# AFNI Message Board

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 May 16, 2022 07:43AM Registered: 1 year ago Posts: 93
Paul,

thank you for the elaborate answer. What you explained about the properties of a time-series and how these properties can affect the power distribution in the frequency-domain when the time-series is cut at the beginning or end is clear to me.

For example, you wrote that “From that point of view, if you are averaging over frequency windows/bands, *and* your time series has constant statistical properties throughout, then chopping away time points might not matter so much.”

I understand this. My question was related to another aspect that is rather related to the internal computations that AFNI proc does, and not related to theoretical considerations, I think.

I am not wondering if the frequency-domain of a time-series can be affected when we compare the Fourier results between a BOLD time-series that was (1) not cut vs. (2) cut. Of course, the resulting frequency-domain (the distribution of power along the spectrum) can change (from minimal to extreme changes), depending on the properties of the time-series and how the cut out part deviates from the rest of the time-series.

Instead, I was wondering if cutting a time-series after preprocessing (after AFNI proc) vs. cutting the time-series before preprocessing (before AFNI proc) can result in different power spectra of the very same time-series. This is to say that the length of the time-series in case 1 is the same as in case 2. Case 1 is a cut time-series, and case 2 is a cut time-series. But in one case the cutting was done before preprocessing, and in the second case it was done after preprocessing.

And now I wonder if THAT can make a difference, precisely because preprocessing in AFNI proc maybe chooses a different fundamental wave or frequency (maybe somehow related to bandpassing or any of the vast amount of other computations), because the run was longer if not cut before preprocessing. The harmonics between case 1 and 2 maybe differ, since the fundamental frequency would not be the same when providing a time-series that is cut vs. non cut into AFNI proc.

Does that make sense? Otherwise, maybe I don't understand if the answer to that question is already provided in your post. Please let me know if I didn't understand you.

Philipp

Edited 1 time(s). Last edit at 05/16/2022 08:02AM by Philipp.
Subject Author Posted

#### Cutting a time-series after vs. before preprocessing

Philipp May 15, 2022 01:54PM

#### Re: Cutting a time-series after vs. before preprocessing

ptaylor May 16, 2022 06:47AM

#### Re: Cutting a time-series after vs. before preprocessing

Philipp May 16, 2022 07:43AM

#### Re: Cutting a time-series after vs. before preprocessing

ptaylor May 16, 2022 08:38AM

#### Re: Cutting a time-series after vs. before preprocessing

Philipp May 16, 2022 08:54AM

#### Re: Cutting a time-series after vs. before preprocessing

ptaylor May 16, 2022 09:27AM

#### Re: Cutting a time-series after vs. before preprocessing

Philipp May 16, 2022 09:46AM

#### Re: Cutting a time-series after vs. before preprocessing

rick reynolds May 16, 2022 10:42AM

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