Quick 1.      

 

SPLIT-BRAIN ESSENTIALS: THEIR RELEVANCE FOR THE

CONCEPT OF ONE'S OTHER MIND

 

 

by

 

 

Joseph E. Bogen, M.D.

 

 

Clinical Professor of Neurological Surgery

University of Southern California

and

Adjunct Professor of Psychology

University of California at Los Angeles

and

Visiting Professor of Biology

California Institute of Technology

 

 

 

        Thenervous system, I repeat, is double . . . I use the word brain to include thecerebral hemisphere and the subjacent motor and sensory tract. [Whereas] I usethe word encephalon to include all parts of the nervous system within theskull. (J.Hughlings Jackson, 1874)

 

        . . .Although some authorities have been reluctant to credit the disconnected minorhemisphere even with being conscious, it is our own interpretation based on alarge number and variety of non-verbal tests, that the minor hemisphere isindeed a conscious system in its own right, perceiving, thinking, remembering,reasoning, willing, and emoting, all at a characteristically human level, andthat both the left and the right hemisphere may be conscious simultaneously indifferent, even in mutually conflicting, mental experiences that run along inparallel. (Nobelist R. W. Sperry, 1974).

 

        It is acommon observation that persons can have misperceptions, motivations and evenbehaviors which we recognize but of which they seem unaware. This lends itselfeasily to what I call the naive theory of "an other mind" (in eachperson so observed). This is not simply the "problem of otherminds" (Shorter, 1967; Churchland, 1984) in that philosophers have oftenquestioned the ascription of mind (including feelings and thoughts) to entitiesother than one's own self. The concept of "an other mind" not onlytakes for granted that each human has mind, including feelings, thoughts,memories, goals and intentions, but moreover claims that these exist induplicate. Restated, it claims that within each person who is less than 100percent insightful, there is goal seeking, self-aware mentation beyond the mindeach person readily acknowledges.

        Is italso true of us, that which we see in others? An empirical answer requiresthat we discover perceptions, motivations and behaviors in ourselves of whichwe are unaware. This is not so easy. For example, hypnosis may makeunrecognized ideation and feelings more apparent to an outside observer but, asFreud concluded, does less for the subject. Freud and his followers thereforeturned to other techniques (e.g., free association) whose conflux made up thepractice of psychoanalysis, which in turn engendered a variety of psychodynamictheories. This paper considers some possible relations between certain aspectsof such theories and the split-brain research which began nearly fifty yearsago and is still ongoing (Tramo et al, 1995; Corballis, 1995; Berlucchi et al,1997; Iacoboni & Zaidel, 1999).

        Observationson human split-brain patients, originally made in the laboratory of Roger W.Sperry at Caltech and leading to his sharing the 1981 Nobel Prize forPhysiology, have long been considered informative for a number of issues inpsychiatry (Bogen, 1969; Bogen and Bogen, 1969, 1999; Galin, 1974, 1988; Hoppeand Bogen, 1977; Hoppe, 1977, 1988, 1989; Benson and Zaidel, 1985; TenHouten etal, 1986, 1987). More recent contributions have been extensively reviewed bySchiffer (1996, 1998).

        Thehuman split-brain results can be summarized in two main conclusions: 1) eachcerebral hemisphere can function to a significant extent independently of theother; 2) in the vast majority of humans the two hemispheres functiondifferently in important respects. The first point, hemispheric independence,had already been shown in split-brain cats and monkeys (Myers & Sperry, 1953;Sperry, 1961); the human results replicated this fact in more dramatic fashion. The second point, complementary hemispheric specialization (Milner, 1971,1980), had previously been shown by studies of patients with lateralizedlesions (Paterson & Zangwill, 1944; Zangwill, 1961; Hcaen et al, 1956). The human lesion results were now reinforced by a different sort of evidence,because it was possible in split-brain persons to demonstrate what eachhemisphere could do on its own, rather than inferring function from deficitsfollowing focal lesions. The confluence of lesion and split-brain evidence gavetheir common conclusion greater credibility (Sperry, 1974; Trevarthen, 1984). This was subsequently supported by a third line of evidence, imaging studies(Mazziotta et al, 1982; Van Lancker, 1997). These efforts gave rise toinnumerable papers, and a plethora of books by psychologists reporting studiesof so-called "normals."

        Theforegoing two main conclusions take on additional import with the understandingthat they obtain in anatomically intact as well as split-brains (Bogen, 1969,1986, 1990). This is based on the expectation that activity in the corpuscallosum can fluctuate, that it is only rarely complete, that it can includeinhibitory components, and that for a majority of fibers, there is a sizeableconduction delay (Ringo et al, 1994). One immediate corollary of complementaryhemispheric specialization is that callosal traffic is asymmetric,right-to-left information being partially but importantly different fromleft-to-right. How various conditions influence the amount and nature ofinterhemispheric communication would appear relevant to circumstances givingrise to conscientiation, catharsis, insight, creativity and other phenomena ofpsychodynamic interest. Further conjectures along this line will besustainable to the extent that they reflect accumulating empirical informationrather than a priori convictions, however rational these latter may be. (Iinclude views like those of the philosopher Frank Jackson (1997) that untilprovided with a convincing rational argument that mind must issue from brain,he remains unimpressed by empirical findings.) Based on a forty yearinvolvement in the split-brain research, it seems to me rather presumptuous forsomeone to opine on the human split-brain without having a reasonablefamiliarity with the simpler case of split-brain cats. In addition, an informedappraisal of the split-brain requires familiarity with hemispherectomizedhumans, of whom there are now many hundreds, often living essentially normallives.

        In thischapter I will begin with the split-brain cat results. I will then review thestandard picture with split-brain humans. A few observations will followregarding hemispherectomy, the "two semantic systems" issue, and abrief discussion of some criticisms of the split-brain research. Thenconsidered is the relevance for creativity of the two main conclusions: partialhemispheric independence and complementary hemispheric specialization. This essayends with a few concluding remarks.

 

THE SPLIT-BRAIN IN CATS AND MONKEYS

        Thecorpus callosum, with over 200 million nerve fibers (Aboitiz et al, 1992),constitutes a nerve fiber collection larger than the sum of all fibersascending to and descending from the cerebral hemispheres. The centrallocation and the large size of the corpus callosum, especially in the human,have for centuries suggested an important role in mentation. During the lastfifty years, a large number and variety of experiments have made it clear thatthe corpus callosum can transfer high level information from one hemisphere tothe other. Moreover, we now know that the hemispheres are not so much"major" and "minor" as they are complementary and that eachhemisphere is capable of thinking on its own, in its own way. Much of thisinformation has come from cutting the corpus callosum, that is, the split-brainoperation.

        Thesplit-brain experiments started with the problem of interocular transfer. (Sperry and Clark, 1949). That is, if one learns with one eye how to solve aproblem, then with that eye covered and using the other eye, one readily solvesthe problem without further learning. This is called "interoculartransfer of learning." Of course, the learning is not in the eye and thentransferred to the other eye, but that's the way it's usually described. Thattransfer occurs may seem obvious, but it is in the questioning of the obviousthat discoveries are often produced. In this case the question is: how canthe learning with one eye appear with use of the other? Experiments showedthat the transfer required the corpus callosum. Sperry's scheme was to splitthe optic chiasm so that the right eye goes to the right cerebral hemisphereand the left eye to the left hemisphere and also to cut the corpus callosumbetween the two hemispheres. This is a "split-brain cat." The catcan be trained with the right eye to choose a cross rather than a square, whilethe left eye is covered. The cat chooses one of two doors at the end of arunway, the cross and square being attached to the doors randomly, only thedoor with cross leading to a food reward. After the cat has learned theproblem (regularly picks whichever door has the cross) one can test the lefteye with the right eye covered; the split-brain cat has to learn all overagain, that is, it starts at 50% (chance). For each cat the learning curve forthe left eye (and left hemisphere) is very similar to the learning curve forthe right eye. Since a split-brain cat has to learn all over from thebeginning with the second eye, the cat can be trained to pick the squareinstead of the cross when using the second eye. It then depends on which eyeis open which choice the cat makes. Thus, each hemisphere has developed a differentmemory about what is correct. Which is to say, each hemisphere has its ownsemantic system (i.e., a system which gives meaning to symbols). That the twohemispheres could be so disparate, giving different, even opposite meanings tosymbols (cross and square) may be surprising since the two thalami in a cat arequite tightly coupled anatomically. Since the anatomical coupling of thalamiin a monkey is a bit less, one might expect a similar duality of mentation insplit-brain monkeys whose visual system is more similar to humans than cats,learn much faster, and have, moreover, a considerable capacity for fine fingermanipulation. Indeed, monkeys show even more than cats a duality of mentation(Sperry, 1964). Two monkey experiments exemplify the large literature:

        1. Oneof the most reliable signs of a bilateral prefrontal lobectomy in monkeys istheir inability to do delayed-alternation tasks (Jacobsen and Nissen, 1937;Sawaguchi and Goldman-Rakic, 1991; Fuster, 1997). It was long supposed thatthis inability could be explained as the result of the distractibility which isalso characteristic of such monkeys. This supposition was tested insplit-brain monkeys. If one hemisphere has a prefrontal lobectomy, it performspoorly on the delayed-alternation task. This poor performance by thelobectomized hemisphere is not accompanied by hyperactivity or distractibility. Apparently, the remaining frontal lobe keeps the monkey quiet and attentiveeven though the intact hemisphere is not participating in the recognition orevaluation of various stimuli (Glickstein et al, 1963).

