I have a quick question regarding the combined use of
-regress_bandpass x y -regress_polort x
in AFNI proc.
By default, AFNI proc automatically adjusts the option “regress_polort” depending on the run’s length. I often read the recommendation here on the AFNI board that IF one really wants to bandpass the data, say from 0.01-0.1 Hz, one should additionally set “regress_polort 2” in case the runs are very long and AFNI proc uses a polort option that is too high.
Lets say I have a TR of 1s and 900 sampling points, and I would like to bandpass from 0.01-0.2 Hz. Wouldn’t the the option “regress_polort 2” be superfluous in this case, or is it advised to use both the bandpass filter and the polort opton set to 2, 3 or any other value?
The help file for 3dDeconvolve (https://afni.nimh.nih.gov/pub/dist/doc/program_help/3dDeconvolve.html) states that
[quote=“For pnum > 2, this type of baseline detrending
is roughly equivalent to a highpass filter
with a cutoff of (p-2)/D Hz, where ‘D’ is the
duration of the imaging run: D = N*TR”]
Let’s say I use “regress_polort 3”. In this case, the polort option regresses frequencies below 0.001111… Hz. But the “bandpass” option, namely 0.01-0.2 Hz, already includes a high-pass filter of 0.01.
Question 1: why is the polort option still recommend when one bandpasses anyway, or is it not and regress_polort 0 should be used?
Question 2: Moreover, the cited 3dDeconvolve formula above only states what happens for a polort option bigger than 2. But what happens when polort 2 is used? Since p-2 (2-2) equals 0 and the resulting frequency of (2-2)/D would always be 0?