Carlos - Filters learned by ICA are both reference free and data adaptive. This is because the information content and spatial sources are not altered by multiplication by a fixed (well-conditioned, dimension-preserving) matrix, which is what re-referencing involves. Both the IC time courses and equivalent dipole source locations should remain the same under re-referencing -- within numerical and statistical error. <br>
<br>I'm sure a laplacian filter plug-in for EEGLAB does exist. To my mind, the shortcoming of Laplacian filters, vis a vis ICA, is that they are not data adaptive. In particular, they cannot well model or represent a source whose equivalent dipole is not radially oriented (e.g., tangential sources located in cortical sulci).<br>
<br>Scott Makeig<br><br><div class="gmail_quote">On Thu, Oct 23, 2008 at 3:16 PM, carlos hamame <span dir="ltr"><<a href="mailto:comediante.x@gmail.com">comediante.x@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
I'm surprised not to see a free-reference option like the laplassian transform here.<br><br><div class="gmail_quote"><div><div></div><div class="Wj3C7c">On Thu, Oct 23, 2008 at 7:29 PM, Steve Luck <span dir="ltr"><<a href="mailto:sjluck@ucdavis.edu" target="_blank">sjluck@ucdavis.edu</a>></span> wrote:<br>
</div></div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div><div></div><div class="Wj3C7c"><div style=""><div>I'd like to make one last set of comments about the choice of the reference electrode:</div>
<div><br></div><div>1) Alex is right that there is no truly correct reference. It is always a matter of balancing the advantages and disadvantages of different reference montages.</div><div><br></div><div>2) He is also correct that there may be statistical power advantages to using the average reference.</div>
<div><br></div><div>3) The major advantage of mastoids (and earlobes, etc.) is that they are widely used in ERP research (although not in all subareas).</div><div><br></div><div>4) The most important thing is to recognize that your data are always influenced by your choice of reference. As long as you don't forget this (and make sure that others don't forget it as well), you will be fine.</div>
<div><br></div><div>5) For experiments with broad, dense, and uniform distributions of electrodes, the wisest choice is usually to look at the data _both_ with an average reference and with a mastoid (or earlobe, etc.) reference. That gives you the best of both worlds.</div>
<div><br></div><div>6) For experiments with a limited or nonuniform distribution of electrodes, avoid using the average reference (unless you are being extremely careful).</div><div><br></div><div>Steve Luck</div><div><br>
</div><blockquote type="cite"><blockquote type="cite"><span style="color: rgb(0, 0, 0);"><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>From: </b></span><span style="font-family: Helvetica; font-size: medium;">"Alexander J. Shackman" <<a href="mailto:shackman@wisc.edu" target="_blank">shackman@wisc.edu</a>><br>
</span></div><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>Date: </b></span><span style="font-family: Helvetica; font-size: medium;">October 21, 2008 12:22:47 PM PDT<br>
</span></div><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>To: </b></span><span style="font-family: Helvetica; font-size: medium;"><a href="mailto:eeglablist@sccn.ucsd.edu" target="_blank">eeglablist@sccn.ucsd.edu</a><br>
</span></div><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>Subject: </b></span><span style="font-family: Helvetica; font-size: medium;"><b>Re: [Eeglablist] ERP analyses and average referencing</b><br>
</span></div><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>Reply-To: </b></span><span style="font-family: Helvetica; font-size: medium;"><a href="mailto:ajshackman@gmail.com" target="_blank">ajshackman@gmail.com</a><br>
</span></div><br><br>Arno prefaced his comments by noting that "average referencing is always incorrect." But as Joe Dien notes in his excellent '98 paper, it would be equally appropriate to say that "mastoids montages are always incorrect" or "ALL referencing schemes are always incorrect."<div>
<br></div><div>Steve and Arno are correct in noting, as Dien did, that the topography and waveforms will differ across montages, making it difficult to compare average to the more commonly used (for ERPs, at least) mastoids montage. </div>
<div><br></div><div>But there are at least two other reasons, at least with high-density recordings, to consider using the average reference. 1) Dien suggests that the average reference, which is employed by both dipolar and distributed source modeling algorithms, potentially provides more insight into the underlying cerebral generators. 2) The average reference is likely to more psychometrically reliable (cf. S. Gudmundsson et al., Clinical Neurophys, 2007). </div>
<div><br></div><div>Alex Shackman<br><div><br></div><div><br></div><div><br><br><div class="gmail_quote">On Sun, Oct 19, 2008 at 2:29 PM, Steve Luck <span dir="ltr"><<a href="mailto:sjluck@ucdavis.edu" target="_blank">sjluck@ucdavis.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0px 0px 0px 0.8ex; padding-left: 1ex;"><div style="">
