[Eeglablist] Time-frequency analysis (subtraction first or analysis first)
Arnaud Delorme
arno at ucsd.edu
Fri Apr 18 13:49:19 PDT 2008
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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