# [FieldTrip] evoked power greater than total power

Rojas, Don Don.Rojas at ucdenver.edu
Thu Jan 24 22:45:04 CET 2013

Russ,

It might help if you posted a bit of your script so that we could see how you are deriving the measures. But, the relationship you state is true only given a couple of assumptions. First it depends on how you define induced power. For some, induced power is the total signal, phase-locked and non-phase-locked, i.e., the average in the frequency domain. So in that sense, induced = total. If you are going with the notion that induced = total - evoked, then yes, you would expect that evoked power would always necessarily be less than or equal to total power. But second, that relationship only holds up if you do not baseline correct the power estimates you are comparing, which is often done in time-frequency analyses. As far as I recall, in ft_freqanalysis there is no particular option to return only induced or evoked power in the way that you mention it, but certainly it could be calculated from the output of that function. So if you let us know what your cfg options were and subsequent calculations, it might be more helpful.

Best,

Don
-----------------------
Don Rojas, Ph.D.
UCD Magnetoencephalography Lab
University of Colorado Denver Anschutz Medical Campus
13001 E. 17th Pl F546
Aurora, CO 80045
USA

On Jan 24, 2013, at 2:03 PM, 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
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