[Eeglablist] CSD maps.

[Christopher Summerfield]

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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