Left Brain, Right Brain: Facts and Fantasies
Michael C. Corballis, Affiliation School of Psychology, University of Auckland, Auckland, New Zealand
Published: January 21, 2014
https://doi.org/10.1371/journal.pbio.1001767 )open access. See original for more.
Handedness and brain asymmetry are widely regarded as unique to humans, and associated with complementary functions such as a left-brain specialization for language and logic and a right-brain specialization for creativity and intuition. In fact, asymmetries are widespread among animals, and support the gradual evolution of asymmetrical functions such as language and tool use. Handedness and brain asymmetry are inborn and under partial genetic control, although the gene or genes responsible are not well established. Cognitive and emotional difficulties are sometimes associated with departures from the “norm” of right-handedness and left-brain language dominance, more often with the absence of these asymmetries than their reversal.
Evolution of Brain Asymmetries, with Implications for Language
One myth that persists even in some scientific circles is that asymmetry is uniquely human . Left–right asymmetries of brain and behavior are now known to be widespread among both vertebrates and invertebrates , and can arise through a number of genetic, epigenetic, or neural mechanisms . Many of these asymmetries parallel those in humans, or can be seen as evolutionary precursors. A strong left-hemispheric bias for action dynamics in marine mammals and in some primates and the left-hemisphere action biases in humans, perhaps including gesture, speech, and tool use, may derive from a common precursor . A right-hemisphere dominance for emotion seems to be present in all primates so far investigated, suggesting an evolutionary continuity going back at least 30 to 40 million years . A left-hemisphere dominance for vocalization has been shown in mice  and frogs , and may well relate to the leftward dominance for speech—although language itself is unique to humans and is not necessarily vocal, as sign languages remind us. Around two-thirds of chimpanzees are right-handed, especially in gesturing  and throwing , and also show left-sided enlargement in two cortical areas homologous to the main language areas in humans—namely, Broca’s area  and Wernicke’s area  (see Figure 1). These observations have been taken as evidence that language did not appear de novo in humans, as argued by Chomsky  and others, but evolved gradually through our primate lineage . They have also been interpreted as evidence that language evolved not from primate calls, but from manual gestures –.
Some accounts of language evolution (e.g., ) have focused on mirror neurons, first identified in the monkey brain in area F5 , a region homologous to Broca’s area in humans, but now considered part of an extensive network more widely homologous to the language network . Mirror neurons are so called because they respond when the monkey performs an action, and also when they see another individual performing the same action. This “mirroring” of what the monkey sees onto what it does seems to provide a natural platform for the evolution of language, which likewise can be seen to involve a mapping of perception onto production. The motor theory of speech perception, for example, holds that we perceive speech sounds according to how we produce them, rather than through acoustic analysis . Mirror neurons in monkeys also respond to the sounds of such physical actions as ripping paper or dropping a stick onto the floor, but they remain silent to animal calls . This suggests an evolutionary trajectory in which mirror neurons emerged as a system for producing and understanding manual actions, but in the course of evolution became increasingly lateralized to the left brain, incorporating vocalization and gaining grammar-like complexity . The left hemisphere is dominant for sign language as for spoken language .
Mirror neurons themselves have been victims of hyperbole and myth , with the neuroscientist Vilayanur Ramachandran once predicting that “mirror neurons will do for psychology what DNA did for biology” . As the very name suggests, mirror neurons are often taken to be the basis of imitation, yet nonhuman primates are poor imitators. Further, the motor theory of speech perception does not account for the fact that speech can be understood by those deprived of the ability to speak, such as those with damage to Broca’s area. Even chimpanzees  and dogs  can learn to respond to simple spoken instructions, but cannot produce anything resembling human speech. An alternative is that mirror neurons are part of a system for calibrating movements to conform to perception, as a process of learning rather than direct imitation. A monkey repeatedly observes its hand movements to learn to reach accurately, and the babbling infant calibrates the production of sounds to match what she hears. Babies raised in households where sign language is used “babble” by making repetitive movements of the hands . Moreover, it is this productive aspect of language, rather than the mechanisms of understanding, that shows the more pronounced bias to the left hemisphere .
Handedness and cerebral asymmetries are detectable in the fetus. Ultrasound recording has shown that by the tenth week of gestation, the majority of fetuses move the right arm more than the left , and from the 15th week most suck the right thumb rather than the left —an asymmetry strongly predictive of later handedness  (see Figure 2). In the first trimester, a majority of fetuses show a leftward enlargement of the choroid plexus , a structure within the ventricles known to synthesize peptides, growth factors, and cytokines that play a role in neurocortical development . This asymmetry may be related to the leftward enlargement of the temporal planum (part of Wernicke’s area), evident at 31 weeks .
