Noise can facilitate the cross-correlation between the injected 40 Hz sine wave current and the resulting average spike rate histogram. A, Spike histogram over 350 trials of the action potential discharge of a cortical pyramidal cell in response to the intracellular injection of the depolarizing phase of a 40 Hz sine wave of various amplitudes. The spikes show little variation from trial to trial in their timing. B, The addition of background noise (4 nS SD of both excitatory and inhibitory components) results in a smoothing of the discharge rate of the pyramidal cell, as well as increased responsiveness to previously subthreshold stimuli (e.g., 0.8 nA). Note that the addition of noise also lessens the phase lag of spike times to threshold stimuli (e.g., 1 nA). C, Plot of the number of spikes generated per trial versus the current amplitude in the presence and absence of added noise. D, The addition of membrane noise results in a significant increase in spike jitter (SD of spike times) and an increase in the peak cross-correlation between the average spike rate and current amplitude.