Studies of Functional Connectivity in White Matter
Resting state functional magnetic resonance imaging (rsfMRI) has been widely used for measuring functional connectivity between cortical regions. However, there have been minimal reports of rsfMRI in white matter, presumably because of the sparse vasculature in white matter relative to gray, and the consistent failure to observe significant hemodynamic responses from tasks within white matter. In this study, we aimed to investigate and assess the nature of temporal variations in rsfMRI signals from human and monkey brains in white matter. Previous studies have reported that the correlations of time course signals in a resting state between voxels are anisotropic in white matter. We therefore constructed functional correlation tensors (FCTs) that quantify the functional relationships between neighboring voxels and their anisotropy in normal brains at rest, and compared these to underlying structural features. Furthermore, we elucidated the underlying biophysical mechanisms that account for their origins by assessing whether MRI signal fluctuations in white and gray matter vary for different baseline levels of neural activity. We found FCTs were capable of visualizing long range white matter tracts as well as short range sub-cortical fibers imaged at rest, suggesting temporal resting state signals may reflect intrinsic synchronizations of neural activity in white matter. Moreover, our monkey studies revealed that fractional power of rsfMRI signals are modulated similarly in regions of SI cortex, gray and white matter as neural baseline activity is varied. Our results imply that neural activity is encoded in white matter, and that that BOLD signal fluctuations in white matter may be detected in a resting state.