Hi!

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Gang
One thing needs clarification: Is the duration for each trial correlated (or confounded) with the pleasantness rating? If so, modulation analysis would be a little bit shaky.

Each Condition (A-D) has 3 onsets per run. They run 2 runs so each condition has a total of 6 onsets. Each onset has a fixed duration of 10 s (no correlated to pleasentness).

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Gang
They would not be the same, but usually they should not differ too much either unless the ratings are very screwed.

Okey! Thanks.

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Gang
Centering would only have impact on the interpretation of the first effect estimate (b0), not that of the rating effect (b1). When comparing b0 across A and C when centering across A and C, you get the difference of b0 between A and C when the pleasantness is held at the common mean of the ratings between A and C.

Not sure I understand why b1 is not affected by a different centering. b1 is the mean minus the rating, right?. If we have two conditions with 4 ratings:
ratings1: 4 4 6 6 (mean=5)
ratings2: 2 2 4 4 (mean=3)
In the case of the default (within) centering the b1 regressors would be (for rating - mean):
ratings1: 4-5 4-5 6-5 6-6 = -1 -1 1 1 (i.e. 4 bumps with amplitude 1)
ratings1: 2-3 2-3 4-3 4-3 = -1 -1 1 1 (i.e. 4 bumps with amplitude 1)
These two would be identical, even if the rating 1 was overall more pleasent.
In the case of acrosss condition centering the b1 regressors would be (for rating - mean):
Across cond mean = 5 +3 / 2 = 4
ratings1: 4-4 4-4 6-4 6-4 = 0 0 2 2 (i.e. 2 bumps with amplitude 2)
ratings1: 2-4 2-4 4-4 4-4 = -2 -2 0 0 (i.e.2 bumps with amplitude -2)
Here you can separate them. Where do I missunderstand this? Thanks!

I'm ofcoruse assuming you are right. So if centering does not matter for b1 then, when constrasting A and C, you would only find differencies if the ratings did not differ the same ammount from the within mean?

So, if they want to find voxels that correlate with rating they should simply compare b1 of e.g. A and C (they found nothing doing this btw)?

What would be the difference/prefered way: Doing this or using the ratings as covariats in the group analysis? Thanks a bunch Gang!



Edited 1 time(s). Last edit at 10/24/2017 01:04PM by Robin.