        2. Adramatic example of mental duality occurs when only one hemisphere of asplit-brain monkey has had a temporal lobectomy. A bitemporal monkey manifeststhe Klver-Bucy syndrome, which includes difficulties in the visualidentification of objects, orality, hypersexuality, and tameness in thepresence of humans. When the intact hemisphere can see, the split-brain rhesusmonkey behaves in the usual rhesus manner, manifesting a fierce fear of humans. But if only the temporal lobectomized hemisphere receives the visualinformation, the split-brain animal acts like a Klver-Bucy monkey, particularlyas regards its relative tameness. When this was reported (Downer, 1961, 1962)it was so amazing that many doubted it. Little room for doubt remains becausethis finding has, in its essentials, been reported by at least seven otherinvestigators including Doty and Overman (1977).

        It isimportant to understand that the duality of minds seen after hemispheredisconnection is not an inference solely from a few clinical cases, and ahandful of surgical patients. Split-brain experiments have been carried outwith many different species by hundreds of investigators around the world. They are virtually unanimous in concluding that each of the disconnectedhemispheres can act independently of the other (Bogen, 1977).

 

THE SPLIT-BRAIN HUMAN

        Experimentswith monkeys paved the way for cutting the corpus callosum in humans as atreatment for severe epilepsy. It has been learned that complete callosotomyis often not needed, so that partial sections have become more common (Bogen,1997a). These partial sections are commonly undetectable. However, whensections involve the posterior part of the corpus callosum they can producepartial aspects of the complete callosotomy (split-brain) syndrome (Bogen 1993,1998).

        The termsplit-brain has several meanings. Applied to the human, it denotes completecallosal section_an operation that has been performed for medically intractableepilepsy, to stop the spread of seizure activity from one hemisphere to theother. It does not imply, as in cat and monkey, a split of the optic chiasm. Restriction of visual input to a single hemisphere in humans therefore requiresrestriction of the stimuli to one visual half-field (and careful monitoring ofthe subject's fixation) rather than restriction of the stimuli to one eye as incats and monkeys. Our own split-brain patients had complete cerebralcommissurotomy including anterior commissure, dorsal and ventral hippocampalcommissures as well as complete callosotomy. But it is now common to use theterm split-brain to refer to human cases of complete callosotomy alone, becausethey show most of the same signs and symptoms. When patients who have had acomplete callosotomy have recovered from the acute operative effects and reacha fairly stable state, they show a variety of phenomena which can be groupedunder five headings.

        SocialOrdinariness: One of the most remarkable results is that in ordinary socialsituations the patients are indistinguishable from normal in spite of thecutting of over 200 million nerve fibers. Special testing methods are neededto expose their deficits. These methods usually involve restricting the inputto only one hemisphere, for example by putting an object into one hand, withvision occluded.

        Lack ofInterhemispheric Transfer: A wide variety of situations have been developed toshow that the human subjects are in this respect the same as split-brain catsand monkeys. A typical example is the inability of the split-brain person (ormonkey) to retrieve with one hand from a collection an object palpated with theother, although same-hand retrieval of the test object is normal.

        HemisphericSpecialization Effects: The hemispheric specialization for language which istypical of human subjects results in phenomena not seen in split-brain animals. A typical example is the inability of right-handers to name or describe anobject in the left hand, even when it is being appropriately manipulated, andmoreover the test object can be reliably retrieved from a collection when thesubject has previously felt it with the same, retrieving hand. This is called"left hand anomia" (in righthanders) but is more than lack of aname_the object cannot be well described, which it is when placed into theright hand of a subject who does not know the name of the object (e.g., a bonerongeur or an unusual pine cone). The unilateral anomia (in the right hand ofour single left-handed patient) is especially important because it is easy totest for, and because it has been present in every subject with completecallosotomy, and because it persists for years in spite of continuingimprovement in other respects; for example, the left-handed apraxia to verbalcommand was almost uniformly present post-op but rapidly subsided in most ofthe patients (Bogen & Vogel, 1975).

        Itdeserved emphasis that the inability to name or describe an object in the handcontralateral to the hemisphere capable of speaking is because the speakinghemisphere does not have the tactile and/or proprioceptive informationnecessary to identify the object. Nevertheless, the object is appropriatelymanipulated (e.g., opening and closing a safety pin) and can be reliablyretrieved (with the same hand) from a collection of ten or twelve other objects(usually on a paper plate) among which the test object has been placedfollowing palpation. The appropriate manipulation and correct same hand retrievalmake it clear that somebody in there (in the right hemisphere) knows what itis.

        CompensatoryStrategies: Split-brain subjects progressively acquire over the years avariety of strategies for circumventing their interhemispheric transferdeficits. A common example is for the patient to speak out loud the name of anobject palpated in the right hand; because the right hemisphere can recognizemany individual words, the object can then be retrieved with the left hand. There is a great deal of variation from one patient to the next in thesecompensatory developments. Even in the best compensated cases, however, anexperienced examiner can parry the strategy and elicit the deficit ininterhemispheric transfer (Zaidel et al, 1990; Reeves, 1991; Tramo et al,1995).

        ImpoverishedFantasy: Although split-brain patients seem rather ordinary during socialactivity (e.g., having lunch or traveling with experimenters), formalpsychiatric interview techniques have repeatedly shown that the patients'fantasy life is reduced, or even absent. Most of them acknowledge dreamingwhen pressed, but the dream content is largely of everyday events, largely freeboth of strong affects and of the condensation, displacement and symbolizationtypical of the dreams of individuals who are anatomically intact (Hoppe &Bogen, 1977; Hoppe, 1977, 1989).

        Eightsplit-brain patients were given the TAT and Rorschach tests by Lewis (1979). The former elicited stories minimally elaborated, if at all. The Rorschachresponses were notable for an absence of movement and a paucity of signssuggesting creativity.

        Theforegoing led to a series of experiments by TenHouten et al (1988, 1989) inwhich split-brain patients (and controls) were studied by interview and EEGwith respect to their reactions (which were matter of fact and uninvolved) to ashort movie containing symbols highly loaded with affective significancerespecting separation and death. In addition to these experimental results,there was evident a similarity between the group of split-brain patients, andpersons with psychosomatic or addictive disorders, both groups being ratherunconcerned with the impoverishment of their fantasy life. These considerationsled to the hypothesis of "functional commissurotomy" by Hoppe (1988,1989). That is, alexithymic patients appeared to be deprived, as were thesplit-brain patients, of what Hoppe has termed "hemisphericbisociation" which facilitates symbolexia, the verbal expression ofempathic identifications with symbolized affective states.

        Much thesame lack of fantasy and of ornate dream content was observed in Hoppe'sinterviews of patients who had right hemispherectomy. This operation,especially in the radical version called total hemispherectomy orhemicerebrectomy, is discussed below.

 

THE FUNDAMENTAL FINDING

        Thelaboratory examination of split-brain patients has undergone a progressivecomplexity and sophistication, with detailed elaboration (Zaidel, Zaidel &Bogen, 1990; Tramo et al, 1995). There are hundreds of technical paperspublished about these patients, which sometimes tends to obscure thefundamental finding.

 

HEMISPHERECTOMY

        The duality of cerebral anatomy raises a basic question: to function effectively, is it necessary to have both members, or can one of the pair suffice? Analogously, one might ask: are the paired structures like the runners of a sleigh such that if one is removed the sleigh founders? Or more like a team of two horses such that if one is removed, the other can still pull the sleigh, albeit not as fast or as far. The latter is correct, as shown by hemispherectomy in both experimental animals and in humans.

        The main lesson learned from experimental hemispherectomy has been that so long as the removal is confined to one side, there is remarkably little effect in the long term on either instinctive or learned behavior (Bogen, 1974). Although hemispherectomized monkeys are capable of learning quite complex problems, they may have difficulties when there are long delays between trials. They may learn more slowly than normal but can attain comparable levels. Patton (1961) observed:

        Perhaps the most striking finding was that the hemicerebrectomized monkey can achieve a level of performance on a complex learning task which equals that of the normal animal.

        Following ten years of extensive studies Kruper, Patton and Koskoff (1971) concluded:

        . . . showing clear learning deficits when comparing the performance of these animals to the performance of normal monkeys continues to be a compelling challenge.

