A, interspike interval histogram for all recorded spikes. B, death rate curve, calculated from A, showing the fraction of surviving intervals that are terminated by a spike within each bin. Thin lines indicate 95% confidence limits. C, distance-to-threshold trajectory, calculated from B using a transform derived from a computational model. D, composite distance-to-threshold trajectory, calculated by combining multiple trajectories from intervals occurring at different firing rates. E, firing rate histogram for all recorded spikes. Shaded area indicates spikes used in composite analysis. F, interspike interval histograms for three subpopulations of spikes at different estimated firing rates, G, death rate curves and H, distance-to-threshold trajectories, calculated from the subpopulation interspike interval histograms shown in F.