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

[Scott Makeig]

Makoto -

When you repeat the claim that EEG sources 'found' by ICA decomposition
must be at least several adjacent gyrii in size, you fail to ask, "How are
LFP signals across each of these gyrii synchronized across the dataset?"
Doesn't this require some physiological basis, and if so, what is it??

Scott Makeig

On Mon, Dec 11, 2023 at 11:17 AM Makoto Miyakoshi via eeglablist <
eeglablist at sccn.ucsd.edu> wrote:

> 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
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-- 
Scott Makeig, Research Scientist and Director, Swartz Center for
Computational Neuroscience, Institute for Neural Computation, University of
California San Diego, La Jolla CA 92093-0559, http://sccn.ucsd.edu/~scott