Figure 3.

Definition of the U(1) phase. (A) The action potential of a spiking neuron is marked by its potential minimum $V_{\min }$ (point a, e), average $\overline{V}$ (point b, d) and maximum $V_{\max }$ (point c). (B) The points from a to e corresponds to the U(1) phase $\phi =\pi ,3\pi /2,0,\pi /2,\pi$ respectively. A complete firing process can thus be viewed as a winding process of $\mathrm{\Delta }\phi =2\pi$ in the phase dynamics.