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For more than 150 years, scientists have known that the left and right sides of the human brain are not identical. Some structures exist in both hemispheres but differ in size, and many functions are different on the left and the right. We refer to this difference as hemispheric specialization or, more generally, brain lateralization. From the time this lateralization was discovered, it was assumed to be unique to humans. Associated by scientists with our use of language and our ability to make tools, the lateralized brain was seen as humanity’s crowning glory, elevating us above all other species.
Initially, as we will see, research seemed to bear out this conclusion, but recent discoveries have not been kind to the theory of uniquely human brain lateralization. Some species of parrots are 90 percent left-footed, ﬁsh of some species all turn the same way when encountering a barrier (the better to school), and monkeys express fear more strongly on the left sides of their faces. This same research, and a great deal more, has immensely complicated our view of lateralization and its signiﬁcance for language, movement, emotions, and attention, to name just a few of the affected functions.
Popular psychology is rife with interpretations—and misinterpretations—of brain lateralization. You might read that people are either “left- or right-brained,” and correspondingly more analytical or intuitive, more cerebral or emotional. You might read, too, that left-handers are right-brained, and vice versa. Further complicating the picture are deep and abiding beliefs about hand preference, including sometimes intense prejudices against left-handed people. At the beginning of the 21st century, the real signiﬁcance of brain lateralization is still being sought.
Discovering Lateralization in Humans
In 1836, a French country physician, Marc Dax, observed that patients who had a stroke, sword wound, or brain tumor that caused paralysis of the right side of their body were more likely to lose the capacity to speak than patients whose injury caused paralysis on their left side. Because the brain’s left hemisphere was known to control the right side of the body, Dax concluded that this hemisphere must be specialized for speech. He also traced the incapacity to speak, a condition called aphasia, to damage in a speciﬁc left-hemisphere region. Dax’s name is little known, today, because, despite valiant efforts of his son, the discovery became associated with the prominent Parisian physician, Paul Broca, after whom the brain region specialized for producing speech is named.
About a half century later, another area in the left hemisphere, also specialized for language (in this case the comprehension of its meaning) was discovered by Carl Wernicke and named in his honor. But subsequent research showed that language was not the only lateralized function of the human brain. In most people, a region called the planum temporale is larger in the left hemisphere than in the right hemisphere and the occipital lobe of the left hemisphere is larger than its equivalent in the right hemisphere. The opposite is true of the frontal lobes.
Interest in brain lateralization mounted in the 1960s with work at the California Institute of Technology by Roger Sperry, Ph.D., and his “split-brain” patients. To stop the spread of massive epileptic seizures, these patients had their corpus callosum—the major connection between the hemispheres—severed. This drastic operation largely isolated their left and right hemispheres from each other, creating a promising natural experiment for studying their separate functions. From this and other research, we now know that there are many lateralized functions.
In general, the left hemisphere is specialized for processing language and producing speech, carrying out sequential processing of information, focusing attention, and inhibiting negative emotions. The right hemisphere is specialized for simultaneous processing of information, attending in a broad or diffuse way, forming and using spatial maps, and expressing intense emotions. Injury to the left hemisphere from stroke, which forces the right hemisphere to take over, leads to deeper depression than does injury of the right hemisphere. Injury to the lateral (temporal lobe) and posterior regions of the left hemisphere causes an increase in feelings of hostility and disgust, which does not result from similar injuries in the right hemisphere. Moreover, the frontal and anterior regions of the right hemisphere are selectively activated in withdrawn social and emotional states. In children, right frontal activation is associated with high reactivity and fearfulness of unfamiliar events, as a 2002 study by Mark McManis, Ph.D., together with Jerome Kagan, Ph.D., and others at Harvard University, has shown. In line with this study, the right hemisphere seems to have more control of the stress hormone system (the pituitary and adrenal cortex) than does the left hemisphere.
Today, the new techniques for imaging blood ﬂow and levels of nerve activity in the brain give us much better ways to relate the anatomy of the brain to its function. One result is that new evidence for left-right specialization is accumulating. For example, scanning brain activity has revealed that panic-prone people have higher levels of activity than normal in their right hemisphere even when they are resting.
These specializations should not be seen as absolute. Some aspects of language processing do take place in the right hemisphere, for example. Moreover, the degree of difference between the hemispheres varies from person to person. In some left-handers, the direction of brain asymmetry is reversed. What do we know about the signiﬁcance of these many differences?
