[FieldTrip] evoked power greater than total power

[Rojas, Don]

Yes, I think it is the baseline correction you are doing. Try the comparison without the correction and see if that makes more sense. If there is still a non-trivial difference, then it might be a more interesting problem.

On Jan 24, 2013, at 3:07 PM, Russell G Port wrote:

So my script generally goes preprocessing:

cfg.trialfun='ft_trialfun_general'
cfg.trialdef.eventvalue=65
cfg.trialdef.prestim=1
cfg.trialdef.poststim=2
cfg.trialdef.eventtype='untitled'
cfg=ft_definetrial(cfg)
cfg.demean='yes'

cfg.outputfile= -------
data=ft_preprocessing(cfg)

then from this output I do two different things, first I produced the time locked data:


[timelock] = ft_timelockanalysis(cfg, data)

For this new variable I then run the ft_freqanalysis with the following settings:


cfg.method='wavelet'
cfg.toi=[-.8:0.001666666666666667:1.5]
cfg.foi= 5:0.95:100
cfg.width=3:0.03:6
cfg.polyremoval=-1

and so this gives me the evoked power

for the total power instead i take the data from the preprocessing function (called data above) and repeat the exact same frequency transformation (same cfg setting/same called function)

Once this is done, I convert both to dB scale using freqbaseline (-.2 to 0) and perforimg log10*10 operation of the data. Its here where suddenly total power is less than evoked power. I assume then its the baseline correction (since the total power has considerably more baseline activity (activity in the -.2 to 0 time window)) that creates an 'artificially' lower total power to evoked power.





On Thu, Jan 24, 2013 at 4:45 PM, Rojas, Don <Don.Rojas at ucdenver.edu<mailto:Don.Rojas at ucdenver.edu>> wrote:
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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> fieldtrip at donders.ru.nl<mailto:fieldtrip at donders.ru.nl>
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-----------------------
Don Rojas, Ph.D.
Associate Professor of Psychiatry
U. of Colorado Denver Anschutz Medical Campus
Director, UCD Magnetoencephalography Lab
13001 E. 17th Pl F546
Aurora, CO 80045
303-724-4994

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