[Eeglablist] Channel interpolation before ICA vs. channel interpolation following ICA

Tarik S Bel-Bahar tarikbelbahar at gmail.com
Tue Jul 25 16:25:31 PDT 2017

Hi Dan, some quick thoughts below, we should hear something from the
experts soon!

I think the current majorly held belief and recommendation from eeglab
experts is that one should drop bad channels before ICA so as to let
it do it's job better, giving it a better chance of focusing on neural
ICs. Other implementations, perhaps via amica or gift-eeg, might
handle or treat the data in special ways that obviate the need to do

Don't think I've not seen this compared empirically in published
papers. And it would be touch to collect and clearly compare results
across multiple extant papers using interpolation before or after ICA.
One could run simulations on some larger datasets. The field (and
eeglab) seems to be needing more data-based empirical consolidation on
a range of truisms (e.g., recent notes of the need for new
publications related to high/low density arrays, clinical vs. research
questions, the nature, if any, of ICA effects on phase metrics,

Dropping channels before ICA and not interpolating before ICA might
not allow for full/accurate representation of the spatial data.

My understanding is that the interpolation before ICA, however, is not
a good idea for ICA because it essentially "makes up" new channel
information. Perhaps biasing effects could depend on the method of

Interpolating channels before ICA does seem to work ok too often in my
experience, though I prefer the interpolate-after method.

I would say the biasing effects on reconstructed eeg data from which
only artifact ICs have been removed depend on how many channels have
been removed (many or few?). I also tend to use post-ICA interpolation
for such analyses.

Note that messages in eeglab are sometimes cryptic or late to be updated.

On Mon, Jul 24, 2017 at 10:55 AM, Daniel Roberts <drobertc at gmu.edu> wrote:
> A question that has frequently come up on this list is whether to perform
> bad channel interpolation before or after ICA. Of course if you interpolate
> prior to ICA it is necessary to indicate the reduced rank of the data to the
> ICA algorithm. In various threads it is recommended to interpolate before
> ICA, or alternatively, interpolate after ICA. I’m curious if anyone is aware
> of any empirical data on which method is preferable, or the potential
> detriments of one method or the other.
> I noticed that EEGLAB generates a warning / error when interpolating then
> average referencing following ICA: ““Error: some channels not used for ICA
> decomposition are used for rereferencing the ICA decomposition has been
> removed” which would seem to suggest interpolating prior to ICA if an
> average reference is required. However, this warning seems to only remove
> ICA decomposition but maintains the channel space data. So perhaps it is not
> an issue if ICA was used only for artifact removal and the rest of the
> analysis will be in channel space.
> Thanks for any thoughts on the issue,
> Dan
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