 

        The first hemispherectomies in humans were done for tumor by Dandy (1933). Subsequent hemisphere removals for tumor or trauma have been done on subjects who developed normally into adulthood (Smith, 1966; Burklund and Smith, 1977; Bogen, 1997c). However, the overwhelming majority of hemispherectomies (many hundreds) have been done for seizure disorders associated with infantile hemiplegia, with gratifying results as indicated by this being a standard, oft-used procedure in 18 of 25 epilepsy centers (Engel, 1993). Some people have dismissed hemispherectomy as providing evidence that one hemisphere (the residual hemisphere) is enough for consciousness on the mistaken belief that the operation is always just a hemidecortication. So eminent a scientist as Larry Weiskrantz has been among these misinformed folk (Weiskrantz, 1997). Indeed, a large majority of operations called "hemispherectomy" leave much behind; but there are enough "total hemispherectomies" to prove the point. In both cats (Bogen, 1974) and monkeys (White et al, 1959; Kruper et al, 1971) the operation was a hemicerebrectomy, right down the midline. Since removal of one-half the hypothalamus can be accompanied by (fatal) bruising of the remaining half, this operation is not appropriate for humans. However, Burklund used the course of the internal cerebral vein to guide his incision (see Burklund & Smith, 1977), removing the basal ganglia. And when both cortex and basal ganglia are removed, the thalamus completely atrophies (Jones, 1985). Similarly, Austin and Grant (1950) did what they called a "total hemispherectomy" which included the basal ganglia, in order to remove tumors. They did not remove the ipsilateral thalamus but knew it would be non-functional and soon atrophy, having lost all of its targets. Of their three normally developed, adult tumor patients who were alert pre-op, all continued speaking and were "acutely aware" of their surroundings throughout the operation which was done under local anesthesia.

        More recent is our study (Bogen et al, 1998) in July 1997 of B.L., who had a hemispherectomy at age 5?. At age 35, he had a verbal IQ of 126 (Smith & Sugar, 1975). When we saw him at age 50 he had been self-supporting for nearly 20 years, appeared to be both computer wise and musically sophisticated, and in person was more lively and charming than most neurologists of my acquaintance. His July 1997 MRI shows the intracranial space is half empty. Whatever Oscar Sugar did not remove in 1953 has atrophied and been absorbed.

        That the significance of hemispherectomy for consciousness is not better known is perhaps surprising in view of the widespread recognition of its therapeutic value and the eminence of its advocates for neurotheory, such as Robert Doty, past president of the Society for Neuroscience, who has repeatedly called attention to its importance (Doty et al, 1986; Lewine, et al, 1994; D;       That thesignificance of hemispherectomy for consciousness is not better known isperhaps surprising in view of the widespread recognition of its therapeuticvalue and the eminence of its advocates for neurotheory, such as Robert Doty,past president of the Society for Neuroscience, who has repeatedly calledattention to its importance (Doty et al, 1986; Lewine, et al, 1994; Doty andRingo, 1994; Doty, 1998).

        Patientswith complete callosotomy are relatively rare. However, persons who have hadhemispherectomy are available in large numbers. Psychoanalytic investigationof these persons would almost certainly be informative. E.g., how general, inhemispherectomy patients is there evidence of repression and the usual defensesbut unaccompanied by fantasy? In any case, one need not be a psychoanalyst toentertain the thought that hemispherectomy evidence for duality of brain mightbe not widely discussed in part because it is unwelcome.

 

SOME RESERVATIONS REGARDING DUALITY

        Havingtwo hemispheres each in a separate head is not the same as having twohemispheres in the same head. And this holds whether or not theinterhemispheric integrating mechanisms in that same head include the cerebralcommissures.

        As notedby Marks (1980), no one nowadays doubts that one hemisphere is enough for amind "comfortably characterizable as human" (p. 47). Can twohemispheres, then, be enough for two minds? When they are in separate heads,the answer is clearly "yes." But what if they are in the same head?

        Unlike twohemispheres in two different heads, the disconnected hemispheres are (and havebeen) always in the same place at the same time, have the same circulatingenvironment (blood and CSF), and communicate a great deal subcallosally.

        Theextracallosal connections make a difference in several ways. For example, theduality of cognition demonstrable in the human split brain seems not to beaccompanied by so great a disparity of affect (Sperry, Zaidel & Zaidel,1979), perhaps because the two limbic systems are so tightly coupled at thehypothalamic level. Furthermore, both human hemispheres, whether in the intactor the split condition, are probably asleep and awake simultaneously. There isno evidence that primate hemispheres (in the same head) can alternate sleep andwaking, as do cetacean hemispheres (Serafetinides et al, 1972; Mukhametov etal, 1977).

        Manypeople seem to believe that the corpus callosum is the main thing that makes usunified (whatever that means). In their view, dividing the corpus callosum"splits the mind" into two, whereas it was previously one. Wouldrestoring the commissures reverse this process? Or, put differently, wouldadding 200 million bridging nerve fibers render a previously complicatedsituation (mental duality) more simple? Brain damage simplifies and rigidifiesthe injured person's behavioral repertoire. The split-brain human is simplerthan the anatomically intact, not more complex. Similar to most scientificexperiments, the simplification exposes circumstances which are hidden orcompensated in the intact individual.

        In myview, the big difference between two hemispherectomized persons and us, withtwo brains in one head, is the presence of many integrative mechanisms, ofwhich the corpus callosum is clearly not the most important for ordinary socialinteractions.

        Ifnumerosity of mind were a continuum, or were even fractionally quantizable, itmight be easier to describe the split brain (and others of us). As it is, ourcurrent language forces us to jump one way or the other. Between "onemind" and "two minds" to describe either the splits or theintact, "two minds" seems to me much closer to the mark.

        Until wehave a better vocabulary we are stuck with anthropomorphizing the hemispheresand with the resistance that such usage must necessarily elicit in those whoknow that they are (whatever their problems) better unified than any pair ofpeople with hemispherectomy.

        Manypersons unhappy with the idea of mental duality (especially for themselves)prefer the committee view of mind (Zeki and Bartels, 1999), the view that ourmentation is a mosaic of multiple modules whose unification is one of the greatmysteries (Doty, 1998). Basic to this dispute is whether consciousness isproduced by specific, dedicated neuronal circuits versus the view thatcomplexity counts more than anatomy and there are no privileged loci (seebelow).

        Since afew cubic mm of damage in the thalamic intralaminar nuclei can cause loss ofconsciousness, markedly contrasting with the retention of consciousness inspite of huge removals of cerebral tissue elsewhere, it is my belief that thereare two privileged loci, one in each hemisphere (Bogen, 1993b, 1995, 1997b). If this is correct, it makes clear why the only brain bisection consistent withneurophysiologic reality is midsagittal, severing the callosal connectionbetween the hemispheres

        Itdeserves emphasis that what is meant by a privileged locus is not some place(or two places, one in each hemisphere) where "it all comestogether." What is meant is that there is a structure in each hemispherecapable of endowing neuronal patterns in cortex with the property of self or"me-ness." Crick and Koch (1990) who have led the current search forthe physiologic basis of consciousness have recently suggested (personalcommunications) that certain structures enable cortical neuronal patterns toattain consciousness. Until the physiologic basis is established, the argumentwill persist between advocates of privileged (anatomically specified) lociversus the view that consciousness arises from sufficiently complex neuronalactivity which, "can transcend traditional anatomic boundaries" (Tononiand Edelman, 1998).

 

TWO SEMANTIC SYSTEMS

        Duringthe past several decades neuropsychology has been progressively influenced bylinguistic concepts. This has been reflected in descriptions and analyses interms of phonology (the sound system), morphology (combining sounds intowords), lexicon (available symbols including words), syntax (combining lexicalitems into phrases), and semantics (the meanings of symbols and phrases), andrecently, increasing attention has been given to pragmatics (the context ofcommunication) for which the right hemisphere is particularly important (VanLancker, 1997). Nowadays the left hemisphere is less often considered dominantfor all of language; rather, left hemisphere dominance is accepted specificallyfor phonology and syntax. Semantics, however, remains controversial: does anormal cerebrum harbor just one semantic system, or two (or more)? One mightsuppose that at least two would be an obvious answer, in view of thesplit-brain results; but not all have seen it this way.

        Thenumerosity of semantic systems occupied the entire first issue (pages 1-150) ofVolume 5 (1988) of the journal Cognitive Neuropsychology. The evidencepresented was almost entirely derived from patients with brain injuries(usually strokes). There are 160 references (with considerable duplication,especially references to T. Shallice and E. Warrington); none are to any of thesplit-brain literature. There is a passing reference by Shallice to anunpublished work by Coslett and Saffran assuming two semantic systems, one ineach hemisphere. (These authors have continued along this line since [Saffran& Coslett, 1998] actually ascribing to each hemisphere not only a semanticbut a conceptual system.) Shallice concluded, "the neuropsychological evidencedoes not at present speak conclusively . . ." . I believe his opinionarose out of 1) the uncertainties attending much of the lesion evidence; 2)his giving little or no weight to the split-brain evidence.

