Dear experts,
I have run an fMRI experiment with auditory stimuli that are all under
1s long. I have modelled these as events (7 conditions A,B,C,D, E,F & G)
in a first level model. Some conditions contain significantly longer
stimuli than others. I included the stimulus length as a modulator for
each condition but from previous messages on the list I understand
that this will covary out length differences within a condition but not
between conditions.
So as I understand this a comparison of conditions A and B (A-B or B-A)
will not show me differences between A & B having co-varied out the
effect of length.
There has been a suggestion on list to model all events as 1 condition
type and to then model event types as modulators. In this case all event
types will be modelled in one column and there will be 8 modulators, the
first 7 indicating whether each event is of type A, B, C,D, E,F or G and
the 8th modulator will be length. The columns indicating trial type
would be modelled with 1s and -1s. Is it the case that A-B in this type
of model will show the differences between the 2 conditions having
co-varied out the effect of length?
Effectively I am trying to find out whether there is a significant
effect once the confound has been removed.
Many thanks for your help,
Emmanuel A Stamatakis PhD Tel: +44 (0) 1223 766454
Department of Experimental Psychology Fax: +44 (0) 1223 766452
University of Cambridge
Downing Street
Cambridge CB2 3EB
U.K. http://csl.psychol.cam.ac.uk
|