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Hi - you want to be a little bit careful with this.

A feature of null hypothesis testing is that "NOT B" isn't very
meaningful. That is, we are not able to say that a voxel does not activate
to stimulus B, we can only say that if this voxel were not activating,
then we wouldn't be all that surprised to see this data - it is really a
very weak statement, and you certainly won't be able to perform null
hypothesis style inference on a mask which is formed as the intersection
of "A" with "NOT B" .

Hence, there isn't an option for this in Feat


Having said that - if you have some sensible reason for performing this
intersection then you can do it with avwmaths.

something like

avwmaths B -bin -mul -1 -add 1 -mas A  A_NOT_B

should give you a mask which defines the intersection of A with NOT B
which you can use for further analysis.

Cheers

T



On Tue, 29 Jun 2004, Andy Smith wrote:

> Is any kind of contrast masking possible in FEAT other than a
> standard exclusive mask?
>
> I have defined two contrasts from the same experiment and I want to
> identify all voxels that are significantly activated in contrast A  AND NOT
> contrast B. It has to be one-way, i.e. I don't want to include voxels that
> are active in B and not A. An exclusive mask gives voxels active in both
> but not in either alone, so that's no good.
>
> Any ideas on how to do this would be much appreciated. (It shouldn't
> be too hard to write a script to do it, but there's no point in re-inventing
> the wheel.)
>
> Thanks,
> Andy Smith
>
> PS. I'm turning to you guys because SPM won't do it, so here's a
> chance for you to gain a convert!
>

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Tim Behrens
Centre for Functional MRI of the Brain
The John Radcliffe Hospital
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Oxford University
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