<div>I would like to echo and expand upon Arno's comments about average referencing. Under the most optimal conditions this can be perhaps a decent approximation (see Dien, 1998). However, under most conditions it is a poor and misleading approximation (and, as Arno pointed out, is is never completely correct). Your waveforms will look completely different depending on what electrodes you happen to be using (see Figure 2 and the related text in chapter 3 of An Introduction the Event-Related Potential Technique). As a result, your data may look quite different from the data of other researchers, even if they are also using the average of all sites as the reference (because they probably don't have exactly the same set of sites that you have).</div>
<div><br></div><div>So, what to do? Lately, my lab has been seeing the same sort of problem, with lots of muscle activity being picked up by mastoid reference electrodes. The best thing to do is to try to get subjects to sit in a more neutral position so that they do not need to contract the neck muscles to keep the head upright. However, if you already have this noise in your mastoid data, you can try referencing to scalp sites that are close to the mastoids (e.g., P9 and P10), which may have less muscle noise. Or, if you have a sufficiently dense array of electrodes, you could use the average of a small cluster around the mastoids on each side as the reference.</div>
<div><br></div><div>The most important thing is to realize that you are _always_ looking at the potential between two electrode sites (or groups of sites). There is no such thing as potential at a single site.</div><div>
<br></div><div>Steve Luck</div><div><br></div><blockquote type="cite"><span style="color: rgb(0, 0, 0);"><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>From: </b></span><span style="font-family: Helvetica; font-size: medium;">arno delorme <<a href="mailto:arno@ucsd.edu" target="_blank">arno@ucsd.edu</a>><br>
</span></div><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>Date: </b></span><span style="font-family: Helvetica; font-size: medium;">October 18, 2008 4:15:53 AM PDT<br>
</span></div><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>To: </b></span><span style="font-family: Helvetica; font-size: medium;">Yvonne Tran <<a href="mailto:Yvonne.Tran@uts.edu.au" target="_blank">Yvonne.Tran@uts.edu.au</a>><br>
</span></div><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>Cc: </b></span><span style="font-family: Helvetica; font-size: medium;"><a href="mailto:eeglablist@sccn.ucsd.edu" target="_blank">eeglablist@sccn.ucsd.edu</a><br>
</span></div><div style="margin: 0px;"><span style="font-family: Helvetica; font-size: medium;"><b>Subject: </b></span><span style="font-family: Helvetica; font-size: medium;"><b>Re: [Eeglablist] ERP analyses and average referencing</b><br>
</span></div><br><br>Dear Yvonne,<br><br>average referencing is always incorrect. The amount of current going in and out of the head is assumed to be 0. Using that properties, average referencing means that the average potential across all electrode is 0 at all times. However, you cannot expect that the electrode spatial distribution will be homogenous over the head (because first you cannot put any within the neck, and there is usually no electrode on the face etc...). It is generally assumed that the current flowing within the neck is negligible (because of high conductances).<br>
<br>As an answer to your question, if your electrode repartition is relatively homogenous on the scalp, then you may use average reference. Nevertheless, average reference will not make it easy to compare between montages.<br>
<br>Best regards,<br><br>Arno<br><br>On 17 sept. 08, at 04:20, Yvonne Tran wrote:<br><br><blockquote type="cite">Dear All<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">We are currently working with spinal cord injured participants and have recorded some oddball data. We have been using A1 and A2 mastoid for reference channels, however, with this particular group we are experiencing increased muscle tension in this region (which cannot be prevented, as some participants are unaware that they are tensing up), and therefore when the data are re-referenced the other EEG channels become flooded with muscle tension noise. This can be overcome when we re-reference using average referencing. My question is how many electrodes (evenly distributed around the scalp) will be ok for average referencing for ERP analyses? We have 26 EEG channels.<br>
</blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Any suggestions/opinions appreciated!<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Thank you<br></blockquote>
<blockquote type="cite">regards<br></blockquote><blockquote type="cite">Yvonne<br></blockquote><div><font color="#540000"><br></font></div></span></blockquote></div></blockquote></div></div></div></span></blockquote></blockquote>
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--------------------------------------------------------------------</div><div style="margin: 0px;">Steven J. Luck, Ph.D.</div><div style="margin: 0px;">
Professor</div><div style="margin: 0px;">Center for Mind & Brain and Department of Psychology</div><div style="margin: 0px;">
University of California, Davis</div><div style="margin: 0px;">267 Cousteau Place</div><div style="margin: 0px;">Davis, CA 95618</div>
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