Number 5 GAZZANIGA

Number 5 GAZZANIGA - The Split Brain in Man by Michael S....

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Unformatted text preview: The Split Brain in Man by Michael S. Gazzaniga August 1967 The human brain is actually two brains. each capable of advanced mental functions. When the cerebrurn is divided surgically. it is as if the cranium contained two separate spheres of consciousness 19 hebminofthehigheranhnalain- eluding man, is a double organ, consisting of right and left herni- spheres connected by an isthmus of nerve tissue called the corpus callosum. Some 15 years ago Ronald E. Myers and R. W. Sperry, then at the University of Chicago, made a surprising discovery: When this connection between the two halves of the cerebnnn was cut, each hemisphere functioned independently as ifitwereacornpletebrainThepha-iom- enonwasfirstinvestigatedinacatin whichmtonlytbebrainbutalsotbe optic chiasm, the crossover of the optic nerves, was divided, so that in- formation from the left eye was dis- patched only to the left brain and infor- mationfromtherighteyeonlytothe right brain. Working on a problem with oneeye,theanimalm.rldrespondnor— mally andlearntoperformatask; when that eye was covered and the same prob- lem was presented to the other eye, the animal evinced no recognition of the problem and had to learn it again from thebegtnnmgwiththeotherhalfofthe The finding introduced entirely new questions in the study of brain mecha- nisms. Was the corpus callosmn respon- sible for integration of the operations of the two cerebral ' in the in- tacthrain?Diditservetolteepeach hemisphere informed about what was gomgoriintheother'r‘Toputtheques- tion another way, would cutting the cor- puscallosumliterallyresultintheright handnotlarowing whattbeleftwasdo- ing? To what extent were the two half— brains actually independent whm they were separated? Could they have sepa— rate thoughts, even separate emotions? Such questions have been pursued by Sperry and hh: co-Workers in a wide- ranging series of animal studies at the California Institute of Technology over the past decade [see “The Great Cere- bral Commissure." by R. W. Sperry; Scrnnrrrrc AMERICAN Offprint 174. Re- cently these questions have been in- vestigated in human patients who underwent the brain-splitting operation for medical reasons. The demonstration in experimental animals that sectioning of the corpus callosum did not seriously impair mental faculties had encouraged surgeons to resort to this operation for people afflicted with uncontrollable epilepsy. The hope was to confine a seizure to one hemiSphere. The opera- tion proved to be remarkably successful; curiously there is an almost total elimina- tion of all attacks. including unilateral ones. It is as if the intact callosum had served in these patients to facilitate seizure activity. This article is a brief survey of investi- gations Sperry and I have carried out atCalTechover-thepastfiveyearawith some of these patients. The operations were performed by P. J. Vogel and I. E. Bogen of the California College of Medi- ' cine. Our studies date back to 1961, when the first patient, a 48-year-old war veteran, underwent the operation: cut- fingofthecorpuscallomandother commissure structures connecting the two halves of the cerebral cortex [see illustration, page 169]. As of today 10 patients have had the operation, and we have examined four thoroughly over a long period with many tests. Fromthebeginntngoneofthemost striking observations was that the opera- tion produced no noticeable change in the patients’ temperament, or general intelligence. In the first case thepafientcouldnotspeakforSOdays after the operation, but he then recov- eredhisspeech.Moretypicalwasthe third case: on awaking from the surgery the patient quipped that he had a “split- ting headache," his still drowsy state he was able to repeat the tongue twister “Peter Piper picked a peck of pickled peppers.” Close observation, however, soon re- vealed some changes in the patients‘ ev- eryday behavior. For example, it could beseenthatinmovingaboutandre- spending to sensory stimuli the patients favored the right side of the body, which iscontrolleclbythe domhiantlefthalfof the brain. For a considerable period after the operation the left side of the body rarely showed spontaneous activity, and the patient generally did not respond to stimulation of that side: when he brushed against something with his left sidehedidnotnoticethathehaddone so, andwhen anobjectwasplaoedinhis left