Hi!
Thanks for the response.
This was also my first idea, but then the contrast (A+C)/2-B would be the same as -1.5 B, which is strange to say the least. Also when I ran FSL I didn't get any significant activation.
My concern is thus also if ány addition can be done with the two vectors (EV1 EV2). To compare with the normal case of 2 time points (A,B): in that case only 1 vector is used (1, -1), with contrasts [1,0,0,0....] or [-1, 0 0 0 0 ] to form either A-B or B-A. A+B cannot be made this way (please correct me if I am wrong). The same way A+C cannot be formed in the triple-case?
One (maybe too simple) method I thought of was to use the vector [1 -2 1] (instead of the standard triple t-test [1 -1 0] and [1 0 -1]), and contrast (1,0,0,0,0..). This way i found some activation, but before interpreting it I would like to be sure that this method is valid.
Thanks again,
Matthijs
-----Original Message-----
From: FSL - FMRIB's Software Library on behalf of Steve Smith
Sent: Fri 4/11/2008 9:21 AM
To: [log in to unmask]
Subject: Re: [FSL] setting up contrast to incorporate time effects
Hi, from the triple-t-test example in the FEAT manual:
A=a+b
B=-a
C=-b
Hence if you want
(A+C)/2-B
you get
(a+b-b)/2+a = 3a/2
so you want the contrast [1.5 0 0 0 0.....]
Cheers.
On 10 Apr 2008, at 09:15, Matthijs Lipperts wrote:
> Dear all,
>
> We are currently working on a project to see the effects of a drug
> (botulinum toxin) on the activation of some brain area's. However to
> prove
> that the decrease in signal is not a time-effect or training-effect,
> the
> measurement is repeated 7 weeks after the injection. We thus have 3
> time
> points: A-->before injection, B--> after injection, C--> 7 weeks after
> injection.
> To see group results I followed the 'tripled T-test' instructions
> with the
> contrasts for A-B, B-C and A-C, and this works perfectly.
> However, we would very much like to see the contrast (A+C)/2-B,
> since this
> gives us the difference between the time effect and the drug-effect.
> So far
> I have not succeeded in this. Any suggestions are helpful!
> Thank you in advance,
> Sincerely, Matthijs
>
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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