[FieldTrip] evoked power greater than total power

[Russell G Port]

thanks everyone, I was starting to get a little nervous

On Thu, Jan 24, 2013 at 5:24 PM, Matt Craddock <matt.craddock at uni-leipzig.de
> wrote:

> 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
>
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