        In thewidely cited book by Shallice (1988) there are a couple of diagrams positingtwo semantic systems. By contrast, there are 19 diagrams explaining humanmentation in which is posited a single semantic or cognitive system. Particular prominence is given to a reprinting of the diagram of Warrington andTaylor (1978). This same diagram (with one semantic system) is also featuredin Rudge and Warrington (1992), an article which briefly refers to callosotomy,citing papers by Akelaitis (1942, 1944). There is no reference to any of thehundred plus refereed articles on the Caltech patients; notably missing arepapers by Sperry responsible for (e.g., Sperry, 1968, 1974) and responding to(Sperry, 1982) his Nobel award; nor is there any reference to split-brainpatients operated and tested elsewhere (e.g., Sidtis et al, 1981; Gazzaniga,1988; Fendrich & Gazzaniga, 1989; Luck et al, 1989; Baynes, 1990).

        Theassiduous avoidance of split-brain data appearing since 1944, in any species,raises interesting questions for sociologists and historians of science, and itmay also reflect psychodynamic processes already well known (on the other handthe avoidance may be deliberate rather than unconscious in which case it mightbe said to be unconscionable).

        The twosemantic or conceptual system issue recurs not only in the context of braininjury but in many studies of "normal" subjects, sometimes quiteexplicitly (Burgess & Skodis, 1993; Hellige, 1993; Burgess & Chiarello,1996; Chiarello, 1998; Atchley et al, 1999). These authors have commonly takennote of the split-brain findings (e.g., Zaidel, 1978).

        Proposalsfor distinguishing the hemispheric predilections include the conclusion that,at least with respect to words, initial processing "is rapid and focusedin the left hemisphere" contrasting with the right hemisphere's"activating a broader range of semantic information [with] increasingsensitivity to pragmatic information" (Burgess & Chiarello, 1996).

 

THE ANARCHIC (ALIEN) HAND

        Duringthe past two decades there has developed increasing interest in a phenomenonnow known as the anarchic hand (Della Sala et al, 1991, 1994; Baynes et al,1997). The anarchic hand (AH) consists of well-coordinated, seeminglypurposeful movements for which the patient denies responsibility, typicallyexclaiming, "That wasn't me that did that." The AH often leads tointermanual conflict. Baynes et al (1997) described a patient as reportinginstances in which the left hand closed doors the right hand had opened,unfolded sheets the right had folded, snatched money the right had offered to astore cashier, and disrupted her reading by turning pages and closing books.

        AH hasbeen seen consequent to callosal lesions at least since the report of Goldstein(1908). Brion and Jedynak (1972) first called the phenomenon "l'tonnementdevant le trouble unilatrale gauche." In their subsequent, admirablydetailed book (Brion & Jedynak, 1975) they called it, "l'autocritiqueinterhmisphrique" (p. 69). It has been a typical aspect of the acutedisconnection syndrome in split-brain patients (Bogen, 1969, 1998). Forexample, a few weeks after one split-brain patient (RY) underwent surgery, hisphysiotherapist said, "You should have seen Rocky yesterday_one hand wasbuttoning up his shirt and the other hand was coming along right behind itundoing the buttons!" (Bogen, 1979).

        The term"alien hand" was erroneously introduced (Bogen, 1979) as the resultof my misreading of Brion & Jedynak (1975). A recent re-reading makesclear that Brion & Jedynak used the term "la main trangre" to describea misidentification resulting from failure of interhemispheric sensory transfer, whereas they used the term ""l'autocritiqueinterhmisphrique" to describe seemingly purposeful actions disavowed bythe patients.

        The term"alien hand" has become common in the neurologic literature, but itis misleading to describe a "wayward" hand as "alien" notonly because it was not what Brion and Jedynak meant, but because (as pointedout by Della Sala et al) the patients do not ascribe the hand to someone else butrecognize it as their own, although out of control

        Theemphasis by Brion and Jedynak (1975) and by Bogen (1979) on callosaldisconnection as the cause of AH was challenged by Goldberg et al (1981) whoreported two right-handed patients with a right AH subsequent to left mesialfrontal infarction. The role of mesial frontal damage is difficult to evaluatesince such lesions typically also involve the corpus callosum. The necessityfor callosal disconnection (particularly for persisting cases) thus resultingin hemispheric independence would be disproved if AH emerged inhemispherectomized individuals suffering subsequent frontal damage. Thisdebate is unresolved. It has been suggested (Feinberg et al, 1992) that thereare two forms of AH, one callosal and the other mesiofrontal. To the extentthat a callosal lesion is essential (Geschwind et al, 1998) the AH supports theidea of an "other mind." Alternatively, it may be that thebehavioral dissociation (between hand action and verbalization) is the resultof disconnecting visually guided motor planning from the mechanism which canendow efferent neuronal activity patterns with awareness (Milner & Goodale,1995; Bogen, 1997).

 

MORE NEGATIVE VIEWS

        Thesplit-brain data, as well as what they implied, were sufficiently dramatic toattract media attention. Much of this was hastily written and oftensensationalized. An article in the New York Times Magazine on September 9,1973, included an artist's fantasy of a split-brain patient wielding a hatchetin the left hand which was being restrained by the right hand. Less esteemedmedia outlets were worse. The media pushed the popularity of the "rightbrain/left brain" story to fad proportions, reaching an almost frenziedpeak by 1979. This led not only to simplistic degradation, probably inevitablewith popularization, but also to exploitation. Commercially motivatedentrepreneurs promised to educate people's right hemispheres in short order,sometimes even overnight, ignoring the lengthy, arduous training for any maturecompetence. This was followed by a negative reaction or backlash, discussedmore fully elsewhere (Bogen, 1993) some mention is appropriate here.

        Somecritics of the split-brain research, were simply ignorant, like the U.C.Berkeley professor who claimed the implications were derived from "a smallgroup of epileptics," quite ignoring the cat and monkey results. Or thelinguist (since better informed and now a friend) who considered thesplit-brain research fallacious because complete section of the corpus callosumwould require "penetration of the third ventricle," which accordingto him would be a neurosurgical error (but see Apuzzo, 1987).

        It hasbeen said that the split-brain results are too variable from one patient to thenext to be meaningful. The variations must no doubt be taken into account (andhave been; see Bogen & Vogel, 1975; Bogen, 1993a, 1998). The results ofgreatest import are those common to all the patients such as the left handanomia which not only has been present in every patient with a complete sectionbut in all of them has persisted indefinitely with over 30 year follow-up onsome.         Other critics have been moresophisticated, often beginning their critique with the claim that anatomicalentities (such as hemispheres) are inappropriate constraints on theirphilosophic or psychologic theorizing. Consider the following example:

        . . .once Descartes' ghostly res cogitans is discarded, there is no longer a rolefor a centralized gateway, or indeed for any functional center to the brain. The brain itself is Headquarters, the place where the ultimate observer is, butit is a mistake to believe that the brain has any deeper headquarters, anyinner sanctum. [A similar disdain for neuroanatomical detail is reflected in Lycan'sassertion, "The central nervous system is as central as it gets"(Lycan, 1997, p. 762)] . . . as a matter of empirical fact, nothing in thefunctional neuroanatomy of the brain suggests such a general meeting place . .. (Dennett & Kinsbourne, 1992).

 

        Dennettand Kinsbourne make the argument that there can be no special structure forconsciousness because such a structure should be, as Descartes (1649) insisted,". . . n'est nullement le coeur, ni aussi tout le cerveau, mais seulementla plus intrieure de ses parties, qui est une certaine glande fort petite,situe dans le milieu de sa substance."(page710) [emphasis added]. Inother words, Descartes believed that the special structure should be "inthe middle". And no such midline structure exists (the pituitary andpineal no longer being credible candidates). This argument has been echoed bymany philosophers. However, the facts of hemispherectomy and completecallosotomy have rendered this "argument about the midline" pass.

        What amodern special structure theory requires is the existence of two suchstructures, one "in the middle" of each hemisphere. It is common forfolks to speak of the functions of the thalamus, singular, in a normalcerebrum. This might be reasonable for rats or cats with sizeable midlinethalamic nuclei; but in humans the two thalami are not in direct communication(even by the massa intermedia when it is present) and the midline nuclei arenegligible in humans (Jones, 1985). In a human, for the thalamus in onehemisphere to communicate with the other thalamus requires pathways down andback up through the brainstem (notably the upper mesencephalon) or up tocortex, across the callosum and down via corticothalamic projections.

        Althoughthe "argument about the midline" is now excluded, other argumentsagainst a special structure continue. Dennet and Kinsbourne (1994) reiterated,

        . . .the limit on the number of consciousnesses that could theoretically be housedin the brain (given suitable disconnections) is the minimal complexity of theneuronal substrate that suffices for this kind of functioning. There could bemany, and certainly more than two. . . Given a lateral (coronal) transection,the posterior sector may be precluded from controlling behavior, while theanterior one be sorely lacking in information to guide spared action.

