Dear Narender, Richard-
aha yes!! I was using F contrasts rather than t. Suddenly these contrast
maps look much more like what I expected them to... How do the F and t
contrasts differ? (I can also look this one up on the archives..)
thanks!!
Rachael
Rachael D. Seidler, PhD
University of Minnesota Dept of Neuroscience
and Brain Sciences Center (11B)
Veterans Affairs Medical Center
One Veterans Drive
Minneapolis, MN 55417
Phone: 612 725-2000 x.1765
Fax: 612 725-2291
Email: [log in to unmask]
>From: Richard Perry <[log in to unmask]>
>To: Rachael Seidler <[log in to unmask]>
>CC: [log in to unmask]
>Subject: Re: contrasts, activations and deactivations
>Date: Mon, 14 Aug 2000 18:58:48 +0100
>
>Dear Racheal,
>
>Sorry, I saw your next question to Narender just after I had sent my
>previous message!
>
>>Dear Narender,
>>thanks for the help.. This is very similar to the suggestion I've
>>just received from Richard Perry. He suggested specifying one
>>condition (task) and including parametric modulation over time. I
>>am unsure, however, how to separate a positive linear expansion
>>(areas with increasing activation over time) from a negative one
>>(areas with decreasing activation over time). Is there some way to
>>put a sign on the parametric modulation? Whether I weight it with a
>>[1] or a [-1] it seems to yield the same map..
>
>Oh dear. Do you mean that the SPM obtained with the contrast 0 1 is
>the same as that obtained with the contrast 0 -1? This seems to be
>impossible, if you are talking about t contrasts. No voxel with a
>significantly positive parameter estimate for your second covariate
>can also have a significantly negative parameter estimate for your
>second covariate. Are you sure that you have chosen t contrasts
>rather than F contrasts?
>
>Best wishes,
>
>Richard.
>--
>from: Dr Richard Perry,
>Clinical Lecturer, Wellcome Department of Cognitive Neurology, Darwin
>Building, University College London, Gower Street, London WC1E 6BT.
>Tel: 0207 679 2187; e mail: [log in to unmask]
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