ft_appenddata and denoise_pca causing problems

[Luisa Frei]

>
> Hi,
> I'm sorry if this has been discussed before and if it has, could
> you please direct me to the relevant emails? Thanks.
>
> I am encountering a problem when using ft_appenddata and
> denoise_pca. My MEG experiment  (4D) consists of 10 blocks per
> session, for each of which I have a separate data set.
>
> During preprocessing (after artifact rejection), I concatenate
> these data sets to find denoising weights per session using the
> function denoise_pca for 4D (I do this on shorter 400 ms epochs to
> save computational power). Afterwards, I apply these weights per
> block, downsample the data and concatenate the resulting data again
> in order to do an ICA per session to remove heart beat artifacts. I
> downsample because I use longer epochs to do that (1.5 sec).
>
> However, when I do an ICA on the denoised and concatenated session,
> I get lots of high variance noise components (first figure):
>

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>
> Trying to find the root of this, I went back and did this analysis
> for one block only (denoising for one block, no concatenating) and
> the result looks much better (second figure):

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>
>
> Then, as a test, I tried to concatenate two blocks, find the
> weights for those two concatenated blocks, apply them to the
> individual blocks, concatenate again and do the ICA on these two
> blocks and I get this (last figure):

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>
>
> So, it seems like when denoising per session (the concatenated
> blocks), I introduce noise when I apply the pca weights to the
> individual blocks. Whereas the whole point of the exercise was to
> reduce noise by denoising per session rather than block.
>
> Why is this? Am I doing something fundamentally wrong? I'm
> wondering because a colleague of mine has done an almost identical
> analysis step a while ago and she doesn't get problems with high
> variance noise components. I'm not sure whether the problem lies
> with append_data or denoise-pca, but I know that append_data has
> been changed recently, so this might be one possible source of the
> problem.
>
> I'm relatively new to MEG analysis, so I would be grateful for any
> pointers.
>
> Thanks,
> Luisa
>
> PS: I know that the different head positions between blocks can
> introduce noise, but when comparing the error introduced by head
> movements and the error introduced by correcting for head
> movements, the error was comparable, so correcting the head
> positions doesn't seem worth the effort.
>
>
>
>
>
>


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and to discuss  new ideas for MEG and EEG analysis.
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and http://www.ru.nl/neuroimaging/fieldtrip.
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