Biopsychology, Global Edition, 11th Ed Chapter 16 PDF

Chapter 16 Lateralization, Language, and the Split Brain The Left Brain and Right Brain Tom Wang/Shutterstock Chapter Overview and Learning Objectives Cerebral Lateralization of LO 16.1...

Chapter 16 Lateralization, Language, and the Split Brain The Left Brain and Right Brain Tom Wang/Shutterstock Chapter Overview and Learning Objectives Cerebral Lateralization of LO 16.1 Summarize early studies of the cerebral lateralization of Function: Introduction function. LO 16.2 Describe three techniques for assessing cerebral lateralization of function. LO 16.3 Outline the discovery of the relationship between speech ­laterality and handedness. LO 16.4 Describe and evaluate the hypothesis that male brains are more lateralized than female brains. The Split Brain LO 16.5 Outline the groundbreaking experiment of Myers and Sperry on split-brain cats. LO 16.6 Describe the method used to demonstrate the hemispheric ­independence of visual experience in human split-brain patients. 431 M16_PINE1933_11_GE_C16.indd 431 22/01/2021 11:44  432 Chapter 16 LO 16.7 Describe the evidence that indicates that the hemispheres of split-brain patients can function independently. LO 16.8 Outline the process of cross-cuing in split-brain patients. LO 16.9 Describe the helping-hand phenomenon and the use of the ­chimeric figures test in experiments on split-brain patients. LO 16.10 Describe a case where the right hemisphere tried to take control of a split-brain patient’s everyday behavior. LO 16.11 Explain how complete hemispheric independence is not an inevitable consequence of split-brain surgery. Differences Between Left LO 16.12 Describe five examples of abilities that have been found to be and Right Hemispheres lateralized. LO 16.13 Discuss how we’ve come to understand that the ­lateralization of function is better understood in terms of individual ­cognitive processes rather than clusters of abilities. LO 16.14 Describe three anatomical asymmetries in the human brain. Evolution of Cerebral LO 16.15 Describe and evaluate three theoretical explanations for why Lateralization and cerebral lateralization of function exists. Language LO 16.16 List those species that display cerebral lateralization and explain what this tells us about when cerebral lateralization evolved. LO 16.17 Describe what the study of nonhuman primates has suggested about the evolution of human language. Cortical Localization of LO 16.18 Describe the historical antecedents of the Wernicke-Geschwind Language: ­ model. Include descriptions of the following disorders: Broca’s Wernicke-Geschwind and Wernicke’s aphasia, conduction aphasia, agraphia, and Model alexia. LO 16.19 Describe the Wernicke-Geschwind model. Wernicke-Geschwind LO 16.20 Identify the effects of cortical damage and brain stimulation Model: The Evidence on language abilities, and evaluate the Wernicke-Geschwind model in light of these findings. LO 16.21 Summarize the current status of the Wernicke-Geschwind model. Cognitive Neuroscience of LO 16.22 Describe the three premises that define the cognitive Language ­ euroscience approach to language, and compare them with the n premises on which the Wernicke-Geschwind model is based. LO 16.23 Describe two influential functional imaging studies of the l­ ocalization of language, and explain what their findings indicate. M16_PINE1933_11_GE_C16.indd 432 22/01/2021 11:44  Lateralization, Language, and the Split Brain 433 Cognitive Neuroscience of LO 16.24 Describe the causes and neural mechanisms of developmental Dyslexia dyslexia. LO 16.25 Describe the difference between the lexical procedure and the phonetic procedure for reading aloud. Then describe the ­difference between surface dyslexia and deep dyslexia. With the exception of a few midline orifices, we humans commonly referred to as lateralization of function. The have two of almost everything—one on the left and one study of split-brain patients—patients whose left and right on the right. Even the brain, which most people view as hemispheres have been separated by commissurotomy—is the unitary and indivisible basis of self, reflects this general a major focus of discussion. Another focus is the cortical principle of bilateral duplication. In its upper reaches, the localization of language abilities in the left hemisphere; lan- brain comprises two structures—the left and right cerebral guage abilities are the most highly lateralized of all cogni- hemispheres—which are entirely separate except for the tive abilities. cerebral commissures connecting them. The fundamental You will learn in this chapter that your left and right duality of the human forebrain and the locations of the cere- hemispheres have different abilities and that they have bral commissures are illustrated in Figure 16.1. the capacity to function independently—to have different Although the left and right hemispheres are similar in thoughts, memories, and emotions. Thus, this chapter will appearance, there are major differences between them in challenge the concept you have of yourself as a unitary function. This chapter is about these differences, a topic being. We hope you both enjoy it. Figure 16.1 The cerebral hemispheres and cerebral commissures. Hippocampal commissure Corpus callosum Massa intermedia Anterior commissure Optic chiasm Posterior commissure Frontal section of the human brain, which Midsagittal section of the human brain, illustrates the fundamental duality of the which illustrates the corpus callosum and human forebrain. other commissures. M16_PINE1933_11_GE_C16.indd 433 22/01/2021 11:44  434 Chapter 16 Figure 16.2 The location of Broca’s area: In the inferior Cerebral Lateralization left prefrontal cortex, just anterior to the face area of the left primary motor cortex. of Function: Introduction Primary motor Central In 1836, Marc Dax, an unknown country doctor, presented Broca’s cortex fissure a short report at a medical society meeting in France. It was area Parietal his first and only scientific presentation. Dax was struck by lobe the fact that of the 40 or so brain-damaged patients with speech problems whom he had seen during his career, not a single one had damage restricted to the right hemisphere. His report aroused little interest, and Dax died the follow- ing year unaware that he had anticipated one of the most important areas of modern neuropsychological research. Discovery of the Specific Contributions of Left-Hemisphere Damage to Aphasia and Apraxia Frontal lobe LO 16.1 Summarize early studies of the cerebral lateralization of function. Lateral fissure Occipital One reason Dax’s important paper had so little impact was Temporal lobe that most of his contemporaries believed that the brain lobe acted as a whole and that specific functions could not be attributed to particular parts of it. This view began to change 25 years later in 1861, when Paul Broca reported his and cognitive processes, and the other plays only a minor postmortem examination of two aphasic patients. Aphasia role. This thinking led to the practice of referring to the left is a brain damage–produced deficit in the ability to produce hemisphere as the dominant hemisphere and the right or comprehend language. hemisphere as the minor hemisphere. Both of Broca’s patients had a left-hemisphere lesion In addition, the discovery that language and motor that involved an area in the frontal cortex just in front of abilities are lateralized to the