Morphological patterns of the collateral sulcus in the human brain.

The collateral sulcal complex is an important landmark on the medial surface of the temporal lobe. Anteriorly, it delineates the limbic regions of the parahippocampal gyrus from the visual-processing areas of the fusiform gyrus. Posteriorly, it continues into the occipital lobe, bearing no relationship to the memory-related limbic regions. Given the considerable extent of the sulcus and functional heterogeneity of the surrounding cortex, an investigation of the morphology of this sulcus was carried out to examine whether it is continuous or a series of sulcal parts, i.e. independent sulci classified together under the name collateral sulcus. We investigated the collateral sulcal complex using magnetic resonance images taking into account the three-dimensional nature of the brain. Our examination demonstrated three separate sulcal segments: (i) an anterior segment, the rhinal sulcus, delineating the uncus from the adjacent temporal neocortex, (ii) a middle segment, the collateral sulcus proper, forming the lateral border of the posterior parahippocampal cortex, and (iii) a caudal segment, the occipital extent of the collateral sulcus, within the occipital lobe. Three relationships exist between the rhinal sulcus and collateral sulcus proper, only one being clearly identifiable from the surface. Posteriorly, the collateral sulcus proper and the occipital collateral sulcus, although appearing continuous on the brain surface, can be separated in the depth of the sulcus in all cases. These results provide quantification of the location and variability within standard stereotaxic space for the three collateral sulcus segments that could be used to aid accurate identification of functional activation peaks derived from neuroimaging studies.