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Cerebral Hemispheric Interactions
Cerebral Hemispheric Interactions MC Corballis, University of Auckland, Auckland, New Zealand r 2014 Elsevier Inc. All rights reserved.
Introduction The two sides of the brain look very much like mirror images of one another, but they function in surprisingly different ways. This was not widely appreciated until the 1860s, when Paul Broca discovered that damage to the third convolution of the left frontal lobe, now known as Broca’s area, disrupts speech but leaves nonspeech movements of the articulators unaffected. Soon thereafter, Carl Wernicke discovered that damage to a region around the juncture of the parietal, temporal, and occipital lobes, again usually in the left hemisphere, results in deficits in comprehension. People with damage in this area, known as Wernicke’s area, can often speak fluently and grammatically, but what they say makes little sense. These and other language deficits caused by brain damage are known as aphasias. Subsequently, more detailed analyses suggest more complex relations between types of aphasia and different brain regions, but what remains clear is that language is predominantly under left hemispheric control. Moreover, because most people are right-handed, again implying left hemispheric control, the left hemisphere came to be regarded as the dominant or major hemisphere, whereas the right hemisphere was regarded as the nondominant, or minor hemisphere. This terminology persisted for around a century, despite some evidence suggesting that the right hemisphere was responsible for some nonverbal functions such as spatial attention or emotion. The most striking evidence for right hemispheric dominance came from so-called hemineglect, in which people with damage to one side of the brain often fail to pay attention to events on the opposite side of space. This is most often observed as left-sided neglect following right hemispheric damage. Left hemispheric damage rarely results in right-sided neglect, and when it does the condition is usually relatively mild and temporary. This asymmetry does not suggest absolute dominance, though, implies that the right hemisphere controls attention to both sides of space, whereas the left hemisphere controls attention only – or largely – to the right side of space.
The Split-Brain Studies A series of studies a century later, in the 1960s, led to a more balanced view of hemispheric differences. These were the splitbrain studies, involving patients who had undergone section of the corpus callosum, by far the largest and most important fiber tract connecting the two cerebral hemispheres, for the relief of intractable epilepsy. In some cases, other smaller connecting fibers were sectioned as well. This operation effectively separated the two hemispheres, allowing scientists to assess the capabilities of each hemisphere with little influence from the other. This research not only confirmed the left-hemisphere dominance for language but also suggested
674
specializations of the right hemisphere for nonverbal functions, including some aspects of spatial and emotional processing. Roger W. Sperry received the Nobel Prize for this work in 1982. Many of the classic studies of the split brain used visual inputs, based on the fact that visual input to the right of visual fixation is projected to the left hemisphere, and input to the left of fixation is projected to the right hemisphere. It followed that split-brained patients were unable to tell whether pairs of letters, digits, colors, or geometrical forms flashed simultaneously to opposite visual fields, and therefore to opposite hemispheres, were the same or different. Patients were also unable to name words or objects flashed to the left visual field, presumably because they were projected to the right hemisphere, which lacks the capacity for speech. Individuals with intact connections between the hemispheres have no difficulty with these tasks. Patients were also unable to name objects felt with the left hand, which projects to and is controlled by the right hemisphere, but were easily able to name those in the right hand. Curiously, some split-brained people showed comprehension of words to the left of visual fixation, suggesting that the right hemisphere has some capacity for receptive language. This seems to contradict the extreme lack of comprehension that often follows damage to the left hemisphere. One possibility is that some split-brained people are atypical in having developed a right hemispheric capacity for comprehension but not for production. Another possibility is that the right hemisphere does normally have some capacity to comprehend but is inhibited by the left hemisphere. This inhibition would be disconnected in the split brain. Later split-brain studies have suggested that the split brain is not quite so ‘split’ as earlier studies had implied. Splitbrained patients do have some capacity to integrate information between the visual fields. For example, if a dot appears in one visual field followed by a second dot in the other, patients can accurately report seeing motion from one side to the other; this is known as apparent motion, or the phi phenomenon. Similarly, patients are quite accurate in telling whether sloping lines in the two visual fields are aligned or not. These results indicate that some lower order-visual processing such as detection of movement or perception of alignment, and perhaps also the directing of spatial attention, can be integrated between hemispheres in the absence of the corpus callosum. This may depend on subcortical connections, such as that of the tectal commissure between the hemispheres. Such findings are also consistent with the notion that we essentially have two visual systems, a relatively primitive subcortical system sensitive to movement, location, and orientation, and a more recently evolved cortical system responsible for the perception of color and form, including perception of words and objects. This subcortical system may be largely unaffected by section of the corpus callosum and other forebrain connections.