 

        Muchearlier, Kinsbourne (1982) wrote that having left and right brains was to beexpected because any bisection would double mentality, including that a coronalsection would also create two minds: a front mind and a back mind. At first, Ithought this a bit of straight-faced drollery. (In the real world, suchsurgery would result in garbage.) When I wrote to him suggesting that hehimself knew that this was unrealistic in the extreme, he replied that it was"an heuristic device." Even now (we remain friends) he considers itmore heuristic than humorous. He has provided a more fully developed viewalong these lines: his "integrated field theory":

        Awarenessis a property of neural networks, not of any particular locus in the brain. Were it possible further to subdivide the human brain, there is no reason inprinciple why several or many independently aware neural systems might notresult. . . . in so far as the cognitive strengths and emotional proclivitiesof each hemisphere are not the same, processing of, and response to, the onrushof events would to some extent differ, further amplifying discrepancies in theexperience of the two hemispheres. This dramatic circumstance does notindicate that the organism housed two distinct personalities (one perhapssuppressed by the other ) prior to the callosal section (Bogen 1969; Puccetti1973). The intact individual's unified awareness [sic] precludes this . . . (Kinsbourne, 1988).

 

        One canunderstand why, fortified by such opinions from well-known, knowledgeableneurologists, most philosophers would ignore the split-brain results, while theremainder would shrug off the suggestion of mental duality. E.g., theneurophilosopher P. S. Churchland wrote:

        Thiswork clearly established that, under suitable conditions, one could show adefinite disconnection between the cognitive activities of the twohemispheres_that each hemisphere had, so to speak, a life or unity or integrityof its own. . . . So much is clear. But hard on the heels of the observationcomes the inescapable inclination to infer that they have two of somethingwhere the rest of us have only one. But two of what! Two minds perhaps, ortwo souls, or two selves, or two persons, two centers of consciousness, twocenters of cognition, two centers of control, two wills, or what? . . .

        Until weknow what we are counting, we cannot begin to count_and we cannot even say withmuch confidence that we have one of whatever it is that split-brain subjectsseem to have two of." (Churchland, 1986).

 

        ThomasNagel was one of the few philosophers to take a serious look at thesplit-brain. He wrote:

        What theright hemisphere can do on its own is too elaborate, too intentionallydirected, and too psychologically intelligible to be regarded merely as acollection of unconscious automatic responses" (p. 403). . . . [And], ifthe patients did not deny awareness of what is being done [by their righthemispheres] no doubts about their consciousness would arise at all." (p.404). . . . [However,] if the idea of a single mind applies to anyone itapplies to ordinary individuals with intact brains, and if it does not apply tothem it ought to be scrapped, in which case there's no point in asking whetherthose with split-brains have one mind or two. (p. 409). (Nagel, 1971).

 

        In fact,the idea of a single mind applies exactly to an individual who has had ahemispherectomy (Bogen, 1977). But Nagel was oblivious to the significance ofconsciousness after hemispherectomy, and in this he has for company almost allauthors on consciousness.

 

 

INTERHEMISPHERIC EXCHANGE AS A

SOURCE OF CREATIVITY

 

        Explanationsof creativity in terms of brain function are uncommon. One brain-based theorywas advanced by us in 1969, updated in 1988, and recently synopsized (Bogen& Bogen, 1968, 1988, 1999). Graham Wallas (1926) suggested that creativityproceeds in four distinct phases: preparation, incubation, illumination, andverification.

        Toproduce something both novel and meaningful one must have a period ofpreparation. This involves acquiring a large fund of information. Next comesa period of incubation during which time the information is rearranged,typically while one is unaware of the process. Then follows illumination. Almost everyone is familiar with the cartoonist's use of a lightbulb tosymbolize the instant illumination of an idea. There is then necessary a phaseof deliberate reorganization and refinement, readily describable by thecreator, to test and refine the final product.

        What canbe the physiologic basis for this succession of stages? During incubation,some very productive thinking goes on which is inaccessible to verbal output(in that one cannot tell how it went on) and whose result can become availablein a sudden insight. Where does this thinking take place? To say that"it comes from the heart" describes the quality rather than theorigin. To say that it comes from "intuition" is merely to rename itrather than to give it a physiologic source. It surely requires a neuronalsystem of a size, complexity, and activity level comparable to thatorgan_namely, the left hemisphere_which produces the phonologic and syntacticrichness of human language. It is likely that much of the thinking that goeson during incubation takes place in the human right hemisphere.

        Bycontrast, the preparation and verification phases seem more left hemispheric. As for illumination, one sees the likelihood of a greater than usualinterhemispheric communication during an individual's more intuitive moments,an interaction dependent upon the corpus callosum. (Atchely et al, 1999).

        Creativity: If leere, so may problem solving. This contributes to a less predictable,that is, a less stimulus-bound, behavior. In other words, specialization ofthe hemispheres for different trains of thought greatly increases theflexibility of the ensemble. Such differentiation probably produces aconcomitant decrease in stability. The successful expansion of the humanspecies (so far) suggests that the loss of stability is less important than thegain in flexibility.

        Creativityhas not only made the human species dominant (and dangerous) on the earth; whatmay be more important for each of us is that it gives value and purpose tohuman existence. Creativity requires more than the propositional skills of theleft hemisphere; it also needs the cultivation and collaboration of the otherhemisphere. There is as yet no clear consensus on how best to describe thecognitive differences between the hemispheres. Whatever terms are used (e.g.,propositional vs. appositional), what is essential for our theory is thewell-established fact that in most humans the two cerebral hemispheres functiondifferently. We believe that this involves, indeed requires, a significantdegree of hemispheric independence.

        There isfirst of all the obvious point that no pathways, including those in the corpuscallosum, function fully at all times. Second, lengthy callosal transmissiontimes encourage hemispheric independence (Ringo et al, 1994) and, third, muchof the callosal activity is inhibitory (Chiarello & Maxfield, 1996). (Thisis because the callosal fibers, themselves glutamatergic, i.e., facilitatory,often terminate on gabaergic interneurons.)

        No onesupposes that all creativity involves the corpus callosum. The left hemispherealone, either following right hemispherectomy or isolated from the right bycallosotomy, is capable of a range of verbal expression, including generationof an unlimited number of novel sentences. But when a solution requirescombining of "dual memory codes, verbal and imaginal" (as BrendaMilner (1971, 1980) put it) one readily recognizes an important role for thecorpus callosum.

        The Lackof Creativity: If some kinds of creativity are dependent on a transcallosalinterhemispheric exchange, there are three obvious explanations for itsabsence. There may be a deficiency of technical competence in a suitablemedium; in the case of literary as well as mathematical creativity this iseasily seen as a lack of propositional skill. Alajouanine averred, "To conceiveis nothing, to express is all." He meant by this that pleasing musicalthemes and poetical images arise in the minds of many people who have notacquired the means for their expression.

        Second,many persons possess technical proficiency in music, drawing, or writing whoseproduction is devoid of those innovative and informative values thatdistinguish an artist from a performer. We are accustomed to hear, these days,of the "culturally disadvantaged," usually referring of properschooling. There is likely a parallel lack of appositional development inpersons whose education has narrowly emphasized reading, writing, and theirconcomitants.

        Third,there must be the possibility of a heightened communication between the twohemispheres, overcoming temporarily the ongoing lack of transfer that hasallowed independent processing; in some cases this may be simply a lifting oftonic inhibition.

        Theconditions for a fluctuation in callosal transmission in the anatomicallyintact individual have yet to be adequately explored. One consideration isthat inspiration occurs not only in repose but often in striving underpressure. In any case, variations in hemispheric exchange are clearlycontingent upon appropriate affective states. What is required is a partial(and transiently reversible) hemispheric independence during which lateralizedcognition can occur and is responsible for the dissociation of preparation fromincubation. A momentary suspension of this partial independence could accountfor the illumination that precedes subsequent deliberate verification. Fromthis point of view, we can understand better the observation of Frederic Bremer(1958), who wrote years ago that the corpus callosum subserves "thehighest and most elaborate activities of the brain"_in a word, creativity.

 

CONCLUDING REMARKS

        For thepractice ofpsychotherapy, postulating the presence of a second mind or personin the same head (perhaps even explicitly addressing it) is now defensible onphysiologic as well as psychologic evidence. However, for psychodynamic theorythere may be a problem in that the hypothesized entities have been induced fromdata obtained from individuals with two brains, each with an ego as well aspotentially conflicting memories.

        It isnot for me to judge the extent to which either the topographic theory of mind(Cs, Pcs and Ucs) or the structural theory (Id, Ego and Superego) might beaffected by the recognition of duplicated mentation. With respect to the Id,basic biologic needs are built into subcerebral (especially hypothalamic)anatomy as well as amygdala and accumbens both of which are present induplicate as are the cortical components of limbic connection. [We often hearpeople talk about "the" limbic system. Fact is, whatever a limbicsystem is composed of, and people still argue about that, there are two limbicsystems, one in each hemisphere.]

