Christy Marshuetz wrote:
> Dear SPMers,
>
> I know this question has been asked before, but it doesn't seem that
> there is a lot of recent consensus.
>
> I have done a second-level analysis in SPM2. I was interested in
> seeing all of the activated voxels in one of my contrast images, so I
> lowered the threshold to T = 1.1 (1.0 results in a crash, incidentally).
>
> When I did that, I noticed a LOT of activity outside the brain. I
> wondered if that activity outside the brain might result from
> smoothing, and if so, if masking it out would help boost power.
First, you need to be sure you understand what is causing the activity
outside the brain. Are you sure it is smoothing? If
it does have that structure to it, then my first guess would be you
wouldn't see much improvement in power by masking
it out, since I believe SPM is taking the smoothness into consideration.
But if it isn't just smoothing, you need to figure out why the numbers
are saying that you are seeing significant
brain activation in the air. Your tests are saying that it is just as
significant as what you are seeing inside the brain!
If you are doing percent signal change, that's a good reason to mask out
the non-brain regions, since you can get large
numbers in non-brain voxels. Otherwise, Lucy, you got some 'splainin'
to do.
:)
Apologies if you're absolutely certain that it's smoothing, but I
occasionally see researchers masking out the non-brain
just because it embarrasses them, with no statistical explanation for
WHY they are seeing that activation.
rich
--
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