[Eeglablist] ICA suggests weirdly low rank

[Евгений Машеров]

Average referenced channels are linearly dependent. Sum of channels equal to 0.  Matrix of values has incomplete rank n-1. 
Except this mathematically cause, may be physical causes - contact between electrodes or electrolyte bridges/

Eugen Masherov, Burdenko NSI


> Dear list members,
> 
> I am experiencing some issues running pop_runica, namely that it
> produces weirdly *low* rank.
> 
> I have a 27-channel infant dataset recorded with a single channel
> reference (Fz), for which I perform the following steps before running
> ICA: filter, remove bad channels, interpolate, re-reference to average
> reference and adding Fz back to the data, epochize (this does not reduce
> the amount of data by much, just truncates the continuous data), reject
> only extremely large artifacts.
> 
> I understand from previous discussions as well as the papers on this
> issue that my data becomes rank deficient by n if I interpolate n
> channels. (Is rank reduced further by -1 due to average re-ferencing
> even if the original reference is added back to the data?) When I run
> pop_runica on a 27 channel data set in which channels have been
> interpolated, however, eeglab returns a substantially lower rank, e.g.
> 22 when two channels have been interpolated (which should be 25; or 24
> if average referencing reduces futher).
> 
> From what I have read in the mailing list, most people experience the
> opposite, i.e. rank() failing to identify rank deficiency, but I haven't
> found anything on weirdly low rank. Does anyone have an idea what may be
> causing this?
> 
> Also, I am unsure whether I should "force" it to compute 25 ICs using
> the gui or the 'pca' option, could this be detremental to ICA results?
> 
> I would appreciate any ideas on this, also let me know if I failed to
> include any details on the data.
> 
> Thank you & best,
> 
> Ivonne
> 
> --
> 
> *Dr. rer. nat. Ivonne Weyers (she/her)*
> Psycholinguistics Group
> Institute of Linguistics
> University of Vienna
> Sensengasse 3A, Room 06.12
> 1090 Wien
> 
> ivonne.weyers at univie.ac.at
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