[Eeglablist] Singular matrix error for Laplacian plugin
josep.marco at starlab.es
Tue Mar 2 12:08:31 PST 2004
One explanation about Spherical Harmonic Expansion: this algorithm computes the laplacian interpolating firstly the data using Spherical Harmonics (and minimizing the total curvature) and then uses a direct relation to compute the laplacian (for more information see http://arxiv.org/abs/physics/0206007). Althoug it is true that the results don't resemble totally the typical scalp current densities (using for example, the Perrin-Pernier splines or the Hjort method) our preliminar studies suggest that in some situations the laplacian computed with the SHE method are more "acceptable" than the data obtained by the other methods. Some other studies reinforce this idea (see for example Lagerlund et al, Electroencephalogr Clin Neurophysiol. 1995 Sep;95(3):178-88)
We hope to build in brief a robust algorithm to compute both SHE and Perrin-Pernier laplacian (the one available has only been tested with our electrode configuration).
----- Original Message -----
From: Arnaud Delorme
To: Scott Laurence Fairhall
Cc: eeglablist at sccn.ucsd.edu
Sent: Tuesday, March 02, 2004 6:10 PM
Subject: Re: [Eeglablist] Singular matrix error for Laplacian plugin
I agree. This plugin is not stable (you are talking about the plugin available on this web page I guess http://184.108.40.206/fbos/projects/kolmogorov.html). I tried to use it and after intense debugging, I still had some conceptual problem (even when I used a high degree of spherical harmonic expansion, I obtained topographic plots that did not totally resemble the classical topoplots (they had some strong similarities and strong dissimilarities)), so there might still be a problem? I can send the edited version of the plugin to whoever is interested (I sent it to the author so he could update his web site with a functional version but he probably did not have time to do it).
Scott Laurence Fairhall wrote:
when I attempt to use the lap_perrin function contained within the EEGLAB laplacian
plugin I receive an error. When matlab attempts to inv(mat) where mat is (i think!) some
form of spline I get a message saying my matrix is badly scaled or singular (and
therefore has no inverse).
Does anyone have any suggestions about what it is in my electrode coordinates that
may cause this to arise?
Research Centre for Cognitive Neuroscience
Department of Psychology
University of Auckland
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Arnaud Delorme, Ph.D.
Computational Neurobiology Lab, Salk Institute
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