[FieldTrip] Combined EEG and MEG source reconstruction

Robert Oostenveld r.oostenveld at donders.ru.nl
Fri Feb 8 14:36:53 CET 2013

Dear Mushfa

In principle it can be made to work, but you'll have to do a little bit of  work outside fieldtrip. You would start with
1) ft_prepare_headmodel and ft_prepare_leadfield for the MEG part
2) ft_prepare_headmodel and ft_prepare_leadfield for the EEG part
Subsequently you would (manually) concatenate the leadfields of the MEG and EEG. Here you have to make sure that the channel order is consistent with that in your combined data.
The resulting structure with combined leadfields can be passed into ft_sourceanalysis as cfg.grid.

However, I foresee a problem with the scaling. At this moment FieldTrip is not (yet) very precise in maintaining the correct units throughout. Consequently you might get different units for the EEG and MEG contribution to each dipole, causing weird cancelation effects. You'll just have to check carefully that the leadfield units and the data units consistently scaled between megmag, megplanar and eeg. 

This brings me to another known problem for the combined vectorview magnetometer and planar gradiometer channels: at this moment the forward computation computes both in T (or fT), whereas the data is represented as T for the magnetometers and T/m for the gradiometers. So the gradiometer leadfields are off by a factor "1/baseline", which is approximately 1/70 if I recall correctly. Many neuromag users use only the planar channels, and therefore don't notice. We are working on fixing the neuromag channel/gradiometer units. That is not as simple as it appears, because conversions of the geometrical dimensions from m to cm or mm also affect the gradiometer baseline. So converting the data from m to mm should result in the planar channel data getting 1000 times smaller, whereas the magnetometer channels shoudl stay the same. See http://bugzilla.fcdonders.nl/show_bug.cgi?id=963 and http://bugzilla.fcdonders.nl/show_bug.cgi?id=963 if you want to follow this.

best regards

On 28 Jan 2013, at 20:20, Mushfa Yousuf wrote:

> Hello Fieldtrippers,
> I am currently doing source analysis using beamformer method in fieldtrip.
> The data i work on is a simultaneous measurement of MEG (306) with EEG
> (128) on PD patients.
> I was successful in calculating the sources for both the modalities alone.
> I tried according the tutorial "Combined EEG and MEG source reconstruction" and was successful till the estimation
> of the lead field. But, then when i want to use the created vol for the
> source analysis i have two structures one for EEG and the second one for
> MEG. Now, i can only define only one to estimate the sources. How to use
> this for both the modalities? Any help would be appreciated.
> With regards
> Mushfa
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip

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