Dear SPM experts,
I’m a SPM beginner, doing a fMRI study with two groups (19 patients & 19 controls). I am using a block design with three different conditions (condition1, condition2 and rest). Right now, I’m doing the 2nd level analyses and I have met a problem that is quite confusing me.
First, I have done the within group comparisons (FWE p=0.05). For the comparison condition1 vs. rest the controls show a very high t-value, T=17.44, in one special region (Temporal Pole Sup left; -48 8 -2; cluster size 525 voxels). However, the patients show absolutely no activation in this region. So, I have expected, that if the controls have such a high t-value whereas the patients show no correspondent t-value for the same area, the between group comparison controls vs. patients has to produce a significant result in this area, indicating a much higher activation for the controls. But the between group comparison - FWE corrected as well as uncorrected! - shows absolutely no group differences in this area. This is not what I have expected and I can’t explain this result. I hope that someone of you has an idea…
A further question refers to the above mentioned t-value. The t-value T=17.44 seems to be extraordinary high to me – especially compared to all the other t-values in my analyses. So, I checked the t-values for every single control subject (n=19) for this special region, in order to get an explanation for this very high t-value. The result was quite confusing, because the single subjects only showed t-values ranging from -0,22945 to 1,05096. This range doesn't explain T=17.44 to me. I hope, that someone of you has an explanation for that high t-value…
Thanks in advance for any help you provide!
Christian
|