> 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?

Yes, your interpretation seems correct. If you meant the default of 3dLME for centering, the center value is actually the overall average, not 0.

> If I want to see the direction of the main effects, should I code GLTs as shown in the script?

They look fine.

> 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)?

Under the conventional ANOVA framework, they would usually end up the same. Under LME, these two types of statistics (F- and Z-statistic) are obtained through a slightly different formulation. So it's expected the two do not exactly match. As long as they are roughly similar, it should be fine. The exact number of voxels in a cluster is defined arbitrarily anyway because of the artificial dichotomization.

Gang