The Morphology and Variability of the Caudal Rami of the Superior Temporal Sulcus

The caudal branches of the superior temporal sulcus (cSTS) have been difficult to characterize because of the considerable degree of morphological variability across individuals. Leading atlases of the human brain are inconsistent with each other in terms of the number of branches identified and the nomenclature used to refer to them. Examination of the magnetic resonance images (MRIs) of 45 human brains (90 hemispheres) demonstrates three branches of the cSTS that ascend into the inferior parietal lobule: an anterior branch (cSTS1), a central branch (cSTS2) and a posterior branch (cSTS3). The cSTS1 is found immediately posterior to the ascending limb of the Sylvian fissure, followed by the cSTS2, and then the last branch, cSTS3, at the parieto-occipital junction. The temporal part of the STS joins most frequently with the cSTS2 (approximately 60% of cases), the cSTS1 (approximately 30% of cases) and least frequently with the cSTS3 (approximately 10% of cases). At the temporo-occipital junction, there is another sulcus that is related to the STS, the ventral anterior occipital sulcus (AOCS-v), a sulcus that has been functionally linked to area MT/v5 in the human brain. While the STS may appear to join AOCS-v from the surface of the brain, it can be established from examination of the depth of the sulci that they are not continuous. The variability in location of each one of the cSTS branches is expressed quantitatively in the MNI standard proportional stereotaxic space.