[Eeglablist] Why most of good 'brain' ICs are 'dipolar' with show 'red'-centerd scalp topos, although 2/3 of the cortex is in sulci?

[Makoto Miyakoshi]

Hello EEGLAB list,

For those who have wondered so, here are my answers.
I asked two questions:

(1) Why do good 'brain' ICs show dipolar scalp topos although 2/3 of the
cortex is in sulci?
(2) Why do these dipolar IC scalp topos show red (positive) centers?

The answer was published a few days ago.

https://urldefense.com/v3/__https://onlinelibrary.wiley.com/doi/10.1002/hbm.26540__;!!Mih3wA!FPOThEiX2hsD7TJBq7WyhlV8v6HSkTe_swsBEoB2RM-Bh-BGerduzZBnmEtDBamyosThbqv9Xrc1gGPSmdm52LpO7jM$ 

The answer to (1): It is because scalp-recorded EEG is insensitive to
sulcal sources compared with gyral sources. This finding justifies the use
of lissencephalic (i.e. no sulci) brain model proposed in Electric Fields
of the Brain (Nunez and Srinivasan, 2006) together with Spline Laplacian.
This also supports the view that the major source of scalp-recordable EEG
is pretty broad (minimum 6.45 cm^2) which requires a continuum of multiple
gyral crowns.

I did not write it in the paper, but the result basically refutes the claim
that ICA is a high-resolution EEG spatial filter because the result
confirms that ICA is mostly blind to 2/3 of the cortex. In fact, it seems
ICA results are always dominated by high-power, low-frequency, and very
broad sources. I will publish this view in the near future.

The answer to (2): It is because EEGLAB's ICA sets the initial topos of all
ICs red centered (i.e. positive dominant). Thus, unless necessary, the
algorithm does not flip the polarities.

Now you wonder--when does the ICA algorithm flip the polarity to produce
'blue' centered (i.e. negative dominant) ICs? I found that those
blue-centered ICs tend to show poor physiological validity with large index
numbers. A known clear exception for this rule is ICs localized for the
motor cortex.

People use ICA to clean EEG. I use EEG to glean ICA, which is more fun.

Makoto