“…Model (1) without the term of the 2nd-order Volterra kernel is the GLM (Friston et al, 1995). There is a vast literature on the estimation of the HRF h i,k ( t ), including parametric methods (e.g., Worsley and Friston, 1995; Friston et al, 1998a; Glover, 1999; Henson et al, 2002; Riera et al, 2004; Lindquist and Wager, 2007; Lindquist et al, 2009) and nonparametric methods (e.g., Aguirre et al, 1998; Dale, 1999; Lange et al, 1999; Woolrich et al, 2004; Zarahn, 2002; Vakorin et al, 2007; Bai et al, 2009; Wang et al, 2011). Estimation of V i , k 1 k 2 ( t 1 , t 2 ) is more challenging than that of the HRF, because the Volterra kernel function, defined on the two-dimensional space, involves many more parameters, while the number of observations, T , for each subject is usually limited.…”