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Salem,

This isn't a mixed effects design as you only have one group and only
one condition. What you want to do is to use the con_001 images in a
one-sample t-test. DO NOT USE the con_002 images in the model. I
suspect that might have been what happened.

Your design (SPM.xX.X)hould have a size of [7 1]; load SPM.mat and
then use size(SPM.xX.X) to find the answer.

A t-contrast of 1 will find the regions that are activated.  A
t-contrast of -1 will find regions that are deactivated. An F-contrast
is simply the square of the 2 t-tests and does not have a direction.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General
Hospital and Harvard Medical School
Office: (773) 406-2464
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On Fri, Feb 25, 2011 at 2:33 PM, SALEM BOUSSIDA
<[log in to unmask]> wrote:
> Dear SPMers,
>
> I have a group of 7 subjects (rats) and I would like to perform a group
> analysis (mixed effects).
> I have done the 1st level analysis (fixed effects) for each subject and  I
> have two con_001 and con_002 images, wich are respectively the contrast
> image for a positive effect (activation) and for a negative effect
> (deactivation) of one condition (current stimulation) .
> Then, I have done a one-sample-t-test : I feed all the con_001 of each
> subject into the 2nd level analysis , then I entered a T contrast [1]  and
> SPM displayed the thresholded T-statistic image. But when I entered a F
> contrast [1], I have got this error:
> "
>
> ??? Inf computed by model function, fitting cannot continue.
> Try using or tightening upper and lower bounds on coefficients.
>
> "
>
> Based on the procedure described in SPM8 manuel concerning the "face group
> data", It is mentionned that I have to enter a F contrast.
> I am  confused by this and I have some questions:
> (1) which contrast should I enter, F or T contrast? and what is the
> differenece between them?
> (2) Is the one-sample-t-test, the appropriate one to run mixed effects
> analysis?
> (3) Do I have to run two separate "one-sample-t-test" for
> positive(activation) and negative-deactivation) effects?
>
>
> Thank you for your help.
>
> Best regards,
>
> Salem
>