Cortical mapping by magnetic resonance imaging (MRI) and quantitative cytological analysis in the human brain: a feasibility study in the fusiform gyrus.

The cerebral cortex is a layered cellular structure that is tangentially organized into a mosaic of anatomically and functionally distinct fields. In spite of centuries of investigation, the precise localization and classification of many areas in the cerebral cortex remain problematic because the relationship between functional specificity and intra-cortical structure has not been firmly established. Furthermore, it is not yet clear how surface landmarks, visible through gross examination and, more recently, using non-invasive magnetic resonance imaging (MRI), relate to underlying microstructural borders and to the topography of functional activation. We have designed a multi-modal neuroimaging protocol that combines MRI and quantitative microscopic analysis in the same individual to clarify the topography of cytoarchitecture underlying gross anatomical landmarks in the cerebral cortex. We tested our approach in the region of the fusiform gyrus (FG) because, in spite of its seemingly smooth appearance on the ventral aspect of both hemispheres, this structure houses many functionally defined areas whose histological borders remain unclear. In practice, we used MRI-based automated segmentation to define the region of interest from which we could then collect quantitative histological data (specifically, neuronal size and density). A modified stereological approach was used to sample the cortex within the FG without a priori assumptions on the location of architectonic boundaries. The results of these analyses illustrate architectonic variations along the FG and demonstrate that it is possible to correlate quantitative histological data to measures that are obtained in the context of large-scale, non-invasive MRI-based population studies.