        Eachhemisphere clearly has the genetically roughed-out circuitry for ego. Bycontrast, the effects of environmental exposure seem most evident in the finelydifferentiated inhibition for which prefrontal cortex is best known. Like manyneurosurgeons, I have had frontal leukotomy patients whose wonderful releasefrom the distress of inoperable malignancy was also accompanied, especially inthe immediate post-op period, by loss of socialization (including excretorycontrol) previously acquired over many years. There is more to superego thanthe ability to consider future consequences of current action, but prefrontalcortex is important in this respect, and it is present in duplicate. Thegrievous consequences of frontal leukotomy require bilateral surgery, and onefrontal lobe (as in hemispherectomized persons) can suffice for attendingschool and holding a job as well as having intelligence well above average(Smith & Sugar, 1975).

        Fornearly a century persons who claimed to be memory experts, commonly on thebasis of their laboratory experiments, poo-pooed the idea of long buriedmemories. One might have expected some concession when Penfield elicited longlost memories with cortical stimulation (Penfield & Jasper, 1954); butdeniers of the unconscious disallowed this evidence on the grounds that thepatients were epileptics and, moreover, susceptible to suggestion (so far as Iam aware, nobody actually called the patients liars).

        Fortyyears ago I naively supposed that resistance to recognizing "one's othermind" would yield to the dramatic results of the split-brain experimentsin cats, especially when confirmed in monkeys, and eventually in humans. Whathas actually happened is that the familiar counter arguments (epilepsy,unreliable patients, variability) have reappeared. For many philosophers, theresults have been dismissed out of hand as not worthy of discussion (Block,1997).

        When thephilosopher Thomas Nagel (1971) considered the split-brain data he wrote,

        "Itis possible that the ordinary, simple idea of a single person will come to seemquaint some day . . . but it is also possible that we shall be unable toabandon the idea no matter what we discover . . . It may be impossible for usto abandon certain ways of conceiving and representing ourselves, no matter howlittle support they get from scientific research."

 

        This isa rather gloomy suggestion; but there is hope. It is the essence of this essaythat the left brain/right brain view and the practice of psychiatry arerelated, a view which has found increasing acceptance. It has been a centuryand a half since The Origin of Species ; considering the intensity ofanti-Darwinism after all this time, "one's other mind" is faringfairly well.

 

SUMMARY

        Frontaland horizontal sections of the cerebrum make plain that with a few exceptions(e.g. pineal, pituitary) all structures are paired. Hemicerebrectomy has madeclear that only one member of the hemispheric pair suffices to sustain thesentiments, emotions, memories and intentions both conscious and unconsciouswhich we call in the aggregate: mind. That the anatomical duality can sustaina duality of mentation is evident from not only the split-brain results in allspecies examined but also the results of appropriate testing of anatomicallyintact individuals. There is no physiological evidence for a plurality of mindbeyond duality.

 

 

REFERENCES

 

Aboitiz, F., Scheibel, A.B., Fisher, et al. (1992). Individual differences in brain asymmetries and fiber composition inthe human corpus callosum. Brain Res. 43:154-161.

 

Akelaitis, A.J. (1942). Studies on the corpuscallosum. VI. Orientation (temporal-spatial gnosis) following section of thecorpus callosum. Arch. Neurol. Psychiat. 48:914-937.

 

Akelaitis, A.J. (1944). A study of gnosis, praxisand language following section of the corpus callosum and anterior commissure. J. Neurosurg. 1:94-102.

 

Apuzzo, M. L. J. (1987). Surgery of the ThirdVentricle. Baltimore: Williams and Williams.

 

Atchely, R.A., Keeney, M., Burgess, C. (1999). Cerebral hemispheric mechanisms linking ambiguous word meaning retrieval andcreativity. Brain and Cognition. 40:479-499.

 

Austin, G.M., Grant, F.C. (1955). Physiologicobservations following total hemispherectomy in man. Surgery. 38:239-258.

 

Baynes, K. (1990). Language and reading in theright hemisphere: Highways or byways of the brain? J. Cog. Neurosci. 2:159-179.

 

Baynes, K., Tramo, M.J., Reeves, A.G., Gazzaniga,M.S. (1997). Isolation of a right hemisphere cognitive system in a patientwith anarchic (alien) hand sign. Neuropsychologia. 35:1159-1173.

 

Benson, D.F., Zaidel, E. (eds). (1985). The dualbrain: Hemispheric specialization in humans. New York, Guilford Press.

 

Berlucchi, G., Aglioti, S., Tassinari, G. (1997). Rightward attentional bias and left hemisphere dominance in a cue-target lightdetection task in a callosotomy patient. Neuropsychologia. 35:941-952.

 

Block, N. (1997). Biology versus computation inthe study of consciousness. Behav. Brain Sci. 20:159-166.

 

Bogen, J.E. (1969). The other side of the brain:II. An appositional mind. Bull. Los Angeles Neurol. Soc. 34:135-162.

 

Bogen, J.E. (1974). Hemispherectomy and theplacing reaction in cats. In Kinsbourne, M. and Smith, W.L. (eds.). Hemispheric disconnection and cerebral function. Springfield: C.C. Thomas.

 

Bogen, J.E. (1977). Further discussion onsplit-brains and hemispheric capabilities. Br J Philosophy Sci 28:281-286.

 

Bogen, J.E. (1979). The callosal syndrome. InK.M. Heilman & E. Valenstein (eds.). Clinical neuropsychology. New York:Oxford University Press.

 

Bogen, J.E. (1986). Mental duality in the intactbrain. Bull Clin Neurosciences. 51:3-29.

 

Bogen, J.E. (1990). Partial hemisphericindependence with the neocommissures intact. In Brain circuits and functionsof the mind: Essays in honor of R. W. Sperry. C. Trevarthen (ed.). Cambridge:Cambridge University Press.

 

Bogen, J.E. (1993a). The callosal syndromes. InHeilman and Valenstein, Clinical neuropsychology, 3rd ed. New York: OxfordUniv. Press.

 

Bogen, J.E. (1993b). Intralaminar nuclei and thewhere of awareness. Proc. Soc. Neurosci. 19:1446.

 

Bogen, J.E. (1995). On the neurophysiology ofconsciousness. Part I. Overview. Consciousness and Cognition. 4:52-62.

 

Bogen, J.E. (1997a). The neurosurgeon's interestin the corpus callosum. In S. H. Greenblatt (ed.). A history of neurosurgery. Park Ridge, IL: Amer. Assoc. Neurol. Surg.

 

Bogen, J.E. (1997b). Some neurophysiologicaspects of consciousness. Seminars in Neurology. 17:95-103.

 

Bogen, J.E. (1997c). Does cognition in thedisconnected right hemisphere require right hemisphere possession of language? Brain & Lang. 57:12-21.

 

Bogen, J.E. (1998). Physiological consequencesof complete or partial commissural section. In Apuzzo, M.L.J. (ed): Surgery ofthe third ventricle. 2d Ed. Baltimore, Willliams and Wilkins.

 

Bogen, J.E., Berker, E., Van Lancker, D., Sudia,S., Lipstad, B., Sadun, A., Weekes, N. (1998). Left hemicerebrectomy: vision,olfaction and mentation 45 years later. Proc. Soc. Neurosci. 24:173 (abstract71.13).

 

Bogen, J.E., Bogen, G.M. (1969). The other sideof the brain III: The corpus callosum and creativity. Bull. L.A. Neurol. Soc.34:191-220.

 

Bogen, J.E., Bogen, G.M. (1988). Creativity andthe corpus callosum. Psychiatric Clinics of North America. 11:293-301.

 

Bogen, J.E., Bogen, G.M. (1999). Split-brains:interhemispheric exchange in creativity. In M.A. Runco & .R. Pritzker(eds.). Encyclopedia of creativity. San Diego: Academic Press.

 

Bogen, J.E., Schultz, D.H., Vogel, P.J. (1988). Completeness of callosotomy shown by magnetic resonance imaging in the longterm. Arch. Neurol. 45:1203-1205.

 

Bogen, J.E., Vogel, P.J. (1975). Neurologicstatus in the long term following cerebral commissurotomy. In Les Syndromes deDisconnexion Calleuse chez l'Homme. (F. Michel and B. Schott, eds.). Lyon:Hpital Neurologique,. pp. 227-251.

 

Bremer, F. (1958). Physiology of the corpuscallosum. Res. Publ. Assoc. Res. Nerv. Ment. Dis.. 36:424-448.

 

Brion, S., Jedynak, C.P. (1972). Troubles dutransfer interhmisphrique (callosal disconnection). A propos de troisobservations de tumeurs du corps calleux. Le signe de la main trangre. Rev.Neurol. 126:257-266.

 

Brion, S., Jedynak, C.P. (1975). Les troubles dutransfert interhmisphrique. Paris: Masson.

 

Burgess, C., Chiarello, C. (1996). Neurocognitivemechanisms underlying metaphor comprehension and other figurative language. Metaphor and Symbolic Activity. 11:67-84.

