Dear all,

I hope someone will be able to help me...
I'm trying to make a parametric modulation analysis but I've got very strange results.
It remains possible that I incorrectly set my design - a technical mistake rather than a theoretical one...
Thus, maybe someone could help me and say to me if I there is an error in my design ?

First, my experiment :
My experiment contains 2 experimental conditions (let's name them A and B) each of these conditions contains sentences stimuli preceded by a Context.
I want to test the linear effect of 2 parametric modulators for each condition,
Beside them, there is some variables from my experiment that I modeled (I think they could have an effect but it doesn't interest me) : fixation crosses and the response of the participants (I think it could generate 'motor' activations).

I set my design as follow:
I modeled the onsets of the Context, then the condition A and the 2 parametric modulators and in the same way the condition B and its 2 PM ( the PM are correlated for each condition but since SPM8 orthogonalizes the PM at 1st level, it's not a problem to model them in the same design, right ?).
I also modeled the onsets of the fixation crosses and responses of the participants.

Finally, in order to test the linear effects, I set contrasts such as [0 0 1] with "1" on my parametric modulators at first level and use the con* files into a one sample 2nd level design for each PM.

The results for at least one PM are very strange and unexpected. It's possible that it comes from an unexpected effect of the PM but I would like to be sure that I did not make any error in my design....

I'm ready for any question and advice ;)

Regards,

Alexandre