Conditional burst firing in a minimal model. Simulations were performed using an LIF model (see Materials and Methods). A, V_m trajectory after the first spike during a just-suprathreshold current step (501 pA; rheobase, 500 pA) in models with and without a fast ADP, both including a medium AHP. After a V_m reset (simulating the fAHP), the combination of ADP and AHP currents causes a rising and falling V_m trajectory. B, Responses to current steps (middle, 0.6 nA; top, 0.7 nA). The ADP⁺ model (right) generates an initial two-spike burst during the 0.7 nA step. C, ISIs in response to 5 ms exponential-filtered noise (σ_I = 0.25 nA, I_DC = 0.50 nA). The ADP⁺ model produces two bands of ISIs, similar to L2–3 PNs. D, ISI histogram for the ADP⁻ model (2 ms bins) shows few short ISIs. E, ISI histogram for the ADP⁺ model has a large peak of short ISIs and a second, broad peak of long intervals.