A study conducted by Slava Karolis, Maurizio Corbetta and Michel Thiebaut de Schotten at the National centre for scientific research (CNRS) in France and the University of Padova in Italy was recently published in the journal Nature Communication. The authors produced the first map of the lateralisation of brain functions, which highlights how the left and the right brain share cognitive functions.
In the late 1800s, Paul Broca, a French surgeon and anthropologist, reported that patients with lesions in the frontal region of the left hemisphere had problems with speech. He suggested, then, that the left hemisphere was dominant for language. The idea that the left hemisphere was dominant remain unchallenged until the work of Roger Sperry who received the Nobel Prize in Medicine in 1981′”for his discoveries concerning the functional specialization of the cerebral hemispheres”. Sperry showed that both the left and right hemisphere were contributing to different functions, the left more verbal, and the right more spatial, hence the idea of hemispheric functional specialization. Since then the idea of functional specialization has been emphasized and even trivialized in popular culture with the idea that some people are more left, i.e. analytic, verbal, and good in math, while others are more right brain, i.e. holistic, creating, and good in arts. However, how many functions are really lateralized to the right or left brain, and what is the complete brain map of lateralized functions was until now unknown.
Slava Karolis, Michel Thiebaut from Schotten and Maurizio Corbetta challenged themselves to map the lateralisation of brain functions. To do this, they collected the data from all functional MRI publications and managed to produce, for the first time, a global map of the functional architecture of lateralized functions.
“By exploring this multidimensional map, we were able to show that brain functions are organized as a pyramid with 4 faces (i.e. a tetrahedron) with 4 peaks representing extremely lateralized functions: symbolic communication (written, oral, language …) strongly lateralized to the left; perception/action, more to the right; emotions strongly lateralized also to the right; and, finally, decision-making, lateralized to right frontal regions involved in reward. This relatively simple architecture was beautiful and surprising” report the authors.
If some functions are more lateralized to the right or left hemisphere, then what happens to the connections between the right and left hemisphere? Prior to this study, two main hypotheses had been proposed. The first proposes that more a brain region is functionally lateralized, the less it will be connected to its counterpart in the opposite hemisphere. This arrangement insures faster processing and higher efficiency by segregating the relevant information only to one place in cortex. The second, radically opposed, suggests that regions that are more specialized will be more strongly connected with their counterparts to inhibit irrelevant activity. In other words, the more lateralized a region is, the more it will inhibit through these connections the opposite hemisphere.
Using a MRI based atlas of anatomical connection, the researchers were able to measure the strength of connections to/from the most functionally lateralized regions of the brain. This analysis showed that brain regions showing the strongest lateralization were less connected with the opposite hemisphere, thus validating the first hypothesis. This discovery also suggest that during evolution increase in brain size and complexity of cognitive function led to this solution of segregating highly specialized information mainly to one side of the brain. This, however, may decrease the chance of recovery as the other hemisphere cannot easily compensate for the lost function.
“These results offer a new way to think about lateralization of cognitive functions and solve some very old puzzles. The architecture of brain functions is beautifully represented in these maps. It will be interesting next to explore the connections between lateralized regions within the same hemisphere, and think of novel ways to compensate the effect of brain injuries. ” concluded the authors.
The architecture of functional lateralisation and its relationship to callosal connectivity in the human brain. Karolis VR, Corbetta M & Thiebaut de Schotten M. Nature Communication. March 2019 10.1038/s41467-019-09344-1