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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).<br>
<br>
Arno<br>
<br>
Stanley Klein wrote:
<blockquote
cite="mid:9016c9f10804161851k278a029egaa676054be320333@mail.gmail.com"
type="cite">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.<br>
Stan<br>
<br>
Dear Brian, <br>
<div class="gmail_quote">
<div class="gmail_quote">
<div>
<div class="Wj3C7c">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. <br>
<br>
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:<br>
<br>
1) Show the old fashioned ERP(t) averaged across trials and subjects.<br>
2) Show the TF plot of the ERP.<br>
3) Show the TF plot of the combined 'evoked' and 'induced' as most
folks on this list suggest<br>
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?). <br>
<br>
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?). <br>
<br>
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. <br>
Stan
<div>
<div><br>
<br>
<br>
<br>
<br>
<br>
<div class="gmail_quote">On Wed, Apr 16, 2008 at 2:22 PM, Brian Roach
<<a moz-do-not-send="true" href="mailto:brian.roach@yale.edu"
target="_blank">brian.roach@yale.edu</a>> wrote:<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div bgcolor="#ffffff" text="#000000">
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 <i>only</i>.
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.<br>
<br>
I was interested in your last comment Stan - do you have any references
about saccades and gamma?<br>
<br>
thanks,<br>
Brian<br>
<br>
<br>
Stanley Klein wrote:
<blockquote type="cite">
<div>
<div>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.]<br>
<br>
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.<br>
<br>
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. <br>
Stan<br>
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