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">sjluck@ucdavis.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"><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-top:0px;margin-right:0px;margin-bottom:0px;margin-left: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-top:0px;margin-right:0px;margin-bottom:0px;margin-left: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-top:0px;margin-right:0px;margin-bottom:0px;margin-left: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-top:0px;margin-right:0px;margin-bottom:0px;margin-left: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-top:0px;margin-right:0px;margin-bottom:0px;margin-left: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="#006312"><br></font></div></span></blockquote><div> <span style="border-collapse:separate;border-spacing:0px 0px;color:rgb(0, 0, 0);font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:auto;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><div style="word-wrap:break-word">
<span style="border-collapse:separate;border-spacing:0px 0px;color:rgb(0, 0, 0);font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:auto;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">
--------------------------------------------------------------------</div><div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">Steven J. Luck, Ph.D.</div><div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">
Professor</div><div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">Center for Mind & Brain and Department of Psychology</div><div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">
University of California, Davis</div><div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">267 Cousteau Place</div><div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">Davis, CA 95618</div>
<div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">(530) 297-4424</div><div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px"><a href="mailto:sjluck@ucdavis.edu" target="_blank">sjluck@ucdavis.edu</a></div>
<div style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px">--------------------------------------------------------------------</div><br></span></div></span> </div><br></div><br>_______________________________________________<br>
Eeglablist page: <a href="http://sccn.ucsd.edu/eeglab/eeglabmail.html" target="_blank">http://sccn.ucsd.edu/eeglab/eeglabmail.html</a><br>
To unsubscribe, send an empty email to <a href="mailto:eeglablist-unsubscribe@sccn.ucsd.edu">eeglablist-unsubscribe@sccn.ucsd.edu</a><br>
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>
<br></blockquote></div><br><br clear="all"><br>-- <br>Alexander J. Shackman, PhD<br>Laboratory for Affective Neuroscience<br>Waisman Laboratory for Brain Imaging & Behavior<br>University of Wisconsin-Madison<br>1202 West Johnson Street<br>
Madison, Wisconsin 53706<br><br>Telephone: +1 (608) 358-5025<br>Fax: +1 (608) 265-2875<br>Email: <a href="mailto:shackman@wisc.edu">shackman@wisc.edu</a><br><a href="http://psyphz.psych.wisc.edu/~shackman">http://psyphz.psych.wisc.edu/~shackman</a><br>
Calendar {still under construction}: <a href="http://www.google.com/calendar/embed?src=ajshackman%40gmail.com">http://www.google.com/calendar/embed?src=ajshackman%40gmail.com</a><br>
</div></div>