Table 1.

Stochastic integrate-and–fire model

	Input				Output		Spike Discharge
Series	qsize, mV	qrate, 1/s	AHP mag, mV	AHP tau, ms	σV, mV	dμV/dt, mV/ms	CV*	Predicted CV*	Galvanic sensitivity, spikes·s−1/mV
1a/reg	0.075	6,000	−6.92	17.5	0.366	0.0526	0.125	0.136	8.26
1b/int	0.2	2,250	−5.00	10.0	0.397	0.0104	0.376	0.767	25.4
1c/irr	0.8	255	−2.73	3.0	0.476	0.00413	0.848	2.305	53.4
2a/reg	0.2	3,450	−6.92	17.5	0.496	0.0457	0.168	0.229	9.39
2b/int	0.2	2,250	−5.00	10.0	0.397	0.0104	0.376	0.483	25.4
2c/irr	0.2	1,940	−2.73	3.0	0.362	0.000856	0.811	8.458	60.4
3a/int	0.075	7,300	−5.00	10.0	0.303	0.00978	0.309	0.620	28.1
3b/int	0.2	2,250	−5.00	10.0	0.397	0.0104	0.376	0.767	25.4
3c/int	0.8	340	−5.00	10.0	0.551	0.00717	0.458	1.537	21.1

Simulation parameters, an integrate-and-fire model for a regular (a/reg), an intermediate (b/int), and an irregular model unit (c/irr). AHP mag, peak afterhyperpolarization (AHP); AHP tau, AHP time constant. Input variables (qsize, qrate, AHP mag, and AHP tau) are set in each simulation to parallel empirical findings (cf. Figs. 9–11) and result in 2 output variables, voltage SD (σ_V) and slope of mean voltage trajectory (dμ_V/dt), both evaluated at the mean interspike interval (50 ms). Spike discharge is characterized by a coefficient of variation (CV)*, the value of CV* predicted from Eq. 11, and the galvanic sensitivity measured as a ratio, spikes·s⁻¹/mV, obtained from regressions between increase in discharge rate and imposed voltage. Series 1 varies both AHPs and qsize so that the 3 units match empirical units differing in discharge regularity. Series 2 varies AHPs but not qsize. Series 3 varies qsize but not AHPs. When qsize or AHP are not varied, they are set to values for the intermediate unit. As a result, series 1b, 2b, and 3b are identical.