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

[Scott Makeig]

Paul Nunez wrote that capacitive effects don't really apply until the mHz
range, not the EEG-relevant <1 kHz... The skull is not a single bone layer,
of course. I have seen models attempting to separately model its 3 layers
(including the central spongiform) - but other analyses concluding that a
single-layer lumped model should be adequate.

Scott

On Tue, Jan 9, 2024 at 12:26 AM Евгений Машеров <emasherov at yandex.ru> wrote:

> 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
> >
>


-- 
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