Dear SPM'ers,
I have a question that is related the the discussion of SVCs. If one were
interested in a general region of the brain, say for the sake of
argument, posterior parietal cortex, is it valid to choose a voxel that is
roughly in the center of posterior parietal cortex and create a sphere
with a rather large radius that covers most of the lobe? Or is the SVC
only appropriate in the case of more specific hypotheses and with smaller
(i.e. 10 mm or fewer) volumes?
Best wishes,
Christy Marshuetz
On Thu, 1 Jun 2000, Matthew Brett wrote:
> Dear Simone,
>
> Just a little note to add to Geraint's answer, which I agree with entirely.
>
> If you have a prior hypothesis, it is perfectly reasonable to specify it
> (beforehand!) in terms of a sphere, box and / or image, which enclose the area
> that you are interested in, and choose the one of these that gives you the most
> significant p value. For example, sometimes an image VOI can give a more
> conservative correction than a sphere of greater volume.
>
> A common mistake, which can be inadvertently encouraged by the SPM interface,
> is to (correctly) decide a sphere radius beforehand, find your peak voxel, and
> choose that as your sphere radius centre. Obviously this is biased, and you
> have to choose the sphere/box radius _and_ centre a priori - perhaps from a
> peak voxel from a previous paper, an orthogonal contrast, or on anatomical
> grounds.
>
> Best regards,
>
> Matthew
>
>
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