Dear SPM users,
I am running the group level analyses for a project where we had subjects do a learning and memory task two times, each time with different but equivalent stimuli. So I am interested in comparing the activation of the group of subject's first scan to that of their second scan. In the past we have used three covariates for the second level model specs: the number of errors committed on each trial of the task, and then two for the ratio of left and right hippocampal volumes of each subject divided by total intracranial volume. We have used these covariates in the past for just looking at activations among subjects who did the test once, so I am not positive what the best way to use covariates when doing a test-retest analysis.
My original idea for the errors, was to do # errors in scan 2- # errors in scan 1. Does this seem logical?
And for the hippocampal volumes, would it be logical to do the same? I don't imagine that they have changed much, but I don't know quite yet as am waiting for the final subject's freesurfer to finish running. So if they haven't, I would imagine then I could simply take the volumes from the most recent scan and use that. But if they have changed, then would it make sense also do a difference between scans and use that?
Any advice would be greatly appreciated!
Many thanks,
Devin Sodums
Research assistant, Jones-Gotman Lab
Montreal Neurological Institute
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