[FieldTrip] ICA on neuromag data - invalid assumption of full rank data & mixed sensors?

[Peter Goodin]

Hi Fieldtrip list,

Firstly I'd like to thank those who have asked questions using ICA on neuromag 306 data and the answers given. It's been extremely informative and taught me a lot about the general principles of ICA (it's no longer completely black magic, only partially...).

I have two and a bit questions about using ICA on neuromag data, the first on the full rank assumption. After maxfiltering, the output data is rank deficient due to removal of the b-out components. I see that fastica is able to detect this and makes adjustments to the amounts of components extracted accordingly, but is doing this justified? What actually happens to the data if ICA is applied to a rank deficient data set? Are there any papers that examine this?

Secondly, I've been including all the meg channels (mags + grads) when running ICA (due to my interpretation of the conversation in http://mailman.science.ru.nl/pipermail/fieldtrip/2012-April/005016.html) then examining the time series components for those related to artifact (specifically ECG and EOG), while using a magnetometer only layout to view component topographies as a backup. My question (well, really a confirmation) is, by removing the artifact components calculated using the mixed sensors, does this remove the associated components from all the sensor types? Viewing pre and post ICA data appears to confirm this, but expert opinion is always good. The bit of a question is - is it acceptable to use a magnetometer only layout to interpret topoplots of the components if the components are based on mixed sensors? I've been using it because the readily identifiable artifact components have a distribution I'd expect from ECG and EOG as does the associated time series but I'm wanting to make sure.

Also, if this is the wrong forum to ask these questions can someone suggest one that might be a better fit?

Thanks again,

Peter.

__________________________
Peter Goodin,
BSc (Hons), Ph.D Candidate.

Brain and Psychological Sciences Research Centre (BPsych)
Swinburne University,
Hawthorn, Vic, 3122

Monash Alfred Psychiatry Research Centre (MAPrc)
Level 1, Old Baker Building
Commercial Road
Melbourne, Vic, 3004
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