left hemisphere triggered a the face area of the primary motor cortex. Broca at first did search for other lateralized functions. In effect, the discov- not recognize the relation between aphasia and the side ery of language and motor lateralization established lateral- of the brain damage; he did not know about Dax’s report. ization of function as a major area of neuroscientific research. ­However, by 1864, Broca had performed postmortem exam- inations on seven more aphasic patients, and he was struck by the fact that, like the first two, they all had damage to Tests of Cerebral Lateralization the inferior prefrontal cortex of the left hemisphere—which by LO 16.2 Describe three techniques for assessing then had become known as Broca’s area (see Figure 16.2). cerebral lateralization of function. In the early 1900s, another example of cerebral lateraliza- Early research on the cerebral lateralization of function com- tion of function was discovered. Hugo-Karl Liepmann found pared the effects of left-hemisphere and right-hemisphere that apraxia, like aphasia, is almost always associated with lesions. Now, however, other techniques are also used for left-hemisphere damage, despite the fact that its symptoms this purpose. The sodium amytal test, the dichotic listening are bilateral (involving both sides of the body). Apraxic test, and functional brain imaging are three of them. patients have difficulty performing movements when asked to perform them out of context, even though they often have SODIUM AMYTAL TEST. The sodium amytal test of lan- no difficulty performing the same movements when they guage lateralization (Wada, 1949) is often given to patients are not thinking about doing so (see Chapter 8). prior to neurosurgery (see Bauer et al., 2014). The neurosur- The combined impact of the evidence that the left hemi- geon uses the results of the test to plan the surgery; every sphere plays a special role in both language and voluntary effort is made to avoid damaging areas of the cortex that are movement led to the theory of cerebral dominance. According likely to be involved in language. The sodium amytal test to this theory, one hemisphere—usually the left—assumes involves the injection of a small amount of sodium amytal the dominant role in the control of all complex behavioral into the carotid artery on one side of the neck. The injection M16_PINE1933_11_GE_C16.indd 434 22/01/2021 11:44  Lateralization, Language, and the Split Brain 435 anesthetizes the hemisphere on that side for a few minutes, damage in World War II (Russell & Espir, 1961), and another thus allowing the capacities of the other hemisphere to focused on neurological patients who underwent unilat- be assessed. During the test, the patient is asked to recite eral excisions for the treatment of neurological disorders well-known series (e.g., letters of the alphabet, days of the (Penfield & Roberts, 1959). In both studies, approximately week, months of the year) and to name pictures of common 60 percent of dextrals (right-handers) with left-hemisphere objects. Then, an injection is administered to the other side, lesions and 2 percent of those with right-hemisphere and the test is repeated. When the hemisphere specialized lesions were diagnosed as aphasic; the comparable figures for speech, usually the left hemisphere, is anesthetized, the for sinestrals (left-handers) were about 30 and 24 percent, patient is rendered completely mute for a minute or two; respectively. These results indicate that the left hemisphere and once the ability to talk returns, there are errors of serial is dominant for language-related abilities in almost all order and naming. In contrast, when the other hemisphere, dextrals and in the majority of sinestrals. In effect, sines- usually the right, is anesthetized, mutism often does not trals are more variable (less predictable) than dextrals with occur at all, and errors are few. respect to their hemisphere of language lateralization. This increased variability also holds for other aspects of brain DICHOTIC LISTENING TEST. Unlike the sodium amytal function; for example, sinestrals show greater variability test, the dichotic listening test is noninvasive; thus, it in their lateralization of attention and face recognition (see can be administered to healthy individuals. In the stan- Vingerhoets, 2019). dard dichotic listening test, three pairs of spoken digits Results of the sodium amytal test have confirmed the are presented through earphones; the digits of each pair relation between handedness and language lateralization are presented simultaneously, one to each ear (see Blass that was first observed in early lesion studies. For example, et al., 2015; Kimura, 2011). For example, a person might Milner (1974) found that almost all right-handed patients hear the sequence 3, 9, 2 through one ear and at the same without early left-hemisphere damage had left-hemisphere time 1, 6, 4 through the other. The person is then asked specialization for speech (92 percent), most left-handed to report all of the digits. Kimura found that most people and ambidextrous patients without early left-hemisphere report slightly more of the digits presented to the right ear damage had left-hemisphere specialization for speech (69 than the left, which is indicative of left-hemisphere spe- percent), and early left-hemisphere damage decreased left- cialization for language. In contrast, Kimura found that hemisphere specialization for speech in left-handed and all the patients who had been identified by the sodium ambidextrous patients (30 percent). amytal test as having right-hemisphere specialization for language performed better with the left ear than the right. Kimura argued that although the sounds from each ear are Sex Differences in Brain projected to both hemispheres, the contralateral connec- Lateralization tions are stronger and take precedence when two differ- ent sounds are simultaneously competing for access to the LO 16.4 Describe and evaluate the hypothesis that same cortical auditory centers. male brains are more lateralized than female brains. FUNCTIONAL BRAIN IMAGING. Lateralization of func- In 1972, Levy proposed that the brains of females and males tion has also been studied using functional brain-imaging differ in their degree of lateralization. Interest in Levy’s techniques. While a volunteer engages in some activity, hypothesis was stimulated by McGlone’s (1977, 1980) such as reading, the activity of the brain is monitored by observation that male victims of unilateral strokes were positron emission tomography (PET) or functional mag- three times more likely to suffer from aphasia than female netic resonance imaging (fMRI). On language tests, func- victims. McGlone concluded that male brains are more lat- tional brain-imaging techniques typically reveal far greater eralized than female brains. activity in the left hemisphere than in the right hemisphere (see Bauer et al., 2014). Journal Prompt 16.1 Discovery of the Relation Between What other clinical implications might this observed sex difference in brain lateralization have? Speech Laterality and Handedness LO 16.3 Outline the discovery of the relationship Levy’s hypothesis of a sex difference in brain lateral- between speech laterality and handedness. ization has been widely embraced, and it has been used to Lesion studies have clarified the relation between the cere- explain almost every imaginable cognitive and behavioral