# What is the output of neuron $y_{2}$ at time step $t$?

In Fundamentals of Neural Networks: Architectures, Algorithms And Applications by Laurene V. Fausett on $$\text{Page:32}$$ it describes Hot and Cold perception modeling with McCulloch-Pitts Net

the book describes the output of neuron $$y_2$$ at time step $$t$$ as below :

$$y_{2}(t)=x_{2}(t-2) \operatorname{AND} x_{2}(t-1)$$ I can't understand why there is $$x_{2}(t-1)$$? I think it should be $$z_{2}(t-1)$$, which one of them is correct?