Hi,
Probably the easiest and most accurate way to do this all is just to
run a second-level FE cross-run within-subject analysis and include
the contrasts (eg A vs C) at that level - this will then generate the
optimal COPEs and VARCOPEs for you, easily extracted with Featquery.
Cheers, Steve.
On 12 Dec 2007, at 22:50, Michelle Voss wrote:
> Hello all,
>
> I'm trying to determine whether it's valid to create an effect size
> of sorts that can compare %signal change within an ROI for separate
> conditions of separate block designs. For example, say I have an
> ROI and I want to examine the degree to which it responds greater
> for a specific condition compared to another condition, but the
> conditions are in separate runs (but same session). E.g., you for
> one fMRI session a blocked run with blocks for stim types A and B,
> and another for stim types C and D, and post hoc you want to compare
> response within an ROI for stims A and C.
>
> I've run featquery for copeA and copeC to extract the mean percent
> signal change for these conditions compared to baseline within the
> ROI. If I want to normalize this difference by a variance term,
> could the correct variance term be given by sqrt(average of varcopeA
> and varcopeC). doing the latter by running featquery just as before
> but 1) not converting to %signal change and 2) extracting the mean
> varcope within the ROI instead of mean cope. from this i have the
> mean varcopeA and mean varcope C within the ROI, from which I can
> take the average, and the sqrt of this average to have a pooled
> standard deviation as a variance term to be used something like:
>
> within an ROI:
>
> (mean %signal change stim A - mean %signal change stim C)/pooled
> standard deviation
>
> but, is this the right variance term for what i want to do here?
> the reason i'm interested in the variance term is because i'm
> comparing two groups with potentially different variances, so i want
> to normalize the difference in response by a variance term..
>
>
> best,
> Michelle
>
>
>
>
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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