[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]

Yes, this electrical stimulation approach was attempted by Don Tucker's EGI
group for many years - with results that were not encouraging, the problem
being that almost all the injected current flows through the scalp, whose
local and time-varying conductivity also then has significant effect... The
SCALE approach treats the independent component signals compatible with an
effective source in cortex (having a strongly dipolar scalp projection with
equivalent dipole located in brain) as cortical stimulations, and
iteratively finds the (single) skull conductivity value that
minimizes reconstruction error of the brain sources to 'sparse, compact,
and smoothly' varying distribution (sometime, two bilaterally
near-symmetric distributions) on the imaged cortical surface - using an
electrical forward problem head model constructed from an individual MR
image - the SCS source inversion algorithm of Cao Cheng. Our next step
should be to build a multidimensional skull conductivity map. Again, we are
attempting to put the SCALE software on NSG (https://urldefense.com/v3/__http://www.nsgportal.org__;!!Mih3wA!BNS_5aZCEDYflkuo2qfpDZgsY0Z-xv87AU3QM1QiNdQklstxqDvGifU6oaUANNcbLSF18LJVuTtkYo2KIrZI$ ) for free
use.

Scott Makeig


On Sun, Jan 7, 2024 at 12:04 PM Евгений Машеров <emasherov at yandex.ru> wrote:

> 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 resistance 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
>
> > Equivalent dipole *depth* is the least well estimated parameter in
> > equivalent dipole (or any other source) estimation, and the one that has
> > the most effect of source estimation. The reason is that skull
> conductivity
> > is currently only given a template value in EEG inverse software, whereas
> > our results suggest it has a range of at least 3:1 across adults - and
> more
> > so in infants, of course. (Tucker's group claimed a possible range of
> > 12:1). Brain-to-skull conductivity ratio ranges in our estimation from
> > ~20:1 to ~70:1 or more. The effects of this uncertainty, as shown clearly
> > in this paper
> > <
> https://urldefense.com/v3/__https://link.springer.com/article/10.1007/s10548-012-0274-6/fulltext.html__;!!Mih3wA!EUn9yGvTSgBO0pIbsPZlgPbP0VXD580VIaATFdFlw8LqObtyVUVFIZ1Tt3y_hzIgfUidyT1YFg1VH75zr671$
> >,
> > are to alter the implied *depth* of the source (by as much as 2 cm). The
> > only noninvasive method for estimating individual skull conductivity
> > without using EEG/MEG or a particular stimulation sequence is the SCALE
> > algorithm, more recently improved
> > <https://sccn.ucsd.edu/~scott/pdf/AkalinAcar_BIBE2020_final.pdf>. We are
> > now attempting to make this method freely available through the
> > Neuroscience Gateway <
> https://urldefense.com/v3/__https://www.nsgportal.org__;!!Mih3wA!EUn9yGvTSgBO0pIbsPZlgPbP0VXD580VIaATFdFlw8LqObtyVUVFIZ1Tt3y_hzIgfUidyT1YFg1VH1rzF2S8$
> >. Meanwhile, think of the
> > zone of uncertainty surrounding an estimated equivalent dipole as shaped
> > like a (large) grain of rice (pointed outward.).
> >
> > Scott Makeig
> >
>


-- 
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