bral lateralization of speech and handedness. For example, difference between the sexes. But there is currently limited one study involved military personnel who suffered brain support for Levy’s hypothesis. For example, some researchers M16_PINE1933_11_GE_C16.indd 435 22/01/2021 11:44  436 Chapter 16 have failed to confirm McGlone’s report of a sex difference to transfer learned information from one hemisphere to in the effects of unilateral brain lesions (see Inglis & Lawson, the other. ­Second, it showed that when the corpus callo- 1982). Even more problematic, a meta-analysis of 17 func- sum is cut, each hemisphere can function independently; tional brain-imaging studies did not find significant effects each split-brain cat appeared to have two brains. If you find of sex on the lateralization of brain function (see ­Hirnstein, the thought of a cat with two brains provocative, you will Hugdahl, & Hausmann, 2019). Accordingly, Levy’s hypoth- almost certainly be bowled over by similar observations esis of sex differences in brain lateralization is no longer ten- about split-brain humans. But we are getting ahead of our- able (see Hirnstein, Hugdahl, & Hausmann, 2019). selves. Let’s first consider the research on cats. In their experiment, Myers and Sperry trained cats to perform a simple visual discrimination task. On each trial, The Split Brain each cat was confronted by two panels, one with a circle on it and one with a square on it. The relative positions of the In the previous module, you were introduced to early circle and square (right or left) were varied randomly from research on the lateralization of function, and you learned trial to trial, and the cats had to learn which symbol to press about four methods of studying cerebral lateralization in order to get a food reward. Myers and Sperry correctly of function: comparing the effects of unilateral left- and surmised that the key to split-brain research was to develop right-hemisphere brain lesions, the sodium amytal test, the procedures for teaching and testing one hemisphere at a time. dichotic listening test, and functional brain imaging. This Figure 16.3 illustrates the method they used to isolate visual- module focuses on a fifth method. discrimination learning in one hemisphere of the cats. There In the early 1950s, the corpus callosum—the larg- are two routes by which visual information can cross from one est cerebral commissure—constituted a paradox of major eye to the contralateral hemisphere: via the corpus callosum proportions. Its size, an estimated 200 million axons, and or via the optic chiasm. Accordingly, in their key experimental its central position, right between the two cerebral hemi- group, Myers and Sperry transected (cut completely through) spheres, implied that it performed an extremely important both the optic chiasm and the corpus callosum of each cat and function; yet research in the 1930s and 1940s seemed to sug- put a patch on one eye. This restricted all incoming visual gest that it did nothing at all. The corpus callosum had been information to the hemisphere ipsilateral to the uncovered eye. cut in monkeys and in several other laboratory species, but The results of Myers and Sperry’s experiment are illus- the animals seemed no different after the surgery than they trated in Figure 16.4. In the first phase of the study, all cats had been before. Similarly, human patients who were born without a corpus callosum or had it damaged Figure 16.3 Restricting visual information to one hemisphere in cats. To restrict visual information to one hemisphere, Myers and Sperry (1) cut the corpus ­callosum, seemed quite normal. In the early (2) cut the optic chiasm, and (3) blindfolded one eye. This restricted the visual 1950s, Roger Sperry, whom you ­information to the hemisphere ipsilateral to the uncovered eye. may remember for the eye-rotation experiments described in Chapter 9, Transected Blindfolded and his colleagues were intrigued corpus one eye by this paradox (see Aboitiz, 2017). callosum Groundbreaking Experiment of Myers and Sperry LO 16.5 Outline the ground- breaking experiment of Myers and Sperry on split-brain cats. The solution to the puzzle of the corpus callosum was provided in 1953 by an experiment on cats by Myers and Sperry. The experiment Transected made two astounding theoretical optic points. First, it showed that one chiasm function of the corpus callosum is M16_PINE1933_11_GE_C16.indd 436 22/01/2021 11:44  Lateralization, Language, and the Split Brain 437 Figure 16.4 Schematic illustration of Myers and Sperry’s (1953) groundbreaking split-brain experiment. There were four groups: (1) the key experimental group with both the optic chiasm and corpus callosum transected, (2) a control group with only the optic chiasm transected, (3) a control group with only the corpus callosum transected, and (4) an unlesioned control group. The performance of the three control groups did not differ, so they are illustrated together here. Control groups Patch on first eye Patch on second eye 100% Correct choices ( or ) 50% Trials Cats with either their optic chiasm transected, corpus callosum transected, or neither transected (shown here) learned the discrimination at a normal rate with one eye blindfolded and retained the task perfectly when the blindfold was switched to the other eye. Experimental group Patch on first eye Patch on second eye 100% Correct choices ( or ) 50% Trials Cats with both their optic chiasms and corpus callosums transected learned the discrimination at a normal rate with one eye blindfolded, but they showed no retention whatsoever when the blindfold was switched to the other eye. learned the task with a patch on one eye. The cats in the transferring the eye patch had a devastating effect on the key experimental group (those with both the optic chiasm performance of the experimental cats. In effect, it blind- and the corpus callosum transected) learned the simple folded the hemisphere that had originally learned the task discrimination as rapidly as did unlesioned control cats or and tested the knowledge of the other hemisphere, which control cats with either the corpus callosum or the optic chi- had been blindfolded during initial training. When the asm transected, despite the fact that cutting the optic chiasm patch was transferred, the performance of the experimen- produced a scotoma—an area of blindness—involving the tal cats dropped immediately to baseline (i.e., to 50 percent entire medial half of each retina. This result suggested that correct); and then the cats relearned the task with no savings one hemisphere working alone can learn simple tasks as whatsoever, as if they had never seen it before. Myers and rapidly as two hemispheres working together. Sperry concluded that the cat brain has the capacity to act More surprising were the results of the second phase as two separate brains and that the function of the corpus of Myers and Sperry’s experiment, during which the patch callosum is to transmit information between them. was transferred to each cat’s other eye. The transfer of the Myers and Sperry’s startling conclusions about patch had no effect on the performance of the intact control the fundamental duality of the cat brain and the cats or of the control cats with either the optic chiasm or the ­information-transfer function of the corpus callosum have corpus callosum transected; these subjects continued to per- been confirmed in a