Hi All,
In an fMRI study with 2 groups, I take the contrast maps generated
from the first level (subject-level) analysis and enter these into a
second level analysis. A contrast of: 1 -1 will tell me regions with
activation that are significantly greater is Group 1 vs Group 2.
However, these regions may not be significantly active in Group 1.
Correct?
If I only want to examine areas where there is a main effect of the
task, I should do the following: with the second level analysis, first
generate a contrast of 1 1 telling me the regions, meaned across all
subjects, that are significantly active (-1 -1 would do the inverse).
Then I use the contrast 1 -1 with an inclusion mask from the contrast
1 1. This will then tell me regions with activation that are
significantly greater is Group 1 vs Group 2 but limiting the analysis
to regions where there is a main, positive effect of the task.
Correct?
It seems to me that interpreting one's findings will be far more
straightforward when one masks for main effects. Are there situation
when I would not want to do this?
Thanks!
Fred
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