 

Burgess, Skodis, J. (1993). LexicalRepresentation and morpho-syntactic parallelism in the left hemisphere. Brainand Language. 44:129-138.

 

Burklund, C.W., Smith, A. (1977). Language andthe cerebral hemispheres. Neurology. 27:627-633.

 

Chiarello, C. (1998). On codes of meaning and themeaning of codes: semantic access and retrieval within and between hemispheres.In M. Beeman and C. Chiarello (eds.). Right hemisphere language comprehension: Perspectives from cognitive neuroscience. Mahwah, NJ: Erlbaum.

 

Chiarello, C., Maxfield, L. (1996). Varieties ofinterhemispheric inhibition, or how to keep a good hemisphere down. Brain andCognition. 30:81-108.

 

Churchland, P.M. (1984). Matter andconsciousness. Cambridge: MIT Press.

 

Churchland, P.M. (1986). Neurophilosophy. Cambridge: MIT Press.

 

Corballis, M.D. (1995). Visual integration in thesplit brain. Neuropsychologia. 33:937-959.

 

Crick, F., Koch, C. (1990). Toward aneurobiological theory of consciousness. Seminars in Neuroscience. 2:263-275.

 

Dandy, W.E. (1933). Physiological studiesfollowing extirpation of the right cerebral hemisphere in man. Bull. JohnsHopkins Hosp. 53:31-51.

 

Della Sala, S., Marchetti, C., Spinnler, H. (1991). Right-sided anarchic (alien) hand: a longitudinal study. Neuropsychologia. 29:1113-1127.

 

Della Sala, S., Marchetti, C., Spinnler, H. (1994). The anarchic hand: a fronto-mesial sign. In Boller, F., Grafman, J.(Eds.) Handbook of neuropsychology, Vol. 9. Amsterdam: Elsevier.

 

Dennett, D.C., Kinsbourne, M. (1992). Time andthe observer. The where and when of consciousness in the brain. Behav. BrainSci. 15:183-247.

 

Dennett, D.C., Kinsbourne, M. (1994). Countingconsciousnesses: None, one, two, or none of the above? Behavioral and BrainSciences. 17:178-180.

 

Descartes, R. (1649). Les passions de l'ame. InBridoux, A. (ed.). Descartes: Oeuvres et Lettres (Bibliothque de la Pliade). Gallimard. 1996.

 

Doty, R.W. (1998). The five mysteries of themind, and their consequences. Neuropsychologia. 36:1069-1076.

 

Doty, R.W., Overman, W.H. (1977). Mnemonic roleof forebrain commissures in macaques. In Harnad, S. et al (eds.). Lateralization in the nervous system. New York: Academic Press.

 

Doty, R.W., Ringo, J.L. (1994). Hemisphericdistribution of memory traces. pp. 636-656 in Delacour, J. (ed.). The memorysystem of the brain. London: World Scientific.

 

Doty, R.W., Ringo, J.L., Lewine, J.D. (1986). Interhemispheric mnemonic transfer in macaques. In Lepor, F., Ptito, M.,Jasper, H. Two hemispheres_one brain: functions of the corpus callosum. New York:Alan R. Liss, Inc.

 

Downer, J.L. de C. (1961). Changes in visualgnostic functions and emotional behavior following unilateral temporal poledamage in the split-brain monkey. Nature 191:50-51.

Downer, J.L. de C. (1962). Interhemisphericintegration in the visual system. In V.B. Mountcastle (ed.). Interhemisphericrelations and cerebral dominance. Baltimore: Johns Hopkins University Press.

 

Engel, J. (1993). Surgical treatment of theepilepsies. 2nd ed. New York: Raven Press.

 

Feinberg, T.E., Schindler, R.J., Flanagan, N.G.,Haber, L.D. (1992). Two alien hand syndromes. Neurology. 42:19-24.

 

Fendrich, R., Gazzaniga, M.S. (1989). Evidence offoveal splitting in a commissurotomy patient. Neuropsychologia. 27:273-281.

 

Forster, B.A., Corballis, M.C. (1999). Interhemispheric transfer of colour and shape information in the presence andabsence of the corpus callosum. Neuropsychologia. 37 (in press).

 

Fourneret, P., Jeannerod, M. (1998). Limitedconscious monitoring of motor performance in normal subjects. Neuropsychologia. 36:1133-1140.

 

Fuster, J.M. (1997). The prefrontal cortex. 2nded. Philadelphia: Lippincott-Raven.

Galin, D. (1974). Implications for psychiatry ofleft and right cerebral specialization. Arch. Gen. Psychiatry. 31:572-583.

 

Galin, D. (1988). Conceptual and methodologicalissues in neuropsychological studies of depression. In M. Kinsbourne (ed.). Cerebral hemisphere function in depression. Washington DC: AmericanPsychiatric Press.

 

Gazzaniga, M.S. (1988). Interhemisphericintegration. pp. 385-405. In Rakic, P., Singer, W. (eds.). Neurobiology ofNeocortex. New York: John Wiley & Sons, Limited

 

Geschwind, D.H., Iacoboni, M., Mega, M.S., Zaidel,D.W., Cloughesy, T., Zaidel, E. (1995). Alien hand syndrome: interhemisphericmotor disconnection due to a lesion in the midbody of the corpus callosum. Neurology. 45:802-808.

 

Glickstein, M., Arora, H.A., Sperry, R.W. (1963). Delayed/response performance following optic tract section, unilateral frontallesion, and commissurotomy. J. Comp. Physiol. Psychol. 56:11-18.

 

Goldberg, G., Mayer, N.H., Toglia, J.U. (1981). Medial frontal cortex infarction and the alien hand sign. Arch. Neurol. 38:683-686.

 

Goldstein, K. (1908). Zur lehre von dermotorischen Apraxie. J. Physiologie und Neurologie. XI, 4/5:169-187. (citedby Brion and Jedynak (1975).

 

Hcaen, H., Penfield, W., Bertrand, C., Malmo, R. (1956). The syndrome of apractognosia due to lesions of the minor cerebral hemisphere. Arch. Neurol. Psychiat. 75:400-434.

 

Hellige, J.B. (1993). Hemispheric asymmetry:What's right and what's left. Cambridge: Harvard Univ. Press.

 

Hoppe, K.D. (1977). Split brains andpsychoanalysis. Psychoanal Q. 46:220-244.

 

Hoppe, K.D. (1988). Hemispheric specializationand creativity. Psychiat, Clin. No. America. 11:303-15.

 

Hoppe, K.D. (1989). Psychoanalysis, hemisphericspecialization, and creativity. J. Amer. Acad. Psychoanalysis. 17:353-69.

 

Hoppe, K.D., Bogen, J.E. (1977). Alexithymia in12 commissurotomized patients. Psychother, Psychosom. 28:148-155.

 

Iacoboni, M., Zaidel, E. (1999). Thecrossed-uncrossed difference in simple reaction times to lateralized auditorystimuli is not a measure of interhemispheric transmission time: evidence fromthe split brain. Experimental Brain Research. In press.

 

Jackson, F. (1997). Finding the mind in thenatural world. pp. 483-491 In Block, H., Flanagan, O., Gzeldere, G. Thenature of consciousness. Cambridge: MIT Press.

 

Jackson, J.H. (1874). On the nature of theduality of the brain. Med. Press Circular 1:19, 41,63. Reprinted in Brain,1915 and in Selected Writings of John Hughlings Jackson, Vol. 2, J. Taylor(ed.). London: Hodder and Stoughton, 1932.

 

Jacobsen, C.F., Nissen, H.W. (1937). Studies ofcerebral function in primates. IV: The effects of frontal lobe lesions on thedelayed alternation habit in monkeys. J. Comp. Physiol. Psychol. 23:101-112.

 

Kinsbourne, M. (1982). Hemispheric specializationand the growth of human understanding. Amer. Psychol. 37:411-420.

 

Kinsbourne, M. (1988). Integrated field theory ofconsciousness. In A.J. Marcel and E. Bisiach (eds.). Consciousness incontemporary science. Oxford: Clarendon Press.

 

Kruper, D.C., Patton, R.A., Koskoff, Y.D. (1971). Visual discrimination in hemicerebrectomized monkeys. Physiol. Behav. 7:173-179.

 

Lewine, J.D., Doty, R.W., Astur, R.S., Provencal,S.L. (1994). Role of the forebrain commissures in bihemispheric mnemonicintegration in macaques. Neurosci. 14:2515-2530.

 

Lewis, R.T. (1979). Organic signs, creativity,and personality characteristics of patients following cerebral commissurotomy. Clinical Neuropsychology. 1:29-33.

 

Luck, S.J., Hillyard, S.A., Mangun, G.R.,Gazzaniga, M.S. (1989). Independent hemispheric attentional systems mediatevisual search in split-brain patients. Nature. 342:543-545.

