I am interested in implementing a notch filter in AFNI for resting state fMRI data. The documentation for 3dFourier suggests that you can implement a notch filter by combining the lowpass and highpass options (although an example of doing this correctly is not provided). Therefore, I attempted a notch filter as follows (I'm using AFNI_19.1.04 on CentOS 7).

3dFourier -prefix ${subj}_task-${task}.fourier.resid \
-retrend -lowpass ${notch_lowpass_hz} -highpass ${notch_highpass_hz} ${subj}_task-${task}.detrend.resid+tlrc

where:
notch_lowpass_hz = 0.06 ## Hz
notch_highpass_hz = 0.14 ## Hz

The resulting call yields a warning:

++ 3dFourier: AFNI version=AFNI_19.1.04 (Apr 21 2019) [64-bit]
** WARNING: lowpass=0.060000 is <= than highpass=0.140000
** -------: results from 3dFourier are suspect!

This leads me to believe that my assumptions about how to implement the notch in AFNI are incorrect. The notch should pass all frequencies below the lowpass and above the highpass (i.e., a bandstop filter eliminating frequencies on the range lowpass-hipass), but the warning suggests that the logical way of combining the lowpass and highpass flags is incorrect. The warning suggests that 3dFourier can only execute a bandpass filter (where lowpass would indeed be higher than the highpass).

Any ideas?



Edited 1 time(s). Last edit at 04/01/2022 04:18PM by kabush_mri.