I am assuming you did not leave out brackets in your expression of the contrast.
So, 1 -1 1 -1 -1 1 tests if:
The sum of activity in tasks 6 4 and 1 is greater than the sum of activity in tasks 5 3 and 2.
It does not provide any specificity as to which tasks are driving the effects, so it is possible to see significance in this case which is driven by a sub-set of your tasks.
In my opinion, in such a case you should ALWAYS plot your beta values afterwards to see if this is the case.
Hope that helps!
Colin Hawco, PhD
Neuranalysis Consulting
Neuroimaging analysis and consultation
www.neuranalysis.com
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-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of MRI Study
Sent: January-13-16 7:31 AM
To: [log in to unmask]
Subject: [SPM] Contrast weights
Dear SPM community
my design is:
Task1 Task2 Task3 Task4 Task5 Task6
and I want to calculate the contrast:
Task6-Task5-Task4+Task3-Task2+Task1
Are the following weights correct:
1 -1 1 -1 -1 1 ?
Thanks in advance for any input.
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