[FieldTrip] Unexpectedly high coherence measure

[Eva Masson]

Hi FieldTrippers !

I am currently implementing a script for computing coherence (and Granger causality, but that’s another topic) measure of iEEG data.

I am aware of two different paths in doing so: the parametric and non parametric approaches. As I am still at the early stages of the method, I am using data that was already tested for coherence in another software as reference and performing the coherence-related steps in FieldTrip, based on the following tutorial :
https://www.fieldtriptoolbox.org/tutorial/connectivity/

To summarize, I want to compute the coherence between electrodes X and Y for each frequency from 0 to 60 Hz. The input data consists in 9 segments of 300ms each after preprocessing, fsample 256.0164 Hz.
The problem is in the coherence measure I compute. The output of the following script is an unexpectedly high coherence measure, sometimes up to 0.9 when there is supposedly no to little coherence as compared to the « reference » measure on the other software. However, after searching through the FieldTrip functions code, the way of measuring the coherence seems to be the same (using the cross and auto Spectrum).

I am very most likely missing a step or a parameter in my script, but I have no clue what it is, where it is, and what I am misunderstanding.

Please find here a simplified version of my code for the parametric way of computing coherence.

% Compute mvar
cfg = [];
cfg.order = 12; % here I have tried different model orders and the problem persists
cfg.toolbox = 'bsmart';
cfg.channel = {‘X’ ‘Y’};
mvar_data = ft_mvaranalysis(cfg, data);

% Passage fréquence
cfg = [];
cfg.method = 'mvar';
cfg.foi = [1 60];
mvar_freq = ft_freqanalysis(cfg, mvar_data);

% Optionnel : cohérence
cfg = [];
cfg.method = 'coh';
mvar_coh = ft_connectivityanalysis(cfg, mvar_freq);

Do you know what is wrong with this ? I would be very thankful if you could help me on this 😊
Thanks a lot !

Cheers !
Eva
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20211011/a44d1413/attachment.htm>