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

[Евгений Машеров]

Could this be a frequency-dependent effect? The conductivity of bones has a significant capacitive component, so the total resistance decreases with increasing frequency.

Eugen Masherov

> Hi Scott,
> 
> This is a very interesting discussion to follow.
> As a comment, we found the breach effect to be negligible when recording high frequency oscillations (HFO > 80 Hz) in the scalp EEG of children.
> The bone defect after epilepsy surgery did not interfere with interpretation of scalp EEG signals in the HFO range.
> https://urldefense.com/v3/__https://www.nature.com/articles/s41598-022-05373-x__;!!Mih3wA!B8Mwi5MMu5aaExAXKUu2fCBUj9bHWbMesKUPcAGEMrVGXUsbTOCCWc-mFQQCeNHwKrFR6SIcRMTtB7J1Vc8RtZAKQrPM_uLtXA$
> 
> Best,
> Johannes Sarnthein
> 
> -----Ursprüngliche Nachricht-----
> Von: eeglablist <eeglablist-bounces at sccn.ucsd.edu> Im Auftrag von ??????? ??????? via eeglablist
> Gesendet: Sonntag, 7. Januar 2024 18:04
> An: smakeig at gmail.com
> Cc: eeglablist at sccn.ucsd.edu
> Betreff: [EXTERN] Re: [Eeglablist] Why most of good 'brain' ICs are 'dipolar' with show 'red'-centerd scalp topos, although 2/3 of the cortex is in sulci?
> 
> Yes, this problem seems to me important and unresolved. Even for an adult with an intact skull, the conductivity of different areas is very different, and an injury or surgical wound changes the EEG picture very strongly (breach effect). Babies generally have open areas in scull (fontanel). A possible solution would be to measure impedance similar to that used to control the quality of electrode placement, but the measurement would be between all pairs of electrodes, 171 pairs for 10-20, or 210 pairs if ear electrodes are included. The current passes through the electrode-skin contact resistance, then branches into a current flowing through the scalp skin and a current passing through the skull, then through the brain tissue, again through the skull and connecting to the first branch of the current, through the contact resistance of the second electrode with the skin. If we assume that the scalp skin has approximately the same thickness and conductivity, we can calculate the resistan ce of the skin area between the two electrodes purely geometrically to within an unknown coefficient. Another assumption is that the brain tissue is homogeneous, and the resistance to current flow through the brain for a selected pair of electrodes can also be calculated to within an unknown factor.
> 
> i,j - electode numbers
> R(i,j) - measured resistance between ith and jth electrodes
> Res(i) - resistance of electrode-skin contact for ith electrode
> Rb(i) - resistance of skull bone under ith electrode Qs - quotients for skin resistance
> L(i,j) - geometric parameter for computation of skin resistance between points i and j, rs=Qs*L(i,j) Qt - quotients for brain tissue resistance
> V(i,j) - geometric parameter for computation of brain tissue resistance between points i and j, rt=Qt*V(i,j)
> 
> R(i,j)=Res(i)+1/(1/(Qs*L(i,j))+1/(Rb(i)+Qt*V(i,j)+Rb(j))+Res(j)
> 
> R(i,j) measured,
> Res(i), Rb(i), Qs, Qt - estimated,
> L(i,j), V(i,j) - precomputed (finite elements method or other
> 
> That is, we have 171 measurements to estimate 2*19+2=40 parameters (or 210 for 2*21+2=44 parameters), which makes the problem mathematically correct. But how correct are the assumptions regarding the conductivity of skin and brain tissue? Technically, this looks feasible, to some extent similar to an impedance tomograph, but, as far as I know, impedance tomography of the brain has not been brought to practical use.
> Some information could also be obtained by comparing the distribution of potential on the scalp caused by a source at a known location with a potential calculated assuming equal conductivity of the skull and meninges. The corneo-retinal potential of the eye can be used as a non-invasive source. Perhaps, by closing the eyes one at a time and asking the subject to look up, down and to the sides, it will be possible to assess the influence of inhomogeneities on the propagation of current. There will likely be simultaneous movements of the other eye, so two dipoles must be taken into account, but if the eye is closed the amplitude will be lower. Of course, the idea is somewhat fantastic, as is the use of the heart's electric field for such sensing, but at least it is completely non-invasive.
> 
> Thanks
> 
> Eugen Masherov
>