to locations on apurely spatial basis, without refer-ence to their object groupings(Egly, Rafal, Driver, &; Starrveveld,1994). These results are revealingabout the functional architecture ofthe attention system. They favorthe view that object-based atten-tion is mediated by a subsystemthat is separable from right-hemi-sphere attention processes.

The recent case of left-handedsplit-brain patient V.J. is particu-larly fascinating. V.j.'s left hemi-sphere can speak, read, and spell,but it cannot write. Instead, herright hemisphere controls the mo-tor output for writing, even thoughit camiot speak, read aloud, or spell(Baynes & Eliassen, 1998). Readingand writing are known to be medi-ated by dissociable mechanisms,because acquired disorders ofreading and writing do not alwaysco-occur in the same patient. How-ever, V.J. raises the more remark-able possibility that in at least someleft-handers, writing skills can belocalized in the right hemispherewhile other major linguistic pro-

cesses can be controlled by the lan-guage-rich left hemisphere. Evi-dence such as this will surelystimulate neuroimaging studies ofthe left-handed brain to establishthe generality of this dissociation,and may prompt a reconsiderationof processing models of writtenlanguage in which graphic func-tions depend on other languageprocesses.

Functional Architecture andLanguage Competence

Even in right-handed split-brainpatients, there is clear variability inthe language competence of theisolated right hemisphere, and, insome cases, speech abilities emergein the right hemisphere manymonths or years after surgery(Baynes & Eliassen, 1998). Suchplasticity in the adult brain is a re-minder that despite lateralizationof function, redundancy also exists,sometimes in the form of latent po-tential.

Nevertheless, the linguistic su-periority of the left hemisphere hasfascinating and diverse cognitiveconsequences. Gazzaniga (1995) re-fers to the capacity that emergesfrom the language-rich left hemi-sphere as the interpreter. The inter-preter is the source of the ongoing,conscious narrative that endeavorsto explain (often erroneously) thecognitions, emotions, and behav-iors produced by nonverbal pro-cessing, including that of the righthemisphere.

The cognitive ramifications ofthe left-brain interpreter have re-*cently been explored in the domainof memory. Metcalfe, Funnell, andGazzaniga (1995) showed that theright hemisphere is superior at re-membering the veridical content ofpreviously presented materials.The left hemisphere, in contrast, ismore likely to elaborate and makeinferences based on what it seesand, consequently, has more diffi-

cfulty distinguishing what was ac-tually seen from what was merelyinferred (see also Phelps & Gazza-niga, 1992; however, see Beeman &Chiarello, this issue, regardingright-hemisphere inference pro-cessing iwith an intact corpus callo-sum). Miller, Kingstone, and Gaz-zaniga (1997) recently extendedthis evidence by showing that theleft hemisphere is better than theright at using elaborative encodingstrategies. The left hemispherereadily uses the meaning or seman-tic category of to-be-remembereditems to form a richer and more du-rable memory trace. Although theright hemisphere is able to catego-rize items, it does not utilize thesesemantic associations in thememory-encoding process. Thus,studies of the bisected brain indi-cate the lateralization of particularspecialized modular abilities whilerevealing the in:iportance of suchmodules for more complex cogni-tive capacities.

CONCIUSION

As neuroscience enters a newera in the study of brain-behaviorrelationships, research on the bi-sected brain will continue to be amajor theoretical and empiricalforce. For decades, methodologicaland technical limitations forced arelian,ce on the hemisphere as theprimary unit of analysis in neuro-psychology. With the advent ofneuroimaging techniques, re-searchers can now draw conclu-sions about the precise functions ofstructurally defined brain regions.But knowing the precise locus of aprocess in the brain still leavesmany questions unanswered. Valu-able insights into the role of theseprocesses within the overall cogni-tive architecture can be gleanedfrom specifying the cognitive capa-bilities and limitations of the sepa-rated hemispheres in the bisectedbrain.

Copyright © 1998 American Psychological Society

Acknowledgments—This work was' supported by a Rackham Faculty Fel-lowship to P.R.-L. and by a National Sci-ence Foundation Graduate Fellowshipto A.M. The authors are grateful toDavid E. Meyer for his thoughtful com-ments on an earlier version of thismanuscript.

Note

1. Address correspondence to Patri-cia A. Reuter-Lorenz, Department ofPsychology, University of Michigan,525 East University, Ann Arbor, MI48109-1109; e-mail: parl@umich.edu.

References

Baynes, K., & EUassen, J.C. (1998). Tt\e visual lexi-con: Its access and orgar\izatior\ in commissur-otomy patients. In M. Beeman & C. Chiarello(Eds.), Right hemisphere language comprehension:Perspectives from cognitive neuroscience (pp. 79-104). Mahwah, NJ: Eribaum,

Baynes, K., Tramo, M.J,, & Gazzaniga, M.S. (1992).Reading with a limited lexicon in the righthemisphere ol a caUosotomy patient. Neuro-psychologia, ,30, 187-200.

Bogen, J.E., & Vogel, P.J. (1975). Neurological sta-tus in the long term following cerebral com-missurotomy. In F. Michel & B. Schott (Eds.},Les syndromes de disconnexion calleause chezI'homme (pp. 227-251). Lyon, France: HospitalNeurologique.

Corballis, M.C, & Sergent, J. (1989). Mental rota-tion in a commissurotomized subject. Neiiro-psychologia, 27, 585-597.

Egly, R., Rafal, R., Driver, J., & Starrveveld, Y.(1994). Covert orienting in the split brain re-veals hemispheric specialization for object-based attention. Psychological Science, 5, 380-383.

