The addition of background noise reduces the spike shadow effect. A, Effect of the generation of an action potential on the subsequent probability of the generation of an additional action potential in response to the second injection of the depolarizing phase of a 40 Hz sine wave at the indicated amplitude and latency (see Fig. 10 D). Graphs are generated by subtracting the probability of generating an action potential with a preceding spike (to a stimulus injected at time 0) from the probability curves when there was no preceding spike. B-D, Increasing the level of added membrane variance (noise) results in a marked decrease in the spike shadow effect, thereby allowing the cell to recover quickly from having generated a preceding action potential. Noise 1, 2, and 4 correspond to 1, 2, and 4 nS of added noise. The same added background conductance was present in all four conditions (A-D) with only the variance changing between conditions. Thus, the average membrane potential and conductance are not different in the four cases shown.