variety of species with a variety of form the task with close to 100 percent accuracy. In contrast, test procedures. For example, split-brain monkeys cannot M16_PINE1933_11_GE_C16.indd 437 22/01/2021 11:44  438 Chapter 16 perform tasks requiring fine tactual discriminations (e.g., and his associate Gazzaniga, and this work was a major rough versus smooth) or fine motor responses (e.g., unlock- factor in Sperry receiving a Nobel Prize in 1981 (see ing a puzzle) with one hand if they have learned them with Table 1.1). Sperry and Gazzaniga began by developing the other—provided they are not allowed to watch their a battery of tests based on the same methodological hands, which would allow the information to enter both ­strategy that had proved so informative in Sperry’s studies hemispheres. This failure of intermanual transfer of fine tac- of ­laboratory animals: delivering information to one tual and motor information in split-brain monkeys occurs hemisphere while keeping it out of the other (see Mancuso because the somatosensory and motor fibers involved in et al., 2019; Uddin, 2011). fine sensory and motor discriminations are all contralateral They could not use the same visual-discrimination and because the hemispheres have lost their ability to com- procedure that had been used in studies of split-brain municate directly. laboratory animals (i.e., cutting the optic chiasm and blindfolding one eye) because cutting the optic chiasm Commissurotomy in Humans with produces a scotoma. Instead, they employed the procedure illustrated in Figure 16.5. Each split-brain patient was Epilepsy asked to fixate on the center of a display screen; then, LO 16.6 Describe the method used to demonstrate the visual stimuli were flashed onto the left or right side of the hemispheric independence of visual experience screen for 0.1 second. The 0.1-second exposure time was in human split-brain patients. long enough for the subjects to perceive the stimuli but short enough to preclude the confounding effects of eye In the first half of the 20th century, when the normal func- movement. All stimuli thus presented in the left visual field tion of the corpus callosum was still a mystery, it was were transmitted to the right visual cortex, and all stimuli known that epileptic discharges often spread from one thus presented in the right visual field were transmitted to hemisphere to the other through the corpus callosum. This, the left visual cortex (see Aboitiz, 2017). along with the fact that cutting the corpus callosum had Fine tactual and motor tasks were performed by each proven in numerous studies to have no obvious effect on hand under a ledge. This procedure was used so that performance outside the contrived conditions of Sperry’s the nonperforming hemisphere—that is, the ipsilateral laboratory, led two neurosurgeons, Vogel and Bogen, to ­hemisphere—could not monitor the performance via the initiate a program of commissurotomy for the treatment of visual system. severe intractable cases of epilepsy—despite the fact that a The results of the tests on split-brain patients have con- previous similar attempt had failed, presumably because firmed the findings in split-brain laboratory animals in one of incomplete transections (Van Wagenen & Herren, 1940). major respect but not in another. Like split-brain laboratory The rationale underlying therapeutic commissurotomy— animals, human split-brain patients seem to have in some which typically involves transecting the corpus callosum respects two independent brains, each with its own stream and leaving the smaller commissures intact—was that the of consciousness, abilities, memories, and emotions (e.g., severity of the patient’s convulsions might be reduced if the Gazzaniga, 1967; Gazzaniga & Sperry, 1967; Sperry, 1964). discharges could be limited to the hemisphere of their origin But unlike the hemispheres of split-brain laboratory ani- (see Mancuso et al., 2019). The therapeutic benefits of com- mals, the hemispheres of split-brain patients are far from missurotomy turned out to be even greater than anticipated: equal in their ability to perform certain tasks. Most notably, Despite the fact that commissurotomy is performed in only the left hemisphere of most split-brain patients is capable of the most severe cases, many commissurotomized patients speech, whereas the right hemisphere is not. do not experience another major convulsion. Before we recount some of the key results of the tests on split-brain humans, let us give you some advice. Some stu- Journal Prompt 16.2 dents become confused by the results of these tests because their tendency to think of the human brain as a single uni- The decision to perform commissurotomies on patients with epilepsy turned out to be a good one. In Chapter 1, tary organ is deeply engrained. If you become confused, you learned that the decision to perform prefrontal think of each split-brain patient as two separate individu- lobotomies on patients with mental illness turned out to als: Right Hemisphere Ray, who understands a few simple be a bad one. Was this just the luck of the draw? (Hint: instructions but cannot speak, who receives sensory infor- Make one list for commissurotomy, and one list for pre- mation from the left visual field and left hand, and who frontal lobotomy, of the evidence for and against each that existed when they were first adopted.) controls the fine motor responses of the left hand; and Left Hemisphere Logan, who is verbally adept, who receives sensory information from the right visual field and right Evaluation of the neuropsychological status of Vogel hand, and who controls the fine motor responses of the right and Bogen’s split-brain patients was conducted by Sperry hand. In everyday life, the behavior of split-brain patients is M16_PINE1933_11_GE_C16.indd 438 22/01/2021 11:44  Lateralization, Language, and the Split Brain 439 Figure 16.5 The testing procedure used to evaluate the neuropsychological status of split-brain patients. Visual input goes from each visual field to the contralateral hemisphere; fine tactile input goes from each hand to the contralateral hemisphere; and each hemisphere controls the fine motor movements of the contralateral hand. APPLE SPOON reasonably normal because their two brains go through life it was an apple either by saying so or by putting the apple together and acquire much of the same information; how- down and picking out another apple with the right hand ever, in the neuropsychological laboratory, major discrep- from the test objects under the ledge. If, however, the non- ancies in what the two hemispheres learn can be created. speaking right hemisphere were asked to indicate the iden- As you are about to find out, this situation has interesting tity of an object that had previously been presented to the consequences. left hemisphere, it could not do so. Although objects that have been presented to the left hemisphere can be accu- rately identified with the right hand, performance is no bet- Evidence That the Hemispheres of ter than chance with the left hand. Split-Brain Patients Can Function When test objects are presented to the right hemisphere Independently either visually (in the left visual field) or tactually (in the left hand), the pattern of responses is entirely different. LO 16.7 Describe the evidence that indicates that A split-brain patient asked to name an object flashed in the the hemispheres of split-brain patients can left