The Planum Temporale (PT) is a region of the temporal lobe involved in processing language sounds that has a larger surface area in the left cerebral hemisphere than in the right. Geschwind, who discovered this asymmetry in 1968, had proposed it to be the anatomical support for the dominance of the left hemisphere for language. However, until now, it had not been possible to find any association between this anatomical asymmetry and the language performance of healthy individuals. We had the idea to use intracortical myelination as a new marker of auditory cortex organization that includes the primary auditory cortex (Heschl’s Gyrus, GH) and the PT (located behind the GH) to re-examine these structure-function relationships.
We measured this new neuroimaging marker on a large number of individuals (database of 445 healthy volunteers) taking into account the individual variability of GH. We have observed that increased performance in a rhyming language task is associated with a decrease in myelination of the right PT, i.e. a greater asymmetry in intracortical myelination of the PT in favour of the left. Because GH, located just in front of PT, can be simple or duplicated, we studied 445 healthy adults whose anatomical variability of GH had previously been identified (Marie 2013). Regions of interest on the first or second HG gyrus (see figure) and on the PT have been defined according to the type of GH duplication allowing us to measure in each hemisphere the in vivo MRI marker of intracortical myelination. This marker, which corresponds to the T1/T2 MRI ratio, was calculated in the regions of interest covering the first GH (H1), the second GH in case of partial duplication (H2CSD), or in case of complete duplication (H2CPD) and in the PT. We were able to describe that the first GH has the highest intracortical myelination values, while the PT has the lowest values.
Illustration of the different configuration of Heschl’s gyrus and of the PT in each hemisphere and corresponding myelination maps. At the top raw, we can see the different anatomical configurations of the average supra-temporal plane in each hemisphere (n=445). From left to right: left complete posterior duplication (CPD), left common stem duplication (CSD) single Heschl’s gyrus (single), right single Heschl’s gyrus (single), right common stem duplication (CSD) and right complete posterior duplication (CPD). The regions of interest are superimposed on each configuration: first Heschl’s gyrus (H1 in red), second Heschl’s gyrus (H2 in yellow) and PT (light blue). At the second raw the average intracortical myelination maps are presented for each configuration.
As the PT is involved in phonological processing, the phonological skills of the participants were evaluated by a rhyme test. We also measured each participant’s average verbal score with other verbal tests. After adjustment for verbal skills, rhyme performance was not associated with intracortical myelination of the left regions, but decreased with increased intracortical myelination of the right PT. These results highlight the importance of neuroimaging markers, such as intracortical myelination, for studying structure-function relationships at the cortical level.
Reference
Tzourio-Mazoyer N, Maingault S, Panzieri J, Pepe A, Crivello F, Mazoyer B , Intracortical Myelination of Heschl’s Gyrus and the Planum Temporale Varies With Heschl’s Duplication Pattern and Rhyming Performance: An Investigation of 440 Healthy Volunteers. Cereb Cortex. 2018 Apr 18. doi: 10.1093/cercor/bhy088.
Marie D, Jobard G, Crivello F, Perchey G, Petit L, Mellet E, Joliot M, Zago L, Mazoyer B, Tzourio-Mazoyer N (2013) Descriptive anatomy of Heschl’s gyri in 430 healthy volunteers, including 198 left-handers. Brain Struct Funct 220: 729-743.