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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).` 

HTH, 
C 

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----- Mail original -----

De: "Joelle Zimmermann" <[log in to unmask]> 
À: "Christophe Phillips" <[log in to unmask]> 
Cc: "[log in to unmask]" <[log in to unmask]> 
Envoyé: Mercredi 9 Septembre 2015 22:12:58 
Objet: Re: [SPM] contrasts 

Thanks Chris, 

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? 



On Wed, Sep 9, 2015 at 4:04 PM, Christophe Phillips < [log in to unmask] > wrote: 




Hi Joelle, 

it depends on what you mean by " compare Trial 1 with the rest of the trials ": 
- if "rest of the trials" = "average response of other 9 conditions", then use c = [ 1, -1/9, -1/9, -1/9, -1/9, 1/9, -1/9, -1/9, -1/9, -1/9] 
- if "rest of the trials" = "sum of responses of other 9 conditions", then use c = [ 1, -1, -1, -1, -1, -1, -1, -1, -1, -1] 
And obviously this will NOT lead to the same answer. :-) 

Cheers, 
Chris 

Christophe Phillips 
Cyclotron Research Centre 
University of Liège, B30 
4000 Liège, Belgium 
T: +3243662316 
F: +3243662946 


De: "Joelle Zimmermann" < [log in to unmask] > 
À: [log in to unmask] 
Envoyé: Mercredi 9 Septembre 2015 21:51:26 
Objet: Re: [SPM] contrasts 


To followup from my previous email, I could also do 1, -1, -1, -1, -1, -1, -1, -1, -1, -1.... 
But this gives me a very different result. 

On Wed, Sep 9, 2015 at 11:47 AM, Joelle Zimmermann < [log in to unmask] > wrote: 

<blockquote>

Hi SPMers, 

I have an idea for a contrast, I just want to check with someone here if I'm doing the right thing: 

I have 10 trials (each one condition), and want to compare Trial 1 with the rest of the trials. is it correct to set up the following contrast: 

1, -1/9, -1/9, -1/9, -1/9, 1/9, -1/9, -1/9, -1/9, -1/9 

Thanks, 
Joelle 






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