Dear
all,
within single
subjects, I want to compare effect sizes for different conditions and between
time points, irrespective of the shape of the hrf (even negative responses and
weird shapes are allowed), as this may be different between regions and change
over time in the subjects I am studying. I have modelled the responses to the
various conditions as FIR functions (10 time bins of 2 seconds) and main effects
are easily calculated that way, with an F-test. However, statistical comparison
between conditions is trickier, without assuming a specific shape or even
time-to-peak of the responses. NB, I do not want to compare specific time bins
to each other - all I want is to compare the effect sizes of the overall
responses, irrespective of shape.
It seems to me I can
subtract the spmF_000x.img files for 2 conditions, the resulting image
giving me the difference in F-values (representing effect size) between
these two conditions, for each voxel. However, I am not sure what height
threshold to use on this difference-image. Should I use the height
threshold, in terms of an F-value, that I obtain from performing a
main-effect analysis for 1 of the conditions, with a specific correction for
multiple comparisons, or is there another calculation that will tell me what
height threshold to use if I want to report, for example, only voxels
with a significantly different effect size at an FWE corrected alpha level
of .05?
Any comments or
advice is much appreciated!
Thanks,
Dirk
************************************
Dirk-Bart den Ouden, Ph.D.
Aphasia & Neurolinguistics Research Laboratory
Dept. of
Communication Sciences & Disorders
Northwestern
University
2240 Campus
Drive, Evanston IL
60208-3066
Phone: (office)
1-847-467-2515; (lab) 1-847-467-7591
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