Encyclopedia of the Neurological Sciences, Volume 1
doi:10.1016/B978-0-12-385157-4.01133-7
Cerebral Hemispheric Interactions
As though in rebound to the notion of left-hemisphere dominance, the split-brain studies led to the two hemispheres being characterized as essentially equal but complementary, with the left brain characterized as analytical and the right as holistic; indeed the right was often considered as the more creative, and the notions of left and right-brain thinking are well established in popular folklore. Scholars too have drawn on the distinction. A recent example is the 2009 book entitled The Master and his Emissary, in which the psychiatrist Iain McGilchrist explains the advance of Western civilization in terms of conflict between the two hemispheres, and in his characterization it is the right hemisphere that plays the more commanding role. Such idealized concepts of hemispheric differences go beyond the facts, and are more metaphorical than real, but have nevertheless provided a useful scaffold for interpreting historical, cultural, and even literary trends. In spite of the rather romanticized view of the right hemisphere, split-brained studies have seldom provided strong evidence of right-hemisphere specialization, perhaps in part because the disconnected right hemisphere may sometimes fail to comprehend what is required in a given task. One task that has been regarded as characteristically right hemispheric is mental rotation, which refers to the capacity to imagine shapes or objects in orientations other than that in which they actually or normally appear. One way to test this is to show people a letter of the alphabet, such as R or F, in various orientations, and ask them to decide whether the letter is normal or mirror reversed. People tend to accomplish this by mentally rotating the letter to the upright before making the decision, and this can be measured by recording the decision time. Decision times increase systematically as the letter is rotated away from the upright, given a measure of the time it takes to mentally rotate the letter to the upright. One splitbrained patient was able to perform this task only when the letter was shown in the left visual field, and not when it was shown in the right visual field, indicating that mental rotation is indeed accomplished by the right hemisphere. Till date, no other split-brained patient has shown this result – indeed no other patient has been able to perform the task at all. This may reflect the inability of the right hemisphere to understand the instructions as to what to do. Other evidence from brain injury or brain imaging nevertheless supports the idea that mental rotation is a specialization of the right hemisphere, and it is often regarded as a prototypical measure of spatial ability, complementary to the verbal abilities of the left hemisphere.
Brain Imaging A more accurate picture of right-brain involvement in human cognition has emerged from brain-imaging studies, and especially from functional magnetic resonance imaging. When people are asked to perform verbal tasks in the scanner, activation is characteristically biased toward the left hemisphere, and largely confirms the role of Broca’s and Wernicke’s areas in the production and perception of language. Some 20 years of brain-imaging research has nevertheless revealed much more detail, involving extensive cortical and subcortical circuits, depending on which aspect of language (e.g., semantics,
675
syntax, phonology, sign language, and reading) is considered. Important areas other than Broca’s and Wernicke’s areas include the supplementary motor area, the left anterior insula, and the anterior cingulate; detailed description is beyond the scope of this article. Nevertheless, the evidence continues to support the view that the left hemisphere is dominant for most aspects of language and this is indeed one of the most robust findings in neuroscience. Brain imaging also shows the left hemisphere to be dominant for signed languages such as those established among deaf communities. Indeed, damage to language areas results in sign language aphasia, and the areas involved in signing are essentially the same as those involved in spoken language, except for those aspects specific to the mode of input. The common neural pathways for speech and signing have sometimes been taken as one piece of evidence that language evolved from manual gestures, rather than from vocalizations. Activation is predominantly in the right hemisphere for some nonverbal operations, such as perception of emotion or spatial judgments. Figure 1 shows brain areas activated by a verbal task and two nonverbal tasks, one involving the perception of faces expressing emotion, and the other involving a spatial judgment. The former illustrates activation predominantly in the left hemisphere, whereas the latter two illustrate activation primarily in the right hemisphere. One aspect of language that appears to depend more on the right hemisphere than on the left is prosody, the tone of voice that signals whether a sentence is a statement, question, or command, or whether the speaker is in an emotional state such as anger or fear. Some studies show the right hemisphere to be the more specialized for music, although there is also evidence that the left hemisphere is dominant for music in experienced musicians, perhaps because they understand music in more ‘linguistic’ fashion. Brain imaging is also beginning to reveal something of the anatomical basis for cerebral asymmetry. It has long been known from anatomical studies that an area of the temporal lobe known as the planum temporal, which partially overlaps with Wernicke’s area, is larger on the left than on the right in most people. This is confirmed by magnetic resonance imaging studies, which also reveal that the arcuate fasciculus, a fiber tract that connects Wernicke’s and Broca’s areas, is generally larger on the left and may completely be absent on the right. Structural brain imaging also suggests that the corpus callosum provides weaker connections in right-handers and those who are left-hemisphere dominant for language, than in those who depart from this pattern. This has led to the suggestion that the corpus callosum may be calibrated, or ‘pruned,’ to allow hemispheric specializations to emerge, while at the same time allowing sufficient interhemispheric communication or the brain to function in an integrated manner.
