In-vivo inference of anatomical connectivity

In order to reconstruct the course of nerve fibres and hence the blueprint of the neuronal circuitry, several classical methods are available. For example, the migration of tracer substances along fibres can be observed or the fibre tracts can be reconstructed using series of polarized light micrographs. These methods can only be applied to dead material or animal models, however. This makes it difficult to directly investigate the networks underlying specific cognitive faculties of humans. In recent years, the emergence of diffusion weighted magnetic resonance imaging has offered the possibility of in-vivo monitoring the direction-dependent mobility of water molecules, which allow for inferences on the microstructure of the tissue, in particular nerve fibre orientation. Based on a mathematical modelling technique called tractography, it is possible to reconstruct the course of nerve fibre tracts. This methodology offers a way of imaging the anatomical connectivity in the living human brain. For example, one can detect differences in fibre bundle properties related to certain diseases or special capabilities. It is also possible to compute a specific connectivity profile for each portion of the cortex. Under the premise that the connectivity of a brain structure with the rest of the brain is of great importance for its function, the comparison of such connectivity profiles yields information on the division of the cortex into functional areas. This methodology is referred to as functional/anatomical parcellation. Using tractography we are able to elucidate the anatomical aspects of both segregation (by reconstructing the parcellations) and integration (by quantifying the connectivity between brain areas). It is of paramount importance that this can be done in a non- invasive way in normal healthy human subjects, so that we can connect this information with the specifically human faculties of the brain.