Being a “Lefty” or a “Righty”
What about hand preference as an indicator of a person’s brain lateralization? Approach this issue with care. It is not as simple as we might expect, and there is a long history of prejudice against left-handed people. Even in recent times, left handers have been stigmatized—as being less intelligent than right handers, for example—and forced to use their right hand for writing. They are also disadvantaged by the almost exclusive manufacture of tools and household implements for right-hand use. The ﬁrst relevant point is that if an individual prefers a hand to perform a given task, this preference means that he is using lateralized output from the opposite hemisphere. The hand used for writing is an example of this. Most people use their right hand, as a consequence of interacting genetic and environmental inﬂuences, one of which is learning to conform—or being actively forced to do so. This right-handedness for writing is logically consistent with the specialization of the left hemisphere for language and speech, but the pattern of hemispheric specialization is not simply reversed in all left-handers; some of them have their language and speech functions in the left hemisphere. In other words, hand preference is just one type of lateralized brain function and need not represent a whole collection of other functions, although this representation is often assumed to be the case.
Moreover, writing is a special case of handedness. Most individuals do not show as strong a hand preference on other tasks, using the left hand for some, the right hand for others. In fact, writing and using certain tools might have biased us toward right-handedness, as a 1995 study by Linda Marchant, Ph.D., and William McGrew, Ph.D., at Miami University, together with Irenäus Eibl-Eibesfeldt, Ph.D., suggests. They scored the hand preferences during everyday activities in three separate groups of people living in societies without a written language. Most of these people used the left hand for some tasks and the right hand for others. Overall, only a weak preference for right-handedness was found, except for tool using, for which the preference was strong. Thus, when we say that a person is right- or left-handed, we should specify on what task or tasks. For example, even people who are strongly right-handed for writing and using tools tend to be better at grabbing a moving target, such as a ball, with their left hand. This preference is consistent with the right hemisphere’s specialized ability for performing spatial tasks and controlling rapid responses.
These complexities and the variation in hand preferences are a strong scientiﬁc argument (if one were needed) against prejudiced views toward left-handers. We also know that hand preference varies from individual to individual, and task to task, much more than does the lateralization of the brain. For any particular task, we would perform better if we used the hand that is controlled by the hemisphere specialized for that task (for example, the right hemisphere and left hand for catching moving targets, and the left hemisphere and right hand for writing and manipulating tools), but practice could override these biases.
It is intriguing that hand-brain inﬂuence works in both directions. Active use of one hand, or one side of the mouth, can call into action its opposite brain hemisphere. Repeated clenching of the left hand or prolonged pulling back of the left side of the mouth produces feelings of sadness—in some subjects to the point of weeping—whereas the same movements of the right side lead to feelings described as positive and “sarcastic, cocky, good, smug.” These results were found by Bernard Schiff, Ph.D., and Mary Lamon, Ph.D., at the University of Toronto.
Who (or What) Lateralized First?
Because brain lateralization was long assumed to be unique to humans, anthropologists set out to discover when humans ﬁrst became right-handed, using as evidence the patterns of ﬂaking ﬂint stone to create cutting edges. Nicholas Toth, Ph.D., at the University of Indiana found that the stones knapped by early humans during the Lower Pleistocene were ﬂaked in a way that suggested that the ﬂint stone was held in the left hand and rotated clockwise, while the right hand held the hammer stone used for striking it. Had hemispheric specialization made its ﬁrst appearance in early humans, perhaps at about the same stage of evolution when language appeared? If so, brain asymmetry and lateralized limb use should be absent in nonhuman animals.
The ﬁrst investigations of hand preferences in nonhuman primates supported this idea. Although individual primates did exhibit hand preferences, there was no evidence that most of the individuals of the same species were biased in the same direction. The right or left preferences seemed to be about half-and-half. Then, in 1987, Peter MacNeilage, Ph.D., of the University of Texas and his colleagues decided to look more closely at the data on hand preferences in primates. Gathering and examining the various published results on this subject, they found that the lower or earlier-evolving primates did show hand preferences. Most reached out for food (usually to catch insects) with their left hand. In the more highly evolved primates, however, there was a trend toward use of the right hand for ﬁne manipulation of food or objects. In line with this pattern, William Hopkins, Ph.D., and colleagues at the Yerkes Institute have found right-hand preferences for manipulating objects in captive chimpanzees.
