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

-


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:
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