howto convert ICA components to SSP projection operators.

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1

Before I potentially reinvent a wheel, I was wondering if there was a way
to compute an ssp projection operator (proj) from the excluded basis set of
an ICA operator.

The reason I'm trying to do this is that I have some data that has an
environmental artifact that is always around 4HZ but whose topology changes
slightly from subject to subject. ICA on a filtered dataset (3.5-4.5Hz)
makes it easy to ID the artifact components but the remaining ICA
components are not really applicable to the broadband signal so using
ica.apply() on the boradband signal doesn't really work for me.

Thanks
Luke
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2

Hi Luke,

we don't have such a function AFAIK and I see a use to it. My first
reaction was to tall you to use a high n_pca_components in ICA but
it's not fully equivalent to an SSP.

proj = make_ica_proj(ica, picks)

?

Alex

3

Luke,
I've created projectors "by hand" like this (caveat: my intent was just to
plot topomaps, not to actually apply the projector):

desc = '4Hz_artifact_ICA'
data = dict(data=my_component, nrow=1, ncol=len(my_component),
            row_names=desc, col_names=epochs.ch_names)
projector = mne.io.Projection(data=data, active=False, kind=1,
                              desc=desc, explained_var=None)
epochs.add_proj(projector)

I've never used it with ICA (only with DSS) but I think in principle it
could work the same way?
-- dan

Daniel McCloy
http://dan.mccloy.info/
Postdoctoral Research Associate
Institute for Learning and Brain Sciences
University of Washington

4

Thanks Dan and Alex.

How are multiple rows of the projector used? In SSP (PCA) the basis vectors
are orthogonal however ICA components may not be.

Would you suggest putting all of the excluded ica components into a single
projector or to build separate projectors for each excluded ICA component?

--Luke

5

hi,

it must be one component per projector. Projector data are 1d.

Alex