Figure 4.

BOLD origin of phase (Φ) and magnitude (M/M ₀) signal changes. (A) Averaged phase (black) and magnitude (magenta) time courses in common positive and negative magnitude and phase activity maps (echo time, TE = 31.5 ms, P < 0.05 Bonferroni corrected), for one subject during visual task and at rest (before averaging across voxels, timeseries in negative activity maps were multiplied by −1). The high temporal correlation between phase and magnitude time‐courses indicates the same BOLD origin of phase and magnitude signal changes. For each condition, we plot respectively in (B) and (C) the amplitude of magnitude and phase signal fluctuations versus TE (average ± standard error across subjects) and a linear fit to each data‐set. For each subject, the amplitude of signal fluctuations was measured as the standard deviation of the average time‐course across voxels pertaining to both magnitude and phase activity maps. The increase in amplitude of signal fluctuations with TE was very similar between magnitude and phase data (the correlation between the two was 0.94 ± 0.03 and 0.99 ± 0.00 for the visual task and rest data, respectively, mean ± standard error across eight subjects). [Color figure can be viewed in the online issue, which is available at http://wileyonlinelibrary.com.]