Individual Differences The foregoing account applies to the majority of individuals but of course not all people conform to the same pattern. Approximately 10–12% of people are left-handed, and a similar but perhaps smaller percentage are right-hemisphere dominant for language or show no consistent dominance. Brain-imaging
676
Cerebral Hemispheric Interactions
Figure 1 Brain areas activated when individuals silently generate words beginning with a designated letter (shown in red), watch videos of faces expressing emotion (shown in blue), and judging whether horizontal lines are bisected by a marker (shown in green).
studies have revealed the rather surprising finding that different asymmetries are poorly correlated, and perhaps even uncorrelated. One study examining asymmetries in the spontaneous activity of the brain suggests four independent dimensions of brain asymmetry, associated with vision, attention, internal thought, and language. More specifically, handedness is correlated with hemispheric asymmetry for language, as has long been known, but seems to be uncorrelated with right-hemisphere dominance for spatial attention. These findings are recent, but may well spell the final end of simple notions of hemispheric ‘dominance.’ The brain appears to be a mosaic of different specialized asymmetries. Although handedness is correlated with hemispheric asymmetry for language, the correlation is imperfect. Approximately 95–99% of right-handers are left cerebrally dominant for language, but so are approximately 65–70% of left-handers. The left hemisphere is also dominant for praxis, or the ability to perform skilled manual actions, such as the use of tools. Damage to the left brain causes a loss of skilled function, a condition known as apraxia. Such findings are perhaps not surprising, because most people are also righthanded, implying dominance of the left hemisphere. What is surprising, though, is that brain imaging shows the left hemisphere to be dominant for praxis in the majority of lefthanders as well as of right-handers. Indeed, the asymmetry for praxis may be closer to that for language than to handedness itself. This has been taken as a further piece of evidence that language may have evolved from manual gestures. It is also clear that left-handers show a more mixed pattern of asymmetry than do right-handers. This provides some support for genetic theories, which postulate that one or more genes dispose most individuals to be right-handed and left cerebrally dominant for language, but that this influence is lacking in some individuals, either because of a genetic allele that cancels the asymmetry instruction or because of some epigenetical influence. In such individuals, asymmetries of handedness or language are then a matter of chance. Although genetic theories with these characteristics can provide reasonably good fits to data on the inheritance of handedness, as well as on the relation of handedness to cerebral asymmetry
for language, the actual gene or genes have not been isolated. One gene that holds some promise is known as LRRTM1 (leucine-rich repeat transmembrane neuronal 1), a paternally inherited gene that also seems to be associated with schizophrenia. The inheritance of handedness and cerebral asymmetry may well turn out to be more complex than proposed by simple genetic theories. One myth that is dispelled by recent studies is that handedness and cerebral asymmetries are unique to humans, and perhaps even define the human condition. It is now apparent that great apes tend to be right-handed, especially for communicative gestures, although the frequency is approximately 65% as against approximately 90% in humans. Cerebral asymmetries abound in many nonhuman species including birds, fish, and marine mammals, as well as nonhuman primates. It is nevertheless widely agreed that human language is unique, with properties not shared by communication in other species, but its lateralization in the human brain may have evolved from asymmetries of action, communication, and brain function that have their roots far back in evolution.
