Dear Karl,

thanks a lot for your answer regarding errors in the predicted
volume of an activated cluster. I have a further question regarding
cluster comparison.

We've used a working memory paradigm which gives strong activation in the
posterior parietal cortex (PPC) and the dorsolateral prefrontal areas,
bilaterally. We performed the study with two subject groups. What I
would like to do is to compare the volume of the activated clusters
in the same active regions (eg., PPC) but for the two subject groups.

In some sense the clusters I want to compare are not so 'different'
because they represent activation in the same brain area for the same
task. Is this approach valid? You mentioned that comparing clusters is not
that easy. Did you mean comparing clusters from different brain regions?
If so, and although I am not doing it now, how would you recommend doing
such a comparison? 

Thanks a lot,
Jorge Jovicich

On Mon, 3 Apr 2000, Karl Friston wrote:
> Dear Jorge,
> > I have calculated the volume of activation of different brain areas
> > using a fixed effects analysis (fMRI data set, SPM99b). How can I
> > obtain an estimate of the error of these volumes?
> The distributional approximations for spatial extent will be found in:

> Friston KJ Worsley KJ Frackowiak RSJ Mazziotta JC & Evans AC Assessing
> the significance of focal activations using their spatial extent Human
> Brain Mapping 1994;1:214-220
> These are used as the basis of inference (i.e. p values) based on
> spatial extent.  Comparing two clusters however is not so simple, if
> that is what you want the error variance for.
> I hope this helps  - Karl