[Mne_analysis] Phase Locking Value and filtering

[Elisabeth Fonteneau]

Hello MNE users,

First thank you for answering my previous questions about PLV.. that was very usefull...
I take the advantage to have some PLV expert to ask another related question about PLV & filtering (low pass)

I am using the function provided by SPM (spm_eeg_morlet) with the following options:
Factor 7 and sampling rate 2 ms from 1 to 80 Hz to start with..
>From my results, I realised that some of my effects were starting in the gamma band (40-80Hz) but could extend in higher gamma range (the clusters were clearly cut at the 80Hz level).

My original data were lowpass filter before the morlet transform to 80 Hz, I then use a different filter that time to a 150 Hz lowpass ... and recompute everything.
Now I was assuming that with my new data, I should find similar clusters within the 40-80hz gamma band and additional ones may be in the higher gamma band..
Indeed, by increasing my frequency range I found some new clusters in high gamma band, but my previous clusters in the lower gamma band completely disappear.
I am puzzle with this and when checking the PLV values, these are closed to be identical in the lower frequencies band (below 40Hz, but with lower value) but on the 40-80hz the PLV values have completely changed...

I am thinking the problem is coming from the low pass filter that will induce some artefact at the edge of frequency band or may be the morlett transform properties?
So far I can not see any other explanation but you might have already one in your head, or may be you came across this problem too..
If my problem is coming from the filter, what will you suggest to compute an accurate PLV value, no filter at all? Or take a larger low pass filter that will not interfere with the frequency of interest?
If by chance you have a paper in mind that could direct me to the solution that will be very helpful

Thanks for your answer
Elisabeth

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Dr Elisabeth Fonteneau
MRC Cognition and Brain Sciences Unit
15 Chaucer Road
Cambridge CB2 7EF
England
Tel:  +44 (0)1223 355294 (extension 263)
Fax:  +44 (0)1223 359062

Email: elisabeth.fonteneau at mrc-cbu.cam.ac.uk
Web: http://www.mrc-cbu.cam.ac.uk/people/elisabeth.fonteneau/
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