Dear SPM'ers,
I hope somebody can help. I plowed through the archives, but as none of the designs seemed equivalent to mine, I didn't find the right answers.
I have had participants performing four distinct tasks. After each trial, they rated the difficulty of the particular task they just performed for the particular stimulus they saw on a four-point scale. Apart from simple contrasts between the conditions (tasks) I would like to see in which areas the activity correlates (negatively or positively) with the parameter, e.g., "during task A, area X was significantly more active as the difficulty parameter increased/decreased". There's some confusion in our lab about how to proceed.
My questions are the following:
1) As the four tasks were qualitatively different, I assume I have to treat the difficulty ratings as four different parameters (one for each condition)?
2) At the first level, I specified four conditions, with one parametric modulator for each. Then in the contrast manager, I defined (apart from the contrasts between conditions) two contrasts for each parameter, a negative and a postive one (e.g., [0 1 0 0 0 0 0 0] and [0 -1 0 0 0 0 0 0]. Is this correct? Between what and what exactly is a contrast calculated in this step?
3) How to precisely interpret the output images of these contrasts? Do these reflect in some sense areas in which activity is positively/negatively correlated to the parameter?
4) How to proceed at the 2nd level? To demonstrate any difference between the conditions after "evening out" the influence of the parameter, I guess I would have to specify the parameters as covariates. But how to calculate, on a group level, "during task A, area X was significantly more active as the difficulty parameter increased/decreased"?
I would greatly appreciate any input.
Regards,
Kris
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