[Eeglablist] dipfit uses average reference?

[Dien, Joseph]

Although other references like mean mastoid have the advantages of being easy to derive and being readily compared across studies, they make no sense from the biophysical perspective.  The problem is that they arbitrarily define the reference point as being zero voltage when in fact there is no inactive site on the head.  The result is potentially severe warping of the solution.  So all source localization algorithms that I am aware of use average reference instead, which has the advantage of being more plausible from a biophysical perspective.  The major weakness of average reference for source solutions is that it requires a complete sampling of the voltages over the head to work perfectly and, of course, we normally do not sample the underside of the head.  I believe that source solution algorithms tend to use an approach comparable to Junghofer's PARE correction which uses interpolation to estimate the voltages on the bottom of the head.  Insofar as source packages like BESA have been reported to produce plausible results even with vertically oriented ERP components like the auditory N1, it appears that these estimates work well enough, although more needs to be done on this topic.  You can read my 1998 paper for more about average reference issues and how it compares to other referencing schemes.  Regarding "tends to highlight shallow sources over deep sources", you're thinking of current source density (CSD) which is something entirely different.

Anyway, bottom line is that I would not recommend trying to disable the average reference solution in Dipfit.

Joe Dien
University of Kansas


-----Original Message-----
From: eeglablist-bounces at sccn.ucsd.edu on behalf of arno
Sent: Wed 11/22/2006 7:18 PM
To: David Groppe
Cc: eeglablist at sccn.ucsd.edu; Robert Oostenveld
Subject: Re: [Eeglablist] dipfit uses average reference?
 
David Groppe wrote:
> I'm using Dipfit 2 to localize independent components using the spherical 
> head model.  Apparently the software requires the data to use the average 
> reference.  Why is this?
Source localization assumes that the data is average reference (I think 
it is because no current should get in or out). I do not think it is 
really an option not to use average reference. Robert might have more 
insight about that.

Arno
> From what I understand, the average reference 
> tends to highlight shallow sources over deep sources (e.g., medial 
> temporal), as deep sources have broad, relatively uniform scalp 
> topographies.  If we're interested in possibly extracting deep sources, is 
> there a way we can get Dipfit 2 to NOT use the average reference?
>    much thanks,
>      -David Groppe
>
> dgroppe at cogsci.ucsd.edu
>   
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