  Task-free connectivity analyses have emerged as a powerful tool in functional
neuroimaging. Because the cross-correlations that underlie connectivity
measures are sensitive to distortion of time-series, here we used a novel
dynamic phantom to provide a ground truth for dynamic fidelity between blood
oxygen level dependent (BOLD)-like inputs and fMRI outputs. We found that the
de facto quality-metric for task-free fMRI, temporal signal to noise ratio
(tSNR), correlated inversely with dynamic fidelity; thus, studies optimized for
tSNR actually produced time-series that showed the greatest distortion of
signal dynamics. Instead, the phantom showed that dynamic fidelity is
reasonably approximated by a measure that, unlike tSNR, dissociates signal
dynamics from scanner artifact. We then tested this measure, signal fluctuation
sensitivity (SFS), against human resting-state data. As predicted by the
phantom, SFS--and not tSNR--is associated with enhanced sensitivity to both
local and long-range connectivity within the brain's default mode network.
