various measures of the synchronization of coupled systems [@ROS01a]. For instance, one can consider the average interspike intervals of each subsystem, *i.e.* $\langle T_1\rangle $ and $\langle T_2\rangle $, calculated from the $x$ variable of the respective subsystem. Their ratio $\langle T_1\rangle /\langle T_2\rangle $ is a measure of frequency synchronization, as depicted in Fig. \[fig2\]. Panel (a) displays the ratio of average interspike intervals in dependence on the noise intensity $D_1$ (green dots) for fixed coupling strength $C=0.07$. One can see that for increasing $D_1$ the ratio $\langle T_1\rangle /\langle T_2\rangle $ decreases. Thus, the two subsystems become less synchronized. Panel (b) shows the dependence on the coupling strength $C$ for fixed noise intensity $D_1=0.25$. Without coupling ($C=0$), the two subsystems operate on their own timescale. For increasing $C$, however, they synchronize as the ratio of the average interspike intervals approaches unity. Panel (c) shows the result as a function of $D_1$ and $C$, where the bright regions indicate a strongly synchronized behaviour of the two subsystems. For small $D_1$ and large coupling strength $C$, the two subsystems display synchronized behaviour, $\langle T_1\rangle/\langle T_2\rangle\approx 1 $. On average, they show the same number of spikes indicated by bright (yellow) 