The question of handedness in apes remains controversial, however, because wild chimpanzees have, so far, shown no evidence of this population bias. Perhaps the captive chimps have acquired their right-hand preferences by mimicking their human keepers. In any case, there are precious few studies of hand preferences in the other apes, especially in situations in which they have to use both hands collaboratively. In fact, the strongest evidence for hand preference in orangutans points to use of the left hand to manipulate parts of the face and head (for cleaning the eyes, ears, or nostrils), as Gisela Kaplan, Ph.D., and I found in semiwild orangutans undergoing rehabilitation in Borneo.
In short, we should examine limb use in a wider range of vertebrates before we conclude that humans have a special claim to handedness (or at least to lateralized limb use). Cockatoos are strongly footed, for example; all species but one studied thus far prefer to hold food in their left foot. Toads are right-handed for wiping pieces of paper from their snouts and for pushing against the experimenter’s hand when righting themselves under water. Even ﬁsh show biases to turn in the same direction when they swim up to a barrier, although the direction of the turning varies with the species. All this behavior hints at very early evolution of brain lateralization, and we ﬁnd still more evidence of this evolution in lateralized behavior apart from hand or foot preferences.
The most comprehensive studies of lateralization in animals have looked at the domestic chick and the rat, but evidence from a wider spectrum of species indicates that lateralization might be a basic pattern in all vertebrates, including humans. A chick can ﬁnd hidden food by using geometrical information (such as the center of an area) or a spatial map (based on the relative locations of conspicuous objects). With a patch over its right eye, the chick can still do so, but shift the patch to its left eye and it cannot. Having to use its left eye means that the chick is processing the visual information primarily in its right hemisphere (the opposite would be the case if it had to use its right eye). This is the way that the neural pathways from eye to hemisphere are connected in all birds and mammals that have eyes on the sides of their heads. Thus, the result shows that the right hemisphere of the chick—as of the human—is specialized for using a spatial map. The same is true for rats; they are far better at ﬁnding a hidden escape platform in the water when they are using their left eye.
Another function of the right hemisphere in humans is involvement in expression of intense emotions, including aggression, and this function also holds true for chicks, toads, lizards, and monkeys. Humans and monkeys both express fear more strongly on the left side of their face (right hemisphere controlling), and chicks, lizards, toads, and baboons direct attacks at other members of their species when they are on the attacker’s left side (and thus seen by their left eye). By contrast, the left hemisphere of these species is specialized to control feeding responses in which the animal has to inhibit responding to stimuli resembling food (pebbles versus grain in the case of chick) or has to manipulate an object to obtain the food. Generalizing, then, the left hemisphere of vertebrates, including humans, is used when decisions have to be made before responding (requiring initial inhibition of the response), whereas the right hemisphere is used when rapid responses are demanded or emotions are expressed.
This pattern is surprisingly similar across vertebrate species, not unique to humans. Indeed, it made its appearance with the earliest vertebrates. Of course, it is likely that the emergence of language depended on brain lateralization, and perhaps the use of tools did, also. Humans certainly have some distinctive forms of lateralization. Even these forms, however, might have had precursors long before humans came on the scene. We know, for example, that many species of songbirds use areas of their left hemisphere in producing their songs and that marmosets and macaque monkeys use their right hemisphere in processing their species-typical vocalizations. Still more relevant, perhaps, is the 2001 discovery by Claudio Cantalupo, Ph.D., and William Hopkins, Ph.D., that in gorillas and chimpanzees a region of the left hemisphere that in humans became Broca’s area is enlarged compared with the equivalent region in the right hemisphere. This size difference looks like a precursor to the asymmetry associated in humans with speech.
Brain, Hand, and Temperament
What evidence supports the popular speculation that we are “right-brained” or “left-brained” and that this is reﬂected in our personality or temperament? The presence of consistent left-right differences in hemisphere activity, shown in individual test subjects, suggests that this belief has some validity. Because hand preference is not an accurate tool for assessing whether a human is left- or right-brained (determining the balance of activity between a person’s hemispheres would be more reliable), investigation of brain lateralization and personality in humans has been difﬁcult. Nevertheless, some research does suggest that hand preference in humans could have some relationship to post-traumatic stress disorder and the personality disorder known as schizotypy. On average, people with these disorders are less right-handed than are control subjects (meaning that they are either left-handed or ambidextrous), as reported by J. Shaw, Ph.D., Gordon Claridge, Ph.D., and Ken Clarl, Ph.D., and coworkers at the University of Oxford. Even if a predisposition to schizophrenia and left handedness are linked genetically, as Clyde Fracks, Ph.D., and coworkers at the Wellcome Trust Centre for Human Genetics, have recently found some evidence to show, this association would be a special case, not representative of left handers in general. It would be incorrect to turn the argument around and use this information as a new way to stigmatize left handers.
