Hi,
Each of the contrasts does a one-tailed t-test and so in A-B you only
see areas where
the response to A is significantly greater than the response to B,
while in B-A it is the
other way around.
In general you want to put in both contrasts in order to understand
more about how the
two conditions relate. The maps produced are normally very different
indeed, and
often one of them is blank (or almost so) if one condition engages a
subset of the
regions (and less strongly) than the other.
All the best,
Mark
On 9 Jul 2007, at 22:45, Dharol Tankersley wrote:
> Dear Listserv:
>
> I am trying to understand how FSL is generating the contrast map A-
> B, and how this is different from the contrast map B-A. What
> happens if there is a deactivation in one of the conditions? The
> two maps do not seem to be precise opposites of one another, and
> even if deactivations in some voxels are rounded to zero, this
> doesn't seem sufficient to explain the large differences we are
> seeing in the A-B and B-A contrast maps being generated.
>
> Thank you,
>
> Dharol
>
>
> Dharol Tankersley
> Dr. Huettel's Laboratory
> Brain Imaging and Analysis Center
> Duke University
> 919-681-9935
>
>
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