An artificial sAHP conductance (G_asAHP) applied by spike-triggered dynamic-current clamp caused noise to increase gain in layer 5 pyramidal neurons. A, Schematic of the dynamic-clamp apparatus. This setup was used to add G_asAHP = ∼2% of the resting conductance of the cell after each spike, with a reversal potential (E_rev) set at E_K and a decay time constant of 1 s. B, Voltage traces showing the sAHP after 30 spikes at 50 Hz, in control solution and while applying G_asAHP. C, Responses to 0.8 nA current steps in control conditions (left) and with the asAHP (right). Bottom panels show the total current injected (I_total = I_step + I_asAHP). I_total was slowly reduced as G_asAHP accumulated. Also note the immediate reduction of I_total during each spike, as V_m moved further from E_K, increasing the driving force for I_asAHP. Middle panels are voltage traces showing the spike trains elicited. Top panels show the corresponding instantaneous firing rate, demonstrating the enhanced slow spike-frequency adaptation caused by the asAHP. D, Steady-state f–I relationships (in the absence or presence of noise) in control conditions and with the asAHP. The asAHP caused the curves obtained with and without noise to diverge. Solid lines are fits to the f–I relationships with noise, as described in Figure 2. E, Bar graph showing the mean additive effect of noise in control conditions and while applying the asAHP. F, Mean gain change by noise in control and with the asAHP. Ctrl, Control.