In a large sample of 428 healthy adults balanced for gender and manual preference (MP), we investigated planum temporale (PT) surface area variability in relation with Heschl's gyrus (HG) duplication pattern, MP, and familial sinistrality (FS), considering different PT definitions. In a sub-sample of 362 participants, we also investigated whether variability of PT asymmetry was associated with differences in verbal abilities. On each participant brain hemisphere MRI, we delineated a posterior PT area (PTpost), excluding the second Heschl gyrus in case of either complete posterior duplication (CPD) or common stem partial duplication (CSD). We then defined a total PT area (PTtot) as the union of PTpost and of the second HG when present, and a HGPT area as the union of PTtot and of the first HG. The HG duplication pattern of one hemisphere was found to significantly affect the PTpost surface area of the same hemisphere, a larger reduction being present in case of CPD than in case of CSD, leading to a strong impact of both left and right HG duplication patterns on PTpost asymmetry. The HG duplication pattern had no effect on PTtot surface areas, while a significant effect of the left HG duplication was present on PTtot asymmetry that was larger in case of a CSD as compared to a single HG. By contrast, the type of HG duplication did not affect HGPT and neither left nor right HG duplication pattern had an effect on HGPT asymmetry. Meanwhile, MP had no effect on PTpost, PTtot, HGPT, or their asymmetries. The absence of a left PTpost was associated with existence of FS (FS+) (7FS+ among nine without PTpost). Removing the nine individuals lacking PTpost, a lower left PTpost surface area was observed in FS+ participants with left CPD. In the sub-sample of 362 participants, we observed a significant interaction between PTpost asymmetry and cognitive abilities due to poorer lexical performances in individuals having a symmetric PTpost as compared to individuals having either a leftward or a rightward asymmetric PTpost. By contrast, there was no significant effect of PTtot or HGPT asymmetry on cognitive abilities. This study shows that HG duplication pattern mainly affects the surface area of the most posterior part of PT and its asymmetry, this PTpost area being specifically associated with variability in verbal performances. This study also shows, for the first time, an association between decreased performances and lack of PTpost anatomical asymmetry, being rightward asymmetrical having no deleterious effect on verbal abilities, thereby supporting the idea that anatomical lateralization is necessary for optimal verbal performances.
Keywords: Asymmetry; Familial sinistrality; Handedness; Hemispheric specialization; Heschl gyrus; MRI; Planum temporale; Verbal abilities; Visuo-spatial abilities.