[FieldTrip] PAC analysis with ft_crossfrequencyanalysis

Schoffelen, J.M. (Jan Mathijs) janmathijs.schoffelen at donders.ru.nl
Wed Apr 13 14:59:59 CEST 2022

Hi Elisa,

Your questions  are not decorated with any insights about code/data, so it’s a bit guessing as to how to compile an answer to your questions that might help you further. I’ll try anyhow:

  1.   What is the minimum epoch length required to compute PAC (is 1s enough to compute the phase of 4 Hz)?

The sliding time window (which is not necessarily the same as the epoch length) should be sufficiently long to get a decent estimate of the phase at 4Hz. Thus, a cfg.t_ftimwin for the low frequency oscillation, to be used in ft_freqanalysis to compute the freqlo should be sufficient.

  2.  Is this computation suitable for tasks with a strong event-related synchronization? I read that ERPs should be subtracted from this analysis to avoid confounding effects but here I have reaching movements.

No, and subtraction of an ERP is not going to be very useful in my opinion. See: https://www.fieldtriptoolbox.org/faq/evoked_vs_induced/

  1.  When computing PAC with Fieldtrip, I get the same value for the entire frequency band of the phase (i.e. same number from 4 to 8 Hz). This is despite the resulting MI matrix being N.channel*N.freqphase*N.freqAmplitude.
  2.  Since I would like to explore which theta frequency phase is mostly coupled with the higher frequency power, I would like to know why I get the same values and how to fix this as I could not find any configuration about averaging frequencies (e.g. cfg.averageoverfreq = 'yes'/'no').

This I don’t understand. I cannot comment on this without knowing more about the exact parameters used for the generation of the input data into the ft_crossfrequencyanalysis function.

  1.  If I manage to compute a meaningful MI, how do I statistically test whether it is significant?

‘Significant’ is always related to a null hypothesis. What’s your null hypothesis?

  1.  Last but not least, is there a physiological or computational explanation for the impossibility of computing PAC on different electrodes?

I don’t understand the question underlying this question: given some data, and a function, one could most of the time produce a result. In that sense it is always possible. Whether or not that result is interpretable, that’s a different question, but it probably does not depend on whether it was computed on electrode O1, or on electrode P6

Best wishes,

Thank you very much for taking the time to read my questions. I really hope to get some answers from you.

Best wishes.


Elisa Tatti, Ph.D.
Post Doctoral Research Fellow
CUNY, School of Medicine
160 Convent ave
New York City, NY

Tel. +1 3472043952

 pronouns: she/her/hers

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