Dear SPM experts,
I am using SPM2 to design a t-test between two groups of PET images.
I encounter errors every time I try to do so with images derived from
a subtraction function in ImCalc. I will outline the procedures and
problems briefly :
1) I used ImCalc to subtract baseline condition from active
2) I prepared a t-test between these two groups under the PET
model. "compare populations: one scan/patient (two sample t-test)",
with proportional scaling global mean=50, threshold masking relative
(.8), Global calculation=mean voxel value, with no non-sphericity
3) After estimating the model, I chose the results option and
defined the contrast. However, when I tried to name the comparison,
the program encountered an error. The MATLAB window read:
??? Attempted to access x(1); index out of bounds because numel(x)=0.
Error in ==> spm_Tcdf at 92
F(ml) = 0.5 + atan(x(mlx))/pi;
Error in ==> spm_getSPM at 445
Ps = (1 - spm_Tcdf(Z,df(2))).^n;
Error in ==> spm_results_ui at 260
[SPM,xSPM] = spm_getSPM;
??? Error while evaluating uicontrol Callback
A few helpful pieces of the puzzle:
-This analysis works when I used small group sizes (4 per group), but
exhibited these errors with larger groups.
-My friend advised me to set up larger group comparisons in this
model: no global normalization, no grand mean scaling, Threshold
masking (absolute) set to –Inf, omitting global calculation, and no
non-sphericity correction. This allowed me to retrieve results,
however they were scattered pixels outside of the brain and did not
appear to be valid.
Is there a way to solve this error so that I may compare two groups of
subjects' "active-baseline" scans. If there is no way to solve the
problems associated with the ImCalc images, then is there another
statistical model that I can use to compare groups in such a way?