Figure 5.

Variance in output firing rate and [Ca²⁺]_i increases as the angle between the FR and 〈g_ADP〉 nullclines decreases. A, Variation of angles between the 〈g_ADP〉 nullcline (dashed) and the FR nullcline (solid). Three representative 〈g_ADP〉 nullclines are shown for whichγ_ADP = 1 (single stable fixed point), 4 (line attractor), and 10 (bistable neuron). B, Stable firing rates of model neurons in the presence of synaptic noise for a range of different g_ADP slopes including the cases illustrated in A. Beyond the line attractor configuration (γ_ADP > 4), neurons exhibit two different stable firing modes corresponding to the top and bottom intersections of the nullclines as for γ_ADP = 10 in A. The gap in the top curve results from the fact that for g_ADP slopes only slightly larger than that of the line attractor configuration, the top fixed points are not stable in the presence of noise (because their basins of attraction might be too small and flat). C, Firing rate as a function of time for the three representative configurations shown in A, labeled by their g_ADP slopes (γ_ADP). For the largest g_ADP slope (10), activity is shown for the system being either in its higher or lower stable activity mode (see B). Note that activity for the line attractor configuration (γ_ADP = 4) is clearly distinguished by its high variance and more prominent slow frequency components. D, Variance in firing rates (σ_FR²; black) and in intracellular Ca²⁺ concentration (σ_Ca²; gray) as a function of the slope of the 〈g_ADP〉 nullcline (γ_ADP). Both signals increase sharply as γ_ADP approaches the line attractor configuration.