Morphology of the collateral sulcal complex and discrimination of functional activation during navigation in the parahippocampal gyrus of the human brain

A comprehensive study of the functional organization of the parahippocampal gyrus (PHG) in the human brain requires a clear understanding of the sulcal-gyral morphology and its variability in individual cerebral hemispheres. This is imperative, given the cytoarchitectonic and functional heterogeneity of the cortical areas that make up the PHG. The collateral sulcal complex comprises a number of sulci that laterally delineate the rostrocaudal extent of the entorhinal cortex and the parahippocampal cortex (PHC) that occupy the surface of the PHG. In non-human primates, the rhinal sulcus refers to a sulcus that laterally delimits the extent of the entorhinal cortex. In contrast, in the human brain, the term rhinal sulcus has been used ambiguously. Some researchers have called a small dimple at the anteriormost part of the PHG the rhinal sulcus, referring to the sulcus laterally bordering the entorhinal cortex as part of the collateral sulcus. This usage of the term rhinal sulcus implies that, in the human brain, the rhinal sulcus bears only a partial relationship with the most anterior part of the entorhinal cortex. In the research carried out for this thesis, the details of the morphology, i.e. the sulcal patterns and variability, of the sulci that make up the collateral sulcal complex are defined in individual subjects on structural magnetic resonance images (MRIs) in the standard stereotaxic space of the Montreal Neurological Institute. We found that the anterior sulcal segment of the collateral sulcal complex, which we refer to as the rhinal sulcus, delimits laterally the entorhinal cortex and, posteriorly, the collateral sulcus proper provides the lateral border of the PHC. In addition, we identified a short sulcus, the parahippocampal extension of the collateral sulcus, which runs transversely into the PHG and provides its posterior border. Further caudally, the occipital branch of the collateral sulcus lay in the lingual gyrus of the occipital lobe, bearing no relationship to the PHG. Next, sulcal probability maps were established for the sulci that laterally delineate the PHG (i.e. the rhinal sulcus, collateral sulcus proper, and parahippocampal extension of the collateral sulcus) to aid accurate identification of the sulcal segments and the location of activation peaks in functional neuroimaging studies. Together these anatomical investigations showed that, in the human brain, a sulcus exists that laterally delineates the extent of the entorhinal cortex and that, therefore, this sulcus should be termed the rhinal sulcus, consistent with the non-human primate literature. The level at which the collateral sulcus proper can be separated from the occipital extent of the collateral sulcus may be a potential candidate for a morphological landmark distinguishing the PHC of the medial temporal lobe, involved in mnemonic information processing, from the lingual gyrus of the occipital lobe, involved in processing visual information.The location of functional activation peaks obtained during the performance of a navigation task in a functional MRI (fMRI) study were related to the morphology of the sulcal segments of the collateral sulcal complex that had been identified in the anatomical studies. Based on these morphological studies, it was possible to show that the middle and posterior parts of the PHC, and not the anterior PHC or entorhinal cortex, were involved when scene-selective information necessary for navigation was processed. Moreover, a similar fMRI study in patients with mild traumatic brain injury revealed that such structure-function relationships could aid the identification of subtle consequences on functional activation in the right posterior and left middle PHC. Without a detailed understanding of the sulcal segments of the collateral sulcal complex, the correct interpretation of the activation peaks observed in the fMRI studies in the healthy and clinical populations would not have been possible.

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