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Christian, The equations and comparison are identical and can be illustrated through rearranging the equation of each test. Your first group contrast was: (G1C1-G1C2) - (G2C1-G2C2) If one rearrange the equation without the parenthesis, the one gets: G1C1-G1C2-G2C1+G2C2 Now we can reorder this: G2C2-G2C1-G1C2+G1C1 Then regroup the equation with parenthesis and get: (G2C2-G2C1) - (G1C2-G1C1) Which is the second group contrast that you did. As you can see the equation is the same in both of your analyses and thus the results will be identical. Hope this helps. Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Postdoctoral Research Fellow, GRECC, Bedford VA Website: http://www.martinos.org/~mclaren Office: (773) 406-2464 ===================== This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406-2464 or email. On Wed, Jan 23, 2013 at 2:36 PM, Christian <[log in to unmask]> wrote: > 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