phase locking value always 1

Jan Mathijs Schoffelen Jan.Schoffelen at FCDONDERS.RU.NL
Thu Jun 7 10:02:25 CEST 2007


Dear Sameer,

cfg.keeptrials = 'no' is strictly forbidden when you want to compute the
plv!

The reason for this comes from the mathematics behind it:
For the computation of the plv, each observation has to be normalised by its
amplitude prior to averaging across all observations. The normalisation step
is implemented in freqdescriptives, therefore it needs an input which still
contains the single trials. I leave it as a quiz for the interested reader
to reason why actually the plv becomes 1, when you only have a 'single'
(that is: averaged) observation.

Yours,

Jan-Mathijs

-----Original Message-----
From: FieldTrip discussion list [mailto:FIELDTRIP at NIC.SURFNET.NL] On Behalf
Of Sameer Walawalkar
Sent: Wednesday, June 06, 2007 11:00 PM
To: FIELDTRIP at NIC.SURFNET.NL
Subject: [FIELDTRIP] phase locking value always 1

Hello,

I use the following setup to calculate PLVs:

cfg = [];
cfg.ouput='powandcsd';
cfg.method='mtmfft';
cfg.keeptrials='no';
cfg.tapsmofrq=4;
cfg.foilim =[05  120];
cfg.channel = 'MEG'  ;
cfg.channelcmb = {'MEG' 'MEG'};
cfg.pad = 2.00 ;


freq_LL_Two_pre = freqanalysis(cfg,dataLL_Two_pre);
cfg = [];
cfg.cohmethod = 'plv';
Phase_neg2_neg1_LL_Two_pre = freqdescriptives(cfg, freq_LL_Two_pre)


The PLVs I get are uniformly 1.000. I am not sure what's going on. the
coherence spectra from cfg.cohmethod = 'coh' are just fine.

thanks,
sameer

p.s: On a side note, for coherences, if I use
cfg.complex = 'angle':
the phase differences I get seem to be distributed from [-2pi 2pi] rather
than [-pi pi] or [0 2pi] (I have not used [] strictly in mathematical
sense).
How do I understand this?

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http://www.ru.nl/fcdonders/fieldtrip.

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.



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