[Eeglablist] ICA on combined M/EEG

[Tatu Huovilainen]

Hello eeglab users,

I have a question regarding ICA on combined M/EEG data, and as John Iversen and Scott Makeig have written a chapter 'MEG/EEG Data Analysis Using EEGLAB' for the book Magnetoencephalography: From Signals to Dynamic Cortical Networks which deals with almost exactly the same sort of data, I feel this is the right place to ask.
The data I'm working with is around an hour of 60 channel unipolar EEG and 306 channel MEG (Elekta Neuromag, 204 planar gradiometer sensors and 102 magnetometer sensors) per subject.

As is explained in the above mentioned chapter, before ICA the data from different sensor types needs to be sphered individually, so in this case separately for EEG, gradiometer and magnetometer sensors. Things would go as in the chapter, but with Neuromag preprocessing the MEG data rank is reduced because of tSSS method (Taulu & Simola, 2006) and further because of continuous head movement correction so that after preprocessing the rank of the MEG part is around 70. So before ICA a PCA reduction is also necessary. So the fist question is at which order should these steps be applied? Do I sphere the gradiometer and magnetometer datas separately, concatenate them, do PCA (with the component count determined by rank()) and then concatenate the sphered EEG data and perform ICA? Secondly, how then are the projections to different sensors calculated from the IC-weights?

Best regards,
Tatu

Taulu, S. & Simola, J. (2006). Spatiotemporal signal space separation method for rejecting nearby interference in meg measurements. Physics in Medicine and Biology, 51,1759–1768.