> S#0 will give me the intercept (and is associated with the major component of the hemodynamic response,
> as the first component from SPMG2, right? ) and S#1 will give me the slope, or the signal modulated by the
> parametric modulation of the function component (please correct me if I am wrong).
Yes, that's right.
> I still don't know what will represent the output for which I ask just the regular glt (S-I) regressor, without
> specifically mentioning which contrast (intercept = 0 or slope =1) to provide? It will provide kind of an average
> between those contrasts (intercept/slope)?
It does not make scientific nor physical sense to average the intercept with the slope. The two have different unit, similar to the difference between distance and speed.
> I understand that S#0 and S#1 will provide the intercepts (S#0 representing the major component of the
> hemodynamic response while S#1 representing the first-order derivative), while S#2 and S#3 are the corresponding
> slope/modulation effects of the two components of the SPMG2 function (again, please correct me if I am wrong).
Correct.
> Similar for SPMG1, what significance will have the output for which I ask just the regular glt (S-I) regressor, without specifically
> mentioning which contrast to provide (0/1=intercept/slope without modulation and 2/3=intercept/slope with modulation)?
Again that's not meaningful nor interpretable when you average the two effects.
Gang