def ddspmt(t):
return (spmt(t) - _spm_dd_func(t)) / <NUM_LIT><EOL>
SPM canonical HRF dispersion derivative, values for time values `t`

    This is the canonical HRF dispersion derivative function as used in SPM.

    It is the numerical difference between the HRF sampled at time `t`, and
    values at `t` for another HRF shape with a small change in the peak
    dispersion parameter (``peak_disp`` in func:`spm_hrf_compat`).