A Cinch for the Brain Our bodies, our behavior and even our brains are anything but symmetrical. And this seems to be an important factor in the seamless functioning of our thought, speech and motor faculties. Researchers at the Max Planck Institute for Psycholinguistics in Nijmegen are currently searching for genetic clues to this phenomenon. They want to decode the fundamental molecular biological mechanisms that contribute to asymmetry in the brain, and to identify possible causes for neurological disorders. TEXT STEFANIE REINBERGER t first glance, the human ture. It’s divided into two halves, both A strong left: Rafael Nadal, for body appears to be com- of which are equal in size and whose many years the world’s number pletely symmetrical: two furrows and bulges follow a similar pat- one men’s tennis player, is right-handed but holds the arms, two legs, two eyes, tern. But the functional centers are ex- racket in his left hand most of two ears. Even features tremely unevenly distributed. The right the time. Researchers are likeA the nose and mouth appear to be and left hemispheres specialize in dif- studying how the brains of left- evenly positioned in both halves of ferent cognitive functions. They essen- and right-handed people differ. the face in most people. On closer in- tially divide up the work between them, spection, though, we see that one leg possibly to expand the total range of is longer than the other, one hand is tasks performed. stronger, or maybe the left ear is posi- “Lateralization is a very distinct phe- tioned lower than the right one. This nomenon in language,” explains Clyde becomes even clearer when we take a Francks, Research Group Leader in the look inside the body: the heart beats Language and Genetics Department at on the left-hand side; the liver and the Max Planck Institute for Psycholin- gallbladder, in contrast, are located in guistics in Nijmegen, in the Nether- the right half of the body. The right lands. “Speech is processed predomi- kidney usually sits slightly lower than nantly in the left half of the brain in the left one, which is generally some- most people.” Only in less than 1 per- what bigger and heavier. cent of the population are the main cen- The external appearance of the ters of speech processing in the right brain would also lead us to believe that half – a phenomenon that occurs almost it is a completely symmetrical struc- exclusively in left-handed people. > 34 MaxPlanckResearch 3 | 16 FOCUS_Symmetry Photo: Issei Kato/Reuters 3 | 16 MaxPlanckResearch 35 Lateralization is expressed differently from person to person – and not only in the » few people whose brain is specialized mirror-inversely to the majority of people. “Lateralization – both in the body early in development. A case in point for Human Genetics in Oxford, UK, structure and in the brain and behavior is handedness – an effect that is also since 2002, initially as a doctoral stu- – is a basic biological principle,” says related to lateralization in the brain, dent and later as an academic staff Francks. “In the brain, however, it and that simultaneously constitutes member. In 2007, he published details seems to develop largely independent- the most striking asymmetric behav- of the discovery of a gene called LR- ly of the body,” he points out. The ioral principle. Even in ten-week-old RTM1 (leucine-rich repeat transmem- asymmetric structure of amino acids, human fetuses, an ultrasound scan brane neuronal 1), which may be which also determines how proteins are shows that 85 percent of the growing linked to the tendency toward left-hand- combined, is primarily responsible for babies move the right arm more fre- edness and is passed down through the the anatomy. Even in the earliest stag- quently than the left. Once the fetus- paternal line. es of embryonic development, the es are 15 weeks old, the thumb that In Oxford, Francks worked for a asymmetric structure of the molecules they prefer to suck is a highly accurate while with Simon E. Fisher, who is cur- determines how the individual compo- indicator of which hand they will fa- rently a Director at the Max Planck In- nents are arranged in the newly devel- vor as an adult. stitute for Psycholinguistics. Fisher, oping organism. however, was focused on other topics. The cause and the mechanisms that DISCOVERY OF AN INITIAL He had discovered the widely known lead to the asymmetry in the brain and LEFT-HANDEDNESS GENE gene FOXP2, which plays a key role in in its function, in contrast, are still speech and language, in 2001. When largely unclear. It is very likely that ge- Clyde Francks has been fascinated by Fisher joined the Max Planck Institute netics play an important role here. the lateralization of the brain for many for Psycholinguistics in 2010 to set up This is suggested by the fact that the years now. A zoologist, he has been the new Language