Confounds in Functional Magnetic Resonance Imaging-Part II: Modifiers.

In the first part of this three-part series, we reviewed the mechanisms that cause nuisance signals in fMRI data. Here, in the second part of the series, we review "human factors" that modulate these confounding mechanisms. The modulators may be trait characteristics such as age or bodyweight, or they may be state dependencies that alter the relative contributions of nuisance signals in the fMRI data. A wide array of psychological (e.g., motivation/arousal, cognitive strategy, task engagement), lifestyle (e.g., exercise, sleep, and dietary habits), experimental (e.g., scanner anxiety, task-related movement), and situational (e.g., prandial state, time of day, menses) factors can affect the brain state directly as well as via changes in systemic physiology mediated by autonomic nervous responses.