In 1968, Geschwind discovered that the planum temporale (PT), a temporal lobe region involved in language sounds processing, had a larger surface area in the left hemisphere than in the right. Because language is hosted by the left hemisphere in more than 90% of human beings, Geschwind then proposed the structural asymmetry of PT as an anatomical marker of the hemispheric lateralization of language. Today, new results allow to revisit this hypothesis.
At that time, it was not possible to test this hypothesis because there was no way to measure brain activity during language tasks in healthy individuals. Thanks to structural magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI), it is nowadays possible to acquire in the same individual images of her/his brain anatomy and her/his brain areas activated during language tasks. Because MRI is a non-invasive method, it is also possible to acquire these brain images in large populations of healthy volunteers. Nearly 50 years after Geschwind’s seminal article in Science, we measured left and right PT surfaces areas, and hemispheric and regional activation asymmetries during language processing of 287 healthy adults. We found that the asymmetry of PT is not a marker of the hemispheric dominance of language. Even the few individuals hosting language regions in their right hemisphere had a larger left PT surface area. We then examined whether the asymmetry of the PT surface area was related to the asymmetries of activation of the auditory areas during word listening. We found that the greater the leftward anatomical asymmetry of the PT, the greater the leftward asymmetry of the activation of the areas located around the PT. However, this positive relationship was due to the fact that the larger the surface area of the right PT, the lower the asymmetry of the auditory regions.
Illustration of the relationships between anatomical asymmetry of the surface of the planum temporale (PT) and the asymmetries of activations during the listening to words in 287 healthy adults. At the top left, we can see that there is no relation between the strength of the anatomical asymmetry of the PT, drawn on the bottom left on an anatomical MRI, and the activation asymmetries measured at the level of the entire hemisphere. At the top right, the graph shows in the same subjects the positive correlation between the asymmetry of the PT surface and the activation asymmetry in the auditory areas. On the right bottom, the figure illustrates the relationship between the right surface of the PT and the right activation of the auditory regions, which is the relationship that best explains the variability of activation between individuals.
We can thus conclude that the asymmetry of the PT surface area is not a marker of the hemispheric dominance of language, but, rather, that right PT surface area is a marker of the functional asymmetries of the speech regions. The fact that it is the right PT surface area that accounts for most of the regional asymmetry variability during speech listening suggests that the afferent inhibitory signals of the dominant hemisphere may be involved in the determination of anatomical asymmetry.
Complete reference
Tzourio-Mazoyer N, Crivello F, Mazoyer B, Is the planum temporale surface area a marker of hemispheric or regional language lateralization? Brain Struct Funct 2017 Nov 03. 10.1007/s00429-017-1551-7