Why Blood Pressure and Body Mass Should be Controlled for in Resting-State Functional Magnetic Resonance Imaging Studies.

Replicability has become an increasing focus within the scientific communities with the ongoing "replication crisis". One area that appears to struggle with unreliable results is resting-state functional magnetic resonance imaging (rs-fMRI). Therefore, the current study aimed to improve the knowledge of endogenous factors that contribute to inter-individual variability. Arterial blood pressure, body mass, hematocrit, and glycated hemoglobin were investigated as potential sources of between-subject variability in rs-fMRI, in healthy individuals. Whether changes in resting state-networks (rs-networks) could be attributed to variability in the BOLD-signal, changes in neuronal activity, or both, was of special interest. Within-subject parameters were estimated utilizing Dynamic Causal Modelling (DCM) as it allows to make inferences on the estimated hemodynamic (BOLD-signal dynamics) and neuronal parameters (effective connectivity) separately. The results of the analyses imply that blood pressure and body mass can cause between-subject and between-group variability in the BOLD-signal and that all the included factors can affect the underlying connectivity. Given the results of the current and previous studies, rs-fMRI results appear to be susceptible to a range of factors, which is likely to contribute to the low degree of replicability of these studies. Interestingly, the highest degree of variability seems to appear within the much-studied Default Mode Network and its connections to other networks.