On Fri, 25 Sep 2009 12:42:51 -0500, Dirk den Ouden <d-
[log in to unmask]> wrote:
>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!
You definitely should _not_ do "statistics on significance" by computing the
significance of differences in the F-statistics.
Instead, you should devise a summary measure for the FIR, such as "sum of
responses for time bins 3 through 7".
>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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