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

[Arnaud Delorme]

In the list of points below, a firth possibility would be to regress out 
the ERP out of the data (Journal of Neuroscience, 27(44):11949-59). 
Also, as other people pointed out, making a distinction between induced 
(ITC=0) and evoked (ITC=1) assumes that the brain is actually making 
this distinction too, which is an hypothesis poorly supported by most 
data I have seen (where the ITC usually varies between 0.2 and 0.6 even 
in independent component that would potentially separate induced and 
evoked processes).

Arno

Stanley Klein wrote:
> Dear EEGlist, First I'd like to comment on how it is cute that this 
> discussion has shifted from the original point of the subject heading, 
> but the same subject heading is relevant to the issue of whether to 
> first subtract off the ERP before doing the time-frequency analysis.
> Stan
>
> Dear Brian,
> On the gamma/microsaccade connection. There was a poster at the 
> Cognitive Neuroscience meeting a few days ago by Shlomit 
> Yuval-Greenberg titled Reassessment of the origins of Induced Gamma 
> Band Responses – A single trial analysis. I didn't see the poster but 
> I heard that that their results do not apply to ECoG or  LFP, only to 
> EEG. It will be interesting to see discussion of this interesting 
> topic when their paper gets published.
>
> On the topic of 'evoked' vs 'induced' responses. I'd like to suggest 
> that when one has a really interesting result, say about gamma, it 
> would be useful to the reader to show the data in more than one way. 
> The following are some possibilities:
>
> 1) Show the old fashioned ERP(t) averaged across trials and subjects.
> 2) Show the TF plot of the ERP.
> 3) Show the TF plot of the combined 'evoked' and 'induced' as most 
> folks on this list suggest
> 4) Show the TF plot after subtracting off the ERP. This has the 
> advantage of possibly emphasizing feedback pathways where the 
> cognitive based time delays can be more variable than the bottom up 
> stream. (Does this make any sense?).
>
> One reason to show ERP(t) is that the filter that is used typically 
> has a very broad bandwidth so one sees something close to the true 
> time course of the response. When one sees (as one often does) some 
> very sharp features in the temporal response one gets excited. The 
> problem with the TF plot is that one typically uses a wavelet filter 
> with more than one cycle so one distorts (adds wiggles to) sharp 
> temporal responses (like a microsaccade?).
>
> The big advantage of methods 3 and 4 are that they capture the 
> non-stationary aspects of the response. It makes a lot more sense to 
> average TF plots across subjects than to average ERPs across subjects 
> because individual differences in the cortical folding can easily 
> produce differences in the scalp potentials. However the folding makes 
> any type of scalp based averaging (including TF plots) across subjects 
> less than ideal. That's why people try to get to source space.
> Stan
>
>
>
>
>
>
> On Wed, Apr 16, 2008 at 2:22 PM, Brian Roach <brian.roach at yale.edu 
> <mailto:brian.roach at yale.edu>> wrote:
>
>     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
>
>
>
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