Problems with fieldtrip toolbox

[Robert Oostenveld]

On 12 Feb 2009, at 17:12, Gatien Hocepied wrote:

> Dear all,
>
> I recently used your toolbox. In order to find the amplitude of  
> several dipoles, I ran different functions of the toolbox. However,  
> even the results seem to be correct for test signals, when I used  
> these functions for my 25-ELEC EEG signals (time window : 10 sec,  
> Fs : 250 Hz à 10000 samples), sometimes, even for only 2 dipoles,  
> the results are not the same  from one experiment to the other  
> (obviously the initial positions are random)…  I used regional non- 
> linear technique. Would you have some clues for me ?

Dear Gatien

The strength of the fitted dipoles depends strongly on the position.  
So if you see different dipole strength/moment, then you should check  
whether the positions are the same.

The position that is found after dipole fitting, and especially for  
multi-dipole models, can depend on the initial starting positions.  
I.e., the dipoles can end up in a local minimum of the error  
landscape, instead of the global minimum. To improve the robustness of  
the dipole fitting approach, you should start the nonlinear search  
with an initial location for your dipoles that is as close as possible  
to the optimal location. So if you have some prior knowledge about  
where your dipoles might be, then you should use that.


best regards,
Robert
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