Fig. 8. Extracellular single-unit recordings of cholinergic cells in vitro. The ISI histogram (B, E, H,K) and autocorrelogram (C,F, I, L) accompanying eachtrace (A, D,G, J) are shown. Bin width is 10 msec. A–C, Regularly spiking cholinergic cells give rise to a narrow, unimodal Gaussian distribution in the ISI histogram (12.5 min sample). The autocorrelogram (2.5 min sample) exhibits multiple, uniformly spaced peaks, owing to the stationarity and regularity of the spike train. D–F, Irregularly firing cholinergic cells exhibit relatively stationary ISIs interspersed with periods of more variable spike timing. The corresponding ISI histogram (15.5 min sample) is unimodal but skewed toward theright. The autocorrelogram (5 min sample) shows a single peak, caused by the increased likelihood of firing at the end of the afterhyperpolarization, and little additional structure.G–I, Cholinergic cells also exhibit bursting or clustered firing that was characterized most clearly by an obvious bimodal distribution in the ISI histogram (10 min sample). The autocorrelogram (10 min sample) exhibits two clear peaks, the first corresponding to the intraburst intervals and the second to the interburst intervals. J–L, Cholinergic cells that fired in a seemingly random manner were also encountered. The ISI histogram (28 min sample) displays a clear modal value but is very skewed, and examination of the autocorrelogram (10 min sample) reveals that, other than the decreased probability of spike generation during the afterhyperpolarization, there is no structure to the spiking pattern.