On 4/16/18 11:56 AM, Sarah Cole wrote:
Hi Doug,
I need your recommendations on two topics. I really appreciate your help.
1.) I have two runs of the same experiment, each for about 12 minutes.
These 2 runs are different only in the presentation order of the
stimuli. I also have one localizer session which first, I calculated
my contrasts in it, found the active voxels with positive values, and
made a mask of those voxels. Then, mapped the masks on those 2 runs
and got the raw data within those voxels. So far, so good. Now, I am
looking at the Beta values in these 2 runs for those active voxels,
they are very different from each other. For example, in the first
run, they are mostly positive but in the second run, they are mostly
negative. My intuition was that they should be in the same ballpark. I
cannot make sense of this. Do you have any suggestions on why this is
happening?
Hard to say without a lot more details. Is this all happening on the
fsaverage surface? Is your localizer the same paradigm as the two task
runs? When you look at the maps of the 2 task runs, do they look the
same? Is your mask in a region where the task has activation?
2.) To calculate the % signal change, is it ok to pick the baseline
whatever is suitable to my research question. For example, if I need
to know the % signal change relative to*the average signal in the
first TR of a specific block*, can I use that as a baseline? Or does
it always need to be the rest or the mean?
I'm not sure there is a hard and fast way to do this. Usually you want
to make sure that there is no task in the baseline. But the task is such
a small part of the overall signal that I don't think it matters.
Thank you so much and looking forward to hearing from you.
Sarah
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