Erin.Oakman at NYUMC.ORG
Tue Mar 30 19:01:25 CEST 2010
Thanks for raising a very relevant question about the difference between induced and evoked activity !
A good discussion of this can be found here, or attached as text
There is not a definite way to separate the induced and evoked activity. The reason is that the sum of the squares is not the same as the square of the sum.
In my very limited experience, I have noticed that researchers sometimes use the term "induced" or "event-related spectral perturbation" to refer to the average of the single-trials power, which has been base-line corrected . At least that is the case in the "induced power" in this paper: Krishnan, G. P., W.P. Hetrick, C.A. Brenner, A. Shekhar, A.N. Steffen and B.F. O'Donnell 2009. Steady state and induced auditory gamma deficits in schizophrenia. Neuroimage.
> A late follow-up to this topic. I have recentrly been musing over how to
> get a "clean" measure of the non-phase locked activity. I have tried
> subtracting the ERF out prior to time-frequency computation but this
> produces quite a bit of artifact...presumably since the single trial data
> will have considerable ;atency "jitter"
The ERF collapses two sources of "jitter"; in the latency of the transient activity (if it exists) and the phase of ongoing oscillatory activity.
> The comments from Christian below make sense ( I think) why simply
> subtracting the two time-frequency power representaions is not valid. But I
> wonder would this subtractive approach be valid if one worked with the
> magnitude of the signal rather than power..omitting all the squaring operations?
Computing the magnitude is still a non-linear operation (square root of a sum of squares, rectification, whatever ... ). The problem for why this won't work either resides in averaging part: in the evoked case you have a linear average followed by a non-linear operation, and in the induced case you have an average of the non-linearly transformed quantity. The "catch phrase" here is: the sum of the squares is not the same as the square of the sum! (or the sum of the rectified data is not the same as the rectified sum)
Hope this helps,
> If this right theoretically, how to achieve this in Fieldtrip?. Would
> setting cfg.output = 'fourier then abs'ing the output work. My suspicion is
> no since the summing is being done first here. Alternatively, does one need
> to hack the code to return the magnitude.
> Thanks for your help on this and sorry for waking old threads :)
> - Suresh
> On Fri, 23 Feb 2007 01:44:59 +0100, Christian Hesse
> <c.hesse at FCDONDERS.RU.NL> wrote:
>> One further comment (please see below):
>>> Hi Thomas,
>>>> Following up on this conversation. It seems that the ‘induced
>>>> activity’ contains both phase-locked and non-phase-locked
>>>> activity, whereby the ‘evoked’ activity contains only phase-locked
>>>> activity. Is it then kosher to separate these components by linear
>>>> subtraction? For example, if we first compute the ‘induced’
>>>> activity by averaging power over individual trials, and from that
>>>> subtract the ‘evoked activity’ (calculated based on average
>>>> response) to get the induced activity without any phase-locked
>>> It is not correct to subtract because computing the induced and
>>> evoked power spectra involves squaring signal amplitudes (a non-
>>> linear operation), and hence, taking your terminology to refer to
>>> the instantaneous amplitudes of the signal components (this applies
>>> to any time-frequency tile)
>>>> Induced = Phase + Non-Phase
>>>> Evoked = Phase
>>>> Non-Phase = Induced – Evoked
>>> what you actually get from spectral or time-frequency analysis is
>>> the power of your MEASURED signal
>>> Induced^2 = (Phase + Non-Phase)^2 = Phase^2 + 2*Phase*Non-Phase +
>>> Evoked^2 = Phase^2
>>> Induced^2 - Evoked^2 = 2*Phase*Non-Phase + Non-Phase^2 AND NOT Non-
>> Note that the other crucial thing to consider here is that you are in
>> one case averaging power over trials over trials:
>> E[ (Induced^2) ] = E[ (Phase + Non-Phase)^2 ] = E[ (Phase^2 +
>> 2*Phase*Non-Phase + Non-Phase^2) ] = E[ (Phase^2) ] E[ (Non-
>> Phase^2) ] + E[ 2*Phase*Non-Phase ]
>> this is why taking the square root of sqrt(Induced^2) does not give
>> (Phase + Non-Phase) but sqrt(E[ (Phase+Non-Phase)^2 ]).
