[FieldTrip] megrealign across runs/sessions

Marc Recasens recasensmarc at gmail.com
Wed May 30 11:47:51 CEST 2012

Hi everyone,

I'm considering the possibility to append the MEG data (CTF-275) from 3
different runs (recorded within the same day but with different
headpositions in the dewar) into one single dataset. That is, combine my
datasets in the sensor-space.
I've been reading about the possibility to use the ft_megrealign function
in order reconstruct the magnetic fields onto a standard gradiometer
However, in the literature this is mainly used to average data across
subjects rather than across runs.

I did a test using the following code:
*cfg= [];*
*cfg.template{1} = run1.grad;*
*cfg.template{2} = run2.grad;*
*cfg.template{3} = run3.grad;*
*cfg.vol = vol;   %  single shell headmodel computed from individual MRI*
*cfg.inwardshift = 3;*
*cfg.verify = 'yes';*
*cfg.feedback = 'yes';*
*[interp1] = ft_megrealign(cfg, run1);  % trial-based data*
*[interp1] = ft_megrealign(cfg, run1);*
[interp1] = ft_megrealign(cfg, run1);

acording to the results (I show the highest RV), the difference between the
original and the realigned data seem really small (which I assume it's good)
*original -> template             RV 2.22 %*
*original             -> original RV 2.11 %*
*original -> template -> original RV 2.14 %*

I'm wondering whether anyone has experience in using ft_megrealign across
runs/sessions and can recommend it (any advise is welcomed).
According to Knosche (2002), the method seems good but I'd like to know
whether someone has test it in real-life situations (especially taking into
account the head position differences in the z axis)

Can affect the accuracy of the subsequent source reconstruction?

Marc Recasens
PhD Student
Universitat de Barcelona
Tel.: +34 639 24 15 98
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