My question is how to obtain the superstring (Type II A and B) vacuum amplitudes on a torus. They are given in Polchinski's String Theory Vol. 2 equation (10.7.9):
Z±ψ=12[Z00(τ)4−Z01(τ)4−Z10(τ)4∓Z11(τ)4]
I understand how each individual Zαβ is obtained but do not understand how they are put together to get Z±ψ.