hand he generally denied its pres- ence. - More tests identified the main features of the bisected-brain syndrome. One of these tests examined respomes to visual stimulation. While the patient fixedhisgareonacentralpointona board,spotsoflightwereflashed(fora tenthofasecond)hiarowacmssthe board that spanned both the left and the right half of his visual field. The patient wasaslcedtotellwhathehadseen. Each patient reported that lights had been flashed in the right half of the visual field. When lights were flashed only in thelefthaflofthefielihowever,thepa- dents generally denied having seen any lights. Since the right side of the visual field is normally projected to the left hemisphereofthebrain andtheleftfield to the right hemisphere, one might have concluded that _ in these patients with divided brains the right hemisphere was in edect We found, however, that thiswasnotthecasewhenthepatimts weredirectedtopointtothelightsthat had flashed instead of giving a verbal re- port. With this manual response they were able to indicate when lights had CORPUS CALLOSUM VISUAL INPUT to bisected brain was limited to one hemisphere by presenting infor- mation only in one visual field. The right and left fields of view are projected. via the optic rhiasm. to the left and right hemisphere: o! the brain respectively. If a person fixes his gaze on a point, therefore, information to the left oi the point goes only to the right hemisphere and information to the right of the point goes to the left hemisphere. Stimuli in the left visual field cannot be described by a split-brain patient'becanse'of the disconnec- tion between the right hemisphere and the speech center, which is in the left hemisphere. been flashed in the left visual field, and perception with the brain's right herni- sphere proved to'be almost equal to per. oeption with the left. Clearly, then, the patients' failure to report the right hemi- sphere's perception verbally was due to the fact that the speech centers of the brain are located in the left hemisphere. Our tests of the patients’ ability to recognize objects by much at first result- ed in the same general findmg. When the object was'held in the right hand, from which sensory information is sent to the left hemisphere, the patient was able .- to name and describe the object; When it was held in the left hand (from which information goes primarily-to the right hemisPhere), the patient could not de- scribe the object verbally but was able to identify it in a nonverbal test—matching it, for example, to the same object in a varied collection of things. We soon real- ized, however, that each hemisphere te- oeives, in addition to the main input from the opposite side of the body, some input from the samestde. This “ipsilater- all" input is crude; it is apparently good mainly for “cuing in” the as to the presence or absence of stimulation and relaying fairly gross information about the location of a stimulus on the surface of the body. It is unable, or a rule, to relay information concerning the qualitative nature of an object. Tests of motor control in these split- brain patients revealed that the left hemisphere of the brain exercised nor- mal control over the right hand but had less than full control of the left hand (for instance, it was poor at directing individ— ual movements of the fingers). Similarly, ' the right hemisphere had full control of the left hand but not of the right hand. When the two hemispheres were in con- flict, dictating different movements for the same hand, the hemisphere on the side opposite the hand generally tool: charge and overruled the orders of the side of the brain with the weaker con- trol. In general the motor findings in the human patients were much the same as those in splibbrain monkeys. We come now to the main question on which we centered our roadie-s, namely how the separation of the hem} spheres alfects the mental capacities of the human brain. For these psychologi- cal tests we used two different devices. One was visual: a picture or written in- formation was flashed (for a tenth of a. second) in either the right or the left visual field, so that the information was transmitted only to the left or to the right brain hemisphere [see illustration on page NO]. The other type of test was CORPUS ‘3 CALLOSUM GAZZANIGA | THE SPLIT BRAIN IN MAN ANTERIOR COMMISSURE __ HIPPOCAMPAL '- COMMISSURE TWO BEHISP oi the human brain are divided by neurosurgeons to control epilep- tic seizures. In this top View of the brain the right