Coherence!

[Muthuraman Muthuraman]

Hello,

Thankyou for the suggestions

As you have said when i change the frequency resolution of the mtmwelch
method to 1Hz from 2Hz

cfg.foi        = [2:1:30];
cfg.t_ftimwin  = ones(1,length(cfg.foi)).*1;

I get the all the values to be Nan's.

Is it possible to load the laplacian transformed data to the fieldtrip and
work on it with the
Frequency analysis, But the actual laplace transformed data is a huge file

for ex:[151000*39] were first 35 channels are the lap.transformed EEG
channels
and the last 4 channels 36,37,38,39 are the Lap.transform EMG channels.

Is there a place where i can upload a example daten file to the fieldtrip
and then you can work on it

I tried importing my own data format as specified in the website of
fieldtrip
was not successfull with the data.trial and data.time

Is there a way to import this daten file to the fieldtrip and then working
on it
for the DICS analysis and then i also think then the results of the
coherence analysis
will be comparable with the fieldtrip methods.

Thanking you

With regards
M.Muthuraman.



>From: Robert Oostenveld <r.oostenveld at FCDONDERS.RU.NL>
>Reply-To: FieldTrip discussion list <FIELDTRIP at NIC.SURFNET.NL>
>To: FIELDTRIP at NIC.SURFNET.NL
>Subject: Re: [FIELDTRIP] Coherence!
>Date: Tue, 9 Jan 2007 17:53:23 +0100
>
>dear Muthuraman
>
>On 9 Jan 2007, at 11:08, Muthuraman Muthuraman wrote:
>>%MTMFFT
>>cfg.channel    = channelselection({'all' 'M1'}, data.label);
>>cfg.channelcmb = channelcombination({'all' 'M1'}, data.label);
>
>for DICS you will also need the cross-spectrum between all EEG  channels.
>Hence you should do cfg.channelcmb = 'all'. The code above  is fine if you
>want to look at coherence on channel level, which is  what you currently
>are looking at.
>
>>cfg.foilim     = [2 30];
>>cfg.taper      = 'hanning';
>>cfg.pad        = 1;
>>cfg.keeptrials = 'yes';
>>cfg.variance   = 'yes';
>>cfg.jackknife  = 'yes';
>>freqmtmfft     = freqanalysis(cfg, data);
>>fdmtmfft       = freqdescriptives([], freqmtmfft);
>
>This uses 1 hanning taper, and you have one-second long data  segments.
>Hence the frequency resolution is 1/1s = 1Hz.
>
>>%MTMWELCH
>>cfg.method     = 'mtmwelch';
>>cfg.output     = 'powandcsd';
>>cfg.foi        = [2:2:30];
>>cfg.t_ftimwin  = ones(1,length(cfg.foi)).*0.5;
>>cfg.taper      = 'hanning';
>>cfg.pad        = 1;
>
>This uses a 500ms window per trial to do the estimation. That gives  you a
>1/0.5 = 2Hz frequency resolution. The lower frequency  resolution implies
>that there is more frequency smoothing, so the  estimate will be different.
>If the true coherence peak is broad-band  (i.e. broader than 1 Hz), then
>the estimate will probably be better  (=higher), if not, then it will be
>worse (=lower).
>
>>cfg.keeptrials = 'yes';
>>cfg.variance   = 'yes';
>>cfg.jackknife  = 'yes';
>>freqmtmwelch   = freqanalysis(cfg, data);
>>fdmtmwelch     = freqdescriptives([], freqmtmwelch);
>
>You probably should do
>   cfg = []
>   cfg.variance   = 'yes';
>   cfg.jackknife  = 'yes';
>and then
>   fdmtmwelch     = freqdescriptives(cfg, freqmtmwelch);
>since the variance options do not apply to freqanalysis, but to
>freqdescriptives instead.
>
>>I have attached a figure with this mail[Coherence between C3 and  M1] in
>>which there is a difference in coherence values
>>for the fieldtrip method and the welch periodogram method with and
>>without laplacian transform
>>
>>Is there any input parameters i can change in the fieldtrip analysis
>>so that i can have the similar values, or is there any major  differences
>>in the methods
>>with fieldtrip and Welch periodogram method.
>
>There is a difference in scalp-current-density (laplacian) data and  EEG
>potential data, so I also expect coherence to be different. It is  better
>to report channel level CSD/laplacian coherence than channel- level
>ordinary potential difference, since SCD/laplacian is reference  free and
>potential is not. Note that you can also use FT to compute  coherence on
>SCD data, assuming that you can export the laplacian  data to a raw file. I
>don't know which software you used to compute  the SCD/laplacian and welsh
>coherence that you have in the figure,  therefore I also cannot comment on
>why the values are different. The  difference between the mtmfft and
>mtmwelsh can be explained (see  above) because the different frequency
>resolutions.
>
>best regards,
>Robert

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