1. The responses of neurons in the ventral cochlear nucleus (VCN) of decerebrate cats are described with regard to their regularity of discharge and latency. Regularity is measured by estimating the mean and standard deviation of interspike intervals as a function of time during responses to short tone bursts (25 ms). This method extends the usual interspike-interval analysis based on interval histograms by allowing the study of temporal changes in regularity during transient responses. The coefficient of variation (CV), equal to the ratio of standard deviation to mean interspike interval, is used as a measure of irregularity. Latency is measured as the mean and standard deviation of the latency of the first spike in response to short tone bursts, with 1.6-ms rise times. 2. The regularity and latency properties of the usual PST histogram response types are shown. Five major PST response type classes are used: chopper, primary-like, onset, onset-C, and unusual. The presence of a pre-potential in a unit's action potentials is also noted; a prepotential implies that the unit is recorded from a bushy cell. 3. Units with chopper PST histograms give the most regular discharge. Three varieties of choppers are found. Chop-S units (regular choppers) have CVs <0.35 that are approximately constant during the response; chop-S units show no adaptation of instantaneous rate, as measured by the inverse of the mean interspike interval. Chop-T units have CVs greater than 0.35, show an increase in irregularity during the response and show substantial rate adaptation. Chop-U units have CVs >0.35, show a decrease in irregularity during the response, and show a variety of rate adaptation behaviors, including negative adaptation (an increase in rate during a short-tone response). Irregular choppers (chop-T and chop-U units) rarely have CVs >0.5. Choppers have the longest latencies of VCN units; all three groups have mean latencies at least 1 ms longer than the shortest auditory nerve (AN) fiber mean latencies. 4. Chopper units are recorded from stellate cells in VCN (35, 42). Our results for chopper units suggest a model for stellate cells in which a regularly firing action potential generator is driven by the summation of the AN inputs to the cell, where the summation is low-pass filtered by the membrane capacitance of the cell. 5. Units with primary-like, primary-like-with-notch, and unusual PST histograms give the most irregular discharge (CVs >0.5); the CV is usually constant during the stimulus, or may decrease with time. All primary-like units and many primary-like-with-notch and unusual units show substantial rate adaptation during the stimulus. These irregular unit types have the shortest latencies in the VCN; their mean latencies are usually <1 ms longer than minimum mean AN latencies. The small group of onset and onset-C units studied have short latencies, like primary-like units, and may give regular or irregular discharge (CVs from 0.3-0.7). 6. The regularity and latency of the irregular units (primary-like, primary-like-with-notch, unusual, and some onset units) are all about the same. These unit types are probably all recorded from bushy cells in VCN, because prepotentials are only seen in the action potentials of these irregular unit types. This conclusion is supported by the results of HRP injection studies (35, 42, 44). The behavior of irregular units probably results from their specialized synaptic connections from small numbers of AN fibers. Their irregularity is consistent with a model in which each arriving AN spike is capable of producing a spike in the VCN cell. One difference among units in this group is the sharpness or precision of their onset responses, as measured by the standard deviation of first spike latency. This difference can be explained by assuming a convergence of different numbers of AN fibers onto bushy cells, with the sharpness of the onset response increasing rapidly as more inputs converge. A difference of only two to three in the number of inputs is sufficient to cover most of the range of latency standard deviations found in this study.
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