hemisphere is retracted and the corpus calloeum and other eonsmissurea, or connectors, that are generallyr out are shown in color. tactile: anobjectwasplacedoutofview in the patient’s right or left hand, again for the purpose of conveying the infor- mation to just one hemisphere—the hemi- sphere on the side opposite the hand. When the information (visual or tac— tile) was presented to the dominant left hemisphere, the patients were able to deal with and describe it quite normally, both orally and in writing. For sample, when a picture of a spoon was shown intherightvisualfieldoraspoonwas placed in the right hand, all the patients readily identified and described it. They were able to read out written messages and'to perform problem in calculation that were presented to the left hemi- sphere. In contrast, when the same informa- tion was presented to the right hemi- sphere,itfailedtoelicitsuchspoltenor written responses. A picture transmitted to the right hemisphere evoked either a haphazard guess or no verbal response at all. Similarly, a pencil placed in the left hand(behindascreenthatcutofivisim) mightbecalledacan openeroraciga- rette lighter, or the patient might not even attempt to describe it. The verbal guesses presumably came not from the right hemisphere but from the left, which had no perception of the object but might attempt to identify it from in- direot clues. - Did this impotence of the right hemi- sphere mean that its surgical separa- tion from the left had reduced its mental powers to an imbecilic level? The ear- lier tests of its nonverbal capacities sug- gested that this was almost certainly not so. Indeed, when we switched to ask- ing for nonverbal answers to the vis- ual and tactile information presented in our new psychological tests, the right hemisphere in several patients showed considerable capacity for accurate per- formance. For example, when a picture of a spoon was presented to the right hemisphere, the patients were able to feel around with the left hand among a varied group of objects (screened from sight) and select a spoon as a match for the picture. Furthermore, when they wre shown a picture of a cigarette they 169 succeeded in selecting an ashtray, from ‘ a group of 10 objects that did not include a cigarette, as the article most closely related to the picture. Oddly enough, however, even after their correct re- sponse, and while they were holding the spoon or the ashtray in their left hand, they were unable to name or describe the object or the picture. Evidently the left hemisphere was completely ‘di- voroed, in perception and knowledge, from the right . Other tests showed that the right hemisphere did possess a certain amount of language comprehension. For exam- ple, when the word “pencil” was flashed to the right hemisphere, the patients were able to pick out a pencil from a group of unseen objects with the left hand. And when a patient held an object in the left hand (out of view), although heconldnotsayitsnameordescrihe ithewaslaterabletopointtoacard onwhichthenameoftheobiectwas written. In one particularly interesting test the word“heart”wasfiashedacrosstheom- terofthevisualfield,withthe“he”por- tiontotheleitofthecenterand“art"to the right. Asked to tell what the word- was, the patients would say they had' seen “art”—the portion projected to the left brain (which is respon- sible for speech). Curiously when, after “heart” had been flashed in the same way, the patients were asked to point uddithelefthandtomeoftwocards— "art" or ‘he"—to identify the word they had seen, they invariably pointed to “he.” The experiment showed clearly that both hemispheres had simulta- neously observed the portioas of the word available to them and that in this particular case the right hemisphere, when it had had the opportunity toes- press itself, had prevailed over the left. - Because an auditory input to one ear goestobothsidesoftbebraimweoon- ducted tests for the comprehension of words presented audiny to the right hemisphere not by trying to limit the original input but by limiting the ability to answer to the right hemisphere. This "was done most easily by having a patient usehiSlefthandtoretrieveJromagrab hag held out of view, an object named by the examiner. We found that the pa- tients could easily retrieve such objects as a watch, comb, marble or coin. The object to be retrieved did not even have to be named; it might simply be de- scribed or alluded to. For example, the command “Retrieve the fruit monkeys like best” results in