Changed sensors types after SSS

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1

Hi all,

I was planning to run a SSS on my MEG data. But before doing so, I’d like to understand the consequences for the subsequent (pre)-processing steps. In, particular I wonder about following passage in Jas et al. (2018):

It is important to highlight that after SSS, the magnetometer and gradiometer data are projected from a common lower dimensional SSS coordinate system that typically spans between 64 and 80 dimensions. As a result, both sensor types contain highly similar information, which also modifies the inter-channel correlation structure. This is the reason why MNE will treat them as a single sensor type in many of the analyses that follow.

I was wondering how are magnetometers and gradiometers treated as the same channel type? Are they a new “generic” type, or are now both called gradiometers, or magnetometers, respectively? From looking at the Raw.info['chs'] it seems like magnetometers have become a new type, but gradiometers haven’t.
Or do I need to actively take this into account at all when doing ICA, going into source space, etc, or will MNE handle things the correct way under the hood anyway ?

Thanks,

Eduard

2

They are still coded and called magnetometers and gradiometers. It’s just that they are no longer independent, and MNE will recognize that Maxwell filtering / SSS has been done and (should) treat them properly during SSP, ICA, covariance estimation, rank estimation, inverse imaging, etc.

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3

Ah, great! So as long as I use MNE functions to run analyses, I don’t need to take special care of SSS’ed data?

Thank you!

4

Yes, this is correct, MNE should always do the right thing automatically.

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