Hi Mark,
I think the motivation of the question was to understand how FSL treats
deactivations. In some other packages, A-B shows regions where A > B in one
color and regions where B > A in another color. So, its redundant in a sense
to do the two contrasts separately. But definitely that's not the case with
FSL. But I am still wondering if the zstat image still has both + and -
values in it, where + values indicate regions where A > B and -ve values
indicate regions where B > A?
Thanks
Vinod
-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Mark Jenkinson
Sent: Monday, July 09, 2007 7:28 PM
To: [log in to unmask]
Subject: Re: [FSL] generation of contrast maps
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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