The Arnold tongues diagram for a Class 1 Izhikevich neuron driven by an external sinusoidal forcing. This plot represents the mode-locked regions as a function of the amplitude and frequency of this sinusoidal forcing. Each colored region represents a different phase-locked regime in terms of an integer ratio. Here, an n:m ratio means the neuron produces n action potentials in response to every m cycles of the stimulus. For example, for stimulus amplitudes and frequencies corresponding to the yellow region, the neuron exhibits 3:1 mode-locking.