[Eeglablist] CSD maps.

[Darren Weber]

http://www.bionic.fr/erpa/Erpa.html

links to Perrin's lab software.  Scroll down a bit and note that SCD plots
are given in units of mA/m^3 (milliAmps, not uV).  I take this as an
authoritative source for the units of SCD.  The m^3 comes from
multiplication of Laplacian (V/m^2) by conductivity (Siemens/m), giving
scalp current density (A/m^3).  Whether you use mA or uA, m or cm is not so
important, the point is that it is amps, not volts (and also m^3, while
Laplacian is m^2).

Best, Darren


----- Original Message ----- 
From: "Christopher Summerfield" <summerfd at paradox.psych.columbia.edu>
To: "Darren Weber" <darren.weber at radiology.ucsf.edu>
Cc: "Arnaud Delorme" <arno at salk.edu>; "eeglablist"
<eeglablist at sccn.ucsd.edu>
Sent: Wednesday, November 05, 2003 9:14 AM
Subject: Re: [Eeglablist] CSD maps.


> thanks very much darren.
>
> if my understanding of the link is correct, then if you want to use CSD
> (or SCD) maps to 'sharpen' your image - or to show activity which is most
> likely due to a cortical source - then you should multiply the result of
> the del2map function by -1 (correct me if I'm wrong)
>
> best
> chris
>
>
>
>
> Christopher Summerfield
> summerfd at psych.columbia.edu
>
> On Wed, 5 Nov 2003, Darren Weber wrote:
>
> >
> > The Laplacian is the inverted polarity of the potential.  I have had
> > problems understanding this, so I went through the process of better
> > understanding the mathematical physics involved.  My notes on this are
here:
> >
> > http://dnl.ucsf.edu/users/dweber/dweber_docs/eeg_scd.html
> >
> > This document has some matlab examples of how the potential polarity is
> > inverted for the Laplacian.  Maybe it will help.  I've been confused by
some
> > figures in the literature, please be careful to get the polarity and
units
> > of your figures correct.
> >
> > I expect the eeglab code is working correctly, in that it is giving the
> > Laplacian.  If my understanding is correct, we should multiply
> > by -conductivity to get the "scalp current density".  In this case, the
term
> > 'scalp current density' actually refers to the divergence of the current
> > density in the scalp.
> >
> > BTW, I guess I don't like the term "current source density" because the
> > surface Laplacian has very little to do with "sources" and "sinks" (the
> > application of Laplace's equation specifically assumes no sources or
sinks,
> > otherwise I think we should be considering Poison's equation).  I
prefer,
> > "scalp current density" but even this is not entirely satisfactory.
> >
> > I must confess that my understanding of the mathematical physics is not
> > perfect.  My qualifications on this topic are inadequate, so please
consider
> > these comments and the notes with reasonable scepticism.
> >
> > Best, Darren
> >
> > ----- Original Message -----
> > From: "Arnaud Delorme" <arno at salk.edu>
> > To: "Christopher Summerfield" <summerfd at paradox.psych.columbia.edu>
> > Cc: "eeglablist" <eeglablist at sccn.ucsd.edu>
> > Sent: Friday, October 31, 2003 5:27 PM
> > Subject: Re: [Eeglablist] CSD maps.
> >
> >
> > > >
> > > >
> > > >strange thing is, the code below plots a CSD map that looks exactly
like
> > > >the inverse of my scalp map.
> > > >
> > > What do you mean? Del2map uses the Matlab del2 function to compute the
> > > 2-D laplacian. I checked the code of del2map and there does not seem
to
> > > be any sign inversion.
> > >
> > > >is there a problem with the code?
> > > >
> > > >
> > > This is a good point. Unfortunatelly, there can always be a problem
with
> > > the code :-). If you have doubts and have the capacity to understand
the
> > > code, you should definetelly check that it does what you expect (if it
> > > doesn't, then contact us).
> > >
> > > Arno
> > > --
> > >
> > > *Arnaud Delorme, Ph.D.*
> > > Computational Neurobiology Lab, Salk Institute
> > > 10010 North Torrey Pines Road
> > > La Jolla, CA 92037 USA
> > >
> > > *Tel* : /(+1)-858-458-1927 ext 15/
> > > *Fax* : /(+1)-858-458-1847/
> > > *Web page *: www.sccn.ucsd.edu/~arno
<http://www.sccn.ucsd.edu/%7Earno>
> > >
> > >
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>