Question about FSL design matrix - unbalanced contrasts

1

Hello all,

I am new to FSL and am creating my design matrix for Level 1 FEAT contrasts. I have a question about “unbalanced contrasts”.

For example, we have three trial types, let’s call them: Very Mean, Moderately Mean, and Nice

I want to have a contrast that looks like so (Very Mean + Moderately Mean) - Nice and call it All Mean - Nice

In FSL would that be: 1 1 -1

OR do I have to do: .5 .5 -1

I’m not sure if I’m able to have two +1’s and only one -1 or if I have to take the two +1s and make them +.5s instead, so that it is balanced with the -1.

Thank you!

2

Hi, if you want the “Mean” stimuli as a whole to be weighted the same as the “Nice” stimuli, then you would want to use the “.5 .5 -1” contrasts.

3

Hi Steven, Thank you so much! I appreciate the help.