Beamformer of correlated sources

[jan-mathijs schoffelen]

Dear Cristiano,

Nice simulation in figure 1!
The way you did the second simulation is just the 'traditional' DICS,
giving you a huge mountain of coherence around the reference location.
This will drown any other true cortico-cortical coherence to the
right hemisphere dipole. (Note that a double dipole approach will not
be of too
much help here, if you do a ccc-analysis with respect to your
reference: this is an important point in the paper you refer to. The
double dipole approach
will hopefully be able to unveil low coherence of the second dipole
to the reference, which is masked by the high power + high coherence
(one of these,
or perhaps both) in the reference dipole.)

As such, the double dipole approach is not yet implemented in a
straightforward way in the release-version of fieldtrip, but I will
be happy to give you some handles:

I would suggest to include the leadfield of the reference location in
the grid with precomputed leadfields which will be passed in the
configuration to sourceanalysis.
You have to concatenate the dipole-location specific leadfields with
the reference dipole's leadfield. This will give you leadfields of
dimensionality Nx6 (or Nx2 if you
have a reason to assume a fixed orientation).
Subsequently you can run sourceanalysis with cfg.keepfilter = 'yes'
and without cfg.refdip, to extract the filter coefficients. These
will have the dimensionality 6xN for each
double dipole, the first 3 rows corresponding to the location
specific location, and the last 3 rows corresponding to the reference
dipole.
To then compute your 'dipole pair and reference channel'-based cross-
spectral density you have to sandwich the sensor-level cross-spectral
density (take care to have
the channel-order identical to the channel order in the filters)
between the augmented filters. You can augment the filters to
accommodate for the reference channel in
the following way: [filter zeros(size(filter,1),1);zeros(1,size
(filter,2)) 1]; (make sure to put the reference channel last in the
channel csd.
 From each sandwiched csd (7x7) you should be able to extract
anything you want. The top-left 6x6 block will contain the between
dipole csd (diagonal 3x3 blocks contain
the within dipole parameters from which power can be estimated: the
off-diagonal blocks contain the between dipole csd). The bottom right
value will contain the reference
signal power, and the 6x1 and 1x6 remaining vectors contain the
reference to dipole csds, in two groups of 3. The coherence can be
estimated (according to Joachim's 2001
PNAS-paper) in this case by: svd( csd(1:3,7))./ sqrt(svd(csd(1:3,1:3))
*csd(7,7));

Good luck,

Jan-Mathijs





On Aug 8, 2008, at 2:05 PM, Cristiano Micheli wrote:

> Dear all
> I have some questions about the use of DICS in the case of two
> correlated
> cortical activations.
> I am using an isometric contraction protocol and i localized the
> reference
> dipole through cortico-muscular coherence with external myography and
> (classic) DICS method. Now I have to calculate cortico-cortical
> activations
> and i use:
>
> cfg               = [];
> cfg.grid         = grid;
> cfg.method    = 'dics';
> cfg.projectnoise = 'yes';
> cfg.lambda    = 0;
> cfg.refdip   = ref_dipole_position;
> cfg.frequency = 20;
> cfg.hdmfile   = 'mymodel.hdm';
> sourcecond    = sourceanalysis(cfg, freq);
>
> According to Schoffelen et al. in order to disentangle the sources a
> modification of the classic DICS algorithm is necessary.
> Am i supposed to extract the two activations if they are correlated
> with the
> above written lines?
> If not, how would it be a FT implementation to get to correlated
> sources
> localization with DICS?
> I tried with simulation and apparently the above implemetation is
> able to
> localize only one source with a spread activation.
> Attachments show:
> '1sourceDICS.png' : simulation with an external reference channel
> correlated
> with a source in the left emisphere
> '2sourcesCCCDICS.png': simulation with reference dipole (same as
> above)
> whose activity is correlated to a simmetric one in the right emisphere
>
> Best
> 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.<1sourceDICS.png><2sourcesCCCDICS.png>

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