visual field is likely to claim that nothing appeared on function independently. the screen. (Remember that it is the left hemisphere who is If a picture of an apple were flashed in the right visual field talking and the right hemisphere who has seen the stimu- of a split-brain patient, the left hemisphere could do one of lus.) A patient asked to name an object placed in the left two things to indicate that it had received and stored the hand is usually aware that something is there, presumably information. Because it is the hemisphere that speaks, the because of the crude tactual information carried by ipsi- left hemisphere could simply tell the experimenter that it lateral somatosensory fibers, but is unable to say what it saw a picture of an apple. Or the patient could reach under is. Amazingly, all the while the patient is claiming (i.e., all a ledge with the right hand, feel the test objects, and pick the while the left hemisphere is claiming) the inability to out the apple. Similarly, if the apple were presented to the identify a test object presented in the left visual field or left left hemisphere by being placed in the patient’s right hand, hand, the left hand (i.e., the right hemisphere) can identify the left hemisphere could indicate to the experimenter that the correct object. Imagine how confused the patient must M16_PINE1933_11_GE_C16.indd 439 22/01/2021 11:44  440 Chapter 16 become when, in trial after trial, the left hand can feel an impression he is above it all by making sarcastic comments. object and then fetch another just like it from a collection Such a student inadvertently triggered an interesting dis- of test items under the ledge, while the left hemisphere is cussion in one of our classes. His comment went something vehemently claiming that it does not know the identity of like this: “If getting my brain cut in two can create two sepa- the test object. rate brains, perhaps I should get it done so that I can study for two different exams at the same time.” The question raised by this comment is a good one. If Cross-Cuing the two hemispheres of a split-brain patient are capable of LO 16.8 Outline the process of cross-cuing in independent functioning, then they should be able to do split-brain patients. two different things at the same time—in this case, learn two different things at the same time. Can they? Indeed Although the two hemispheres of a split-brain patient have they can. For example, in one test, two different visual no means of direct neural communication, they can com- stimuli appeared simultaneously on the test screen— municate neurally via indirect pathways through the brain let’s say a pencil in the left visual field and an orange in stem. They can also communicate with each other by an the right visual field. The split-brain patient was asked external route, by a process called cross-cuing. An example to simultaneously reach into two bags—one with each of cross-cuing occurred during a series of tests designed hand—and grasp in each hand the object that was on the to determine whether the left hemisphere could respond screen. After grasping the objects, but before withdraw- to colors presented in the left visual field. To test this pos- ing them, the patient was asked to tell the experimenter sibility, a red or a green stimulus was presented in the left what was in the two hands; the patient (i.e., the left hemi- visual field, and the split-brain patient was asked to ver- sphere) replied, “Two oranges.” Much to the bewilder- bally report the color: red or green. At first, the patient per- ment of the verbal left hemisphere, when the hands were formed at a chance level on this task (50 percent correct); withdrawn, there was an orange in the right hand and a but after a time, performance improved appreciably, thus pencil in the left. The two hemispheres of the split-brain suggesting that the color information was somehow trans- patient had learned two different things at exactly the ferred over neural pathways from the right hemisphere to same time. the left. In another test in which two visual stimuli were pre- However, this proved not to be the case: sented simultaneously—again, let’s say a pencil to the left If a green light was presented and the patient happened visual field and an orange to the right—the split-brain to correctly guess green, she would be correct and the trial patient was asked to pick up the presented object from an would end. However, if the green light was presented and assortment of objects on a table, this time in full view. As the patient guessed red, after a pause, she would frown, the right hand reached out to pick up the orange under shake her head, and then change her answer: “Oh no, the direction of the left hemisphere, the right hemisphere I meant green.” The right hemisphere saw the green light saw what was happening and thought an error was being and heard the left hemisphere guess “red.” Knowing that red was wrong, the right hemisphere initiated a frown made (remember that the right hemisphere saw a pencil). and shook her head, no. This signaled to the left hemi- On some trials, the right hemisphere dealt with this prob- sphere that the answer was wrong and that it needed to lem in the only way that it could: The left hand shot out, be corrected. grabbed the right hand away from the orange, and redi- rected it to the pencil. This response is called the helping- This example demonstrates how neurological patients hand phenomenon. can use different cognitive strategies to perform the same The special ability of split brains to do two things at task. The fact that neurological patients can perform the once has also been demonstrated on tests of attention. Each same task in different ways often clouds their deficits and hemisphere of split-brain patients appears to be able to greatly complicates the assessment of their neurological maintain an independent focus of attention (see ­Gazzaniga, status. 2005). This leads to an ironic pattern of results: Split-brain patients can search for, and identify, a visual target item Doing Two Things at Once in an array of similar items more quickly than healthy controls can (Luck et al., 1989)—presumably because the LO 16.9 Describe the helping-hand phenomenon two split hemispheres are conducting two independent and the use of the chimeric figures test in searches. experiments on split-brain patients. Yet another example of the split brain’s special abil- In many of the classes we teach, a student fits the follow- ity to do two things at once involves the phenomenon of ing stereotype: He sits—or rather sprawls—near the back visual completion. As you may recall from Chapter 6, of the class; and despite good grades, he tries to create the individuals with scotomas are often unaware of them M16_PINE1933_11_GE_C16.indd 440 22/01/2021 11:44  Lateralization, Language, and the Split Brain 441 split-brain patients—see Figure 16.6. Figure 16.6 The chimeric figures test. When a split-brain patient focuses on a The patients were then asked to ­ himeric face, the left hemisphere sees a single normal face that is a completed c ­version of the half face on the right. At the same time, the right hemisphere sees a describe what they saw or to indi- single normal face that is a completed version of the half face on the left. cate what they saw by pointing to it in a series of photographs of intact A Chimeric Face faces. Amazingly, each patient (i.e., each left hemisphere) reported