Taking the sum with so many (9) conditions wouldn't make much sense indeed.
I could image a design where you either sent auditory or visual stimuli, or both simultaneoulsy. In this example, you could be interested in comparing the sum of the single activations (auditory + visual) with the activation of the joint stimulations (auditory & visual).
A contrast just translates the question you have (expressed in words) into numbers given the design matrix (which expresses you experimental design).`
"Joelle Zimmermann" <[log in to unmask]
"Christophe Phillips" <[log in to unmask]
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Mercredi 9 Septembre 2015 22:12:58Objet:
Re: [SPM] contrasts
I understand the average concept: ie Null: activation in trial 1 = average activation in rest of the trials
But finding the sum a bit hard to wrap my head around, would it ever really make sense to sum the activation in the trials 2-9?