In these prenatal brain asymmetries, around two-thirds of cases show the leftward bias. The same ratio applies to the asymmetry of the temporal planum in both infants and adults . The incidence of right-handedness in the chimpanzee is also around 65–70 percent, as is a clockwise torque, in which the right hemisphere protrudes forwards and the left hemisphere rearwards, in both humans and great apes . These and other asymmetries have led to the suggestion that a “default” asymmetry of around 65–70 percent, in great apes as well as humans, is inborn, with the asymmetry of human handedness and cerebral asymmetry for language increased to around 90 percent by “cultural literacy” .
Variations in Asymmetry
Whatever their “true” incidence, variations in handedness and cerebral asymmetry raise doubts as to the significance of the “standard” condition of right-handedness and left-cerebral specialization for language, along with other qualities associated with the left and right brains that so often feature in popular discourse. Handedness and cerebral asymmetry are not only variable, they are also imperfectly related. Some 95–99 percent of right-handed individuals are left-brained for language, but so are about 70 percent of left-handed individuals. Brain asymmetry for language may actually correlate more highly with brain asymmetry for skilled manual action, such as using tools ,, which again supports the idea that language itself grew out of manual skill—perhaps initially through pantomime.
Even when the brain is at rest, brain imaging shows that there are asymmetries of activity in a number of regions. A factor analysis of these asymmetries revealed four different dimensions, each mutually uncorrelated. Only one of these dimensions corresponded to the language regions of the brain; the other three had to do with vision, internal thought, and attention —vision and attention were biased toward the right hemisphere, language and internal thought to the left. This multidimensional aspect throws further doubt on the idea that cerebral asymmetry has some unitary and universal import.
Handedness, at least, is partly influenced by parental handedness, suggesting a genetic component , but genes can’t tell the whole story. For instance some 23 percent of monozygotic twins, who share the same genes, are of opposite handedness . These so-called “mirror twins” have themselves fallen prey to a Through the Looking Glass myth; according to Martin Gardner , Lewis Carroll intended the twins Tweedledum and Tweedledee in that book to be enantiomers, or perfect three-dimensional mirror images in bodily form as well as in hand and brain function. Although some have argued that mirroring arises in the process of twinning itself ,, large-scale studies suggest that handedness , and cerebral asymmetry  in mirror twins are not subject to special mirroring effects. In the majority of twins of opposite handedness the left hemisphere is dominant for language in both twins, consistent with the finding that the majority of single-born left-handed individuals are also left-hemisphere dominant for language. In twins, as in the singly born, it is estimated that only about a quarter of the variation in handedness is due to genetic influences .
The manner in which handedness is inherited has been most successfully modeled by supposing that a gene or genes influence not whether the individual is right- or left-handed, but whether a bias to right-handedness will be expressed or not. In those lacking the “right shift” bias, the direction of handedness is a matter of chance; that is, left-handedness arises from the lack of a bias toward the right hand, and not from a “left-hand gene.” Such models can account reasonably well for the parental influence –, and even for the relation between handedness and cerebral asymmetry if it is supposed that the same gene or genes bias the brain toward a left-sided dominance for speech ,. It now seems likely that a number of such genes are involved, but the basic insight that genes influence whether or not a given directional bias is expressed, rather than whether or not it can be reversed, remains plausible (see Box 1).
Genetic considerations aside, departures from right-handedness or left-cerebral dominance have sometimes been linked to disabilities. In the 1920s and 1930s, the American physician Samuel Torrey Orton attributed both reading disability and stuttering to a failure to establish cerebral dominance . Orton’s views declined in influence, perhaps in part because he held eccentric ideas about interhemispheric reversals giving rise to left–right confusions , and in part because learning-theory explanations came to be preferred to neurological ones. In a recent article, Dorothy Bishop reverses Orton’s argument, suggesting that weak cerebral lateralization may itself result from impaired language learning . Either way, the idea of an association between disability and failure of cerebral dominance may be due for revival, as recent studies have suggested that ambidexterity, or a lack of clear handedness or cerebral asymmetry, is indeed associated with stuttering  and deficits in academic skills , as well as mental health difficulties  and schizophrenia (see Box 1).
Although it may be the absence of asymmetry rather than its reversal that can be linked to problems of social or educational adjustment, left-handed individuals have often been regarded as deficient or contrarian, but this may be based more on prejudice than on the facts. Left-handers have excelled in all walks of life. They include five of the past seven US presidents, sports stars such as Rafael Nadal in tennis and Babe Ruth in baseball, and Renaissance man Leonardo da Vinci, perhaps the greatest genius of all time.