see- ing a complete, bilaterally sym- metrical face, even when asked such leading questions as “Did you notice anything peculiar about what you just saw?” When the patients were asked to describe what they saw, they usually described a com- pleted version of the half that had been presented to the right visual Fixation field (i.e., the left hemisphere). point Dual Mental Functioning and Conflict in Split-Brain Patients LO 16.10 Describe a case where the right hemisphere tried to take control of a split-brain patient’s everyday behavior. In most split-brain patients, the right hemisphere does not seem to have a strong will of its own; the left hemisphere seems to control most everyday activities. However, in a few split-brain patients, the right hemisphere takes a more active role in controlling behavior, and in these cases, there can be serious Left hemisphere of a split-brain patient Right hemisphere of a split-brain patient conflicts between the left and right sees this. sees this. hemispheres. One patient (let’s call ASDF_MEDIA/Shutterstock; Jason Stitt/Shutterstock him Peter) was such a case. because their brains have the capacity to fill them in (to complete them) by using information from the surrounding The Case of Peter, the Split-Brain areas of the visual field. In a sense, each hemisphere of a split-brain patient is a participant with a scotoma covering Patient Tormented by Conflict the entire ipsilateral visual field. The ability of the hemi- At the age of 8, Peter began to suffer from complex partial spheres of a split-brain patient to simultaneously and inde- seizures. Antiepileptic medication was ineffective, and at 20, pendently engage in completion has been demonstrated he received a commissurotomy, which greatly improved his in studies using the chimeric figures test—named after condition but did not completely block his seizures. A sodium Chimera, a mythical monster composed of parts of differ- amytal test administered prior to surgery showed that he was ent animals. Levy, Trevarthen, and Sperry (1972) flashed left-hemisphere dominant for language. photographs composed of fused-together half-faces of Following surgery, the independent mischievous behav- two different people onto the center of a screen in front of ior of Peter’s right hemisphere often caused him (his left M16_PINE1933_11_GE_C16.indd 441 22/01/2021 11:44  442 Chapter 16 Experimenter: “How do you know?” hemisphere) considerable frustration. He (his left hemisphere) complained that his left hand would turn off television shows Patient: “By the E on the back of my hand.” that he was enjoying, that his left leg would not always walk in Another factor that has been shown to contribute sub- the intended direction, and that his left arm would sometimes stantially to the hemispheric independence of split-brain perform embarrassing, socially unacceptable acts (e.g., striking patients is task difficulty (Weissman & Banich, 2000). a relative). As tasks become more difficult, they are more likely to In the laboratory, Peter (his left hemisphere) sometimes involve both hemispheres of split-brain patients. It became angry with his left hand, swearing at it, striking it, and trying to force it with his right hand to do what he (his left appears that simple tasks are best processed in one hemi- ­hemisphere) wanted (Joseph, 1988). sphere, the hemisphere specialized for the specific activ- ity, but complex tasks require the cognitive power of both hemispheres. Independence of Split Hemispheres: Current Perspective Differences Between Left LO 16.11 Explain how complete hemispheric and Right Hemispheres independence is not an inevitable So far in this chapter, you have learned about five methods consequence of split-brain surgery. of studying cerebral lateralization of function: unilateral Discussions of split-brain patients tend to focus on cases lesions, the sodium amytal test, the dichotic listening test, in which there seems to be a complete separation of left-­ functional brain imaging, and studies of split-brain patients. hemisphere and right-hemisphere function, and this is what This module takes a look at some of the major functional we have done here. But complete hemispheric independence differences between the left and right cerebral hemispheres is not an inevitable consequence of split-brain surgery. In most that have been discovered using these methods. Because the split-brain patients, it is possible to demonstrate some com- verbal and motor abilities of the left hemisphere are readily munication of information between hemispheres, depending apparent, most research on the lateralization of function has on the particular surgery, the time since surgery, the partic- focused on uncovering the not-so-obvious special abilities ular information, and the method of testing. For example, of the right hemisphere. feelings of emotion appear to be readily passed between the Before we introduce you to some of the differences hemispheres of most split-brain patients. This is easily dem- between the left and right hemispheres, we need to clear onstrated by presenting emotionally loaded images to the up a common misconception: For many functions, there right hemisphere and asking patients to respond verbally to are no substantial differences between the hemispheres; the images. Their verbal left hemisphere often responds with and when functional differences do exist, these tend to be the appropriate emotion, even when the left hemisphere is slight biases in favor of one hemisphere or the other—not unaware of the image (Sperry, Zaidel, & Zaidel, 1979). absolute differences. Disregarding these facts, the popular Consider the following remarkable exchange (para- media inevitably portray left–right cerebral differences as phrased from Sperry, Zaidel, & Zaidel, 1979, pp. 161–162). absolute. As a result, it is widely believed that various abili- The patient’s right hemisphere was presented with an array ties reside exclusively in one hemisphere or the other. For of photos, and the patient was asked if one was familiar. He example, it is widely believed that the left hemisphere has pointed to the photo of his aunt. exclusive control over language and the right hemisphere has exclusive control over emotion and creativity. Experimenter: “Is this a neutral, a thumbs-up, or a thumbs-down person?” Patient: With a smile, he made a thumbs-up sign and Journal Prompt 16.3 said, “This is a happy person.” Prior to delving into this module, list your Experimenter: “Do you know him personally?” ­preconceptions about cerebral lateralization of function. Patient: “Oh, it’s not a him, it’s a her.” What side of the brain does what? Experimenter: “Is she an entertainment personality or an historical figure?” Patient: “No, just... ” Language-related abilities provide a particularly good Experimenter: “Someone you know personally?” illustration of the fact that lateralization of function is sta- Patient: He traced something with his left index finger on tistical rather than absolute. Language is the most lateral- the back of his right hand, and then he exclaimed, “My ized of all cognitive abilities. Yet, even in this most extreme aunt, my Aunt Edie.” case, lateralization is far from total; there is substantial M16_PINE1933_11_GE_C16.indd 442 22/01/2021 11:44  Lateralization, Language, and the Split Brain 443 language-related activity in the right hemisphere. For functional brain-imaging studies. When