Leftward interhemispheric asymmetry of macaque monkey temporal lobe language area homolog is evident at the cytoarchitectural, but not gross anatomic level.

The most marked leftward interhemispheric asymmetry of the human and great ape brain is present in a multisensory association area of the superior temporal gyrus. This cortical region, the planum temporale (part of Brodmann's area 22), has a cytoarchitectural homolog, area Tpt, in Old World monkeys. Reports in non-human primates vary widely in descriptions of area Tpt-associated indices of asymmetry at the gross anatomic level, but such asymmetry has not yet been considered at the cellular level. Here we assessed a mixed sex sample of perfusion-fixed adult macaque monkey brains to determine whether purported interhemispheric asymmetry of Tpt is manifested at the gross anatomic level by consideration of the length of the lateral sulcus. A separate sample was used to consider interhemispheric asymmetry by volumetric assessment of the cytoarchitectural profile of area Tpt. There was no significant hemispheric asymmetry of lateral sulcus length at the gross anatomic level. Previous studies that used endocasts may not have factored in opercularization of the sylvian point, shown here as being pronounced. Conversely, there was a significant leftward volumetric asymmetry of area Tpt at the cytoarchitectural level. Furthermore, the intrahemispheric profile of area Tpt topography was more varied than expected. These results indicate that leftward hemispheric asymmetry of these critical human language areas may have been based on a functional substrate with a long evolutionary history. The leftward cytoarchitectural asymmetry and highly varied topography of Tpt should be factored into investigative approaches that consider this region of the cerebral cortex.