[What to expect from MRi in the investigation of the central nervous system?].

Functional Magnetic Resonance Imaging (fMRI) has appeared as a new tool which is very powerful for cognitive neuroscience, offering the potential to look at the dynamics of cerebral processes underlying cognition, noninvasively and on an individual basis. Work remains to be done to optimize the technique and to better understand its basic mechanisms, but one may expect to build in a few years a functional list of the brain cortical networks of regions implicated in sensory-motor or cognitive processes. Still, the real understanding of brain function requires direct access to the functional unit made of the neuron, so that one may look at the transient temporal relationships that exist between largely distributed groups of hundreds or thousands of neurons. Furthermore, communication pathways between networks must be identified to establish connectivity maps at the individual scale, taking into account individual variability resulting from genetic factors and cerebral plasticity. In this respect, MRI of molecular diffusion is very sensitive to water molecular motion and, thus, to tissue dynamic microstructure, such as cell geometry. Preliminary data suggest that diffusion MRI visualizes dynamic tissue changes associated with large neuronal activation and space orientation of large bundles of myelinated axons in the white matter.