[FieldTrip] evoked power greater than total power

[Matt Craddock]

On 24/01/2013 22:03, Russell G Port wrote:
> Hi All,
>
> I am currently trying to understand something and I think fieldtrip
> people will already know whats going on. I run my data, from both
> in-vivo EEG recordings in animals and MEG in children, through
> freqanalysis. I have the strange result though that my total power (in
> either mV^2 or nAm^2) is less than my evoked power. How can this be, if
> total=evoked+induced. I am assuming that something that I have done
> while handling the data is causing this error, but I have check my work
> very carefully. Has anyone ever seen anything like this before, or can
> suggest what I am doing to help point me in the right direction?
>
> Cheers - Russ
>

Hi Russ,

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-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.

Cheers,
Matt
-- 
Dr. Matt Craddock

Post-doctoral researcher,
Institute of Psychology,
University of Leipzig,
Seeburgstr. 14-20,
04103 Leipzig, Germany
Phone: +49 341 973 95 44