3dDeconvolve basis function

Hello there,
I am trying to build a proper general linear model for an experiment, I have timings and durations of stimuli. I want to include the HRF with its time and dispersion derivatives. As far as I understood, SPMG1 is the HRF, and SPMG2 is the HRF function with its time derivative, but I'm not sure what the third parameter produced when I use SPMG3 is. My question is whether this is the dispersion derivative I'm looking for, and if not then how can I include it?
Thank you in advance.

The third basis function represents the dispersion curve. To confirm this, you can use the following command:

3dDeconvolve -nodata 32 1.0 -num_stimts 1 -polort -1 \
-local_times -x1D stdout: -stim_times 1 '1D: 0' 'SPMG3(2)' \
| 1dplot -thick -one -stdin -xlabel Time -ynames SPMG3

Executing this command generates the following output:

If applicable, you might also consider an alternative and potentially more sensitive approach, which is discussed in detail in this blog post.