source localization neuromag 122 system.

[Joachim Gross]

Dear Hanneke,

I assume that t(1:3,4) is the center of the figure of 8 coil (the
crossing point). Adding or subtracting 0.008 along the correct
dimension seems to be necessary to arrive at the center of each of the
two loops that make up the planar gradiometer.

Joachim


On 10 Sep 2009, at 10:37, Hanneke van Dijk wrote:

> 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
>
>
> ----------------------------------
>
> The aim of this list is to facilitate the discussion between users
> of the FieldTrip toolbox, to share experiences and to discuss new
> ideas for MEG and EEG analysis.
>
> http://listserv.surfnet.nl/archives/fieldtrip.html
>
> http://www.ru.nl/fcdonders/fieldtrip/
>
> <headmodel1.jpeg><alphasource.jpeg>


----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
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