Results of PTM model comparisons.

M1: the most reduced model with all four PTM parameters constant across SFs; M2: reduced model with only internal additive noise varying across SFs; M3: reduced model with changing template gain over SFs; M4: another reduced model with both Nadd and β varying across SFs; M5: the full model with Nadd, β, Nmul, and γ changing with SFs. The Chi-square test for nested model analysis shows that the fourth model (M4) is the best fitting model.