Abstract
Studies of resting state functional MRI (rs-fRMI) are increasingly focused on “dynamics”, or on those properties of brain activation that manifest and vary on timescales shorter than the scan’s full duration. This shift in focus has led to a flurry of interest in developing hypothesis testing frameworks and null models applicable to the dynamical setting. Thus far however, efforts in this direction have been weakened by a number of crucial shortcomings, several of which are outlined and discussed in this short paper. The discussion below is important because the weaknesses exhibited by many of the recently proposed models are not merely quantitative or parametric. Rather they are fundamental, falling into categories that ultimately neutralize the ability of the resulting models to play a scientifically clarifying role, while simultaneously injecting distracting new vectors of murkiness and confusion into a young but promising research enterprise these efforts are surely intended to support. One key point emphasized in this paper is that since functionally relevant temporal variations in brain activation are undeniably continual in living human rs-fMRI subjects (e.g., the period of time during which all neural activity throughout a human brain carries no functional relevance whatsoever is something whose existence, even during stroke or seizure, seems highly improbable), “absence of brain dynamics” as a null hypothesis for fMRI imaging studies is neither scientifically interesting nor well-defined. As we learn more about the phenomenon of functionally relevant brain dynamics (FRBD) and its imaging correlates, scientifically meaningful null hypotheses and well-tuned null models will naturally emerge. In the course of our discussion, we revisit the important concept of stationarity, what it is, how to assess it from multiple realizations of a process and finally attempt to parse through the benefits and limitations of applying this property in models of rs-fMRI data. We hope that the discussions herein are useful, and promote thoughtful consideration of these important issues.