Leftward asymmetry of the planum temporale and its association with language

Several cortical brain regions show structural left-right asymmetries. One of the most pronounced forms of structural asymmetry in the human brain is the leftward macrostructural asymmetry of the planum temporale, the posterior part of the superior surface of the temporal lobe. The planum temporale overlaps with Wernicke's area, a core structure in the brain network involved in sensory language processing. Therefore, several studies have investigated the association between macrostructural leftward asymmetries of the planum temporale and functional leftward asymmetries in language processing. However, the results of these studies have been ambiguous and sometimes contradictory. In this mini-review article, we argue that asymmetric structure-function associations in the language system cannot be fully understood by only examining averaged asymmetries obtained from macrostructural measures such as volume, cortical thickness, or surface area. Recent in-vivo neuroimaging studies, along with earlier post-mortem histological studies, suggest that the planum temporale also shows substantial leftward asymmetries in its microstructural organization. These microstructural asymmetries concern the columnar organization of the planum temporale and the density of neurites. Importantly, recent studies have shown that microstructural asymmetries in the planum temporale exhibit stronger associations with functional hemispheric lateralization of the language system than macrostructural ones. Based on these findings, we suggest that the association between structural and functional asymmetries in the language system can only be understood if macrostructural and microstructural asymmetries are both considered.