Dear John,
> In the diagram of my efMRI model basis set (hrf with time and dispersion
> derivatives), it looks as though the dispersion derivative is the 2nd
> derivative of the hrf reflected about zero. Is that the case? If not what
> is it?
It is the derivative of the HRF with respect to dispersion (see
spm_hrf) for how dispersion is parameterized. This is actually very
similar to the second derivative.
With best wishes - Karl
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