complex, cogni- example, on the dichotic listening test, people who are left- tively driven movements are made by one hand, most of hemisphere dominant for language tend to identify more the activation is observed in the contralateral hemisphere, as digits with the right ear than the left ear, but this right-ear expected. However, some activation is also observed in the advantage is only 55 to 45 percent. Furthermore, the right ipsilateral hemisphere, and these ipsilateral effects are sub- hemispheres of most left-hemisphere dominant split-brain stantially greater in the left hemisphere than in the right (see patients can understand many spoken or written words and Bundy & Leuthardt, 2019; Hervé et al., 2013). ­Consistent simple sentences (see Gazzaniga, 2013). with this observation is the finding that left-hemisphere lesions are more likely than right-hemisphere lesions to Examples of Cerebral Lateralization produce ipsilateral motor problems—for example, left- hemisphere lesions are more likely to reduce the accuracy of Function of left-hand movements than right-hemisphere lesions are LO 16.12 Describe five examples of abilities that have to reduce the accuracy of right-hand movements. been found to be lateralized. SUPERIORITY OF THE RIGHT HEMISPHERE IN SPATIAL Table 16.1 lists some of the abilities that are often found to ABILITY. In a classic early study, Levy (1969) placed a be lateralized. They are arranged in two columns: those that three-dimensional block of a particular shape in either the seem to be controlled more by the left hemisphere and those right hand or the left hand of split-brain patients. Then, after that seem to be controlled more by the right hemisphere. they had thoroughly palpated (tactually investigated) it, she Let’s consider several examples of cerebral lateralization of asked them to point to the two-dimensional test stimulus function. that best represented what the three-dimensional block would look like if it were made of cardboard and unfolded. SUPERIORITY OF THE LEFT HEMISPHERE IN She found a right-hemisphere superiority on this task, and ­CONTROLLING IPSILATERAL MOVEMENT. One unex- she found that the two hemispheres seemed to go about the pected left-hemisphere specialization was revealed by task in different ways. The performance of the left hand and right hemisphere was rapid and silent, whereas the performance of Table 16.1 Abilities that display some degree of cerebral lateralization. the right hand and left hemisphere was hesitant and often accom- RELATIVE panied by a running verbal com- DOMINANCE Left-Hemisphere Right-Hemisphere mentary that was difficult for the patients to inhibit. Levy concluded that the right hemisphere is supe- VISION Words Faces rior to the left at spatial tasks. This Letters Geometric patterns conclusion has been frequently Emotional expression confirmed (see Dietz et al., 2014; AUDITION Language sounds Nonlanguage sounds Hirnstein, Hugdahl, & Hausmann, Music 2019; Zaidel, 2013), and it is consis- TOUCH Tactile patterns tent with the finding that disorders Braille of spatial perception (e.g., contra- MOVEMENT Complex movement Movement in spatial patterns lateral neglect—see Chapters 7 and Ipsilateral movement 8) tend to be associated with right- hemisphere damage. MEMORY Verbal memory Nonverbal memory Finding meaning in memories Perceptual aspects of memories SPECIALIZATION OF THE LANGUAGE Speech Emotional content RIGHT HEMISPHERE FOR Reading EMOTION. According to the Writing old concept of left-hemisphere Arithmetic dominance, the minor right hemi- SPATIAL Mental rotation of shapes sphere is not involved in emo- ABILITY Geometry tion. This presumption has been Direction proven false. Indeed, analysis Distance of the effects of unilateral brain lesions indicates that the right hemisphere may be superior to M16_PINE1933_11_GE_C16.indd 443 22/01/2021 11:44  444 Chapter 16 the left at performing some tests of emotion—for e­ xample, What Is Lateralized? Broad Clusters in accurately identifying facial expressions of emotion (see Bernard et al., 2018; Gainotti, 2018, 2019a, 2019b; of Abilities or Individual Cognitive Mitchell & Phillips, 2015; Prete et al., 2015). Although the Processes? study of unilateral brain lesions suggests a general right-­ LO 16.13 Discuss how we’ve come to understand hemisphere dominance for some aspects of emotional that the lateralization of function is better processing, functional brain-imaging studies have not pro- understood in terms of individual cognitive vided unambiguous support for this view (see Costanzo et processes rather than clusters of abilities. al., 2015; Gainotti, 2019a). Early theories of cerebral laterality tended to ascribe SUPERIOR MUSICAL ABILITY OF THE RIGHT HEMI- complex clusters of mental abilities to one hemisphere SPHERE. Kimura (1964) compared the performance or the other. The left hemisphere tended to perform bet- of 20 right-handers on the standard digit version of ter on language tests, so it was presumed to be domi- the dichotic listening test with their performance on a nant for language-related abilities; the right hemisphere version of the test involving the dichotic presentation tended to perform better on some spatial tests, so it was of melodies. In the melody version of the test, Kimura presumed to be dominant for space-related abilities; and simultaneously played two different melodies—one to so on. Perhaps this was a reasonable first step, but now each ear—and then asked the participants to identify the the consensus among researchers is that this conclusion two they had just heard from four that were subsequently is simplistic. played to them through both ears. The right ear (i.e., the The problem is that categories such as language, emo- left hemisphere) was superior in the perception of dig- tion, musical ability, and spatial ability are each composed its, whereas the left ear (i.e., the right hemisphere) was of dozens of different individual cognitive activities, and superior in the perception of melodies. This is consistent there is no reason to assume that all those activities associ- with the observation that right temporal lobe lesions are ated with a general label (e.g., spatial ability) will necessar- more likely to disrupt music discriminations than are left ily be lateralized in the same hemisphere. Indeed, major temporal lobe lesions (see Casey, 2013). exceptions to all broad categories of cerebral lateralization HEMISPHERIC DIFFERENCES IN MEMORY. Early stud- have emerged (see Cai & Van der Haegen, 2015; Crepaldi ies of the lateralization of cognitive function were premised et al., 2013; Turner et al., 2015). How is it possible to argue on the assumption that particular cognitive abilities reside that all language-related abilities are lateralized in the left in one or the other of the two hemispheres. However, the hemisphere when the right hemisphere has been shown to results of research have led to an alternative way of think- be involved in speech perception and the understanding of ing: The two hemispheres have similar abilities that tend to word meaning (see Kreitewolf, Friederici, & von Kriegstein, be expressed in different ways. The study of the lateraliza- 2014; Poeppel, 2014)? tion of memory was one of the first areas of research on Many researchers are taking a different approach to the cerebral lateralization to lead to this modification in think- study of cerebral lateralization. They are basing their stud- ing. You see, both