Source coherence

[Robert Oostenveld]

Hi Masao,

I had a look at your data, and at the computations that you perfomed
for the analysis. Your frequencies of interest are 4.6 and 5Hz, and
your time windows are 10 seconds long (right?).

That means that you have an intrinsic frequency resolution of 0.1 Hz,
which is enough to separate the two frequencies. But looking at your
configuration, you seem to be using multitapering with a frequency
smoothing of 0.5 Hz. That means that you are using ~5 tapers and that
the two frequencies of interest will be smeared into each other. If you
want to use multitapering, you should decrease the cfg.tapsmofrq to
less than 0.4 (e.g. 0.2). But I doubt whether you should use frequency
smoothing here at all, since the frequencies are so low and only
separated by a small amount. Better not use multitapering: in
freqanalysis do cfg.taper='hanning' instead of the default 'dpss'.

Furthermore, I suggest that you also look at the coherence between all
MEG channels and the REF21 for the 46Hz and the REF11 for the 50Hz
(i.e. the reference channel with which there probably should not be any
coherence). You did not include those in your crsspctrm channel
combinations.

I started by looking at power. There seems to be quite some temporal
and low-occipital (neck?) power in both conditions, which might be
indicative of artifacts (although EMG typically peaks at much higher
frequencies). That is something that you might want to check.
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The topography of channel level coherence with REF22 looks nicely
dipolar for the 46Hz, but with REF21 it does not look so clear for the
50Hz. AT 50Hz, there seems to be a bilateral blob that also extends
into the right hemisphere.


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I have recomputed the dics source reconstructions (just to be sure that
they are done with the latest version). If I look at the 4.6 Hz, it
looks nice. If I look at the 5 Hz, there is a relatively high and very
sharp blob in the occipital midline, very close to the surface of the
scanned volume. Given it's spatial sharpness, I don't trust it too
much. But given the bilateral distribution of the coherence with REF21
in that condition, it also does not surprise me that a source at that
location could be relatively highly coherent.

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So, to me the outcome of the analysis looks more or less like I would
expect from this data. I hope that you can further improve your
analysis with the hints that I have given.

best regards,
Robert


P.S. I executed the following lines of matlab code:

load Freq_01_046
load Freq_01_050
load Source_01_046
load Source_01_050

frqdes46 = freqdescriptives([], freqDICS01_046);
frqdes50 = freqdescriptives([], freqDICS01_050);

cfg = [];
cfg.xparam = 'foi';
cfg.zparam = 'powspctrm';
figure; topoplotER(cfg, frqdes46);
figure; topoplotER(cfg, frqdes50);

cfg.zparam = 'cohspctrm';
cfg.cohrefchannel = 'REF22'
figure; topoplotER(cfg, frqdes46);
cfg.cohrefchannel = 'REF21'
figure; topoplotER(cfg, frqdes50);

cfg = [];
cfg = source01_046.cfg;
cfg = rmfield(cfg, 'hdmfile');
cfg.vol = source01_046.vol;
cfg.method = 'dics';

cfg.refchan = 'REF22';
source46 = sourceanalysis(cfg, freqDICS01_046);

cfg.refchan = 'REF21';
source50 = sourceanalysis(cfg, freqDICS01_050);

cfg = [];
cfg.funparameter = 'avg.coh';
figure;
sourceplot(cfg, source46)
figure;
sourceplot(cfg, source50)




On 19 May 2005, at 20:14, Matsuhashi, Masao (NIH/NINDS) wrote:

> Hi Robert,
>
> I am posting this message to the mailing list, with the original
> message
> added to the bottom.
>
> Thank you for your response.
>
> 1. Actually, the two frequencies are very close to each other. One is
> 5 Hz
> and the other is 4.6 (60/13) Hz. Sampling is 600/sec. I used 6000 trial
> length so the frequency resolution should be 0.1 Hz, but I am not sure
> because MTM decreases the frequency resolution in terms of W.
> The two reference signals are not independent, as they share the common
> carrier frequency of 60Hz, but when we calculated the coherence
> sensor-wise,
> they were separated relatively nicely.
>
> 2. I pushed the data to your ftp site. I hope it's working ok.
> The filename is DICS_Matsuhashi_20050519.zip, containing 4 files.
> Freq_01_050 and Source_01_050 are from condition 1, foi=5.0 Hz.
> Freq_01_046
> and Source_01_046 are from condition 2, foi=4.6 Hz.
>
> Regards,
> Masao