CTF275 coherence

Cristiano Micheli michelic72 at GMAIL.COM
Fri Jun 27 16:11:21 CEST 2008

Dear Jan-Mathijs
In the preprocessing i performed baseline correction for meg data and 10 Hz
hi-pass filtering for emg (sampling at ~300 Hz).
However since normally the correlated activities for isometric pinch take
place in the beta band, and eye-blinks and other artifacts are in other
bands or uncorrelated with the emg, i did not think about applying artifacts
For sure i will have a look at it but what for me constitutes the signal of
a good emg-meg correlation is not only the peak in the spectrum but also two
controlateral dipolar 'spots' in the topography, which i took as a good
result. And i expect dics to be more sensible than emg-meg coherence in
detecting sources, with the right settings.
I specify the settings of frequency analysis like that:
    refch    =  'EMG_lH';
    cfg       = [];
    cfg.output     = 'powandcsd';
    cfg.method     = 'mtmfft';
    cfg.foilim     = [max_f max_f]; % peak of coherence spectrum
    cfg.tapsmofrq  = 5;
    cfg.keeptrials = 'yes';
    cfg.channel    = {'MEG' refch};
    cfg.channelcmb = {'MEG' 'MEG';'MEG' refch};
    freqcond       = freqanalysis(cfg,data);
    cfg = [];
    fd             = freqdescriptives(cfg,freqcond);

Maybe i should use another method for csd calculations or another smoothing
Alltogether i have 250 subtrials and 9 tapers. I collect 25 trials of 10
seconds each and then i cut each trial in segments of 1 second each. 
The reason why i use my own gradiometers definitions is because i already
have this information ready in my code from a CTF routine. Also i did not
get how to look for it in the documentation. How can i do it easier?
Then I run the dics with the following parameters:
    cfg           = [];
    cfg.grad     = grads; 
    cfg.grid      = grids_;
    cfg.method    = 'dics';   
    cfg.projectnoise = 'yes';
    cfg.lambda    = 0; 
    cfg.refchan   = refch;
    cfg.frequency = max_f;
    cfg.hdmfile   = 'myhead.hdm';
    sourcecond    = sourceanalysis(cfg, freqcond);

In the simulations i get the same problem whereas the spectra look
reasonable and the dipole fit localizes the source in the right place from
coherence peak.
The simulations were done with one source (virtual emg) coherent in phase
and amplitude with the source time course. Source is modelled by a dipole in
position [0  5  10] cm (head coordinates) and moment [10 0 0] nA.m
How do i perform a good frequency analysis in order to have a good localization?
If it does not depend on fr. analysis which other factors can cause this
Thank you for the patience


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