What is the canonical pdf used in R's fmri.stimulus in the 'fmri' package?

Sure, the documentation below shows the function, but it doesn’t specify d1 and d2. The reference paper shows di=ai*bi, but I’m getting slightly different results when writing out that pdf and comparing it to the package. Where does duration fit into this?

Using the canonical HRF with default parameters, I wrote the following pdf:

Thank you in advance!

fmri.pdf (r-project.org)