the patients’ pulling outahananairomagrabbagfullof plastic fruit; at the command “Sunkist 170 sells a lot of them” the patients retrieve an orange. We lcnew that touch informa- tion from the left hand was going exclu- _ sively to the right hemisphere because moments later, when the patients were asked to name various pieces of fruit placed in the left hand, they were unable to score above a chance level. The upper limit of linguistic abilities in each hemisphere varies from to subject. In one case there was little or no evidence for language abilities in the right hemisphere, whereas in the other three the amount and extent of the ca- pacities varied. The most adept patient showed’ some evidence of even being able to spell simple words by placing plastic lettersona'tahlewithhisleft hand. The Subject was told to spell a wordsuchas"pie,”andtheesaminer then placed the three appropriate letters, oneatatimeinarandomorder,inhis left hand to be arranged on the table. The patient was able to spell even more abstract wards such as “how,” “what” and “the.” In another test three or four letters were placed in a pile, again out of view. to be felt with the left hand. The letters available in each trial would spell only one word, and the instructions to the subject were “Spell a word." The pa- tient was able to spell such words as “cup” and “love.” Yet after he had corn- this task, the patient was unable to name the word he had just spelled! III | MEMORY. LEARNING. AND THINKING The possibility that the right hemi- sphere has not only some language but even some speech capabilities cannot be ruled out, although at present there is no firm evidence for this. It would not be surprising to discover that the patients are capable of a few simple eaclamatory remarks, when under emo- tional stress. The possibility also remains, ofcourse, thatspeechofsometypecould be trained into the right hemisphere. Tests aimed at this question, however, would have to be closely scrutinized and controlled. Thereasonisthathere. asinmanyof the tests, “cross-cuing' from one hemi- sphere to the othercouldbeheld respon- sible for any positive findings. We had a case of such cross-citing during a series of tests of whether the right hemisphere could respond verbally to simple red or greensfirnuli.Atfirst,a£tereitherared oragreenlightwasflashedtotheright hemisphere, the patient would guess the colorat achancelevel, asmightbeex- pected if the speech mechanism is solely represented in the left hemisphere. After a few trials, however, the score im- proved whenever the examiner allowed a second guess. We soon caught on to the strategy the patient used. If a red light was dashed and the patient by chance guessed red, he would stick with that answer. If the flashed light was red and the patient by chance guessed green, he would frown, shake his head and then say, “Oh no, I meant red.” What was happening was that the right hemisphere saw the red light and heard the left hemisphere make the guess “green.” KnOWing that the answer was wrong, the right hemisphere precipitated a frown and a shake of the head, which in turn cued in the left hemisphere to the fact that the answar wasan andthatithadbet‘tercon-ect itself! We have learned that this cross- cuing mechanism can become exh‘emely refined. The realization that the neuro- logical patient has various strategies at his command emphasizes how dill'icult it is to obtain a clear neurological descrip- tionofahmanbeingwithbraindam- age. _ Is the language comprehension by the right hemisphere that the patients ex- hibited in these tests a normal capability ofthathemisphere orwas it acquiredhy learning after their operation, perhaps during the cause of the experhnents themselves? The issue is diflicult to de- cide.Wemustrememberthatweare examhfingahalfofthehtnnanbramlt system easily capable of learning hum a singletrialinatest.Wedolmowthatthe right hemisphere is decidedly inferior to theleftinitsoverallcommandoflan- gnage. We have established, for in- stance, that although the right hemi- sphere can respond to a concrete m such as “pencil,” it cannot do as well with verbs; patients are unable to re- RESPONSE TO VISUAL STIMULUS is tested by flashing a Word or a picture of an object on a translucent screen. The examiner first chechtbe subject’lgaaetohe lureitiafixedon a dotlhatmarka the eter of the visual field. The examiner my call {or a verbal response—reading the flashed .WMIL for example—or for a non- verbal one, each u picking up the object that is named from among a number of things spread on the table. The objects are hidden fromthe subject’s view so thither canhe identified onlyhy touch. 