[FieldTrip] code change potentially relevant for MEG people

Stolk, A. a.stolk at fcdonders.ru.nl
Thu Jan 6 11:06:49 CET 2011

Hey JM,

Ik plot groepsgemiddelde aboslute powers (zie plotje) waarbij de voxel van interesse gekozen wordt aan de hand van significant verschillen. Dus blobs die gevonden zijn na indep T-testen op de subject level en dep T-test op groepslevel.

De prepare-single-shell algoritme is dezelfde voor elke conditie in elke proefpersoon. 

Ik struikel over deze zin 'comparing raw power is not correct'. Je bedoelt 'comparing raw powers computed with separate (old and new implementation) is not correct'? 


----- "jan-mathijs schoffelen" <jan.schoffelen at donders.ru.nl> schreef:

> Van: "jan-mathijs schoffelen" <jan.schoffelen at donders.ru.nl>
> Aan: "FieldTrip List" <fieldtrip at donders.ru.nl>
> Verzonden: Donderdag 6 januari 2011 09:43:47
> Onderwerp: [FieldTrip] code change potentially relevant for MEG people
> Dear all,
> Happy new year to all of you! I just made a change to the code which 
> may be relevant to some. This change will be present in tonight's  
> release version, and will be immediate for the in-house users at the 
> Donders Centre.
> If you are not doing MEG research you can stop here.
> If you never use FieldTrip to compute leadfields for MEG data you can 
> stop here.
> If you never use the 'singleshell' method for your MEG leadfield  
> computation, or if you don't know what that means, you can stop here.
> There appeared to be a huge difference in magnitude of MEG-leadfields 
> computed with FieldTrip using different algorithms. Particularly,  
> there was a big discrepancy between the 'singleshell' method on the  
> one hand, and the localspheres/singlesphere method on the other hand. 
> It turned out to be the case that there was a scaling factor in the  
> singleshell method, which (assuming the geometrical units were in cm) 
> would yield a magnetic field/gradient in ft or ft/cm. At the moment  
> FieldTrip does not everywhere explicitly impose particular physical  
> units on the data and therefore also should not make particular  
> assumptions. To make the three methods of leadfield computation more 
> equivalent we changed the implicit unit-assuming code.
> What does this all mean to you? Probably not an awful lot. Yet, you  
> need to be aware that by scaling the leadfields, the magnitude of the 
> inverse solution will probably also change (unless you use norm- 
> normalized leadfields). It is not correct to compare analysis results 
> using old-scale leadfields with analysis results using new-scale  
> leadfields, if the respective analysis results have not been  
> normalized in a meaningful way. In other words, comparing raw power is
> not correct, comparing neural activity indices, relative changes, t- 
> statistics etc. is probably alright.
> Sorry for the inconvenience,
> Best wishes
> Jan-Mathijs
> Dr. J.M. (Jan-Mathijs) Schoffelen
> Donders Institute for Brain, Cognition and Behaviour,
> Centre for Cognitive Neuroimaging,
> Radboud University Nijmegen, The Netherlands
> J.Schoffelen at donders.ru.nl
> Telephone: 0031-24-3614793
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
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