>> in the evoked case you are taking the power of the average amplitude
>> Evoked^2 = E[ Phase ]^2 (---> note the ^2 on the outside of the sum)
>> so in subtracting you are actually assuming that E[Phase]^2 = E
>> [(Phase)^2] which is unlikely to be accurate the case in finite samples.
>> Hope I have not confused others (or myself) here.
> This is indeed the approach that I have followed succesfully a couple of
> times (e.g. Bastiaansen et al., JOCN 2006), although the terminology that
> you are using is somewhat confusing. I (and I guess most people) would refer
> to induced activity as that part of the EEG that is non-phase-locked, so I
> would restate your equation to:
> induced = EEG - evoked.
> However, there is a drawback to this approach, since it assumes that the ERP
> is absolutely stationary over trials. This is not the case in reality (e.g.
> subjects' attentional level or other states may change from trial to trial,
> giving rise to variability in the single-trial ERPs). This means that by
> subtracting the average ERP, one may introduce frequency components in the
> residual EEG that were not present before. Klimesch, and Kalcher and
> Pfurtscheller, have come up with ways of scaling the average ERP so as to
> yield a best fit of the average with each single-trial ERP, but also that
> approach may be sub-optimal.
> My latest way around the problem is to run a TF analysis on the untreated
> EEG (containing both evoked and induced activity), and comparing this to a
> TF analysis of the subject-averaged ERPs (the evoked activity alone).
> Qualitative differences between the two analyses can now only be attributed
> to induced activity.
> Thomas Thesen wrote:
>> Hi FieldTrippers,
>> Following up on this conversation. It seems that the ‘induced activity’
> contains both phase-locked and non-phase-locked activity, whereby the
> ‘evoked’ activity contains only phase-locked activity. Is it then kosher to
> separate these components by linear subtraction? For example, if we first
> compute the ‘induced’ activity by averaging power over individual trials,
> and from that subtract the ‘evoked activity’ (calculated based on average
> response) to get the induced activity without any phase-locked activity?
>> So if
>> Induced = Phase + Non-Phase
>> Evoked = Phase
>> Non-Phase = Induced – Evoked
>> Or does the fact that this is a linear operations on data that have been
> constructed through a non-linear process render this somehow invalid? It has
> certainly been done before. Your comments would be much appreciated.
From: FieldTrip discussion list [FIELDTRIP at NIC.SURFNET.NL] On Behalf Of Bobby Stojanoski [stojanoski at UTSC.UTORONTO.CA]
Sent: Thursday, March 25, 2010 1:33 PM
To: FIELDTRIP at NIC.SURFNET.NL
Subject: [FIELDTRIP] Induced activity
I am a relatively new user of fieldtrip and am very impressed!
I am interested in comparing differences at certain frequencies – induced 40-100 Hz – between 2 experimental conditions. My understanding was that I can calculate induced activity in the gamma range by calculating the power for each trial (subject average -- freqanalysis) and then averaging across subjects (grand average -- freqdescriptives/freqgrandaverage).
To my dismay, when I plotted the results of my grandaverage I found a band of power at 55 - 65 Hz for the entire duration of my epoch. I should add this was not the case when I plotted power across trials for each participant.
Earlier discussions mention computing induced+evoked (using freqanalysis and freqgrandaverage) and subtracting that from evoked (using timelockanalysis+freqanalysis) to get extract induced only activity. However, later posts suggest that this is not a valid approach.
1. Where have I made my mistake?
2. If ((induced+evoked)-evoked)) is not valid, what is the correct approach to calculating induced activity at 40 - 100 Hz?
Any help would be greatly appreciated!
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