Because many aspects of lateralization are similar in human and nonhuman brains, we can study animals to ﬁnd out more about this feature of brain function, and what we learn will likely be relevant to the human condition. In fact, studying primates could be particularly enlightening because it seems that there might be a closer correlation between their hand preferences and the activities of their brain hemispheres than is the case in humans.
One study in my laboratory discovered that marmosets with a right-hand preference for picking up food when they are in a familiar cage are more likely than those with a left-hand preference to explore a new environment or to interact with novel toys. To elaborate: Marmosets are able to use either hand to pick up food, but about half consistently use their left hand and half their right. It appears that a marmoset’s hand choice reﬂects its relative balance of activity between the hemispheres. This preference, in turn, appears to reﬂect a range of behavior made in a new environment.
A study of chimpanzees found a similar result. Left-handed primates are also known to show higher stress levels, in terms of both their behavior and their physiology. When stressed, levels of the hormone cortisol are elevated to higher levels in left-handers compared with right-handers. A study of EEG activity in rhesus monkeys conﬁrmed the role of hemispheric activity in these results. Individual monkeys had stable differences in baseline levels of activity in the left versus right hemispheres, and the monkeys with more activity in the right hemisphere had higher cortisol levels and were more fearful than those with the opposite EEG balance.
Thus, it looks very much as if individuals who are more dependent on using their left hemisphere could have temperaments that are different from those who are more dependent on their right hemisphere. Many popular psychology books associate right-and left-brain differences with patterns of thought and creativity, with left-brained individuals being more verbal, linear, and analytical in their thinking, and right-brained individuals being more intuitive, creative, and non-verbal. This division is very likely to be correct, but the inaccuracy is in determining who is left- or right-brained because hand preference is far from an accurate indicator of this. Also, we should not assume that these differences are immutable. Other studies on animals (chicks, in particular) have emphasized that the dominant hemisphere changes during development and does so on a precise schedule. In chicks, the left hemisphere is dominant during the ﬁrst week of life, when the chick has to inhibit some of its innate responses as it learns to recognize food and other things. Then, at days 10 and 11 of life, the right hemisphere brieﬂy assumes dominance, precisely at a time when the chick ﬁrst becomes independent enough to leave its mother and ﬁnd its way around (and therefore has to use the spatial abilities of its right hemisphere). Likely, similar changes occur during development in other species, including humans, although they might not be as precise as in the rapidly developing chick. Moreover, as we know from experiments on animals, experiences in later life could shift the balance between the hemispheres for either brief or longer periods.
The Lateralized Brain: When It Hurts, When It Helps
There are distinct disadvantages in having a lateralized brain. Not the least of these are the bias to respond to food items—prey— on the right and so miss a potential meal on the left and the bias to attack others on the left side and so miss a potential rival on the right. In addition, animals respond to a predator more rapidly and strongly when it approaches on the left side, leaving them more vulnerable to predators on their right side. In fact, predators could learn to exploit this bias by striking on their prey’s right side. Although these side biases are not absolute, they would put a species at an evolutionary disadvantage, unless they were balanced by some advantages of being lateralized. So what might be some of those advantages?
Researchers studying the human brain have long maintained that the advantage of having a lateralized brain is increased brain capacity, because lateralization means that neural circuits do not have to be duplicated in each hemisphere. Each hemisphere can have its own specialized circuits and functions. Although so far there is no empirical evidence to support this claim about humans, researchers are studying it in animals. For example, comparing weakly lateralized with strongly lateralized animals yields some clues. Very new evidence shows that being more lateralized enables each hemisphere to carry out different functions at the same time, and the more lateralized the brain, the better it is able to do this.
