Dear Colin,
I am working on a first-level analysis myself and really liked your comment on the fact that you better not specifically model the OFF conditions.
I added to my model the movement parameters estimated in the motion correction preprocessing steps as multiple regressors. What is your opinion on this? Do you think this is a good idea or would you leave it out?
Kind regards,
Nina
Nina Goossens
PhD researcher
KU Leuven - Department of Rehabilitation Sciences
Tervuursevest 101 box 1501
3001 Leuven
tel. +32 16 37 90 40
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Van: SPM (Statistical Parametric Mapping) [[log in to unmask]] namens [log in to unmask] [[log in to unmask]]
Verzonden: maandag 14 december 2015 23:20
Aan: [log in to unmask]
Onderwerp: Re: [SPM] define contrasts
Firstly, it is not generally considered necessary, or even wise, to specifically model the OFF conditions. If you contrast this with any other condition, you'll see a lot of stuff like negative findings in default and positive activity in visual regions or general cognitive control areas, and unless you have a really specific reason to do so, not much that would be very interesting or interpretable. If you leave the OFF condition out of your model, it is still IMPLICITLY modeled as 'everything else'.
Your post provides very minimal info, but presumably you need to hit the "Define New Contrast" button. Then enter your contrast. For example Positive Vs. Negative t-contrast would be 0 -1 1 0 assuming the even order below. If you are attempting an F-test then you need to enter a valid F-contrast, which is structured differently.
Again I think you should really consider spending some more time going through the tutorials as most of these issues is covered in there
: )
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 Bel Rach
Sent: December-14-15 5:01 PM
To: [log in to unmask]
Subject: [SPM] define contrasts
dear SPMers,
when i am trying to define contrasts in spm contrast manager gui it says "no valid contrast defined"
I HAVE
4 conditions (negative / positive / neutral / rest or off)
- OFF: onsets at 0, 36, 72, 108, 144, 180, 216, 252, 270, 288, 306 with a duration of 18 seconds
- Positive: 18, 162, 306 with a duration of 18 seconds
- Negative: 54, 198, 234 with a duration of 18 seconds
- Neutre: 90, 126, 270 with a duration of 18 seconds.
help please
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