Optogenetic Stimulation Shifts the Excitability of Cerebral Cortex from Type I to Type II: Oscillation Onset and Wave Propagation
Steady stimulation was applied to both the excitatory and inhibitory cell populations near the origin (−0.25 < x < 0.25). The stimulation applied to the inhibitory cells (Ji = 4) was chosen so that the medium operated as a type II medium at the stimulation site. Absorbing boundary conditions (not shown) were imposed at |x| = 3 mm in all panels. A: Weak stimulation of the excitatory cells (Je = 1.5) elicited localized gamma oscillations that emerged via a supercritical hopf bifurcation. In this case, the gamma oscillations failed to propagate as waves. B: Moderate stimulation (Je = 2.2) evoked a propagating wave on every third gamma cycle. C: Strong stimulation (Je = 3) evoked propagating waves on every second gamma cycle.