[Mne_analysis] SSP vs. ICA for Artifact Correction

Marijn van Vliet w.m.vanvliet at gmail.com
Fri Sep 9 02:41:46 EDT 2016
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Dear Matthew,

in my current experience, the differences between SSP and ICA are 
negligible, *when the methods are applied correctly*. The paper Tuomas 
linked contains two nice example of how not to apply SSP correctly.

(1) The authors didn't chance the default of removing 2 SSP components, 
where they should probably have used 1 in certain cases (EEG usually 
only has 1 EOG component). So it's no surprise they found that the 
signal was reduced as well as the noise. Take home message: always check 
that the number of components you remove is appropriate.

(2) The authors never mention checking whether the extracted EOG epochs 
were correct. It is likely that they used the automated methods provided 
by MNE-Python blindly and just went with it. This could be acceptable in 
other studies (as long as the data is 'good enough' for the intended 
purpose), but certainly not in a study with the explicit focus on 
comparing noise reduction methods! Take home message: always check the 
EOG epochs found with the 'create_eog_epochs' function. Some things in 
the data (excessive movements by the subject, concatenating raw files 
that cause 'jumps' in the signal) can really mess up the automated 
algorithm.


In conclusion, I recommend the ICA pipeline in MNE-Python because that 
method is easier to apply correctly :) Notably because it doesn't rely 
on EOG event detection and because it has an automated manner of 
selecting the number of components to remove (plus the components are 
not orthogonal, so removing a second component is safer). But even with 
the ICA pipeline: double check the results! At the very least, check 
whether the noise components marked by the automated method make sense.

regards,
Marijn.

On 09/08/2016 11:21 PM, Tuomas Puoliväli wrote:
> Dear Matthew,
>
> There is a recent study by Haumann and co-authors that compares ICA 
> with SSP for rejecting artefacts 
> (http://www.ncbi.nlm.nih.gov/pubmed/27524998). They conclude that ICA 
> performs better than SSP but one should be careful with ICA when 
> dealing with low SNR data.
>
> Best regards,
> Tuomas
>
>
>
> On 8 September 2016 at 21:42, Boggess, Matthew Jozsef 
> <MBOGGESS at mgh.harvard.edu <mailto:MBOGGESS at mgh.harvard.edu>> wrote:
>
>     Hello,
>
>     MNE provides both SSP and ICA methods for correcting artifacts
>     (eye blinks, saccades, and heartbeat). However, I haven't ever
>     seen these methods compared and was curious if anyone has any
>     reasons to prefer one method over the other? I remember seeing on
>     one of the MNE documentation pages a while back that ICA was
>     recommended over SSP, but I haven't been able to find this
>     statement anymore (perhaps it was removed?).  Was there a reason
>     behind that recommendation?
>
>     Thanks!
>     Matt
>
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