and Genetics Depart- differences between the two halves of searching for genes underlying hand- ment, he recruited his former colleague. the brain are already very apparent edness at the Wellcome Trust Centre The subject of “asymmetry in the brain Photo: Frank Vinken 36 MaxPlanckResearch 3 | 16 FOCUS_Symmetry Left Tulya Kavaklioglu is a member of Clyde Francks’ research group. As part of her doctorate, she is looking for genes that influence left-handedness. Researchers are interested in the links between handed - ness and the various functionalities of the two cerebral hemispheres. Right Studies using magnetic resonance imaging (MRI) show that more than 90 percent of right-handed people process language more in the left cerebral hemisphere than in the right one. The same applies to around 80 percent of L % of study participants R left-handed people. The rare phenomenon of increased language processing in the 50 80 right cerebral hemisphere manifests itself almost exclusively in left-handed people. and behavior” thus arrived at the Max 1874. This region of the brain plays an nounced the asymmetry is. This may Planck Institute in Nijmegen together important role in language comprehen- even affect just individual areas of the with Clyde Francks. sion – and just like Broca’s area, the brain. But how does this affect an in- French physician Paul Broca had al- Wernicke region is located on the left dividual’s cognitive functions? And ready discovered in the early 1860s in most people. how does it affect the differences be- that the important functional centers tween men and women? for language and speech are distribut- DIFFERENCES BETWEEN MEN Previous research findings provided ed asymmetrically across the brain. He AND WOMEN very different answers to the gender had stumbled across a strange phe- question. In 2008, for example, a team nomenon: if a particular area in the More recent scientific studies based on of researchers from the University Med- left half of the brain was destroyed, functional imaging, which can be used ical Center Utrecht in the Netherlands those affected were still able to under- to depict the active regions of the brain carried out a meta-analysis. The scien- stand what was being said to them, but based on blood circulation or sugar me- tists analyzed data from 13 studies on they were no longer capable of ex- tabolism, have shown that the relevant handedness and the lateralization of pressing themselves verbally. Patients areas for language and language pro- certain regions and functions of the whose injuries were located on the op- cessing are distributed across the brain, brain. They concluded that, although posite side of the brain, however, thus often even in regions that are far apart men are more frequently left-handed affecting the right hemisphere, did not from one another. Researchers thus also than women, they did not identify any display these deficits. report language and speech activity in differences whatsoever between the The Frenchman had discovered one the right hemisphere – albeit less than sexes in the regions and the functions of the main centers of speech, named in the left hemisphere. of the brain that the Dutch researchers Broca’s area after him, which is now Moreover, lateralization is expressed had included in their study. considered to play an important role in differently from person to person – Francks wasn’t convinced. As in the speech production as well as in the pro- and not only in the few people whose past, he suspected that there were slight cessing of sentence structure and gram- brain is specialized mirror-inversely to differences between the sexes. Togeth- mar. Another important center of lan- the majority of people. The brains of er with doctoral student Tulio Guada- guage, Wernicke’s area, was discovered people whose center of language pro- lupe, he therefore decided to investi- by German neurologist Carl Wernicke cessing is located principally on the gate it again himself. The two scientists Photo: Frank Vinken Graphic: PLOS ONE, 2014 (1848–1905) just a few years later, in left also differ in terms of how pro- analyzed images of brain scans of more 3 | 16 MaxPlanckResearch 37 FOCUS_Symmetry Below Gender-specific differences are found in the asymmetry of the planum temporale, an area that is located in both cerebral hemispheres: left-right asymmetry is generally less pronounced in women than in men. Men with less-pronounced left-right asymmetry often experience reading difficulties. Right The molecular biology lab at the Max Planck Institute for Psycholinguistics offers researchers the opportunity to study at cellular level the effect of certain genes on the development of the brain. Distribution of planum temporale asymmetry by gender 12 10 Women Men 8 6 Thickness 4 2 0 0.00 0.05 0.10 0.15 0.20 0.25 Asymmetry index than 2,300 healthy men and women.