CTF275 coherence

[jan-mathijs schoffelen]

Dear Cristiano,

I took the liberty to change the subject of your posting into a more
appropriate one.
Did you have a look at the topography of the coherence on the scalp,
using fieldtrip.
This is a first check I would do, to get an indication that at least
the other important ingredient
to the beamformer (apart from the leadfields), i.e. the csd, has been
computed more or less
ok.
Two additional points: you mention something about the coregistration
with the MRI. It is of
course extremely important that your volume conductor model is
aligned with the MRI.
Checking the topography of the scalp level CMC using CTF's software
does not guarantee this.
Second point: spatial filters are biased towards the depth and
therefore people often use
quantities like pseudo-T/F/Z, neural activity indices, constrast
between conditions, or normalised
leadfields in order to make sense out of the volumetric images.
However, coherence is normalised
for the power by definition, so I would not expect a problem per se
in the visualisation.

Yours,
Jan-Mathijs


On Jun 25, 2008, at 12:26 PM, Cristiano Micheli wrote:

> Hi All
> I checked the tutorials:
> "Localizing oscillatory sources using beamformer techniques" and
> "Analysis
> of corticomuscular coherence" and it seems that cortico-muscular
> coherence
> calculations and dics have to do with sourceanalysis.m routine.
> I managed to have it working after having calculated frequency
> power spectra
> and cross-spectral densities as inputs for this function.
> After i select the other inputs (head model, gradiometer's positions,
> lambda) i have to set a frequency upon which to let the dics run (i
> selected, like in the examples, the peak of maximal coherence
> spectra).
> The result of the run is a 3D map of coherence or power activities
> for every
> point inside the head grid and i used a scatter3.m to visualize it,
> not to
> mess up with the co-registration of MRI images (the next step i
> will speak
> about).
> I compared the result with a coherence topography calculated with CTF
> software and i get an activation in the controlateral area with a
> clear
> dipolar shape.
> Unfortunately i do not see an equivalent in the same position for
> the DICS
> output.
> Reading the documentation i realised that it can be because of a
> biasing of
> the spatial filter toward the center of the sphere. I recalculated
> the dics
> with a baseline condition and after all i subtracted it from the task
> activation, still the result is far away from being a localized source
> activity, according to me. I also tried with different lambda but
> the result
> is not localized.
> How can i fix it?
>
> Thanks in advance for the help
> 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.

----------------------------------
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.