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

[Евгений Машеров]

Hi Makoto,

Thanks for the valuable ideas.
I try to take into account that the field of a collection of sources can be very different from the field of a single source. In a 2019 article, I try to explain the peak recorded on the scalp by summing up action potentials, although a single action potential is not detectable at such a distance.
On the other hand, a dipole can be approximated by a pair of monopoles at a short distance, and at a recording distance greater than the distance between the monopole sources, it is almost impossible to distinguish their field from the dipole field.
That is, we have an ambiguity in the representation of the field on the scalp by the system of sources.
Purely mathematically, there are advantages to representing EEG sources as a system of monopoles. First of all, this is that a monopole is one parameter, and a dipole with fixed coordinates is three parameters. Then, the decrease in the monopole potential is not so rapid compared to the dipole, so there is no need to introduce such a strong correction so that the found sources are not concentrated on the convexital surface. Of course, mathematical convenience is insignificant compared to physiological validity, but I still hope, despite solid arguments against, to show the presence of real, and not just calculated, monopole sources.
Regarding the question of dipoles that are placed by the program in the white matter or in the cerebrospinal fluid inside the ventricles, I see at least three possible reasons for this.
The first is that the accuracy of calculating dipole coordinates is not absolute; the error can be quite high, especially if the number of EEG channels is insufficient. At the same time, increasing the number of channels does not lead to the desired increase in accuracy due to signal shunting by the scalp and meninges. In addition, the idea of matter inside the brain as homogeneous and isotropic is extremely simplified. The resistance of different structures can vary by an order of magnitude and depend on the direction of current flow. As a result, the dipole is shown not in the place where it actually is, but where it is unrealistic to expect it.
The second is that inhomogeneities in the conductivity of brain tissue can form a fictitious dipole. The third is that the sources may not be dipoles, but since we can only look for dipoles, then dipoles are found, but they are shifted.

Eugen

> Hi Eugen,
> 
> I'm not a super specialist on this topic, but let me share my experience
> with you as a possible alternative explanation.
> The pitfall may not be in excluding monopole and higher-order ones in the
> multipole expansion from the picture. Basically I agree with Ramesh that in
> the case of scalp EEG recording, the contribution of monopolar source must
> be zero in theory. Again, I'm not a specialist here, but at least the
> current consensus is like that.
> 
> Instead, I want to draw your attention to another more humanly problem.
> A dipole sheet, which is defined as a massive number of dipoles aligned in
> parallel in a 2-D plane, has counterintuitively different spatial
> projection profile. I call it a transducer array effect. You don't need any
> complicated phase modulation controlled over the distribution. Just a
> simple array placements on a plane can let it project the electric fields
> much further. You would think as if a monopole would be contributing on top
> of a modeled dipole. See the following data.
> https://sccn.ucsd.edu/wiki/Makoto's_preprocessing_pipeline#Physiologically_invalid_deep_dipoles.3F_.28For_130.2C000_page_views.2C_07.2F26.2F2021_Update.29
> https://sccn.ucsd.edu/mediawiki/images/c/cb/SupplementaryFiguresForSimuUDL_BSCR80.pdf
> I found this problem when I asked why my single dipole estimation almost
> always (1 out of 4 estimates!) goes to subcortical locations, which makes
> anatomically no sense. I summarized my opinion here
> https://urldefense.com/v3/__https://onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1111*2Fejn.15131&file=ejn15131-sup-0001-SupInfo.docx__;JQ!!Mih3wA!G4zHbSFIhBoPBMHAap-1JemDZ2t7VObfAPTWa1BFqhWZM92642zQ5lxjborLV-gWU3_MYZXf2aiArM7JlEqkiTWngxw$
> 
> See the section titled 'What does it mean to have dipole sources in ‘Upper
> Basal’?. 4 <#_Toc45876597>'
> 
> From the hindsight, I think it is very misleading, at least psychologically
> (I'm a psychologist), to use a single dipole to model a dipole sheet with a
> substantial area. It seems this 'pitfall' is in Riera et al. (2012),
> otherwise how can you conclude that monopole contribution is present in the
> scalp EEG recording? But again, I'm not a specialist here. I'm open to your
> criticism.
> 
> Makoto
>