Fig. 1.

Power spectra of the GLM residuals in native space averaged across brain voxels and across subjects for the assumed boxcar design of 10 s of rest followed by 10 s of stimulus presentation (boxcar10). The dips at 0.05 Hz are due to the assumed design period being 20 s (10 s + 10 s). For some datasets, the dip is not seen as the assumed design frequency was not covered by any of the sampled frequencies. The frequencies on the x-axis go up to the Nyquist frequency, which is 0.5/repetition time. If after pre-whitening the residuals were white (as it is assumed), the power spectra would be flat. AFNI and SPM’s alternative method: FAST, led to best whitening performance (most flat spectra). For FSL and SPM, there was substantial autocorrelated noise left after pre-whitening, particularly at low frequencies