I have tested this hypothesis by comparing chicks with relatively strong and weak brain lateralization. The comparison is made by requiring the chicks to engage their left hemisphere in ﬁnding grains scattered among pebbles and, at the same time, to engage their right hemisphere in detecting a model predator overhead (a silhouette of a hawk). The more lateralized chicks performed better on both tasks than did the weakly lateralized ones. The more strongly lateralized chicks were ones hatched from eggs exposed to light during the ﬁnal stages of incubation. At this stage, the chick embryo is positioned in the egg so that its head is turned to the left side and its body occludes, or blocks, its left eye. The right eye remains next to the shell and, therefore, can be stimulated by light. This asymmetrical stimulation of the eyes translates into lateralized activity in the developing hemispheres and establishes some forms of visual lateralization that are not present in chicks hatched from eggs incubated in the dark. From the results of my experiment, we can predict that chicks hatched from eggs exposed to light— and therefore with strong lateralization—have better chances of survival in the natural environment, where there would be many times when different demands are made on both hemispheres at the same time. This advantage might, therefore, be part of the answer to why lateralization of the brain is so common among vertebrates.
It is very possible that other species have this advantage; more research is needed. We know, however, that wild chimpanzees with greater differences in the way they use their hands (that is, who are more lateralized) are more efﬁcient in obtaining food that requires the skillful coordination of both hands. In 2000, William McGrew and Linda Marchant observed chimpanzees as they “ﬁshed” for termites from a mound, a skill that they perform by inserting a piece of grass or a twig into the holes in the mound. The termites bite the inserted probe and the chimpanzee pulls them out of the nest. The termites are then scraped from the twig as the chimpanzee draws it across its other hand or arm, and the chimp scoops them up with its lips. This is a tough task for a chimpanzee to learn. The more efﬁcient chimpanzees consistently use the same hand to insert the twig and the other to draw the twig across their arm. Chimpanzees that have no preferred hand for either aspect of this task obtain fewer termites to eat. This, of course, is a motor skill, whereas the experiment with chicks tested perceptual lateralization. Both approaches, however, could be applied to research on humans, and the ﬁndings would be very interesting.
Why Do Population Biases Occur?
So we see that the lateralized brain is more efﬁcient at performing two tasks that make different simultaneous demands on neural processing. But a brain efﬁcient in processing of this kind could be lateralized in either direction; there is no need for all or most members of a species to prefer the same direction for a particular action. When they do, we call it a “population bias.” Why, then, do we ﬁnd so many examples of population-level bias in animals and humans?
Giorgio Vallortigara, Ph.D., of the University of Trieste, and I believe that it has something to do with social behavior. When the lateralized individual must interact with other lateralized individuals, there could be an advantage in being lateralized in the same way. A simple example in humans might be the gesture of shaking (right) hands, and another is sharing tools made for using with the right hand. In a group of chicks, it might be the predictability of behavior made possible by a shared direction of lateralization, because one chick might avoid being attacked by approaching another on its right side. This principle of predictability would apply to other vertebrate species as well.
Elegant studies of schooling and nonschooling ﬁsh highlight the role of social behavior in population-biased lateralization. Vallortigara and Angelo Bisazza, Ph.D., of the University of Padua, found that only the schooling species have a consistent bias to turn in the same direction at a barrier, even when individual ﬁsh are tested alone. Obviously, always turning in the same direction would help keep a school together and so aid survival when predators attacked. Fish of non-schooling species showed individual preferences to turn left or right, but there was no bias at the group level to turn in the same direction. It seems, therefore, that it is members of social species that tend to be lateralized in the same direction. Humans certainly ﬁt this pattern.
The Importance of a Lateralized Brain
The more we discover about lateralization, the more essential it appears to be to brain structure and function, and the more of our behavior it helps to explain. The same, as we have seen, is proving true in other vertebrates. We see now that lateralization goes far beyond any simple dominance of one hemisphere over the other, as was once thought. Lateralization may vary in strength, and even direction, during development; it may vary in different tasks and in different types of neural processing; and it may vary in more social versus less social species. Indeed, given the seeming ubiquity of lateralized brain function in vertebrates, we must wonder why it took so long to discover it in nonhuman species. The answer lies in that initial decision, more than a century ago, to appropriate lateralization for ourselves, as evidence of our self-proclaimed mental superiority. It is an attitude that persists in some quarters to this day, with researchers on humans who are reluctant to accept the mounting evidence that nonhuman species are lateralized, or that their forms of lateralization have any equivalence to our own.
We need have no fears. Recognizing the existence of brain lateralization in nonhuman animals gives us more powerful ways of investigating its function, evolution, causation, and development. As we understand the complexities of the different forms of lateralization in other species, how the nature and inﬂuence of lateralization changes with development, and how it differs in different tasks and contexts, we will deepen our understanding of many patterns of thinking and behaving. That, in turn, will greatly enhance our appreciation of lateralization and its effects, subtle as well as broad, in those tantalizingly complex creatures—ourselves.