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SPM  August 2011

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Subject:

Re: Induced / Evoked and Lenght of Time Window

From:

Vladimir Litvak <[log in to unmask]>

Reply-To:

Vladimir Litvak <[log in to unmask]>

Date:

Mon, 15 Aug 2011 15:49:34 +0100

Content-Type:

text/plain

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text/plain (69 lines)

Dear Urs,

Maybe you have better eyes but with my limited vision I cannot see
much difference between evoked and induced options in your results.
This actually makes a lot of sense because the induced option is only
different from the evoked if you specify a frequency band. As you can
see in the plot the projector in both cases is just a gaussian window
and that can be applied either before or after averaging with the same
result. Perhaps there is just some kind of minor scaling difference
that should be removed later anyway when exporting to images.

Since as implied from your second question, you are interested in ERPs
and want to relate your results to specific peaks in the ERP, it
probably makes more sense for you to use the 'evoked' option because
that corresponds exactly to this way of thinking. The induced option
is more suitable for questions formulated in terms of time-frequency
analysis, like 'gamma activity related to face processing'.

Regarding your second question I don't think there is a way to answer
it on a principled basis. I think your choice should be based on two
non-principled criteria (1) you can show what you want to show (2) you
can justify your choice in your paper so that it doesn't look
arbitrary. That's what I think most people do. If you want to be
really rigorous, you can use bootstrap i.e. resample with replacement
the same number of trials from your data 100 times, compute the peak
latency every time, get a distribution and take 95% confidence
interval as your window.

Best,

Vladimir



On 15 Aug 2011, at 13:32, Urs Bachofner <[log in to unmask]> wrote:

> Dear Vladimir, dear SPM Users,
>
> In 3D Source Localisation I'm unsure about the choices to be made in the window section.
>
> I'm still working with my ERPs (ca. 180 good trials, -100 to 500 ms) and I'm expecting activation in the frontal regions.
>
> 1. Induced vs. Evoked:
>
> "The projectors generated at the previous step can either be applied to each trial and the results averaged (induced) or applied to the averaged trials (evoked)."
>
> When I'm applying this to the whole time span of the trial (0 - 500 ms), I get much nicer activations in frontal regions when I choose the "induced" option.
> Now since I'm now interested in P200 and P300 specifically, I choose a timespan +/- 20ms of the peaks of those components.
> When doing so, all of a sudden I get nicer frontal results (darker and more definite spots) when choosing "evoked".
> Could someone please explain what the advantages of the two modes are and in what situation which one should be applied?
>
>
>
> 2. This leads to the question about the time window. I happen to know the exact ms of the Peak of my P200 and P300 when all trials are averaged.
> What would be the best compromise between a short time window (+/- 4ms, maybe some peaks of some trials are missed) and a long time span (like +/- 50ms, other peaks and eeg components could be included)?
>
> Thank you for your inputs if you've got any too share with me.
>
>
> Best,
>
> Urs Bachofner
>
>
>
> --
> NEU: FreePhone - 0ct/min Handyspartarif mit Geld-zurück-Garantie!	
> Jetzt informieren: http://www.gmx.net/de/go/freephone

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