We analyze the phenomenon of anticipating synchronization of two excitable systems with unidirectional delayed coupling which are subject to the same external forcing. We demonstrate for different paradigms of excitable system that, due to the coupling, the excitability threshold for the slave system is always lower than that for the master. As a consequence the two systems respond to a common external forcing with different response times. This allows to explain in a simple way the mechanism behind the phenomenon of anticipating synchronization.The synchronization of nonlinear dynamical systems is a phenomenon common to many fields of science ranging from biology to physics [1], and it has been an active research subject since the work by Huygens in 1665. Recently, the synchronization of chaotic systems in a unidirectional coupling configuration has attracted a great interest due to its potential applications to secure communication systems [2]. Particular attention has been payed to the so-called anticipating synchronization regime, an idea first proposed by Voss in [3]. He showed that, in some parameter regions, two identical chaotic systems can be synchronized by unidirectional delayed coupling in such a manner that the "slave" (the system with coupling) anticipates the "master" (the one without coupling). More specifically, the coupling scheme proposed in [3] for the dynamics of the master, x(t), and slave, y(t) is the following:where y τ ≡ y(t − τ ). For appropriate values of the delay time τ and coupling strength K, the basic result is that y(t) ≈ x(t+τ ), i.e. the slave "anticipates" by an amount τ the output of the master. This regime and its stability has been theoretically studied in several systems, from the simplest ones described by linear differential equations and maps where the mathematical details can be fully worked out [4,5], to the more complicated ones such as semiconductor lasers [6] operating in the chaotic regime. Experimental evidence of anticipating synchronization has been shown in Chua circuits [7] and in semiconductor lasers with optical feedback [8].This same phenomenon has recently been shown to occur also when the dynamics, instead of chaotic, is excitable. In refs.[9] the effects of unidirectional delayed coupling between two identical excitable systems was studied for both the FitzHugh-Nagumo [10] and Hodgkin-Huxley [11] models. It was shown that, when both systems are excited by the same noise, and for a certain range of coupling parameters, the randomly distributed pulses of the master are preceded by those of the slave. This allows for predicting the occurrence of excitable pulses in the master. Since many biological systems (as neurons and heart cells) exhibit excitable behavior and they often operate in feedback regime in a noisy environment, the study of the delayed coupling effects in a presence of noise is certainly of wide concern.The anticipating synchronization regime has been often described as a rather counterintuitive phenomenon because of the possibility of the s...