 

Lycan, W.G. Consciousness as internal monitoring. In H. Block, O. Flanagan and G. Gzeldere (eds.). The nature of consciousness. Cambridge: MIT Press.

 

Marks, C.E. (1980) Commissurotomy, consciousness,and unity of mind. Montgomery, VT: Bradford monographs.

 

Mazziotta, J.C., Phelps, M.E., Carson, R.E., Kuhl,D.E. (1982). Tomographic mapping of human cerebral metabolism: Auditorystimulation. Neurology. 32:921-937.

 

Milner, B. (1971). Interhemispheric differencesin the localization of psychological processes in man. Br. Med. Bull. 27:272-277.

 

Milner, B. (1980). Complementary functionalspecializations of the human cerebral hemispheres. In Nerve Cells,Transmitters and Behavior. R. Levi-Montalcini (ed.). Rome: Pontifica AcademiaScientarium.

 

Milner, A.D., Goodale, M.A. (1995). The visualbrain in action. Oxford: Oxford University Press.

 

Mukhametov, L.M., Supin, A.Y., Polyakova, I.G. (1977). Interhemispheric asymmetry of the electroencephalographic sleeppattern in dolphins. Brain Res. 134:581-584.

 

Myers, R.E., Sperry, R.W. (1953). Interoculartransfer of a visual form discrimination habit in cats after section of theoptic chiasma and corpus callosum. Anat. Record. 115:351-352.

 

Nagel, T., (1971). Brain bisection and the unityof consciousness. Synthese. 22:396-413.

 

Paterson, A., Zangwill, O.L. (1944). Disorders ofvisual space perception associated with lesions of the right cerebralhemispheres. Brain. 67:331-358.

 

Patton, R.A. (1961). Hemicerebrectomy andadaptive behavior in the rhesus monkey. In H.W. Brosin (ed.). ExperimentalPsychiatry. Pittsburgh: U. of Pittsburgh Pr.

 

Penfield, W., Jasper, H. (1954). Epilepsy and thefunctional anatomy of the human brain. Boston: Little, Brown and Company.

 

Perlmuter, L.C., Monty, R.A. The importance ofperceived control: fact or fantasy? Amer. Scientist. 65;759-765.

 

Puccetti, R. (1973). Brain bisection and personalidentity. Brit. J. Philosophy of Science. 24:339-355.

 

Reeves, A.G. (1991). Behavioral changes followingcorpus callosotomy. In Smith, D., Treiman, D., Trimble, M. (eds.). Advancesin Neurology, Vol. 55. New York: Raven Press.

 

Reeves, A.G., Roberts, D.W., (eds.). (1995). Epilepsy and the corpus callosum II. New York: Plenum Press.

 

Ringo, J.L., Doty, R.W., Demeter, S, Simard, P.Y. (1994). Time is of the essence: A conjecture that hemispheric specializationarises from interhemispheric conduction delay. Cerebral Cortex. 4:331-343.

 

Rudge, P., Warrington, E.K. (1991). Selectiveimpairment of memory and visual perception in splenial tumours. Brain. 114:349-360.

 

Saffran, E.M., Coslett, H.B. (1998). Implicit vs.letter-by-letter reading in pure alexia: a tale of two systems. Cognitiveneuropsychology. 15:141-165.

 

Sawaguchi, T, Goldman-Rakic, P.S. (1991). D1dopamine receptors in prefrontal cortex: involvement in working memory. Science. 251:947-950.

 

Schiffer, F. (1996). Cognitive activity of theright hemisphere: Possible contributions to psychological function. HarvardReview of Psychiatry. 4:126-138.

 

Schiffer, F. (1998). Of two minds. New York: TheFree Press.

 

Serafetinides, E.A., Shurley, J.T., Brooks, R.E. (1972). Electroencephalogram of pilot whale Globicephala scammoni inwakefulness and sleep: Lateralization aspects. Int. J. Psychobiol. 2:129-135.

 

Shallice, T. (1988). From neuropsychology tomental structure. Cambridge: Cambridge University Press.

Shorter, J.M. (1967). Other minds. In Edwards,P. (ed.). The Encyclopedia of Philosophy, Vol. 6. New York: Macmillan.

 

Sidtis, J.J., Volpe, B.T., Holtzman, J.D., Wilson,D.H., Gazzaniga, M.S. (1981). Cognitive interaction after staged callosalsection: evidence for transfer of semantic activation. Science. 212:344-346.

 

Smith, A. (1966). Speech and other functionsafter left (dominant) hemispherectomy. Neurol., Neurosurg. and Psychiatry. 29:467-471.

 

Smith, A., Sugar, O. (1975). Development ofabove-normal language and intelligence 21 years after left hemispherectomy. Neurology. 25:813-818.

 

Sperry, R.W. (1961). Cerebral organization andbehavior. Science. 133:1749-1757.

 

Sperry, R.W. (1964). The great cerebralcommissure. Sci. American. 210:42-52.

 

Sperry, R.W. (1974). Lateral specialization inthe surgically separated hemispheres. In Schmitt, F.O., Worden, F.G. (eds). The Neurosciences: The Third Study Program. New York: Rockefeller UniversityPress.

 

Sperry, R.W. (1982). Some effects ofdisconnecting the cerebral hemispheres. Science. 217:1223-1226.

 

Sperry, R.W., Clark, E. (1949). Interoculartransfer of visual discrimination habits in teleost fish. Physiol. Zool. 22:372-378.

 

Sperry, R.W., Gazzaniga, M.S., Bogen, J.E. (1969). Interhemispheric relationships: The neocortical commissures, syndromes ofdisconnection. In Vinken, J.J., Bruyn, G.W. (Eds.) Handbook of ClinicalNeurology, Vol. 4. Amsterdam: Elsevier, pp. 273-290.

 

Sperry, R.W., Zaidel, E., Zaidel, D. (1979). Selfrecognition and social awareness in the disconnected minor hemisphere. Neuropsychologia. 17:153-166.

 

TenHouten, W., Hoppe, K., Bogen, J. and Walter, D. (1986). Alexithymia: an experimental study of cerebral commissurotomy patientsand normal control subjects. Am. J. Psychiatry. 143:312-16.

 

TenHouten, W., Walter, D.O., Hoppe, K.D., Bogen,J.E. (1987). Alexithymia and the Split Brain V: EEG alpha-bandinterhemispheric coherence analysis. Psychother. Psychosom. 47:1-10.

 

Tononi, G., Edelman, G.M. (1998). Consciousnessand complexity. Science. 282:1846-1851.

 

Tramo, M.J., Baynes, K., Fendrich, R., Mangun,G.R., Phelps, E.A., Reuter-Lorenz, P.A., Gazzaniga, M.S. (1995). Hemisphericspecialization and interhemispheric integration: insights from experiments withcommissurotomy patients. In Reeves, A.G., Roberts, D.W. (eds.). Epilepsy andthe Corpus Callosum II. New York: Plenum Press.

 

Trevarthen, C.B. (1984). Hemisphericspecialization. Handbook of physiology_The nervous system, III, Ch 25. Washington, DC, American Physiological Society.

 

Van Lancker, D. (1997). Rags to riches: ourincreasing appreciation of cognitive and communicative abilities of the humanright cerebral hemisphere. Brain and Language 57:1-11.

 

Wallas, G. Stages of control. In Rothenberg, H.,Hausman, C. (Eds.) The Creativity Question. Durham, NC: Duke UniversityPress, pp. 69-73.

 

Warrington, E.K., Taylor, A.M. (1978). Twocategorical stages of object recognition. Perception. 7:695-705.

 

Weiskrantz, L. (1997). Consciousness lost andfound. Oxford: Oxford Univ. Press.

 

Wertheimer, M. (1961). Psychomotor coordinationof auditory and visual space at birth. Science. 134:1692.

 

White, R.J., Schreiner, L.H., Hughes, R.A.,MacCarty, C.S., Grindlay, J.H. (1959). Physiologic consequences of totalhemispherectomy in the monkey. Neurology. 9:149-159.

 

Zaidel, D.W. (1994). A view of the world from asplit-brain perspective. In C.M.R. Critchley, (ed.). The neurologicalboundaries of reality. London: Farrand Press.

 

Zaidel, E. (1978). Lexical organization in theright hemisphere. In P.A. Buser & A. Rougel-Buser (eds.). Cerebralcorrelates of conscious experience. Amsterdam: Elsevier/North Holland.

 

Zaidel, E., Zaidel, D.W., Bogen, J.E. (1990). Testing the commissurotomy patient. (In Boulton, A.A., Baker, G.B., Hiscock,M. (eds.). Neuromethods, Vol, 17, Neuropsycyhology. Clifton, NJ: The HumanaPress, Inc.

 

Zangwill, O.L. (1961). Asymmetry of cerebralhemisphere function. In Garland, H., (ed.). Scientific Aspects of Neurology. London: E. & S. Livingstone.

 

Zeki, S., Bartels, A. (1999). Toward a theory ofvisual consciousness. Consciousness and Cognition. 8:225-259.

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