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?
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?
Thank you so much and looking forward to hearing from you.
Sarah
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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Sarah Cole