Fig. 2.

Method for estimating arousal fluctuation in resting-state fMRI. A spatial template, derived a priori and representing the degree to which voxels across the brain display increased or decreased signal change with behavioral arousal, is projected onto each fMRI volume via spatial correlation. This correlation, quantifying the similarity between the template and the spatial distribution of signal intensity at each preprocessed fMRI volume, traces out a time course of estimated arousal fluctuation, the fMRI arousal index, across the measurement period. Two examples of time frames from monkey S are shown, illustrating the instantaneous patterns (single volumes of the normalized fMRI signal) at frames having high positive and negative spatial correlation with the template. These frames correspond to estimates of increased and decreased levels of arousal, respectively.