Figure 1

Bi-stability in a simplified neural-mass version of the Hutt & Longtin model [3], with neuron firing rates functions of Heaviside kind. A. 2D phase-portrait in the PSPs membrane potentials, showing the vector field (arrows) and sample free-noise trajectories (blue) that converge to either of two existing attractors. An oblique line, containing unstable (saddles) states, largely separates the attraction basins. B. Noise-induced alternations between a larger (up state) and a smaller (down state) value can be tracked according to the current distance of the trajectory to the saddle line. C, D. Histograms of the residence times in each of the stable states. E, F. The mean residence times as a function of the propofol level (p) and the inverse of the excitatory synaptic impulse response time-scale (α).