See also: Broca, Paul. Callosotomy. Corpus Callosum. Handedness and Cerebral Laterality. Magnetic Resonance Imaging. Sperry, Roger. Wernicke, Carl
Further Reading Corballis MC (1995) Visual integration in the split brain. Neuropsychologia 33: 937–959. Corballis MC (1997) Mental rotation and the right hemisphere. Brain and Language 57: 100–121. Corballis MC, Badzakova-Trajkov G, and Ha¨berling IS (2012) Right hand, left brain: Genetic and evolutionary bases of cerebral asymmetries for language and manual action. WIREs Cognitive Science 3: 1–17. Francks C, Maegawa S, Lauren J, et al. (2007) LRRTM1 on chromosome 2p12 is a maternally suppressed gene that is associated paternally with handedness and schizophrenia. Molecular Psychiatry 12: 1129–1139. Frey SH (2008) Tool use, communicative gesture and cerebral asymmetries in the modern human brain. Philosophical Transactions of the Royal Society B 363: 1951–1957.
Cerebral Hemispheric Interactions
Gazzaniga MS (2000) Cerebral specialization and interhemispheric communication: Does the corpus callosum enable the human condition? Brain 123: 1293–1326. Haberling IS, Badzakova-Trajkov G, and Corballis MC (2011) Callosal tracts and patterns of hemispheric dominance: A combined fMRI and DTI study. NeuroImage 54: 779–786. Liu H, Stufflebeam SM, Sepulcrea J, Heddena T, and Buckner RL (2009) Evidence from intrinsic activity that asymmetry of the human brain is controlled by multiple factors. Proceedings of the National Academy of Sciences of the United States of America 106: 20499–20503.
677
McGilchrist I (2009) The Master and his Emissary. New Haven, CT: Yale University Press. Price CJ (2012) A review and synthesis of the first 20 years of PET and fMRI studies of heard speech, spoken language and reading. NeuroImage 62: 816–847. Rogers LJ and Andrew RJ (2002) Comparative Vertebrate Lateralization. Cambridge, UK: Cambridge University Press.
Introduction The two sides of the brain look very much like mirror images of one another, but they function in surprisingly different ways. This was not widely appreciated until the 1860s, when Paul Broca discovered that damage to the third convolution of the left frontal lobe, now known as Broca’s area, disrupts speech but leaves nonspeech movements of the articulators unaffected. Soon thereafter, Carl Wernicke discovered that damage to a region around the juncture of the parietal, temporal, and occipital lobes, again usually in the left hemisphere, results in deficits in comprehension. People with damage in this area, known as Wernicke’s area, can often speak fluently and grammatically, but what they say makes little sense. These and other language deficits caused by brain damage are known as aphasias. Subsequently, more detailed analyses suggest more complex relations between types of aphasia and different brain regions, but what remains clear is that language is predominantly under left hemispheric control. Moreover, because most people are right-handed, again implying left hemispheric control, the left hemisphere came to be regarded as the dominant or major hemisphere, whereas the right hemisphere was regarded as the nondominant, or minor hemisphere. This terminology persisted for around a century, despite some evidence suggesting that the right hemisphere was responsible for some nonverbal functions such as spatial attention or emotion. The most striking evidence for right hemispheric dominance came from so-called hemineglect, in which people with damage to one side of the brain often fail to pay attention to events on the opposite side of space. This is most often observed as left-sided neglect following right hemispheric damage. Left hemispheric damage rarely results in right-sided neglect, and when it does the condition is usually relatively mild and temporary. This asymmetry does not suggest absolute dominance, though, implies that the right hemisphere controls attention to both sides of space, whereas the left hemisphere controls attention only – or largely – to the right side of space.
