Left Brain-Right Brain Study Debunks a Decades-Old Neuromyth

Our left brain hemisphere is not always the seat of language function in humans.

Posted Dec 20, 2017

Neuromyths are common misconceptions about the structure and function of the human brain. Unfortunately, left brain-right brain neuromyths have become deeply embedded in our public consciousness and are unwittingly perpetuated like "urban legends" on a regular basis.

Pixabay/Creative Commons
Source: Pixabay/Creative Commons

The good news: State-of-the-art neuroscientific research is slowly beginning to debunk many age-old neuromyths. For example, a consortium of neuroscientists from the European Multilateral Project recently discovered that left brain-right brain asymmetry of a cortical region called the “planum temporale” is not a marker of language lateralization in the left cerebral hemisphere.

Their study, “Is the Planum Temporale Surface Area a Marker of Hemispheric or Regional Language Lateralization?” was published in the latest issue of the journal Brain Structure and Function. The main takeaway of this study is that, contrary to popular belief, the leftward asymmetry of a particular brain region isn't necessarily a marker of left-brain lateralization of language function in humans.

The European Multilateral Project is a collaborative effort by neuroscientists in France, Spain, and the Netherlands. In recent years, this group began collecting massive data sets which they share with one another. Their goal is to answer questions related to the lateralization of language processing and potentially debunk other left brain-right brain neuromyths. 

For this study, the team of neuroscientists investigated the link between planum temporale (PT) surface area asymmetry and the hemispheric processing of language in 287 healthy adults who were matched for sex and right-handedness or left-handedness.

"It is the first study with such a large sample of individuals and includes the entire range of language variability implemented in the brain," according to Nathalie Tzourio-Mazoyer, who is head of the Neurofunctional Imaging Group at the Institute of Neurodegenerative Diseases at the University of Bordeaux in France. She is also the principal author of this study.

What and Where Is the Planum Temporale?

Wikipedia/Creative Commons
Diagram labeling planum temporale (PT) in green.
Source: Wikipedia/Creative Commons

The planum temporale is a cortical region that forms the heart of “Wernicke’s area” in the cerebral cortex. For decades, Wernicke’s area has been considered the seat of language processing in the brain. Early studies of the planum temporale in the mid-twentieth century found that it was one of the most laterally asymmetrical cortical regions in the cerebrum. Notably, the PT can be as much as 10 times larger in the left cerebral hemisphere in comparison to the "right brain."

On average, left planum temporale surface area is much bigger than right PT in about two-thirds of the population. On the flip side, right planum temporale surface area is bigger than the left side in only about 11 percent of the population.

This is where things get interesting: Regardless of PT surface area size, the European Multilateral Project found, the left hemisphere was specialized for language processing in most study participants. However, in some people (for reasons that are not completely understood) language processing sometimes flipped and most of their linguistic functions occurred in the right cerebral hemisphere. Surprisingly, the size of someone's left or right planum temporale was not directly correlated with which side of the brain he or she used to process language. 

As the authors explain in the study abstract: “Regardless of the anatomical definition used, we observed no correlations between the left surface areas or asymmetries and the hemispheric or regional functional asymmetries during the language production task. The PT asymmetry thus appears to be associated with the local functional asymmetries in auditory areas but is not a marker of inter-individual variability in language dominance.”

A 20th-Century Hypothesis: "The Organization of Language and the Brain"

In 1968, a neurologist named Norman Geschwind—who wrote the seminal paper, "The Organization of Language and the Brain," published in the journal Science—discovered that the planum temporale tends to be much larger in the left hemisphere of the cerebral cortex than in the right hemisphere. Based primarily on this size difference and other factors, he concluded that language processing always occurred in the left hemisphere. 

Now, five decades later, the European Multilateral Project was able to use 21st-century neuroscientific techniques to show that asymmetry of DT surface area between the left brain-right brain is not an accurate marker of language lateralization.

During this study, participants had to perform various language-related and auditory tasks such as describing an image with a sentence and listening to phrases or lists of words as their brain activity was monitored using fMRI. As mentioned earlier, the unexpected finding was that participants who processed language using the right hemisphere didn't always have more planum temporale surface area in the corresponding hemisphere and vice versa.

"The results of the study show that the planum temporale does not explain the rare but strong individual variability of the language domain that exists in humans and, therefore, cannot be considered as a marker of language asymmetry at an individual level," the researchers conclude.

References

Tzourio-Mazoyer, Nathalie, Fabrice Crivello, Bernard Mazoyer. "Is the Planum Temporale Surface Area a Marker of Hemispheric or Regional Language Lateralization?" Brain Structure and Function (First published online: November 3, 2017) DOI: 10.1007/s00429-017-1551-7

Geschwind, Norman. "The Organization of Language and the Brain." Science (1970) DOI: 10.1126/science.170.3961.940

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