[Eeglablist] Beyond good and evil of ICA

[Makoto Miyakoshi]

Dear List,

Recently there was a criticism against ICA on the list. Unfortunately it is
technically invalid so I remained unsatisfied. Let me share real problems
of the ICA model (Onton and Makeig 2006) to re-do it. This is a continued
discussion from the one titled 'How phase is calculated in linear
decomposition' and now this is my turn to criticize ICA!

As far as I know, there are three known limitations in ICA model.

   1. Spatial stationarity. I have seen a nice traveling waves in ECoG grid
   data during Joaquin Repela's presentation at SCCN. This clearly violates
   the assumptions of spatial stationarity in ICA.
   2. Temporal stationarity. Shawn Hsu at SCCN presented time-series data
   of ICA model likelihood during drowsy driving task. Also, Jason Palmer's
   AMICA also demonstrated temporal changes in model likelihood. So one model
   per data does not fit the truth (unless the task has a strong control over
   a subject's cognitive and behavioral states).
   3. Dipolar source model. Although most of ICA results are fit with
   dipole models, it seems ICA also returns (probably) non-point sources. When
   one fits a dipole model to such a non-point source, the location tend to
   end up with physiologically invalid depth (this is the most annoying thing
   about ICA today)

I'd like to hear detailed criticism about these points. Note I saw these
critical counterevidence in SCCN; we are not a boring ICA cult who have
blind belief in it.

Nonetheless, ICA model has a critical merit. I named it
*Independence-Dipolarity
Identity (I-D Identity, or IDId)*. I-D Identity means that when ICA solves
temporally problem, it also solves spatial problem at the same time *without
using ANY spatial constraint*. Dipolarity can be thought of, in short,
*biophysical
origin-ness*. Hence I believe that this is evidence that ICA hits
*some *physiological
truth of EEG generation.

There could be multiple criticisms against the limitations of ICA model,
but at the same time any criticism, at least so far, was NOT strong enough
to deny *I-D Identity *of the ICA model*. *After all, because of this *I-D
Identity*, I still advocate ICA (but similar dipolarity can be achieved by
using very different approach, such as SOBI... so independence is not the
only requirement to reach the biophysical validity. It's still a mystery to
me.)

All models are wrong, but some are useful... but I want to go beyond this
statement to reach the ground truth of EEG!

-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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