A few more thoughts on one- vs. two-dipole fitting: <div><br></div><div>A general two-equivalent-dipole model can always fit a given (IC or other) scalp map better than a one-dipole model, but in the case of a very-near dipolar source reflecting synchronous activity across a cortical patch, the second dipole fit may only reduce errors produced by the head model, etc... There may also be (incorrect) local minima in the 'error surface' for general two-dipole fits. </div>
<div><br></div><div>For symmetric two-dipole fitting, the chance of false local minima is low to none and the two-dipole solution might not reduce the residual error beyond the one-dipole model in some instances. However, there is a chance that the second dipole might just be used to correct for head model or numeric error, and not reflect physiological reality (cortical synchrony). </div>
<div><br></div><div>As Arno responded, in many instances an IC map with two bilaterally symmetric foci clearly calls for a symmetric two-dipole model fit. Other instances are less clear, however, and so far we have not worked on a automated test for deciding to fit one versus two dipoles for a given IC. One of the SCCN group and/or some other researchers using ICA may be interested in working on that.</div>
<div><br></div><div>-Scott Makeig</div><meta charset="utf-8"><div><br><div class="gmail_quote">On Sun, Jun 5, 2011 at 9:31 AM, Arnaud Delorme <span dir="ltr"><<a href="mailto:arno@ucsd.edu">arno@ucsd.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div style="word-wrap:break-word">Dear Igor,<div><br></div><div>we fit 95%-99% of the components using a single equivalent dipole. We only fit 2 symmetrical dipoles for bilateral occipital alpha ICA components which are easily recognizable. Note that a large number of components cannot be fitted well with a single dipole (or 2 bilateral dipoles). These include artifactual components, or components that do not account for much of the data variance and reflect noise in the EEG data or in the ICA algorithm optimization procedure.</div>
<div><br></div><div>Best regards,</div><div><br></div><div>Arno</div><div><br><div><div>On May 27, 2011, at 6:06 AM, Igor Riecansky wrote:</div><br><blockquote type="cite"><span style="border-collapse:separate;font-family:Helvetica;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;font-size:medium"><div lang="EN-US" link="blue" vlink="purple">
<div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"> </div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif">
Dear EEGLAB community,</div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"> </div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif">
I am starting to use DIPFIT2 to localize brain sources of PCA/ICA EEG components. I face a problem to decide how many dipoles to fit, i.e. one unilateral vs two bilateral symmetrical. Some papers simply always assume 2 symmetrical sources. However, if component's topography is highly asymmetric, this seems not to be justified. Furthermore, in case of a symmetric topography one could often reasonably assume both one unilateral or two bilateral sources both originating from brain structures close to the midline. Of course, solutions may substantially differ for one and two sources. What is you experience and practice? How do you solve this problem? Do you use any quantitative test? Residual variance is always lower when two sources are fitted compared to one source. Is there some rule to consider this reduction as significant, or can the reduction of residual variance be statistically tested?</div>
<div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"> </div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif">
I will greatly appreciate any feedback and help. Thanks.</div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"> </div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif">
</div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif">Igor</div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif">
</div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"><span style="font-size:9pt">-----------------------------</span></div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif">
<span style="font-size:9pt">Igor Riecansky, MD PhD</span></div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"><span style="font-size:9pt">SCAN-Unit</span></div>
<div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"><span style="font-size:9pt">Institute of Clinical, Biological and Differential Psychology</span></div>
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<span style="font-size:9pt">University of Vienna</span></div><div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"><span style="font-size:9pt">Liebiggasse 5, A-1010 Austria</span></div>
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<div style="margin-top:0in;margin-right:0in;margin-bottom:0.0001pt;margin-left:0in;font-size:11pt;font-family:Calibri, sans-serif"><span lang="DE" style="font-size:9pt">E-mail:<span> </span><a href="mailto:igor.riecansky@univie.ac.at" style="color:blue;text-decoration:underline" target="_blank">igor.riecansky@univie.ac.at</a></span></div>
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For digest mode, send an email with the subject "set digest mime" to <a href="mailto:eeglablist-request@sccn.ucsd.edu">eeglablist-request@sccn.ucsd.edu</a><br></blockquote></div><br><br clear="all"><br>-- <br>Scott Makeig, Research Scientist and Director, Swartz Center for Computational Neuroscience, Institute for Neural Computation & Adj. Prof. of Neurosciences, University of California San Diego, La Jolla CA 92093-0559, <a href="http://sccn.ucsd.edu/~scott" target="_blank">http://sccn.ucsd.edu/~scott</a><br>
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