Iím trying to compare pre and post treatment brain activity in 11 subjects,
who were exposed (during scan) to 3 different categories of stimuli, presented
through visual and auditory modalities (individually). Stimuli for each subject
were presented in a 12-run block design (6 visual, with 2 runs of each of 3
stimuli categories; 6 auditory, with 2 runs of each of 3 stimuli categories). I
wish to analyze separate, and conjoined (V + A), activation over three time
points (1 pre- and 2 post- treatment scans).
I set up a 2nd level 3-way within subject repeated-measures ANOVA modeling
only the subject factor and the interaction between time and condition (visual
vs. auditory X 3 stimuli categories = 6 possible conditions). From this I can
still get main effects by using the appropriate contrasts (corresponding to the
way it's done in a recent between group contrast-weights 2008 SPM tutorial).
However, I get no suprathreshold voxels (i.e. p = 0.001 sets lower t value at
2,000 something). Sometimes it works (i.e. I got normal looking results with 10
subjects on my home computer) but most of the time it doesn't. Can anyone
offer any advice? Thank you!