AFNI Message Board

Dear AFNI users-

We are very pleased to announce that the new AFNI Message Board framework is up! Please join us at:

https://discuss.afni.nimh.nih.gov

Existing user accounts have been migrated, so returning users can login by requesting a password reset. New users can create accounts, as well, through a standard account creation process. Please note that these setup emails might initially go to spam folders (esp. for NIH users!), so please check those locations in the beginning.

The current Message Board discussion threads have been migrated to the new framework. The current Message Board will remain visible, but read-only, for a little while.

Sincerely, AFNI HQ

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RWCox
October 26, 2021 11:28AM
These are hard questions to answer, for a couple of reasons.
First, it has been a LONG time since I used periodograms seriously (say 1985).
Second, since I don't have access to your data, I'm flying blind here.

Here's my first stab at suggestions, in no particular order:
  • Look at the outputs of 3dPeriodogram to see what could be causing the weirdness in the fit.
  • Consider using 3dTfitter with the -L1 option to do a least-absolute-sum fit rather than a least-squares fit -- this will reduce the impact of "outliers" in the periodogram "data".
  • More intricate: use 1dNLfit to fit the power law decay formula "a*f^(-p)" directly, rather than using the log transformation to make a linear fitting problem. Fitting the desired curve directly is usually preferred over transforming the problem to make the regression linear. However, 1dNLfit only operates on 1D text files, as it is pretty slow to run on a whole 3D dataset -- so you'll either have to ROI average the periodograms first, or extract the periodograms from and ROI, run 1dNLfit a lot, and then average.
I don't think it is "nfft" itself which is your problem, unless indirectly you happen to be cutting off some wacky data when you shrink nfft.

Something else you might consider would be doing global signal regression during the pre-processing, and eliminating the bandpassing.
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Subject Author Posted

3dPeriodogram and the Fast Fourier Transform: nfft question

Philipp October 23, 2021 09:23AM

Re: 3dPeriodogram and the Fast Fourier Transform: nfft question

Philipp October 26, 2021 04:59AM

Re: 3dPeriodogram and the Fast Fourier Transform: nfft question

RWCox October 26, 2021 11:28AM