🟢 Neural mass equation - MTK

• 🟢 Neural mass equation - MTK
• • Branch of periodic orbits with Collocation method
  • • Plot of some of the periodic orbits as function of $E_0$
  • References

In this tutorial, we study the following model taken from ^[Cortes]. It is essentially the same tutorial as in Neural mass equation but treated with ModelingToolkit.jl.

\[\left\{\begin{array}{l} \tau \dot{E}=-E+g\left(J u x E+E_{0}\right) \\ \dot{x}=\tau_{D}^{-1}(1-x)-u E x \\ \dot{u}=U E(1-u)-\tau_{F}^{-1}(u-U) \end{array}\right.\]

The model is interesting because the branch of periodic solutions converges to an homoclinic orbit which is challenging to compute with our methods.

It is easy to encode the ODE as follows

using Revise, ModelingToolkit, LinearAlgebra
using DifferentialEquations, Plots
using BifurcationKit
const BK = BifurcationKit

indexof(sym, syms) = findfirst(isequal(sym),syms)

@variables t E(t) x(t) u(t) SS0(t) SS1(t) 	# independent and dependent variables
@parameters U0 τ J E0 τD U0 τF τS α    		# parameters
D = Differential(t) 				# define an operator for the differentiation w.r.t. time

# define the model
@named NMmodel = ODESystem([SS0 ~ J * u * x * E + E0,
	SS1 ~ α * log(1 + exp(SS0 / α)),
	D(E) ~ (-E + SS1) / τ,
	D(x) ~ (1.0 - x) / τD - u * x * E,
	D(u) ~ (U0 - u) / τF +  U0 * (1 - u) * E],
	defaults = Dict(E => 0.238616, x => 0.982747, u => 0.367876,
	α => 1.5, τ => 0.013, J => 3.07, E0 => -2.0, τD => 0.200, U0 => 0.3, τF => 1.5, τS => 0.007))

# get the vector field and jacobian
odeprob = ODEProblem(structural_simplify(NMmodel), [], (0.,10.), [], jac = true)
odefun = odeprob.f
F = (u,p) -> odefun(u,p,0)
J = (u,p) -> odefun.jac(u,p,0)

id_E0 = indexof(E0, parameters(NMmodel))
par_tm = odeprob.p

# we collect the differentials together in a problem
prob = BifurcationProblem(F, odeprob.u0, par_tm, (@lens _[id_E0]); J = J,
    record_from_solution = (x, p) -> (E = x[1], x = x[2], u = x[3]))
nothing #hide

We first compute the branch of equilibria

# continuation options
opts_br = ContinuationPar(p_min = -10.0, p_max = -0.9,
	# parameters to have a smooth result
	ds = 0.04, dsmax = 0.05,
	nev = 3)

# continuation of equilibria
br = continuation(prob, PALC(tangent = Bordered()), opts_br; normC = norminf)

scene = plot(br, plotfold=false, markersize=3, legend=:topleft)

With detailed information:

br

Branch of periodic orbits with Collocation method

We then compute the branch of periodic orbits from the last Hopf bifurcation point (on the right). We use finite differences to discretize the problem of finding periodic orbits. Obviously, this will be problematic when the period of the limit cycle grows unbounded close to the homoclinic orbit.

# newton parameters
optn_po = NewtonPar(tol = 1e-8,  max_iterations = 10)

# continuation parameters
opts_po_cont = ContinuationPar(dsmax = 0.15, ds= -0.0001, dsmin = 1e-4, p_max = 0., p_min=-5.,
	max_steps = 150, newton_options = optn_po,
	nev = 3, plot_every_step = 10, detect_bifurcation = 0)

# arguments for periodic orbits
# this is mainly for printing purposes
args_po = (	record_from_solution = (x, p; k...) -> begin
		xtt = get_periodic_orbit(p.prob, x, p.p)
		return (max = maximum(xtt[1,:]),
				min = minimum(xtt[1,:]),
				period = getperiod(p.prob, x, p.p))
	end,
	plot_solution = (x, p; k...) -> begin
		xtt = get_periodic_orbit(p.prob, x, p.p)
		plot!(xtt.t, xtt[1,:]; label = "E", k...)
		plot!(xtt.t, xtt[2,:]; label = "x", k...)
		plot!(xtt.t, xtt[3,:]; label = "u", k...)
		plot!(br; subplot = 1, putspecialptlegend = false)
		end,
	normC = norminf)


Mt = 30 # number of time sections
	br_pocoll = @time continuation(
	# we want to branch form the 4th bif. point
	br, 4, opts_po_cont,
	# we want to use the Collocation method to locate PO, with polynomial degree 5
	PeriodicOrbitOCollProblem(Mt, 5; meshadapt = true);
	# regular continuation options
	args_po..., callback_newton = BK.cbMaxNorm(10))

scene = plot(br, br_pocoll, markersize = 3)
plot!(scene, br_pocoll.param, br_pocoll.min, label = "")

We plot the maximum (resp. minimum) of the limit cycle. We can see that the min converges to the smallest equilibrium indicating a homoclinic orbit.

Plot of some of the periodic orbits as function of $E_0$

We can plot some of the previously computed periodic orbits in the plane $(E,x)$ as function of $E_0$:

plot()
# fetch the saved solutions
for i_sol in 1:2:40
	# periodic orbit
	traj = BK.get_periodic_orbit(br_pocoll, i_sol)
	plot!(traj[1,:], traj[2,:], xlabel = "E", ylabel = "x", label = "")
end
title!("")

References