MEG realign

Mikhail Zvyagintsev MZvyagintsev at UKAACHEN.DE
Wed Oct 4 12:38:00 CEST 2006

Dear Robert / other fieldtrippers,
I have a question regarding MEG realign procedure. I actually found it 
very useful and trying to learn how to deal with it.
I am using 4 D Neuroimaging mashine (BTi), and, therefore, I had to 
change the routine a bit - at least change settings, concerning the 
sensors name. In my particular case I got the message 'mean distance 
towards template gradiometers is 25.45 mm'. (I actually attached a 
picture regarding sensors' position's difference derived from Matlab, 
but probably it is not so easy to recognize what is what on this picture 
- points are sensors and different colors - positions in different 
sessions, the view is from front).
And later in this routine I use the approach based on calculation of 
brain surface from headshape. As a headshape we use here points from 
bridge (of nose) to nape (of the neck).
I get 644 dipoles from it 'prunedinv' subfunction gives me 'pruning 49 
out of 148 singular values' (so, we use 148 channels mashine). After 
calculation of realign matrix and applying it for 'realigning trial' the 
residual variance (RV) which I get for one trial is arround 30%. I have 
a feelling that it is rather to much, or might be it's normal, becouse 
difference between sensors is rather big one.
Because in case 'mean distance towards template gradiometers' was 6.48 
mm - residual variances were arround 5-6%.
distance      RV
6.48 mm     5-6%
25.45 mm   ~30%.
My question is - is such RV difference correspondent to the difference 
distance I have and how many points should be like a 'control value' for 
'prunedinv' routine?
Thank you in advance,
Mikhail Zvyagintsev


Mikhail Zvyagintsev  MD, BSc
Klinik für Psychiatrie & Psychotherapie
UK Aachen
Pauwelsstr. 30
D-52074 Aachen
tel +49-0241-80 85580
FAX +49-0241-80 82401
MZvyagintsev at

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