Eviatar, Z., & Zaidel, E. (1994). Letter matchingwithin and between the disconnected hemi-spheres. Brain and Cognition, 25, 128-137.

Fendrich, R., Wessinger, CM., & Gazzaniga, M.S.(1996). Nasotemporal overlap at the retinalvertical meridian: Investigations with a callo-sotomy patient. Neuropsychologia, 34, 637-646.

Franz, E.A., Eliassen, J.C., Ivry, R.B., & Gazzaniga,M.S, (1996). Dissociation of spatial and tempo-ral coupling in the bimanual movements ofcallosotomy patients. Psychological Science, 7,306-310.

Gazzaniga, M.S. (1995). Principles of human brainorgaruzation derived from split-brain studies.Neuron, 14, 217-218.

Gazzaniga, M.S., Bogen, J,E,, & Sperry, R.W.(1962). Some functional effects of sectioningthe cerebral commissures in man. Proceedingsof the National Academy of Sciences, USA, 48,1765-1769.

Ivry, R.B., Eranz, E.A., Kingstone, A., & Johnston,J.C. (1998). The psychological refractory pe-riod effect following callosotomy: Uncouplingof lateralized response codes, journal of Experi-mental Psychology: Human Perception and Perfor-mance, 24, 463-480.

Kosslyn, S.M., & Koenig, O. (1992). Wet mind: Thenew cognitive neuroscience. New York: FreePress.

Levy, ]. (1977). Manifestations and implications ofshifting hemi-inattention in commissurotomypatients. In E.A. Weinstein & R.P. Friedland(Eds.), Advances in neurology (Vol. 18, pp. 33-39). New York: Raven Press.

Mangun, G.R., Hillyard, S.A., Luck, S.J., Handy,T., Plager, R., Clark, V.P., Loftus, W., & Gaz-zaniga, M.S. (1994). Monitoring the visualworld: Hemispheric asymmetries and subcor-tical processes in attention, journal of CognitiveNeuroscience, 6, 267-275.

.Metcalfe, J,, Funnell, M., & Gaz/.aniga, M.S. (1995).Right-hemisphere memory superiority: Stud-ies of a split-brain patient. Psychological Science,6, 157-164.

Miller, M.B., Kingstone, A., & Gazzaniga, M.S.(1997)- HERA and the split-brain. Society forNeuroscience Abstracts, 22, 1579.

Moscovitch, M., Winocur, G., & Behrmann, M.(1997). What is special about face recognition?Nineteen experiments on a person with visualobject agnosia and dyslexia but normal facerecognition, fournai of Cognitive Neuroscience, 9,555-604.

Petersen, S.E., Fox, P.T., Snyder, A.Z., & Raichle,M.E. (1990). Activation of extrastriate andfrontal cortical areas by visual words andword-Iilce stimuli. Science, 249, 1041-1044.

Phelps, E.A., & Gazzaniga, M.S. (1992). Hemi-

spheric differences in mnemoriic processing:The effects of left hemisphere interpretation.Neuropsychologia, 30, 293-297.

Polk, T.A., & Farah, M.J. (1998). The neural devel-opment and organi^iation of letter recognition:Evidence from functional neuroimaging, com-putational modeling, and behavioral studies.Proceedings of the National Academy of Sciences,USA, 95, 847-852.

Reuter-Lorenz, P.A. (in press). Attention and thebisected brain: Implications for an analysis ofcallosal function. In E. Zaidel & M. Iacoboni(Eds.), The role of the carpus callosum in sensory-motor integration: Anatomy, physiology and be-havior. New York: Plenum Press.

Reuter-Lorenz, P,A,, & Baynes, K. (1992). Modes oflexical access in the cailosotomized brain, jour-nal of Cognitive Neuroscience, 4, 155-164.

Reuter-Lorenz, P.A., Nozawa, G., Gazzaniga, M.S.,& Hughes, H.C. (1995). The fate of neglectedtargets in the caliosotomized brain: A chrono-metric analysis, journal of Experimental Psychol-ogy: Human Perception and Performance, 21, 211-230.

Robertson, L.C, Lamb, M.R,, & Zaidel, E. (1993).Interhemispheric relations in processing fuer-archical patterns: Evidence from normal andcommissurotomized subjects, Neuropsychology,7, 325-342.

Sergent, J. (1990). Furtive incursions into bicameralminds. Integrative and coordinating role ofsubcortical structures. Brain, 113, 537-568,

Seymour, S,, Reuter-Lorera, P.A., & Gazzaniga,M.S. (1994). The disconnection syndrome: Ba-sic findings reaffirmed. Brain, 117, 105-115.

Singer, J,, Chedd, G., & Angier, j . (1997). Pieces ofmind: inside the human body [Videotape].(Available from Connecticut Public Television,Hartford, CT)

Smith, E,E,, & Jonides, J. (1997), Working memory:A view from neuroimaging. Cognitive Psychol-ogy, 33, 5^2.

Stone, V.E., Nisenson, L., Eliassen, J.C, & Gazza-niga, M.S. (1996). Left hemisphere representa-tions of emotional facial expressions. Neuro-psychologia, 34, 23-29.

Van Kleeck, M. (1989). Hemisphere differences inglobal versus local processing of hierarchicalvisual stimuli by normal subjects: New dataand a meta-analysis of previous studies. Neu-ropsychologia, 27, 1165-1178,

Weekes, N.Y., Carusi, D., & Zaidel, E. (1997), In-terhemispheric relations in hierarchical per-ception: A second look. Neuropsychologia, 35,37^4.

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