[FieldTrip] code change potentially relevant for MEG people

[Stephan Moratti]

Hi Jan-Mathijs,

Happy new year! Thanks for the update! I think the MEG community  
appreciates a lot the Fieldtrip work you all do!

Best,

Stephan

El 06/01/2011, a las 9:43, jan-mathijs schoffelen escribió:

> 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
>
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> fieldtrip at donders.ru.nl
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________________________________________________________
Stephan Moratti, PhD

see also: http://web.me.com/smoratti/

Department of Basic Psychology
Universidad Complutense de Madrid

Centro de Tecnología Biomédica CBT,
Universidad Politécnica de Madrid,

en la actualidad (currently at) en el
Centro de Magnetoencefalografía Dr. Perez Modrego,
Universidad Complutense de Madrid,
Faculdad de Medicina,
Pabellón 8,
Avda. Complutense, s/n,
28040 Madrid,
Spain,

email: smoratti at psi.ucm.es
Tel.:    +34 91 394 2186
Fax.:   +34 91 394 2294

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