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
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Anders Eklund
Phd student
Medical Informatics, Department of Biomedical Engineering
CMIV, Center for Medical Image Science and Visualization
Tel: +46 73 6003790 mail: [log in to unmask]
Fax: +46 13 101902 web: http://www.wanderineconsulting.com/
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