Hello SPM experts,
Our group have been analysing data from a 3 (drug) x 2 (condition: normal music, scrambled music) within subjects design. We have initially looked at how the three drugs influence neural response to music (normal music > scrambled music).
The task used a boxcar design (normal music/scrambled music). After each section of music participants also gave pleasantness ratings. We are interested in using these behavioural ratings as parametric modulators. If I understand correctly, this should enable the model to account for greater neural activation for sections that are rated more pleasantly.
I have created con images at the first level, but in a small number of cases the design matrix has rank deficiency. This is because all pieces of music were given the same pleasantness ratings. As a result it is not possible to enter a first level contrast including parametric modulation (grey sections in design matrix). I am interested to know how to account for this (e.g. by entering a completely "neutral" con image, reflecting no parametric modulation, for the following contrast: parametrically modulated normal music > normal music. It would be great if there is a way to do this rather than excluding participants because this would result in all of their sessions (3 drugs) being thrown out.
Thanks very much for your help!
Tom
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