[Eeglablist] Time-frequency analysis (subtraction first or analysis first)

[Brian Roach]

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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