Average discharge rate of single auditory-nerve fibers in cats was measured in response to 400-msec tone bursts. For each fiber, rate versus stimulus-level functions were constructed for a number of frequencies. For tones at a fiber's characteristic frequency (CF), rate increases rapidly over a range of 20 to 30 dB above threshold. For higher stimulus levels, a range of behaviors is observed. For some fibers, rate saturates completely at higher levels; i.e., there are no further systematic increases in rate when level is increased beyond about 30 dB above threshold. For other units there is a noticeable bend in the rate-level function at 20 to 30 dB above threshold; however, rate can continue to increase gradually over another 30 to 40 dB. For frequencies above fiber CF, the slope of rate-level functions measured near the midpoint between maximum and spontaneous rates is a decreasing function of frequency. For frequencies below CF, slope is either approximately constant and equal to the slope at CF or increases to some maximum value as frequency is decreased from the CF. These properties of rate-level functions are well accounted for by a simple model consisting of a mechanical stage followed by a saturating nonlinearity (transducer stage). The input (pressure) versus output (basilar membrane displacement} functions for the mechanical stage are taken directly from the measurements of Rhode. The input-output function for the transducer is developed empirically.

1. 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. 2. When STBs are presented synchronously (constant phase at onset), PSTHs of responses to very-low-frequency (less than 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. 3. 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 spke 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. 4. 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. 5. 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 (less than 10 ms) decrease in instantaneous rate accompanied by a sharp increase in discharge irregularity.(ABSTRACT TRUNCATED AT 400 WORDS)

This paper is concerned with the representation of the spectra of synthesized steady-state vowels in the temporal aspects of the discharges of auditory-nerve fibers. The results are based on a study of the responses of large numbers of single auditory-nerve fibers in anesthetized cats. By presenting the same set of stimuli to all the fibers encountered in each cat, we can directly estimate the population response to those stimuli. Period histograms of the responses of each unit to the vowels were constructed. The temporal response of a fiber to each harmonic component of the stimulus is taken to be the amplitude of the corresponding component in the Fourier transform of the unit's period histogram. At low sound levels, the temporal response to each stimulus component is maximal among units with CFs near the frequency of the component (i.e., near its place). Responses to formant components are larger than responses to other stimulus components. As sound level is increased, the responses to the formants, particularly the first formant, increase near their places and spread to adjacent regions, particularly toward higher CFs. Responses to nonformant components, exept for harmonics and intermodulation products of the formants (2F1,2F2,F1 + F2, etc), are suppressed; at the highest sound levels used (approximately 80 dB SPL), temporal responses occur almost exclusively at the first two or three formants and their harmonics and intermodulation products. We describe a simple calculation which combines rate, place, and temporal information to provide a good representation of the vowels' spectra, including a clear indication of at least the first two formant frequencies. This representation is stable with changes in sound level at least up to 80 dB SPL; its stability is in sharp contrast to the behavior of the representation of the vowels' spectra in terms of discharge rate which degenerates at stimulus levels within the conversational range.