Dear SnPM'ers,
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). 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.edu/~nichols University of Michigan
> [log in to unmask] 1420 Washington Heights
> -------------------------------------- Ann Arbor, MI 48109-2029
>
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