In my last blog, What's Right for You May not be Right for Others, I introduced Iain McGilchrist's notion of the 5% of the worldwide population who have atypical lateralization for language. Whereas his book, The Master and His Emissary, is so inclusive in describing left/right hemispheric differences within the brain and in successive generational proclivities, from my point of view, he left out the best bit about the highly creative minority of right-hemisphere or mixed-dominant individuals.
In this post, I'm going to discuss handedness and atypical lateralization for language, a predisposing factor for psychosis. The information is based on the newest research articles found in the medical textbook, Language Lateralization and Psychosis (2009), edited by Iris E. C. Sommer and René S. Kahn. The other names that appear below are contributing chapter authors or researchers they cite in this text.
First of all, left-right asymmetry, whether in the brain (cerebral) or in the body (visceral), happens during the baby's development in the womb. The leftward asymmetry (greater than right size) that produces left-hemispheric dominance for language occurs between weeks 29 and 31 of gestation. The bodily effects are evident later on. Your feet, for instance, are probably different in size, the right is the larger foot in males, and the left in females. My left hand is much longer than my right. Typical left-hemispheric cerebral dominance is not just found in humans: it is in animals too, stretching back phylogenetically several hundred million years.
Children of two left-handed parents have a 50% chance of becoming left-handed, while for children of right-handed parents it's less than 10%. The language lateralization of the parents also determines to a large extent that of their children. I may be an anomaly with two right-handed parents and the only left-handed child amongst six siblings; but I'd say two of my sisters are right- or mixed-dominant despite their handedness. In furtherance of this scenario, I recently had dinner with a good friend and her teenage son. Both were right-handed. But, when I asked them to fold their arms across their chests, they both put the left hand up, showing some mixed-dominance. My friend is an actress and a singer and her son's father is left-handed; so, this makes sense for both mother and son. In fact, enhanced right-brain dominant folk seem to find each other as a quick tally of my friends attests.
Great apes are typically right-handed and the parts of the brain used in human language, Broca's and Wernicke's areas, are associated with tool use in chimpanzees. So, grasping with the hand was probably the precursor for "grasping" an idea using language. Even birds use their feet preferentially: the right foot exerts strong force and the left uses fine manipulations. There is evidence that in preindustrial societies right-handedness only became apparent to researchers when analyzing the inhabitants' fine manipulations. Handedness specialist I. C. McManus says that “8% -10% of the population has been left-handed for at least the past 200,000 years or so.” This percentage has remained constant. Again, there are gender differences: men are 25% more likely to be left-handed than women. Compared to the 5% to 6% of right-handers who show RH language dominance, 30% to 35% of left-handers do.
Visuospatial functions are normally assigned to the right hemisphere. Birds too show right-eye (LH) superiority for discriminating visual patterns, left-eye (RH) for spatial tasks. It's well known that men have better visuospatial ability than women and this is because men are generally more left-brain dominant for language than women, whose language functions are more spread out, leaving less room for the visuospatial. But all combinations of language vs. visuospatial functions can occur: L/L, L/R, R/R. In women who are R/R like me, their language function can practically eclipse their visuospatial ability. I have no map reading ability and am generally lost in environmental space. Forget the old adage that left-handers die sooner than righties. New research shows that “the very oldest respondents have a higher rate of left-handedness than those who are somewhat younger,” according to McManus.
Whereas an intolerance for left-handedness arose with the industrial revolution, as fine work needed to be done and equipment and writing pens were designed for right-handers, a French study by Faurie and Raymond (2004) showed that the rate of left-handedness has not actually changed since the Upper Paleolithic. France has been antipathetic to left-handedness, historically tying the left hand behind the back to force right-handedness. There are no left-handers in China, as it is forbidden. My European friends might be interested in the fact that the highest rates of left-handedness are in Britain, the Netherlands, and Belgium, I assume because of more tolerance. Yet, even if forced to write with the right hand, the bearer of the left-handed gene will carry it over to their own children while continuing to use the left for other non-writing functions.
In this History of Left-handers, note that Eve is picking the apple
with her left (evil) hand
What about language lateralization and psychosis?
Quoting Somers, Sommer and Kahn, "non-right-handed subjects, but not strong left-handers, had higher scores on schizotypy questionnaires than right-handed subjects. Mixed-handers showed a trend towards high schizotypy in comparison to left-handers." So, it's safer to be extremely left-handed than mixed, because the major language function is segregated to the right, not just increased language activity in the frontal and temporal areas of the right hemisphere that can account for hemispheric indecision and a predisposition to psychosis of the schizophrenic or bipolar variety. The authors state further that "bilateral language representation facilitates magical and delusional ideas by means of the more diffuse semantic activation to the right hemisphere compared to the left." Bilateral language representation is also associated with autism, dyslexia and ADHD.
In fMRI studies, right-handed thought-disordered patients showed activation in the right-hemisphere homolog of Wernicke’s area during speech production, while controls performing the same task showed left-lateralized activity, supporting Julian Jaynes's view. Diederen and Sommer studied 24 psychotic patients who actually experienced auditory verbal hallucinations (AVH) in the scanner. All were strongly right-handed and continued to experience AVH frequently despite using antipsychotic medication. The results showed increased activity in the right homolog of Broca's area, which the researchers connected to negative emotion and compared to inappropriate "release" language following damage or surgery to the left hemisphere.
Overall, the studies indicate that decreased language activation on the left and increased activity on the right are associated with auditory hallucinations, while extreme left-handers are not prone to psychosis. Timothy Crow goes so far as to say that “schizophrenia is the price Homo sapiens pays for language (Crow, 1997a).” It's the lack of complete dominance, the failure to inhibit the non-dominant hemisphere that is problematic. Patients with the most severe hallucinations were the least leftward lateralized.
The final chapter by Clyde Francks lays the genetic blame on dad rather than mom. He claims that the same paternally inherited LRRTM1 gene associated with mixed/left-handedness, which suppresses the maternal gene, was also over-transmitted to schizophrenic patients in a large family study. Interestingly, he found the same father-child enhanced (roughly five times the mother's) genetic transmission in dyslexics.
These studies, of course, do not paint the whole picture. The genetics are well described, but no mention is made of the traumatic incidents needed to trigger psychotic and/or dissociative episodes in those predisposed. The creative influence of increased right-hemispheric input is touched upon, especially as an argument for sustaining the genetic transfer of sometimes disabling mental illness, but not nearly enough. But then their lack is an opening for the kind of study I am doing on the genetics, early traumas and environmental influences on the minds of great poets, prophets and mediums whose voices have brought great art, guidance and shifting paradigms of consciousness to a general population less endowed for hearing them.