During mammalian embryogenesis, determination of three body axes takes place. First, the embryo assumes an AP axis, soon to be followed by the development of the dorsal-ventral axis. Then, after quite a delay, the third and final symmetry break happens: that between left and right. The node coordinates determination of the left-right body axis.
Hemispheric asymmetries pervade practically all major neural systems of the human brain. On a functional level, neuroimaging studies indicate that the most pronounced asymmetries in brain activity exist in the language system.
Our behavior is also asymmetrically organized. We have a favorite direction to turn the head when kissing (Güntürkün, 2003).
1865: Broca presents lesions of the left hemisphere – Broca’s area. There were deficits of language production. Such deficits did not occur after right hemisphere damage.
Observations fostered that brain asymmetry represents an advanced organizational pattern and is thus typical of the human brain.
In the 1970’s, publications about chaffinches, rats, and chicks reported in quick succession that asymmetries of the brain and function also exist in non-human animals.
Asymmetries of the brain were even found in the caenorhabditis elegans, a nematode with a nervous system that contains just 302 neurons.
We believe that most species differences in lateralization are not explained by phylogenetic relations but rather are shaped by local ecological adaptations.
In 2006, Vallortigara suggested that the major fitness benefit of lateralization is to increase neural capacity, because specialization of one hemisphere for a cognitive function avoids useless duplication of cognitive functions between the two hemispheres.
Another factor that might enhance the evolutionary pressure to make efficient use of neural tissue is the fact that the human skull is limited in size by the restrictions of the birth canal.
Studies show that chimpanzees with stronger individual handedness were more successful when fishing for termites using a tool. Similarly, pigeons with stronger visual asymmetry were more successful when foraging for grain scattered among pebbles. Similarly, individuals with pronounced language lateralization have higher verbal IQs and better reading skills.
If certain functions are lateralized to one hemisphere or the other, efficient gating via commissural systems becomes essential. This inter-hemispheric interaction can be both inhibitory as well as excitatory. It is possible that inhibition is necessary for activities that are lateralized while excitation is required to learn a new skill.
In humans, a hereditary basis for handedness was suggested as early as in the 1930’s. From the 1960’s onward, the dominant view was that handedness and language lateralization were determined genetically, with a single gene for both phenotypes. This perspective was refuted in the early 2000’s.
Today, most investigators agree that language and lateralization are complex phenotypes determined by several, possibly interacting, genetic and non-genetic influences.
Because there are no substantial nervous, muscular or other differences between the left and tight hand, handedness is thought to be a brain phenotype.
A subtle but significant sex difference exists in the distribution of left- and right-handedness. There would be 10 female left-handers for 12 male ones.
It was found that human embryos preferentially turn their heads to the right and suck at their right thumbs already at 14 weeks after conception. This behavioral asymmetry strongly correlates with later handedness at school age. After delivery and when in a supine position, most newborn infants prefer to lie with their heads turned to the right, resulting in greater perceptual experience related to the right hand. This head-turning preference predicts handedness in later life and thus has been suggested to contribute to handedness development.
Interestingly, adults also show a preference to turn their heads to the right when kissing. We also know that visual experience, birth stress, cultural pressures can contribute to handedness.
I believe it is important to keep these facts in mind as we are working towards balancing an individual.