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Hi Michael, but this also assumes that the null data is normally distributed, right? Is there any general opinion whether fMRI null data is normally distributed or not? /Anders Michael Harms skrev 2010-10-28 16:10: > HI Anders, > Keep in mind that the distribution of the statistic is under the null > hypothesis of no signal. If you are examining the distribution (across > voxels) of the t-statistic on actual data from an experimental task, the > distribution may very well not be a t-distribution (e.g., say half your > voxels contain a strong, real response -- then your distribution of t- > values will be skewed positive relative to the t-distribution for null > data of the same dof). > > If you input true null data into a processing stream, then by definition > you better get the expected distribution (otherwise, that points to the > existence of a problem/bug in your processing stream). > > cheers, > -MH > > On Thu, 2010-10-28 at 13:16 +0200, Anders Eklund wrote: >> Dear SPMers, >> >> I've implemented my own single subject fMRI analysis in Matlab (slice >> timing correction, motion compensation, smoothing, detrending, GLM& >> t-test) and get activity maps that seem reasonable. I now want to >> calculate a threshold that is corrected for multiple comparisons and >> have for that purpose used two approaches, Bonferroni correction and >> random field theory. As I understand it, both these methods rely on the >> fact that the test statistics, under the null hypothesis, follows a >> Student's t-distribution (since I calculate a t-test value in each voxel). >> >> For most of my datasets the test statistics seems to approximately >> follow a Student's t-distribution with the same degrees of freedom, but >> it is often slightly wider at the bottom or slightly skewed. For one >> dataset, the test statistics is rather far from the t-distribution. >> >> 1) How much work has been done on validating the assumption that the >> test statistics follow a certain distribution? Is there any paper where >> this is discussed? >> >> 2) Is there any specific preprocessing step, that I might have missed, >> that ensures that the test statistics follows a certain distribution? >> >> 3) Does SPM check the distribution of the test statistics? >> >> Best regards, >> Anders Eklund >>