What to expect from MRI in the investigation of the central nervous system?

Functional magnetic resonance imaging (fMRI) has appeared as a new tool tha t is very powerful for cognitive neuroscience, offering the potential to lo ok at the dynamics of cerebral processes underlying cognition, non-invasive ly and on an individual basis. Work remains to be done to optimize the tech nique and to better understand its basic mechanisms, but one may expect to build in a foreseeable future a functional list of the main brain cortical networks implicated in sensory-motor or cognitive processes. Still, the rea l understanding of brain function requires direct access to the functional unit consisting of the neuron, so that one may look at the transient tempor al relationships that exist between largely distributed groups of hundreds or thousands of neurons. Furthermore, communication pathways between networ ks, which are carried by brain white matter, must be identified to establis h connectivity maps at the individual scale, taking into account individual variability resulting from genetic factors and cerebral plasticity. In thi s respect, MRI of molecular diffusion is very sensitive to water molecular motion and, thus, to tissue dynamic microstructure, such as cell size and g eometry. Preliminary data suggest that diffusion MRI visualizes dynamic tis sue changes associated with large neuronal activation and space orientation of large bundles of myelinated axons in the white matter. (C) 2000 Academi e des sciences/Editions scientifiques et medicales Elsevier SAS.