[FieldTrip] ICA on elekta data

[Robert Oostenveld]

Hi Craig,

I am CCing the FT mailing list, as there is a lot of expertise there as well (as well as interest in the answer probably).

You would for sure combine the gradiometers. They have their signals expressed in the same units (fieldtrength/distance). But you would probably also benefit from including the magnetometers, as they reflect the same sources that linearly mix on the channel level. However, the magnetometers are expressed in another unit (fieldstrength) which results in them being potentially weighted differently. E.g. imagine that you express distance in mm, then the planar signals are 1000 times smaller than if you express distance in meter, whereas the magnetometer signals (the 102 rows of the same data matrix) don't change. 

If you were to do PCA, it would make a large difference since the channel-level variances are hugely different. But I think that if you do ICA, the "sphering" step that is done prior to the unmixing matrix estimation takes care of the different scaling and it would not matter if you were to to it in meter or milimeter. As I don't have the hands-on experience, it is something I would explicitly check by doing the ICA decomposition twice on differently scaled gradiometer data (keeping the magnetometer rows in the data the same).

best regards,
Robert


 


On 30 Sep 2013, at 15:33, Richter, Craig wrote:

> Hi Robert,
> 
> How would you approach ICA on data from the Elekta system?  I have the 102 chips with 1 magnetometer, and two orthogonal planar gradiometers, as you know.  It seems to me that the magnetometers should be done separately, but how would you handle the gradiometers?  Do these need to be combined?
> 
> I wanted to see how you would proceed.
> 
> Best,
> 
> C.
>