The Split-Brain Studies A series of studies a century later, in the 1960s, led to a more balanced view of hemispheric differences. These were the splitbrain studies, involving patients who had undergone section of the corpus callosum, by far the largest and most important fiber tract connecting the two cerebral hemispheres, for the relief of intractable epilepsy. In some cases, other smaller connecting fibers were sectioned as well. This operation effectively separated the two hemispheres, allowing scientists to assess the capabilities of each hemisphere with little influence from the other. This research not only confirmed the left-hemisphere dominance for language but also suggested
674
specializations of the right hemisphere for nonverbal functions, including some aspects of spatial and emotional processing. Roger W. Sperry received the Nobel Prize for this work in 1982. Many of the classic studies of the split brain used visual inputs, based on the fact that visual input to the right of visual fixation is projected to the left hemisphere, and input to the left of fixation is projected to the right hemisphere. It followed that split-brained patients were unable to tell whether pairs of letters, digits, colors, or geometrical forms flashed simultaneously to opposite visual fields, and therefore to opposite hemispheres, were the same or different. Patients were also unable to name words or objects flashed to the left visual field, presumably because they were projected to the right hemisphere, which lacks the capacity for speech. Individuals with intact connections between the hemispheres have no difficulty with these tasks. Patients were also unable to name objects felt with the left hand, which projects to and is controlled by the right hemisphere, but were easily able to name those in the right hand. Curiously, some split-brained people showed comprehension of words to the left of visual fixation, suggesting that the right hemisphere has some capacity for receptive language. This seems to contradict the extreme lack of comprehension that often follows damage to the left hemisphere. One possibility is that some split-brained people are atypical in having developed a right hemispheric capacity for comprehension but not for production. Another possibility is that the right hemisphere does normally have some capacity to comprehend but is inhibited by the left hemisphere. This inhibition would be disconnected in the split brain. Later split-brain studies have suggested that the split brain is not quite so ‘split’ as earlier studies had implied. Splitbrained patients do have some capacity to integrate information between the visual fields. For example, if a dot appears in one visual field followed by a second dot in the other, patients can accurately report seeing motion from one side to the other; this is known as apparent motion, or the phi phenomenon. Similarly, patients are quite accurate in telling whether sloping lines in the two visual fields are aligned or not. These results indicate that some lower order-visual processing such as detection of movement or perception of alignment, and perhaps also the directing of spatial attention, can be integrated between hemispheres in the absence of the corpus callosum. This may depend on subcortical connections, such as that of the tectal commissure between the hemispheres. Such findings are also consistent with the notion that we essentially have two visual systems, a relatively primitive subcortical system sensitive to movement, location, and orientation, and a more recently evolved cortical system responsible for the perception of color and form, including perception of words and objects. This subcortical system may be largely unaffected by section of the corpus callosum and other forebrain connections.
Encyclopedia of the Neurological Sciences, Volume 1
doi:10.1016/B978-0-12-385157-4.01133-7
Cerebral Hemispheric Interactions
As though in rebound to the notion of left-hemisphere dominance, the split-brain studies led to the two hemispheres being characterized as essentially equal but complementary, with the left brain characterized as analytical and the right as holistic; indeed the right was often considered as the more creative, and the notions of left and right-brain thinking are well established in popular folklore. Scholars too have drawn on the distinction. A recent example is the 2009 book entitled The Master and his Emissary, in which the psychiatrist Iain McGilchrist explains the advance of Western civilization in terms of conflict between the two hemispheres, and in his characterization it is the right hemisphere that plays the more commanding role. Such idealized concepts of hemispheric differences go beyond the facts, and are more metaphorical than real, but have nevertheless provided a useful scaffold for interpreting historical, cultural, and even literary trends. In spite of the rather romanticized view of the right hemisphere, split-brained studies have seldom provided strong evidence of right-hemisphere specialization, perhaps in part because the disconnected right hemisphere may sometimes fail to comprehend what is required in a given task. One task that has been regarded as characteristically right hemispheric is mental rotation, which refers to the capacity to imagine shapes or objects in orientations other than that in which they actually or normally appear. One way to test this is to show people a letter of the alphabet, such as R or F, in various orientations, and ask them to decide whether the letter is normal or mirror reversed. People tend to accomplish this by mentally rotating the letter to the upright before making the decision, and this can be measured by recording the decision time. Decision times increase systematically as the letter is rotated away from the upright, given a measure of the time it takes to mentally rotate the letter to the upright. One splitbrained patient was able to perform this task only when the letter was shown in the left visual field, and not when it was shown in the right visual field, indicating that mental rotation is indeed accomplished by the right hemisphere. Till date, no other split-brained patient has shown this result – indeed no other patient has been able to perform the task at all. This may reflect the inability of the right hemisphere to understand the instructions as to what to do. Other evidence from brain injury or brain imaging nevertheless supports the idea that mental rotation is a specialization of the right hemisphere, and it is often regarded as a prototypical measure of spatial ability, complementary to the verbal abilities of the left hemisphere.
