Methods for Connectivity Analysis in fMRI
Functional magnetic resonance imaging (fMRI) has become one of the most prominent neuroimaging tools for the in vivo study of brain function. Since the BOLD effect was described by Ogawa et al. (1990), the number of studies involving fMRI has increased exponentially. It is a noninvasive technique with high spatial resolution, and these factors have contributed greatly to its success. MRI does not require the use of radiation or radioactive isotopes, as is the case in computed tomography (CT) and positron emission tomography (PET). As such, the risk factors associated with
the protocol are significantly smaller, allowing more comprehensive use within both clinical and nonclinical populations. Acquisition of data within fMRI is based on the paramagnetic properties of deoxyhemoglobin and hemodynamic coupling, producing the BOLD contrast. Logothetis et al. (2001) revealed that the BOLD signal can be considered as an indirect measure of local synaptic activity, as it shows greater correlation with local potentials than multi-unit activity. The spatial resolution of fMRI is greater than both PET and EEG, allowing finer scale analysis of this local synaptic activity. In addition, an MRI scanner allows a variety of neurological data to be obtained, from fMRI to structural (T1/T2 images; diffusion tensor imaging, DTI) and even metabolic (magnetic resonance spectroscopy, MRS) data sets. As such, its utility in a research and clinical setting is growing. This is evidenced by several hospitals now acquiring MRI systems with magnetic fields greater than 1.5, as well as software and hardware capable of acquiring fMRI data, at least for resting state protocols.