source localization neuromag 122 system.

[Hanneke van Dijk]

Dear all,

At the moment I am using the MNE toolbox and Fieldtrip to localize an alpha
source (1.5 seconds spontaneous data recorded with a Neuromag 122 system).

With data from the new 306 channel system it seems to work out well. However
with data from the old system the source ends up in the cerebellum (attached
a picture of Neural Activity index, cfg.lambda = '10%'). Of course it should
be in the parieto-occipital cortex or the sensorimotor cortex. I have the
feeling there is problem with the position of the gradiometers relative to
the headmodel or the other way around. The head model seems to be too far
away from the sensors (see attached headmodel1), if my hunch is correct I
don't know how to fix this or where exactly I can find the problem.

How I made the headmodel you can find on:
http://fieldtrip.fcdonders.nl/example/read_neuromag_mri_and_create_single-subject_grids_in_individual_head_space_that_are_all_aligned_in_mni_space

I'm constructing the grad files (to do sourceanalysis with) seperately using
read_header and mne2grad (with orig.chs(n).coil_type=2, because the
coil_type is 2 for the old system).

Another related question: in mne2grad on line 104 and 108 it says

   104 >> grad.pnt(kCoil,:)=100*(t(1:3,4)-0.008*t(1:3,1)); % multiply with
100 to get cm
   .....
   108 >> grad.pnt(kCoil,:)=100*(t(1:3,4)+0.008*t(1:3,1));

Where does this 0.008 come from? I have changed this number to try it out
but did't see any clear differences in the gradiometer positions.

Thanks in advance for your help,
Wishing you all the best,

Hanneke

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