Brain Imaging A more accurate picture of right-brain involvement in human cognition has emerged from brain-imaging studies, and especially from functional magnetic resonance imaging. When people are asked to perform verbal tasks in the scanner, activation is characteristically biased toward the left hemisphere, and largely confirms the role of Broca’s and Wernicke’s areas in the production and perception of language. Some 20 years of brain-imaging research has nevertheless revealed much more detail, involving extensive cortical and subcortical circuits, depending on which aspect of language (e.g., semantics,
675
syntax, phonology, sign language, and reading) is considered. Important areas other than Broca’s and Wernicke’s areas include the supplementary motor area, the left anterior insula, and the anterior cingulate; detailed description is beyond the scope of this article. Nevertheless, the evidence continues to support the view that the left hemisphere is dominant for most aspects of language and this is indeed one of the most robust findings in neuroscience. Brain imaging also shows the left hemisphere to be dominant for signed languages such as those established among deaf communities. Indeed, damage to language areas results in sign language aphasia, and the areas involved in signing are essentially the same as those involved in spoken language, except for those aspects specific to the mode of input. The common neural pathways for speech and signing have sometimes been taken as one piece of evidence that language evolved from manual gestures, rather than from vocalizations. Activation is predominantly in the right hemisphere for some nonverbal operations, such as perception of emotion or spatial judgments. Figure 1 shows brain areas activated by a verbal task and two nonverbal tasks, one involving the perception of faces expressing emotion, and the other involving a spatial judgment. The former illustrates activation predominantly in the left hemisphere, whereas the latter two illustrate activation primarily in the right hemisphere. One aspect of language that appears to depend more on the right hemisphere than on the left is prosody, the tone of voice that signals whether a sentence is a statement, question, or command, or whether the speaker is in an emotional state such as anger or fear. Some studies show the right hemisphere to be the more specialized for music, although there is also evidence that the left hemisphere is dominant for music in experienced musicians, perhaps because they understand music in more ‘linguistic’ fashion. Brain imaging is also beginning to reveal something of the anatomical basis for cerebral asymmetry. It has long been known from anatomical studies that an area of the temporal lobe known as the planum temporal, which partially overlaps with Wernicke’s area, is larger on the left than on the right in most people. This is confirmed by magnetic resonance imaging studies, which also reveal that the arcuate fasciculus, a fiber tract that connects Wernicke’s and Broca’s areas, is generally larger on the left and may completely be absent on the right. Structural brain imaging also suggests that the corpus callosum provides weaker connections in right-handers and those who are left-hemisphere dominant for language, than in those who depart from this pattern. This has led to the suggestion that the corpus callosum may be calibrated, or ‘pruned,’ to allow hemispheric specializations to emerge, while at the same time allowing sufficient interhemispheric communication or the brain to function in an integrated manner.
Individual Differences The foregoing account applies to the majority of individuals but of course not all people conform to the same pattern. Approximately 10–12% of people are left-handed, and a similar but perhaps smaller percentage are right-hemisphere dominant for language or show no consistent dominance. Brain-imaging
676
Cerebral Hemispheric Interactions
Figure 1 Brain areas activated when individuals silently generate words beginning with a designated letter (shown in red), watch videos of faces expressing emotion (shown in blue), and judging whether horizontal lines are bisected by a marker (shown in green).
