Figure 8. Dependence of factitious respiratory motion on phase encoding direction.

To show that motion related findings secondary to B₀ are distinct from true brain motion, we obtained additional data in a single participant known to be a very low mover. Four BOLD fMRI runs (TR = 1.1s) were acquired with phase-encoding (PE) directions AP, PA, LR, RL. The participant was blind to PE condition. (Additional data acquired in a separate session with distinct scan parameters are shown in SI Figures S2–S4). Note principal respiration peak (~0.23 Hz; black arrows) in the motion parameter capturing translation in the PE direction: y-translation for AP and PA (Panels A and B) and x-translation for LR and (panels C and D). This effect is highly consistent with factitious head motion induced by B₀ modulation. ~0.23 Hz signals largely invariant with respect to PE direction occur in the z-translation and x-rotation parameters. These signals most likely represent true translation along the long body axis and head nod about the ear-ear axis arising from mechanical linkage of the head to the chest through the neck. Respiratory motion in other parameters (y-rotation, z-rotation) could represent true head motion or “leakage” (see Appendix A).