- Describe hemispheric asymmetries
- Describe the role of testosterone and stress in brain lateralization and handedness
- Describe sex differences in brain lateralization and suggest an explanation
- Describe lateralization in other species and briefly discuss implications for possible origins of human brain lateralization
Handedness, testosterone, and lateralization for language appear to be related. Approximately 10% of people are left-handed and 2/3 of left-handers are male. 95% of right handed men and women have language lateralized to the left hemisphere. The greatest asymmetries are in the posterior language areas including the temporal planum and angular gyrus. If the left hemisphere is damaged or defective prenatally the right hemisphere can acquire language functions.
Handedness, Language, and Lateralization
The brain’s anatomical asymmetry, its lateralization for language, and the phenomenon of handedness are all clearly interrelated, but their influences on one another are complex. Though about 90% of people are right-handed, and about 95% of right-handers have their language areas on the left side of their brains, that still leaves 5% of right-handers who are either right-lateralized for language or have their language areas distributed between their two hemispheres. And then there are the left-handers, among whom all of these patterns can be found , including left-lateralization.
Some scientists suggest that the left hemisphere’s dominance for language evolved from this hemisphere’s better control over the right hand. The circuits controlling this “skillful hand” may have evolved so as to take control over the motor circuits involved in language. Broca’s area, in particular, is basically a premotor module of the neocortex and co-ordinates muscle contraction patterns that are related to other things besides language.
Brain-imaging studies have shown that several structures involved in language processing are larger in the left hemisphere than in the right. For instance, Broca’s area in the left frontal lobe is larger than the homologous area in the right hemisphere. But the greatest asymmetries are found mainly in the posterior language areas, such as the temporal planum and the angular gyrus.
Two other notable asymmetries are the larger protrusions of the frontal lobe on the right side and the occipital lobe on the left. These protrusions might, however, be due to a slight rotation of the hemispheres (counterclockwise, as seen from above) rather than to a difference in the volume of these areas. These protrusions are known as the right-frontal and left-occipital petalias (“petalias” originally referred to the indentations that these protrusions make on the inside of of the skull).
The structures involved in producing and understanding language seem to be laid down in accordance with genetic instructions that come into play as neuronal migration proceeds in the human embryo. Nevertheless, the two hemispheres can remain just about equipotent until language acquisition occurs. Normally, the language specialization develops in the left hemisphere, which matures slightly earlier. The earlier, more intense activity of the neurons in the left hemisphere would then lead both to right-handedness and to the control of language functions by this hemisphere. But if the left hemisphere is damaged or defective, language can be acquired by the right hemisphere. An excess of testosterone in newborns due to stress at the time of birth might well be one of the most common causes of slower development in the left hemisphere resulting in greater participation by the right.
This hypothesis of a central role for testosterone is supported by experiments which showed that in rats, cortical asymmetry is altered if the rodents are injected with testosterone at birth. This hormonal hypothesis would also explain why two-thirds of all left-handed persons are males.
Interindividual variations, which are essential for natural selection, are expressed in various ways in the human brain. Volume and weight can vary by a factor of two or even more. The brain’s vascular structures are extremely variable; the deficit caused by an obstruction at a given point in the vascular system can vary greatly from one individual to another. At the macroscopic anatomical level, the folds and grooves in the brain also vary tremendously from individual to individual, especially in the areas associated with language . Variability in the language areas can also be observed at the microscopic level, for example, in the synaptic structure of the neurons in Wernicke’s area.
Interindividual variability is also expressed in the brain’s functional organization, and particularly in the phenomenon of hemispheric asymmetry. For instance, some data indicate that language functions may be more bilateral in women than in men. The percentage of atypical lateralization for language also varies with handedness: it is considerably higher among left-handers than among right-handers.
Lastly, as if all this were not enough, there is also such a thing as intraindividual variability. In the same individual, a given mental task can sometimes activate different neuronal assemblies in different circumstances—for instance, when the individual is performing this task for the first time, as opposed to when he or she has already performed it many times before.
Even in many species that are quite distant from humans in evolutionary terms (frogs, for example), the brain is left-lateralized for the vocalization function.
In chimpanzees, lateralization for the anatomical areas corresponding to Broca’s and Wernicke’s areas already exists, even though it does not yet correspond to the language function. And like the majority of humans, the majority of chimpanzees use their right hand in preference to their left.
These asymmetries in the other primates represent persuasive evidence of the ancient phylogenetic origin of lateralization in the human brain. The expansion of the prefrontal cortex in humans might in part reflect its role in the production of language.
An asymmetrical lateralized brain may have evolved in order to reduce redundancy of information processing functions in favor of greater processing capacity in a brain whose volume is limited by the size of the human female birth canal (Corballis, 2017). To increase processing capacity if brain volume is fixed, lateralization of function may more efficiently utilize the processing resources available.
Women have the reputation of being able to talk and listen while doing all sorts of things at the same time, whereas men supposedly prefer to talk or hear about various things in succession rather than simultaneously. Brain-imaging studies may now have revealed an anatomical substrate for this behavioural difference, by demonstrating that language functions tend to place more demands on both hemispheres in women while being more lateralized (and mainly left-lateralized) in men. Women also have more nerve fibers connecting the two hemispheres of their brains (via a broader corpus callosum), which also suggests that more information is exchanged between them.
Most people are right-handed and language in most people is lateralized to the left hemisphere. Lateralization of the brain's functions may have been one way to increase the brain's processing capacity by reducing redundant usage of available processing resources. Vocalization is left-lateralized in species ranging from frogs to chimpanzees, suggesting ancient evolutionary origins of brain lateralization in humans.
Corballis, M. C. (2017). The evolution of lateralized brain circuits. Frontiers in psychology, 8, 1021.
"Handedness, Language, and Lateralization" adapted by Kenneth A. Koenigshofer, Ph.D., Chaffey College, from Broca's Area, Wernicke's Area, and other Language-Processing Areas in the Brain by Bruno Dubuc in The Brain from Top to Bottom, under a Copyleft license.