172 ‘ with the right hand. they were capable of matching a test stimulus to the correct design when it appeared among five re- lated patterns presented in their right visualfieldThisshowedthatthedomi- nant left hemisphere is capable of dis- _ criminating between correct and incor- rectstimuli. Sinceitis alsotruethatthe patients have no motor problems with their right hand, the patients' inability to perform these tasks must reflect a break- down of an integrative process some- where between the sensOry system and the motor system. We foundthatincertain othermental processes the right hemisphere is on a parwiththeleft. In particulaLitcanr'n- dependently generate an emotional reac- tion. In one of our experiments exploring the matter we wonld t a series of ordinary objects and then suddenly flash a picture of a nude woman. This evoked an amused reaction regardless of wheth— er the picture was presented to the left hemisphere or to the right. When the picture was flashed to the left hemi- sphere of a female patient, she laughed and verbally identified the picture as a nude. When it was later presented to the right hemisphere, she said in reply to a question that she saw nothing, but al- most immediately a sly smile spread over her face and she began to chuckle. Asked what she was laughing at, she said: "I don't know . . . nothing . . . oh-that funny machine.” Althongh the right hemi- sphere could not describe what it had seen, the sight nevertheless elicited an emotional response like the one evoked from the left Taken together, cur studies seem to demonstrate conclusively that in a split- brain situation we are really dealing with III l MEMORY. LEARNING. AND THINKING two brains, each separately capable of mental functions of a order. This implies that the two brains should have twice as large a span of attention—that is,sh0uldbeabletohandletwiceas much information—as a camel whole brain. We have not yet tested this pre- ciser to human patients, but E. D. Young and l have found that a split- brain monkey can indeed deal with near- ' 1y twice as much informittion as a normal . animal {see intonation below]. We have so far determined also that brain-bisect- 'edpatientscantnrryouttwotasksas fast-asanormalpersoncandoone. Just how does the corpus callosum of the intact brain combine and inte- grate the perceptions and knowledge of the two cerebral henuspheres? This has been investigated recently by Giovanni - Berlucchi, Giacomo Rizaolati and me at thelstitutodiFisiologia UmanainPisa. Wemaderecordingsofneuralacfivityin theposteriorpartofthecalloaumofthe catvdththehopeofrelatingthere- sponsesoithatstructuretosfimulationof theanimal'sfisualfields.Theldndsofre— sponsosrecordedturnedouttobesimilar tothoseobservedinthevisualon‘texof the cat. In other words, the results sug- gestthat visual pattern information can be transmitted through the callosum. This finding militates against thenotion that learning and memory are trans- ferredacrossthecallosum,ashasusually other,withtheresu1tt]:atbothhemi- spheres can learn together a tionpresentedtcjuxtmehemisphereln the split-brain animal this extension of the visnal pathway is cut 0E; this would explain rather simply why no learning proceeds in the visually isolated hemi- sphereandwhyithastolearnthedis- unnination from scratch. Curiously, however, the neural activ- ityinthecallosumcameonlyinresponse tastimuliatthemidlineofthefisual field. This finding raises dificult ques- tions. How can it be reconciled with the well-atablished observation that the left hemisphere of a normal person can give a running description of all the visual in- formation presented throughout the en- tire half-field projected to the right hemi- sphere? For this reason alone one is wearily driven back to the conclusion that somewhere and somehow all or part ofthecallosumtransmitsnotonlyavis- ual scene but also a complicated neural code of a higher order. ‘ All the evidence indicates that separa- tionotthehemispherescreatestwohide— pendent spheres oi consciousness within asinglecranilmr,thatistosay,withina single organism. This concludon is dis- trubingtosomepeoplewhoviewcon— mess as an indivisible property of the human brain. Itseerns prernahrreto others.whoinsistd:atthempacitiesre— vealedthus farfortherighthemispbere are at the level of an automaton. There is, to be sure, hemispheric inequality in thepresenteases,butitmaywellbea' characteristic