An investigation of functional and anatomical connectivity using magnetic resonance imaging.

This article examines functional and anatomical connectivity in healthy human subjects measured with magnetic resonance imaging methods. Anatomical connectivity in white matter is obtained from measurements of the diffusion tensor. A Monte-Carlo simulation determines the probability that a particle diffuses between two points, with the probability of a jump in a particular direction from a given voxel being based on the local value of the diffusion tensor components. Functional connectivity between grey matter pixels is assessed without recourse to a specific activation paradigm, by calculating the correlation coefficient between random fluctuations in the blood oxygenation level-dependent signal time course in different pixels. The methods are used to examine the anatomical and functional connectivities between crowns of adjacent gyri. A high functional connectivity was found between grey matter pixels, with white matter displaying only very low correlation. A comparison of the measurements of anatomical and functional connectivity found that there is no simple correlation between these measures, except that low values of functional connectivity were not found together with high values of anatomical connectivity. Furthermore pairs of regions situated around the central sulcus indicated a dependence of the two connectivity measures on each other. These results are in accordance with an interpretation that regions which are clearly directly linked by white matter fiber tracts should show high functional connectivity, but that the inverse need not be true as functional connectivity may also be indirectly mediated via more distant grey matter regions.