Modern Views on the Anatomy of Planum Temporale

Abstract

Background: The planum temporale (PT) is a workhorse for auditory processing. This important brain region for audition has been studied extensively in both healthy and disordered populations (Shapleske et al., 1999). PT demonstrates the most pronounced hemispheric asymmetry out of all structures in the human brain (Toga and Thompson, 2003), often larger in the left hemisphere. This anatomical asymmetry has been related to the left-hemisphere dominance of language function (Geschwind & Levitsky, 1968). Occupying the posterior portion of the dorsal plane of the superior temporal gyrus, the PT is concealed within the Sylvian fissure. It is bounded anteriorly by Heschl's gyrus, and posteriorly by the posterior ascending ramus (PAR) of the Sylvian fissure. However, these two anatomical boundaries are considerably variable in their morphology. PARs vary significantly in length and angle, and sometimes are absent (St. George et al., 2016). In addition, Heschl's gyrus often demonstrates a partial or complete duplication (Marie et al., 2015). Although previous literature attempting to quantify the anatomy of PT has considered some of these complexities, it has not considered the implications of new research refining its boundaries.Specifically, redefining of the boundaries of primary auditory cortex (Heschl’s gyrus) and the PAR has motivated the current research which allows an accurate view PT. Accounting changes in boundaries of primary auditory cortex, as well as newly developed anatomical criteria based on angle and length of the PAR, the following are hypothesized: 1) duplication of Heschl’s gyrus, and 2) the presence of a PAR will significantly and independently reduce the size of PT and 3) these findings will explain anatomically-based sources of PT asymmetry found within the human brain. Methods: High-resolution T1-weighted MRIs of 28 healthy dextral adults were examined. The superior temporal plane was exposed via the traditional knife-cut method using modern neuroimaging software. The surface area of PT was measured on each hemisphere’s cortical mesh, accounting for the natural curvature of the surface of the brain. Presence of PAR was determined based on criteria established by St. George et al., 2016. Heschl’s gyrus patterns were identified via visual inspection of the superior temporal plane. Duplications of Heschl’s gyrus were classified according to the three traditionally recognized variants (Rademacher et al., 1993). The effect of PAR presence and Heschl’s gyrus variant on PT surface area was examined as a linear model using statistical modeling software. Results and Conclusions: We found that PT surface area was significantly larger in left compared to right hemisphere, and there no significant difference in PT surface area between males and females (both consistent with previous literature). Additionally, PT area was significantly larger in hemispheres without PAR compared to hemispheres with PAR. Furthermore, PT area was significantly larger in hemispheres with single Heschl’s gyrus morphology compared to hemispheres exhibiting Heschl’s gyrus duplications. Our results confirm that the surface area of PT depends on the morphology of neighboring perisylvian structures which helps to explain its structural asymmetry. This type of translational research affects site of lesion interpretations, topographic evoked potential mapping and how we draw structure-function correlations to MRI and fMRI.