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From: Thomas Nichols <[log in to unmask]>
Date: Fri, 14 Oct 2005 9:17:41 -0400
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Esa,
> How does the search volume affect the severity of correction for multiple
> comparisons in SnPM? In SPM the severity of the multiple comparison
> correction is proportional to the volume of search volume (to the number
> of resels).
The short answer is that yes, a nonparametric approach does account for
> What kind relation can be expected in SnPM?
>
> We know that the permutation distribution of the maximal statistic deals
> with the multiple comparisons correction. Is it just that with a small
VOI
> the critical threshold of the permutation distribution of maximal
statistic
> gets lower as compared to the whole brain analysis thus enhancing
sensitivity?
>
> Best regards,
> Esa Wallius
>
> On Tue, 4 Oct 2005, Thomas E Nichols wrote:
>
> > Victor,
> >
> >> I am doing a non-parametric second-level analysis (12 con images) with
> >> SnPM-99.
> >>
> >> I understand that there is no "SVC" option in SnPM, however could I
> >> still anyhow benefit by having a spatially-constrained hypothesis
> >> about activation?
> >
> > Definitely! Anytime you can constrain your analysis you can generally
> > increase your sensitivity. Specifically, by using a ROI you are
> > reducing the severity of your multiple comparisons problem, and hence
> > will be able to use a lower threshold, and so get more power.
> >
> >> For the sake of experiment I tried to perform an SnPM99 analysis on
> >> con-images previously masked by a VOI. I noticed that the "critical
> >> threshold" reported at the bottom of Table is lower for the VOI
> >> analysis then for the whole brain analysis, both were done with the
> >> same variance smoothing, number of permutations, thresholds (voxel
> >> p<0.05, no extent threshold), etc. Moreover the VOI-based analysis
> >> revealed a significant activation that did not show up during the
> >> whole-brain analysis.
> >
> > Yup... this is totally consistent with what I wrote above. The VOI is
> > yielding a lower threshold and hence greater sensitivity. Makes
> > sense, no?
> >
> > -Tom
> >
> >
> > -- Thomas Nichols -------------------- Department of Biostatistics
> > http://www.sph.umich.ed
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