Dear SPMers,
I have a somewhat simple fMRI design question.
A highly-experienced fMRI friend has already weighed in, but, I thought that I would toss it out to the SPM community ...
I'd like to run an experiment with 2 runs, each run will consist of:
4 neutral video clips (15 s each)
4 cocaine-use video clips (15 s each)
4 scrambled video clips (15 s each)
(8 clips total per condition)
We would randomize the order of the video clips (with the exception of starting with the neutral video clip per run).
I will get the participants to do a simple task during each video (i.e., what is the color of the frame) just to ensure that they are paying attention.
My question is this: do I include a fixation cross (15 s) after each video clip in order to allow the signal to go back down to baseline/close to baseline or do I not bother? Ultimately, I will be most interested in the contrasts of cocaine vs neutral, cocaine vs scramble.
Additionally, is there any benefit to jitering the onset of the video clips (e.g., having a short fixation cross of variable length 1-3 s between each clip, thus, the periodicity isn't always 15s). Or, is there no benefit to this in a block design.
Any thoughts would be appreciated.
Cheers,
Natalia
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