[Simnibs-discuss] Differences in Efield depending on Processing

Axel Thielscher axelt at drcmr.dk
Wed Nov 2 17:21:31 CET 2022

Hi Joel,

a question for better understanding: When you set up the 2nd pathway, do 
you then model ALL electrodes of the EEG10-20 system, or only those 
which you actually need for the montage?

I am asking because adding many well-conducting electrodes to the model 
will slightly increase the amount of current shunted through the 
scalp(+electrodes). Think of it as covering many parts of the scalp with 
well-conductive paste. We never quantified the size of this effect, but 
it will cause the leadfield e-fields in the brain to be consistently lower.

Best regards,


On 19-10-2022 21:07, Joel Upston wrote:
> Hi there,
> I have noticed a systematic difference between the two different lines 
> of simulating the same currents, electrodes and underlying dataset. 
> One is through processing through the leadfield and using only the 
> electrodes and currents as given and the other is through only 
> modeling those electrodes and currents without the leadfield. Now the 
> 2^nd  pathway (no leadfield) is giving higher E_mag than the leadfield 
> version (~7-12%), the spatial maps look similar but just the intensity 
> differences. I think it has to do with the scaling that is done after 
> the solve by scaling the estimated current calibration error, which if 
> I understand correctly isn't done in the leadfield processing.  If 
> this is the case I am trying to establish which would be more accurate 
> to use in your view if I need a specific target value of (0.3 V/m for 
> example). I have attached an image of a case where this is shown with 
> the leadfield processed is on the left and the non-leadfield is on the 
> right. I have tried this on about 5 different datasets and the pattern 
> is consistent.
> Thanks,
> Joel
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Dr. Axel Thielscher
Professor of Neurophysics and Neuroimaging
Danish Research Center for Magnetic Resonance
Copenhagen University Hospital Hvidovre
DK-2650 Hvidovre, Denmark
Department of Health Technology
Technical University of Denmark
DK-2800 Kgs. Lyngby
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