Dear SPM list,
I conducted an event-related study (TR=2.0 sec) in which
participants looked at pictures of common objects during three scanning
runs. I later had a separate group of participants rate the stimuli on
several dimensions (e.g., how easily a picture can be imagined). I now
want to see if the imagery ratings predict the intensity of activity within
the clusters of activity in the standard fixed effects analysis of the FMRI
data. I assume that this information should be entered as a user-specified
regressor in the design matrix. I have two questions though:
First, should I mean correct the vector describing the imagery ratings?
The problem with mean correcting the scores is that some of the values are
now negative. In this case, I expect that activity should increase with
each picture presentation, but that it should be greater based on how
easily it can be imagined. When I mean correct, half of the scores are
negative, which might suggest deactivation for a given stimulus
presentation. While researchers often mean correct their regressors, is
mean correction always necessary?
Second, if I do mean correct the scores, should I use the mean rating of
the stimuli within the run, or should I use the overall mean rating for the
stimuli? In other words, should I use the same mean value in all three
runs, or should I use a calculate a mean rating for just the stimuli in a
given run, and subtract that value from each individual picture’s rating in
that particular run?
Thanks for any suggestions!
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