Classification of unit types in the anteroventral cochlear nucleus: PST histograms and regularity analysis

The responses of neurons in the anteroventral cochlear nucleus (AVCN) of barbiturate-anesthetized cats are characterized with regard to features of their responses to short tone bursts (STBs; 25 ms). A 'decision tree' is presented to partition AVCN units on the basis of post-stimulus time histogram (PSTH) shape, first spike latency, and discharge rate and regularity calculated as functions of time during responses to STBs. The major classes of AVCN units (primary-like, primary-like-with-notch, chopper and onset) have been described previously; in this paper, special attention is given to clarifying and systematizing boundaries between classes. Certain types of 'unusual' units that may be confused with units in one of the major classes are also examined. When STBs are presented synchronously (constant phase at onset), PSTHs of responses to very-low-frequency (<1.0 kHz) tones are difficult if not impossible to resolve into the classes listed above because all unit types phase-lock to low-frequency tones. However, when STBs are presented asynchronously, the responses of units with low best frequencies can be categorized on the basis of PSTH shape and first spike latency. Primary-like, primary-like-with-notch, and onset units are distinguished primarily on the basis of PSTH shape. These three unit types have comparable minimum first spike latencies and synchronization to tones. One type of 'unusual' response poses a particular hazard with respect to the generation of uncontaminated primary-like populations. Such 'unusual' units have PSTHs that appear primary-like; these units are, however, distinguished by their unusually long first spike latencies. Unlike primary-like units, these 'unusual' units show extremely poor synchronization to tones. Chopper units are defined as having an initial response that is highly regular, resulting in the characteristic multimodal PSTH. 'Unusual' units with multimodal PSTHs but whose initial responses are not highly regular (measured by the reproducibility of the initial firing pattern in response to multiple repetitions of a STB) are eliminated from the chopper populations. In barbiturate-anesthetized cats, at least three patterns of chopper response can be distinguished on the basis of temporal patterns of rate and regularity adaptation. 'Sustained' choppers show no adaptation of instantaneous rate (measured by the inverse of the mean interspike interval), and their discharge remains highly regular throughout the response. 'Transiently adapting' choppers undergo a very rapid (<10 ms) decrease in instantaneous rate accompanied by a sharp increase in discharge irregularity. After the initial transient, the rate and regularity stabilize at the final value; these units thus have sustained discharge patterns that are comparatively irregular. 'Slowly adapting' choppers also undergo a decrease in rate and an increase in firing irregularity; however, these changes occur continuously over a longer time (>20 ms). Choppers that exhibit this third rate adaptation pattern may have discharge regularities spanning the combined range of the other two patterns; the chopper population as a whole thus exists as a continuum with respect to discharge regularity. The chopper population appears to be homogeneous with respect to first spike latency and synchronization to tones; there are, however, a number of response features that change systematically with discharge regularity. A hypothesis concerning the source of the systematic variation of these response properties within the chopper population is presented.