Hello SPM users and experts,
I would like to compute t-contrasts of single beta coefficients to use in multivariate pattern analyses, following Miyaki et. al (2010) showing superior performance decoding with t-values compared to beta coefficients. I'd like to do it on the fly rather than creating spmT images, since this seems wasteful.
I believe the basic operation is to divide the voxelwise beta weight by the sqrt of the ResMS image. I have compared the output of this computation with the output of a formal spmT contrast made with the batch editor. They are related by a constant scale factor (in my test case, 73.55).
My question is: what is the constant scale factor? It should not matter for any subsequent calculations, this is simply so that I can properly say I am dealing with t-values, and for my understanding.
Thank you.
Nick
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