thanks everyone, I was starting to get a little nervous <br><br><div class="gmail_quote">On Thu, Jan 24, 2013 at 5:24 PM, Matt Craddock <span dir="ltr"><<a href="mailto:matt.craddock@uni-leipzig.de" target="_blank">matt.craddock@uni-leipzig.de</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5">On 24/01/2013 22:03, Russell G Port wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi All,<br>
<br>
I am currently trying to understand something and I think fieldtrip<br>
people will already know whats going on. I run my data, from both<br>
in-vivo EEG recordings in animals and MEG in children, through<br>
freqanalysis. I have the strange result though that my total power (in<br>
either mV^2 or nAm^2) is less than my evoked power. How can this be, if<br>
total=evoked+induced. I am assuming that something that I have done<br>
while handling the data is causing this error, but I have check my work<br>
very carefully. Has anyone ever seen anything like this before, or can<br>
suggest what I am doing to help point me in the right direction?<br>
<br>
Cheers - Russ<br>
<br>
</blockquote>
<br></div></div>
Hi Russ,<br>
<br>
I'm guessing you mean after baseline correction. Evoked and total have very different baselines: with evoked power, pre-stim-onset power is usually pretty much 0, since there shouldn't really be anything time/phase-locked in this period. That's not the case for total power. If you're doing relative measures of change from baseline this difference can be ENORMOUS - for example, I often see evoked power 500 times greater than baseline compared to, say, a 20% increase in total power (this is in the gamma band range). On the single trial level, the evoked, such as it is, contributes very little to the overall signal - averaging out the noise/background/non-phase-<u></u>locked activity is what makes it stand out. one way people sometimes isolate induced activity is to remove the ERP from each trial before doing the TF; if you do it that way and compare it to total power, you should see that it makes very little difference (which is one reason why I prefer not to do it, another being that it rests on a faulty assumption - that the ERP is stationary and the same on each trial). Basically, there's nothing wrong here.<br>
<br>
Cheers,<br>
Matt<span class="HOEnZb"><font color="#888888"><br>
-- <br>
Dr. Matt Craddock<br>
<br>
Post-doctoral researcher,<br>
Institute of Psychology,<br>
University of Leipzig,<br>
Seeburgstr. 14-20,<br>
04103 Leipzig, Germany<br>
Phone: <a href="tel:%2B49%20341%20973%2095%2044" value="+493419739544" target="_blank">+49 341 973 95 44</a></font></span><div class="HOEnZb"><div class="h5"><br>
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