Hello All,
This question involves first-level analysis in FSL using the Full Model Setup for statistics.
I’ve been trying to figure out a way of comparing 1 conditions activation, against the summed activation of 2 other conditions. Right now each condition is represented by an original EV. Initially, I was going to do this by collapsing the conditions in timing files (make a timing file for conditions 2 and 3, rather than 2 separate files). But after looking through the FSL documentation, I think it can be done via contrasts.
Let’s say I have 9 conditions. I want to compare Condition 1 to the activation in condition 2+3. This is to find where the activation in condition 1 differs from only conditions 2+3 (ignoring the other conditions 4-9).
The documentation seems to suggest it would be set-up by specifying the contrast this way [1 -1 -1 0 0 0 0 0 0]. So I set to zero all the EVs that I don’t care about (conditions 4-9). Set to -1 the conditions that I’m comparing against and set-to 1 the condition I want the activation for (that is different than the activation across the -1 EVs).
The entire design matrix would be as follows (y axis =contrast, x axis=original EV)
1 -1 -1 0 0 0 0 0 0
-1 1 -1 0 0 0 0 0 0
-1 -1 1 0 0 0 0 0 0
0 0 0 1 -1 -1 0 0 0
0 0 0 -1 1 -1 0 0 0
0 0 0 -1 -1 1 0 0 0
0 0 0 0 0 0 1 -1 -1
0 0 0 0 0 0 -1 1 -1
0 0 0 0 0 0 -1 -1 1
Though, perhaps the -1 should actually be specified as -.5 (since there are 2 EVs involved).
My worry is that what I’ve found in the documentation doesn’t involve ignoring some EVs. See this excerpt:
“An important point to note is that you should not test for differences between different conditions (or at higher-level, between sessions) by looking for differences between their separate individual analyses. One could be just above threshold and the other just below, and their difference might not be significant. The correct way to tell whether two conditions or session's analyses are significantly different is to run a differential contrast like [1 -1] between them (or, at higher-level, run a higher-level FEAT analysis to contrast lower-level analyses); this contrast will then get properly thresholded to test for significance.”
So, I’m not sure that this is actually the correct usage. From the documentation, an F-Test would not seem to be appropriate. The F-Test gets used mainly when the total activation of multiple conditions needs to be tested against baseline (not against other conditions).
Thank you all in advance for the help!
-Jennifer
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