Dynamical Systems in Neuroscience:

458 Synchronization (see www.izhikevich.com)membrane potential, V (mV)x( )x( ) x( )(c)phase of oscillation(a)200x(t)-20-40-60-800 2 4 6 T2T3Ttime (ms)T00 T 2T 3TK + activation gate, n(b)0.60.40.20V-nullcline-80 -60 -40 -20 0 20membrane potential, V (mV)(d)T/43T/85T/8T/23T/4 7T/8T/8n-nullclineT/43T/8T/80,T x 0limit cycle attractorT/20,T5T/8 7T/83T/4Figure 10.1: Definition of a phase of oscillation, ϑ, in the I Na + I K -model with parametersas in Fig. 4.1a and I = 10.having exponentially stable limit cycles and experiencing pulsed stimulation at timest s that instantaneously increases the state variable by constant A. The Dirac deltafunction δ(t) is a mathematical shorthand notation for resetting x by A. The strengthof pulsed stimulation, A, is not assumed to be small. Most of the results of this sectioncan also be applied to the case when the action of input pulse is not instantaneous,but smeared over an interval of time, typically shorter than the period of oscillation.10.1.1 Phase of oscillationMany types of physical, chemical, and biological oscillators share an astonishing feature:They can be described by a single phase variable ϑ. In the context of tonic spiking,the phase is just the time since the last spike, as in Fig. 10.1a.In general, the notion of the phase is related to the notion of parametrization ofa limit cycle attractor, as in Fig. 10.1b. Take a point x 0 on the attractor and plotthe trajectory x(t) with x(0) = x 0 . Then, the phase of x(t) is just ϑ = t. As tincreases past the period T , then 2T , etc., the phase variable ϑ wraps around theinterval [0, T ], jumping from T to 0; see Fig. 10.1c. Gluing together the points 0 andT , as in Fig. 10.1d, we can treat the interval [0, T ] as a circle, denoted as S 1 , withcircumference T . The parametrization is the mapping of S 1 in Fig. 10.1d into the