I have a few questions about the 3dLME.
My design is originally a 2x2 with two within-subject factors (Session: T1, T2) and Velocity (Slow, Fast).
I realized that one of the two sessions make subjects moving significantly more than the other (this was quantified with average movement per TR).
Therefore, in order to exclude that the motion, even after motion correction, could explain the variance between the levels of the factor Session, I decided to add a covariate of no interest “Movement”. Since this would mean that every subject would have a different value for each session, but the same value for each velocity in each session, I decided to use 3dLME. The script looks as follow (data for one subject is shown as example in -dataTable):
3dLME -prefix lme_qvars0 -jobs 30 \ -model "Session*Velocity*Movement" \ -mask simulation_mask+tlrc \ -qVars "Movement" \ -qVarsCenters 0 \ -ranEff "~1+Movement" \ -SS_type 3 \ -num_glt 4 \ -gltLabel 1 'S_F_mov' \ -gltCode 1 'Velocity : 1*Slow -1*Fast Movement :' \ -gltLabel 2 'T1_T2_mov' \ -gltCode 2 'Session : 1*T1 -1*T2 Movement :' \ -gltLabel 3 'S_F' \ -gltCode 3 'Velocity : 1*Slow -1*Fast' \ -gltLabel 4 'T1_T2' \ -gltCode 4 'Session : 1*T1 -1*T2' \ -dataTable Subj Session Velocity Movement InputFile 501 T1 Fast 0.011 path/S2_sub501.results/stats.S2_sub501_REML+tlrc \ 501 T1 Slow 0.011 path/S2_sub501.results/stats.S2_sub501_REML+tlrc \ 501 T2 Fast -0.008 path/S1_sub501.results/stats.S1_sub501_REML+tlrc \ 501 T2 Slow -0.008 path/S1_sub501.results/stats.S1_sub501_REML+tlrc \ ...continues...
Here are my questions:
[li] Since a t test of Movement values between T1 and T2 comes significant, I first centered around the mean of each session. Then in the 3dLME script I am testing what the effects would be if Movement was not happening, i.e. =0 (which I guess would be the default anyway). Is this interpretation correct?
[/li][li] The output of the 3dLME shows the F tests for main effects and interactions. If I want to see the direction of the main effects, should I code GLTs as shown in the script? And why in that case the clusters I amobtaining with the Z tests (corresponding to glt) do not correspond 100% to those of the F tests (e.g., in terms of cluster size)? As you can see I have both tried with Movement effect specified (GLT 2) and not (GLT 4, which I guess would be the way to go), but none of these match 100% the results of the Main Effect.
Thank you in advance for your help!