Hello everyone,

I have a couple of questions about the best way to design my experiment.

I have two tasks and am doing block design. I am interested in each task separately and also their contrast. Both tasks will activate the same parts of the brain, but to different levels. [The tasks are high force and low force - they will drive the BOLD response up in M1 (and other areas too) but to different levels].

I could put each task in a separate scan or I could intermix them so that I have both tasks in both scans and then concatenate the scans. From what I have read before, it is recommended that tasks-to-be-contrasted be put in the same scan. So, the task sequence will look like - task_1 rest task_2 rest task_1 rest task_2 rest ...

There is a slight problem with this, at least in cases where the tasks-to-be-contrasted activate the same brain region. The BOLD response of this common brain region will go up both tasks albeit to different levels. Hence, this brain region will ideally not be picked either by the regressor for task_1 or by the regressor for task_2. Since I will eventually contrast the two tasks, it is only appropriate that the common regions be not picked. However, I am also interested in each task separately. For that purpose, putting them in the same scan seems the wrong thing to do.

What should I do, put the tasks in separate scans or put the tasks in both scans followed by concatenation of scans?
What are the downsides of putting them in separate scans and contrasting them later in AFNI?

My guess is that I can't have both - high precision in individual tasks and high precision in the contrast. I have to chose one. But I would like to hear from an expert.

Thanks and regards,
G