Artifactual time‐course correlations in echo‐planar fMRI with implications for studies of brain function

Brain function is widely investigated with functional magnetic resonance imaging (fMRI) in humans and animals. In fMRI, the time courses of voxels typically reflect the local blood‐oxygen level, which is taken as an indicator of neuronal activity. Voxel time‐course correlations are often explicitly modeled and interpreted in terms of neuronal interactions. They also affect standard analyses that do not explicitly target neuronal interactions. As a consequence, time‐course correlations between voxels influence conclusions about cognitive and physiological brain processes in many studies. However, voxel correlations are known to arise not only from cognitive and physiological processes, but also as artifacts of fMRI techniques such as the commonly used echoplanar imaging. We empirically demonstrate this phenomenon by plotting time‐course correlation as a function of voxel separation for a human brain in resting state and for a water‐filled sphere (called a “phantom”). The phantom served as a test object known not to contain any interacting neuronal systems. The plots for brain and phantom are surprisingly similar. The correlational structure found in the phantom must be artifactual. Artifactual correlations spanning many centimeters occur within and between different imaging slices. Correlations between voxel time courses do not necessarily reflect brain processes. Instead, fMRI is affected by artifactual correlations, which are very strong for neighboring voxels and clearly present even at large distances. This needs to be taken into account in the neuroscientific interpretation of voxel correlations. © 2008 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 18, 345–349, 2008