Hi, much thanks in advance for your assistance.
I have some basic questions about setting up gltCode tests in 3dLME and interpreting them. I’ve tried most of these things out (except Q5), and things seem to be working. I just want to make sure, though, that I’m not doing anything incorrectly, given that I can’t find the answers to these questions online.
]Background. I’m using a fully-crossed balanced design with no missing data of the form A x B X C. A is a group variable with 2 levels, A1 and A2 (20 participants at each level). B and C are repeated measures, each with two levels, B1 and B2, and C1 and C2, respectively.
The output that I’m getting for this design from 3dLME shows F briks for each main effect and interaction, and for the intercept. Nothing else.
Q1. When I add gltCode contrasts, I obtain a z brik and an unlabeled brik. I assume contains betas (looks like betas)? This is all fine, but I had been expecting a t brik, not a z brik. I just want to make sure that this is ok, and if so, I’d like to better understand why a z brik has been generated and not a t brik. Is there a brief reason?
Q2. I would like to obtain z briks for each level of each factor in the design (e.g., A1). Am I correct that the appropriate code for generating these 6 maps in my design is:
-gltCode # ‘A : +1A1’
-gltCode # 'A : +1A2’
-gltCode # ‘B : +1B1’
-gltCode # 'B : +1B2’
-gltCode # ‘C : +1C1’
-gltCode # 'C : +1C2’
If this is not correct, can you let me know if there’s a way to generate these maps using -gltCode statements. I realize that I can always run 3dDeconvolve on only the trials for each level of a factor in the individual analyses, and then analyze these at the group level with 3dLME. If there’s any easier way using -gltCode, though, it would be preferable.
Q3. I would also like to obtain a brik for the grand mean. Am I correct that any of the following would work, and should be equivalent (again the design is fully crossed and balanced, with no missing data)?
-gltCode # ‘A : +1A1 +1A2’
-gltCode # ‘B : +1B1 +1B2’
-gltCode # ‘C : +1C1 +1C2’
If this is not correct, can you let me know if there’s another way to generate this map with a -gltCode statement. And again, if necessary, I assume that I could compute it by aggregating all trials at the individual level and then analyzing the individual maps at the group level.
Q4. When testing a simple effect of one factor at one level of a second factor, I’ve been using statements like:
-gltCode # ‘A : +1A1 -1A2’ : B +1B1’
-gltCode # 'A : +1A1 -1A2’ : B +1B2’
I want to be sure that when specifying the level of B, regardless of whether it’s B1 or B2, the correct weight in BOTH cases is +1 (i.e., NOT +1 in one case and -1 in the other).
Q5. Imagine that I add a quantitative variable for an individual difference measures to the design, say Q. In a -gltCode statement, what would the following code (adapted from the 3dLME examples) request:
-gltCode # ‘A : +1*A1 Q :’
Is this asking for the interaction between A and Q at the level of A1? Or something else?
If I wanted the interaction between A and Q overall, how would I request it? I realize that there may be an F brik for this, but if I wanted a directional t brik, how would this be requsted in in gltCode statement?
Please feel free to add any further advice or suggestions that you see relevant.
Much thanks for your patience and assistance!