CTF275 coherence

[jan-mathijs schoffelen]

Dear Cristiano,

I don't expect that the exact specifications of your single sphere
(in particular the slight shift of the origin) will cause the
localization to fail totally. I would guess that the problems lie
more in the quality of the estimated cross-spectral density matrix.
In a previous reply I mentioned about the topography only giving an
indication about the correctness of the computation of the csd's.
Obviously, this only gives a hint about the EMG-MEG coherence.
The beamformer needs the csd between all MEG-sensor pairs. It could
be that this is badly estimated. On how many repetitions is it based
(trials x tapers)? Did you remove artifacts from the data? The trial
number and presence of artifacts might severely compromise the
quality of the csd-matrix.
Additionally, I am not sure which gradiometer specification you use.
 From your previous mails it seems that you generate it yourself.
However, fieldtrip should do it for you. If you are using your own
hack, is there any reason for it?

Yours,

JM

On Jun 26, 2008, at 12:42 PM, Cristiano Micheli wrote:

> Thank you for the feedback
> The coherence is ok. I attach the topography for coherence's peak.
> I took a
> head model not corresponding to the subject to test the algorithm
> quickly.
> This can be the matter. Anyway the single lead field should be
> influenced
> only by head model center, not by its radius, if i am not wrong.
> The radius
> defines only the grid i think. The particular case i consider has
> center (in
> head coordinates) [0.9 0 4.3] cm. Can these shifts from the nominal
> position
> [0 0 5] influence so much the result? Are there documents about it?
> About
> errors depending on the conductor position? I'm trying now to
> simulate it
> with surrogate data with corresponding gradiometers-head model
> relative
> position.
> I really would like to keep the topic active
>
> Sincerely
> Cristiano
>
>
> ----------------------------------
> 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.<field.png>

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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.