of the individuals we have studied. It is entirely possible that if a human brain were divided in a very young person, both hemispheres could- as a result separately and independe develop mental functions of a high order atthelevelattainedonlyintheld‘t hemisphere of normal dairdrowh-omtheholtombeioreimgettingwhichpenelsmlit (a). Split-brain mkeys complete the entire task with the panels lit only an milliseconds. The monkeys look at the panels throng: flhmdncetheopficchiannkwtindieseanimalgtheflhefl allow each hemisphere to see the colored panels on one side only. SPLIT-BRAIN MONKEYS can handle more visual inhalation than normal animals. When the monkey pulls a knob {l}, eight - of the 16 panels light momentarily. The monkey must then start at thebottommdpunchdwlighlsdratmlitandmothersfilWith the panels lit for 000 milliseconds rial-oral monkeys get up to the spond appropriately to simple printed instructions, such us “smile” or “frown,” whenthBSewordsareilashedtotheright hemisphere, norcantheypointtoapic— me that corresponds to a flashed verb. SomeofourreoentstudiesattheUni- varsity of California at Santa Barbara also indicate that the right hemisphere has a very poorly developed grammar; it seems to be incapable of forming the plural of a given ward, for example. In general, then, the extent of Language present in the adult right hemisphere in no Way conipares with that present in the left hemisphere or, for that matter, ' with the extent of language present in the child's right hemisphere. Up to the ageoffmnorso,it would appear-from a variety of neurological observations, the right hemisphere is about as procient in handling language as the left Moreover, _ studies of the child’s development of lan- guage, particularly with respect to gram- mar, strongly suggest that the founda- tions ‘of grammar-a ground plan for language, so to speak—are somehow in- herent in the human organism and are Eullyrealizedbetweentbeagesoftwo- and three. Inotberwords, inthe young child each hemisphere is about equally developed with respect to langua e and speech function. We are thus f with the interesting question of why the right hemisphere at an early age and stage of development possesses substantial language capacity whereas at a more adult stage it possmses a rather poor ca- pacity. It is diflimlt indeed to cmoeive of the underlying neurological mecha- nism that would allow for the establish- mentofacapacityofahighorderhia particular hemisphere on a temporary basis. The implication is that during ma- turation the processes and systems active in making this capacity manifest are smnehow inhibited and dismantled in the right hemisphere and allowed to re- sideonlyintbedominantlefthemi- sphere. Yettherighthemisphereisnotmall respects inferior or subordinate to the left. Tests have demonstrated that it ex- cels the left in some ' func- tions. Assnexample, testshyusandby Began have shown that in these patients the left hand is capable of arranging blocks to match a pictured design and of drawing a cube in three dimensions. whereas the right hand, deprived of hash-notions from the right hemisphere could not perform either of these tasks. It is of interest to note, however, that although the patients (our first subject in particular) could not execute such tasks GAZZANIGA I THE SPLIT BRAIN IN MAN 13’1 VISUALTACTILE ASSOCIATION is performed by I. split-brain patient. A picture oi a Ipoon is flashed to the right hemisphere; with the left hand he retrieves l spoon iron be; hind the men. The touch information from the left hand projects (color) mainly to the right hemisphere, but a weak “mallard-:3" component goes to the lei! hemisphere. This is usually not enough to enable him to say (using the left hemisphere} what‘he has picked up. . EXAMPLE RIGHT HAND "VISUALCONSTRUCTIONAL" tasks are handled better by the right hemisphere. This was seen mo'st clearly in the first patient, who had poor ipsilsienl control of his right hand. Although right-handed, he could copy the examples only with his left hand. have long been interested in the problems of perception in lower animals such as the honey bee, species of birds, and the green turtle. they are also interested in studying human perception in order to help man understand the capacities and limitations of his perception under different circumstances. ...
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Number 5 GAZZANIGA - The Split Brain in Man by Michael S....

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