Briefly, filtered time series of voxels were subjected to frequency domain transformation using the fast Fourier transform (FFT). The power spectrum was calculated by squaring the amplitude of each frequency component. The square root of this power spectrum was then averaged across 0.01–0.08 Hz at each voxel to derive the amplitude of low-frequency fluctuations (ALFF). Non-brain tissues were removed using the MRIcro software. The ALFF of each voxel was standardized by dividing it by the global mean ALFF value for each subject, resulting in a relative ALFF. Fractional ALFF (fALFF) represents the proportion of ALFF within the frequency band of 0.01–0.08 Hz compared to the entire detectable frequency range of 0.01–0.25 Hz in the signal. Finally, the individual-level ALFF and fALFF were converted into a z-score map, reflecting spontaneous neuronal activity.
