I am having a debate with one of my graduate students. We have 6 runs
(sessions) in this experiment. We have 6 different lists with the
conditions in different orders in each list. Because of counterbalancing
the order the lists is different across subjects (some do list 1 1st, some
list 2, etc). While doing single subject 1st level analysis, for pure
convenience reasons, she did not model the sessions in the order they
actually occurred, but rather based on the list they were doing.
My thought is that the session effects not only represent the variance that
is different between each session, but should also represent the actual
order the subject did the sessions because there is meaning in the order of
them. The question is - does the way SPM deals with this regressor take
into account sessions as an ordinal value where their number has meaning or
does it treat them as nominal so it really doesnt matter what order they are
put in?
It still seems to me one's model should match as closely as possible what
you think actually is going on - but if its purely for aesthetic reasons she
probably doesnt have to reanalyze. On the other hand, if the order is
meaningful and by shuffling them we are not taking advantage of how
effective the estimate of these regressors are then reanalysis is necessary
even if no visible differences are seen between the results.
BTW: We are using SPM8.
best,
elise temple
Elise Temple, Ph.D.
Educational Neuroscience Laboratory
http://www.dartmouth.edu/~templelab/
Assistant Professor
Department of Education
Graduate faculty in Psychological & Brain Sciences
Dartmouth College
Hanover, NH 03755
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