Dear SPM experts,
as a SPM beginner, I am doing a fMRI study with two groups (patients & controls). I am using a block design with two different conditions (condition1 & condition2). On the 1st level analysis I created for every single subject the following difference con-images:
- con1: condition1 – condition2
- con2: condition2 – condition1
On the 2nd level analysis I ran a 2-sample t-test in SPM to investigate differences in activation between patients and controls for the above mentioned contrasts. For con1 (condition1 - condion2) I compared patients -controls [1 -1] as well as controls -patients [-1 1] and got the following results:
- patients show 2 clusters with higher activation
- controls show 5 clusters with higher activation
After that I also compared patients and controls for the second contrast-images, con2 (condition 2 - condition 1). As I did before I compared patients -controls [1 -1] as well as controls -patients [-1 1] and got the following results:
- patients show 5 clusters with higher activation,
but EXACTLY the same clusters/statistics like the controls did for comparision of con1
- controls show 2 clusters with higher activation,
but EXACTLY the same clusters/statistics like the patients did for comparision of con1
So, I am really confused about these results.... I can not explain why patients and controls should show exactly the same clusters and even the same statistics, when I compare both experimental conditions between patients and controls. And I also would absolutely not expect such a connection. Has any one of you an explanation for that???
Thanks in advance for any help you provide!
Christian
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