[Eeglablist] Time-frequency analysis (subtraction first or analysis first)
Andrei Medvedev
am236 at georgetown.edu
Fri Apr 18 13:07:52 PDT 2008
Dear Brian,
I must say that I COMPLETELY agree with your way of thinking about subtracting the 'evoked' activity from each trial. I've been thinking similarly that there is no good way to separate 'evoked' from 'induced' activity completely.
Another thought is that, actually, you may not need to subtract the ERP from each trial because the contribution of the evoked component into the 'total' TF (as done on each trial without ERP subtraction) is relatively small. Apart from some theoretical considerations about that, you can often see that empirically if you do TF of the ERP, compare it to the 'total' spectrogram and see that, quite often, spectral components well seen in the ERP's TF are not seen or very weak in the 'total' TF (and vice versa). Therefore, what we now call the 'total' TF, represents MOSTLY the 'induced' activity in many real cases (as it was originally defined/used in Tallon-Baudry et al 1996, I believe).
Regards,
Andrei.
Andrei Medvedev, Ph.D.
Assistant Professor,
Center for Functional and Molecular Imaging
Georgetown University Medical Center
3900 Reservoir Rd, NW
Preclinical Science Building LM-14
Washington, DC 20057-1488
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There is an article by Truccolo et al in Clinical Neurophysiology
(Trial-to-trial variability of cortical evoked responses: implications
for the analysis of functional connectivity) which examines such a
subtraction of the evoked response. I believe the idea is that your
averaged ERP does not exactly represent all potential evoked activity in
a given trial. Therefore some residual evoked activity will survive a
"single trial minus ERP" routine. ERSP data contains both evoked and
induced power, which are tough to separate without making some
assumptions about the data. I don't think that you can subtract the ERP
from every single trial, subject it to TF analysis, and say that the
resulting ERSP map shows phase varying (or induced) responses /only/.
To do that, you would have to assume that the ERP subtraction accounted
for exactly all of the evoked power activity in every single trial.
I was interested in your last comment Stan - do you have any references
about saccades and gamma?
thanks,
Brian
Stanley Klein wrote:
> It looks like there is some consensus on whether to subtract first and
> then the TF or vice versa. That's nice. [On the other hand subtracting
> first is a nice way to get rid of ERP, but there are better ways, as
> described next.]
>
> Andrei, I'm not sure I understood your last comment or question, but I
> have a related question. Whenever one does time-frequency power plots
> I would think that one should ALWAYS first get the time locked average
> and subtract it off of all the individual trials. Then one could do a
> TF plot of each. How many on this list do that? I suspect that people
> mix together the standard evoked response and also the phase varying
> response. Why do that since its so easy to show the the two TF plots
> separately.
>
> Also I've heard rumors that saccades and microsaccades are responsible
> for most EEG gamma oscillations. So one should also put those events
> into a separate category too. Too bad things are complicated. But it
> makes life interesting.
> Stan
>
> On Tue, Apr 15, 2008 at 9:38 AM, Andrei Medvedev <am236 at georgetown.edu
> <mailto:am236 at georgetown.edu>> wrote:
>
> Hi All,
>
> I think this was just a small mistake confusing options 1 and 2. I
> believe so because it is option 1 (not 2) which would require
> pairing of trials to do EEG subtraction first, which is indeed a
> rare possibility.
>
> To me, it also looks like option 2 is more correct because TF
> analysis (in its most common 'spectral perturbation' or 'induced
> activity' version) looks for changes in spectra regardless of
> phase. This is why if you analyze only one condition, you do TF
> first and then average trials. Similar thing should then be done
> when comparing two conditions, that is, TF first.
>
> With one condition, you can also do averaging first and then TF,
> in this case you would have the so-called 'evoked' responses in
> the frequency domain (instead of 'induced' responses mentioned
> above). Evoked activity shows you the frequency components
> phase-locked to the stimulus (a more strict form of time locking).
> If you try to do similar thing with two conditions (trials should
> be paired somehow but there is no 'natural' way to pair them, only
> in some special circumstances), you will have a problem of phase
> relations between conditions and may get different answers (such
> as sum/subtraction of in-phase/out-of-phase sine waves, as other
> people point out). This would be a very different response and I
> believe nobody is doing this. But theoretically, this type of
> response can be explored as well (if you have a 'natural' way of
> pairing trials).
>
> BTW, I haven't tried to use TF decomposition in EEGLAB applied to
> the averaged ERP (i.e., averaging of trials first, then TF
> resulting in an 'evoked' response for one condition). Has anyone
> tried this?
>
> Regards,
> Andrei.
>
> Georgetown University
>
>
> ----- Original Message -----
> From: Arnaud Delorme <arno at cerco.ups-tlse.fr
> <mailto:arno at cerco.ups-tlse.fr>>
> Date: Sunday, April 13, 2008 1:53 pm
> Subject: Re: [Eeglablist] Time-frequency analysis (subtraction
> first or analysis first)
>
> > Dear Hsu,
> >
> > only your first statement is correct. The second one could be
> > correct if
> > you could pair the trials, but it would be very rare that you would
> > want
> > to do this (since trials are recorded at different times and are
> > usually
> > not paired between conditions). Look up the help of the newtimef
> > function which allows computing differences between power between
> > different conditions and newcrossf which allows computing
> > difference
> > between phase coherence images.
> >
> > Best,
> >
> > Arno
> >
> > Hsu, Shen-Mou wrote:
> > > Dear list-memebers,
> > >
> > > Suppose that I am interested in comparing two conditions A and B
> > in terms of their power and phase coherence. I was wondering which
> > one of the following steps is more theoretically correct. 1. After
> > segmentation, calculate the EEG differences between the condition A
> > and B and then perform time-frequency analysis on the differences.
> > 2. After segmentation, perform time-frequency analysis on the EEG
> > data of the condition A and B respectively and then compute the
> > power or phase coherence differences between two conditions. Any
> > comments would be much appreciated.
> > >
> > > Many thanks,
> > >
> > > Shen-Mou Hsu
> > >
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