Figure 4.

Initiation of afterdischarge by multiple perturbations. Bifurcation analysis in figure 3(B) showed that a single spike may be insufficient to provoke afterdischarge despite the system being bistable. We predicted that a stronger perturbation, such as multiple evoked spikes, would trigger afterdischarge if that cumulative perturbation was sufficient to force the V–z trajectory across the saddle point. As predicted, three spikes evoked at 15 ms intervals were sufficient to initiate afterdischarge. On the time series, note that z did not fall below the saddle point after the third evoked spike. Similarly, the V–z trajectory approached the stable branch of the bifurcation diagram from the near side of the saddle point after the first two evoked spikes, but approached from the far side after the third evoked spike, after which point afterdischarge was inevitable.