[FieldTrip] DICS: fixed orientation of sources option

jan-mathijs schoffelen jan.schoffelen at donders.ru.nl
Thu Nov 15 20:35:45 CET 2012

Hi Andrea,

If you specify fixedori = 'no', FT will not project the leadfield onto the orientation of maximum power, prior to computing the spatial filter, and this indeed will yield 'lambda-like' behaviour. Note that the essential difference is that with fixedori='yes' a scalar beamformer is computed, with fixedori='no' a vector beamformer is computed. With the powmethod option you can specify whether to take the 'trace', or the first singular value 'lambda1' of the vector beamformer output. As of yet there is no functionality that takes one of the two options depending on the 'singular value spectrum'. 



On Nov 15, 2012, at 5:28 PM, andrea brovelli wrote:

> Dear all,
> I have a question regarding the option "fixedori" of DICS (the code from lines 292 to 306 in beamformer_dics.m).
> If I get it right, you implemented the situation when the first singular value lambda1 is much greater that the second singular value lambda2 of Cs (Cs is given by eq. 4 of Gross et al., 2001, PNAS).
> I checked in my data and this is not the case. Maybe my data is just a very special case, but I find that the first is a little larger that the second. Here they are:
> lambda1 = 0.5380;
> lambda2 = 0.2921;
> lambda3 = 0.0000;
> After eq. 9 in Gross (2001), it says that if lambda1 >> lambda2 does not hold, the trace of Cs should be used instead.
> So, has it been implemented in Fieldtrip somewhere ? And, if not, would you know how to do it ?
> Thanks a lot
> bye
> Andrea
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Jan-Mathijs Schoffelen, MD PhD 

Donders Institute for Brain, Cognition and Behaviour, 
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands

Max Planck Institute for Psycholinguistics,
Nijmegen, The Netherlands

J.Schoffelen at donders.ru.nl
Telephone: +31-24-3614793

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