studies have revealed the rather surprising finding that different asymmetries are poorly correlated, and perhaps even uncorrelated. One study examining asymmetries in the spontaneous activity of the brain suggests four independent dimensions of brain asymmetry, associated with vision, attention, internal thought, and language. More specifically, handedness is correlated with hemispheric asymmetry for language, as has long been known, but seems to be uncorrelated with right-hemisphere dominance for spatial attention. These findings are recent, but may well spell the final end of simple notions of hemispheric ‘dominance.’ The brain appears to be a mosaic of different specialized asymmetries. Although handedness is correlated with hemispheric asymmetry for language, the correlation is imperfect. Approximately 95–99% of right-handers are left cerebrally dominant for language, but so are approximately 65–70% of left-handers. The left hemisphere is also dominant for praxis, or the ability to perform skilled manual actions, such as the use of tools. Damage to the left brain causes a loss of skilled function, a condition known as apraxia. Such findings are perhaps not surprising, because most people are also righthanded, implying dominance of the left hemisphere. What is surprising, though, is that brain imaging shows the left hemisphere to be dominant for praxis in the majority of lefthanders as well as of right-handers. Indeed, the asymmetry for praxis may be closer to that for language than to handedness itself. This has been taken as a further piece of evidence that language may have evolved from manual gestures. It is also clear that left-handers show a more mixed pattern of asymmetry than do right-handers. This provides some support for genetic theories, which postulate that one or more genes dispose most individuals to be right-handed and left cerebrally dominant for language, but that this influence is lacking in some individuals, either because of a genetic allele that cancels the asymmetry instruction or because of some epigenetical influence. In such individuals, asymmetries of handedness or language are then a matter of chance. Although genetic theories with these characteristics can provide reasonably good fits to data on the inheritance of handedness, as well as on the relation of handedness to cerebral asymmetry
for language, the actual gene or genes have not been isolated. One gene that holds some promise is known as LRRTM1 (leucine-rich repeat transmembrane neuronal 1), a paternally inherited gene that also seems to be associated with schizophrenia. The inheritance of handedness and cerebral asymmetry may well turn out to be more complex than proposed by simple genetic theories. One myth that is dispelled by recent studies is that handedness and cerebral asymmetries are unique to humans, and perhaps even define the human condition. It is now apparent that great apes tend to be right-handed, especially for communicative gestures, although the frequency is approximately 65% as against approximately 90% in humans. Cerebral asymmetries abound in many nonhuman species including birds, fish, and marine mammals, as well as nonhuman primates. It is nevertheless widely agreed that human language is unique, with properties not shared by communication in other species, but its lateralization in the human brain may have evolved from asymmetries of action, communication, and brain function that have their roots far back in evolution.
See also: Broca, Paul. Callosotomy. Corpus Callosum. Handedness and Cerebral Laterality. Magnetic Resonance Imaging. Sperry, Roger. Wernicke, Carl
Further Reading Corballis MC (1995) Visual integration in the split brain. Neuropsychologia 33: 937–959. Corballis MC (1997) Mental rotation and the right hemisphere. Brain and Language 57: 100–121. Corballis MC, Badzakova-Trajkov G, and Ha¨berling IS (2012) Right hand, left brain: Genetic and evolutionary bases of cerebral asymmetries for language and manual action. WIREs Cognitive Science 3: 1–17. Francks C, Maegawa S, Lauren J, et al. (2007) LRRTM1 on chromosome 2p12 is a maternally suppressed gene that is associated paternally with handedness and schizophrenia. Molecular Psychiatry 12: 1129–1139. Frey SH (2008) Tool use, communicative gesture and cerebral asymmetries in the modern human brain. Philosophical Transactions of the Royal Society B 363: 1951–1957.
Cerebral Hemispheric Interactions
Gazzaniga MS (2000) Cerebral specialization and interhemispheric communication: Does the corpus callosum enable the human condition? Brain 123: 1293–1326. Haberling IS, Badzakova-Trajkov G, and Corballis MC (2011) Callosal tracts and patterns of hemispheric dominance: A combined fMRI and DTI study. NeuroImage 54: 779–786. Liu H, Stufflebeam SM, Sepulcrea J, Heddena T, and Buckner RL (2009) Evidence from intrinsic activity that asymmetry of the human brain is controlled by multiple factors. Proceedings of the National Academy of Sciences of the United States of America 106: 20499–20503.
677
McGilchrist I (2009) The Master and his Emissary. New Haven, CT: Yale University Press. Price CJ (2012) A review and synthesis of the first 20 years of PET and fMRI studies of heard speech, spoken language and reading. NeuroImage 62: 816–847. Rogers LJ and Andrew RJ (2002) Comparative Vertebrate Lateralization. Cambridge, UK: Cambridge University Press.