the left and right hemispheres have the ies on the work of cognitive psychologists, who have broken ability to perform on tests of memory, but the left hemi- down complex cognitive tasks—such as reading, judging sphere is better on some tests, whereas the right hemisphere space, and remembering—into their constituent cognitive is better on others. processes. Once the laterality of the individual cognitive ele- There are two approaches to studying the cerebral lat- ments has been determined, it should be possible to predict eralization of memory. One approach is to try to link par- the laterality of cognitive tasks based on the specific cogni- ticular memory processes with particular hemispheres—for tive processes that compose them. example, it has been argued that the left hemisphere is spe- cialized for encoding episodic memory (see Chapter 11). The other approach is to link the memory processes of each Anatomical Asymmetries hemisphere to specific materials rather than to specific pro- of the Brain cesses. In general, the left hemisphere has been found to LO 16.14 Describe three anatomical asymmetries in the play the greater role in memory for verbal material, whereas human brain. the right hemisphere has been found to play the greater role in memory for nonverbal material (e.g., ­Willment & Golby, The discovery of cerebral lateralization of function led to 2013). Whichever of these two approaches ultimately a search for anatomical asymmetries in the brain. In par- proves more fruitful, they represent an advance over the ticular, it led to a search for those anatomical differences tendency to think that memory is totally lateralized to one between the hemispheres that are the basis for their func- hemisphere. tional differences. For example, do anatomical differences M16_PINE1933_11_GE_C16.indd 444 22/01/2021 11:44  Lateralization, Language, and the Split Brain 445 between the left and right hemispheres make the left hemi- structure of these cortical language areas (see Amunts & sphere more suited for the control of language? Zilles, 2012). Given these two difficulties, it is not surpris- Most efforts to identify interhemispheric differences in ing that reports of their anatomical asymmetry have been brain anatomy have focused on the size of three areas of variable (see Amunts & Zilles, 2012; Kong et al., 2018). cortex that are important for language, the most lateralized In many cases, the predicted size advantage of the left- of our cognitive abilities: the frontal operculum, the planum hemisphere language areas is reported, but in other cases temporale, and Heschl’s gyrus (see Figure 16.7). The frontal there is no asymmetry, or even a right-hemisphere size operculum is the area of frontal lobe cortex that lies just in advantage. front of the face area of the primary motor cortex; in the left Any report that one of the three cortical language areas hemisphere, it is the location of Broca’s area. The planum tends to be larger in the left hemisphere typically leads to temporale and Heschl’s gyrus are areas of temporal lobe the suggestion that the anatomical asymmetry might have cortex. The planum temporale lies in the posterior region caused, or have been caused by, the lateralization of lan- of the lateral fissure; it is thought to play a role in the com- guage to the left hemisphere. However, there is little sup- prehension of language and is often referred to as Wernicke’s port for such conjectures (see Bishop, 2013). The fact that a area. Heschl’s gyrus is located in the lateral fissure just ante-particular cortical area is on the average larger in the left rior to the planum temporale in the temporal lobe; it is the hemisphere does not suggest that it is causally linked to lan- location of primary auditory cortex. guage lateralization, even if the cortical area has been linked Many anatomical differences between the average to language. At a bare minimum, it must be shown that the left and right hemispheres of the human brain have been anatomical and functional asymmetries are correlated—that reported, and the nature of those asymmetries is a function the degree of anatomical lateralization in a person reflects of sex and age (see Kong et al., 2018). There is no question the degree of language lateralization in the same person. that the average human cerebral hemispheres tend to be To our knowledge, no such correlations have been found. anatomically different, but the functional consequences We want to end this section by highlighting the results of the differences have not been apparent (see Kong et of an important recent study that bears directly on the al., 2018). Let’s consider research on the three cortical question of whether there are anatomical asymmetries ­language areas. in language-related brain areas. In 2018, Kong et al. con- There are two serious difficulties in studying ana- ducted a large-scale analysis of the MRI data from over tomical asymmetry of the language areas. First, their 17,000 healthy individuals. They reported large asym- boundaries are unclear, with no consensus on how best metries in the size of the frontal operculum and Heschl’s to define them (see Hagoort, 2014; Poeppel, 2014). Second, gyrus. Heschl’s gyrus was larger on the left, as predicted. there are large differences among healthy people in the However, different parts of the frontal operculum showed different directions of asymmetry: Its anterior portion was larger on the Figure 16.7 Three language areas of the cerebral cortex that have been the right, and its posterior portion was focus of studies on neuroanatomical asymmetry: The frontal operculum, the planum larger on the left. Clearly, if there is temporale (Wernicke’s area), and Heschl’s gyrus (primary auditory cortex). a relationship between anatomical asymmetries and language function, it is far more complex than previously Frontal operculum thought. In short, the search for ana- tomical differences between the two hemispheres has been only partially successful. Many anatomical asym- metries have been discovered, but few have been clearly related to functional asymmetries (see Kong et al., 2019). Several researchers have suggested that studies of differences in the microstruc- ture (e.g., differences in cell type, syn- apses, and neural circuitry) between the Heschl’s gyrus two hemispheres may prove more infor- mative than comparisons of differences Planum temporale in the size of vaguely defined areas (see Chance, 2014). M16_PINE1933_11_GE_C16.indd 445 22/01/2021 11:44  446 Chapter 16 the motor theory of cerebral asymmetry is that it does not Evolution of Cerebral suggest why motor function became lateralized in the first place (see Beaton, 2003). Lateralization and LINGUISTIC THEORY. A third theory of cerebral asymmetry, the linguistic theory of cerebral asymmetry, posits that the primary Language role of the left hemisphere is language; this is in contrast to the You have already seen in this chapter how the discussion analytic–synthetic and motor theories, which view l­anguage of cerebral lateralization inevitably leads to a discussion of as a secondary specialization residing in the left hemisphere language: Language is the most lateralized cognitive func- because of that hemisphere’s primary specialization for ana- tion. This module considers the evolution of cerebral later- lytic thought and skilled motor activity, respectively. alization and then the evolution of language. The linguistic theory