[FieldTrip] Normalization of beamformer leadfields

[Marc Lalancette]

Normalizing columns of the leadfield separately is not recommended.  It is not a rotationally invariant operation, meaning you will get different results depending on your choice of coordinate system, which in short means that it introduces a physically meaningless bias, thus potentially amplitude and localization distortions.  Note that this is also true of the unit-noise-gain normalization formula for the vector beamformer of Sekihara (which may still be used in some software, but is not in Fieldtrip).

I was planning on writing a short paper on this, but unfortunately never found the time.  I had a poster at Biomag 2014.  Here's the link, but note that I later found errors in the computations for the "source bias and resolution figures" so it's probably best to ignore them, though the general idea that there are orientation and possibly location biases in most vector formulae is still valid http://dx.doi.org/10.6084/m9.figshare.1148970 .  Maybe I'll redo the figures and post a "corrected" version at some point.

Cheers,

Marc Lalancette
Lab Research Project Manager, Research MEG
The Hospital for Sick Children, 555 University Avenue, Toronto, ON, M5G 1X8
416-813-7654 x201535




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