“…In 1989, Marcus and Westervelt [22] introduced for the first time a discrete delay in the Hopfield model (1.2), and they observed that the delay can destabilize the system. In fact, the delays can affect the dynamic behavior of neural network models [3,22] and, for this reason, stability of delayed neural network models has been investigated extensively (see [1,2,4,6,7,9,11,18,19,20,23,25,26,27,28,29,30], and the references therein). Another relevant fact to take into account is that the neuron charging time, the interconnection weights, and the external inputs often change as time proceeds.…”