Murray B. Sachs

Rate versus level functions for auditory‐nerve fibers in cats: tone‐burst stimuli

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 fibers 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.

Encoding of steady-state vowels in the auditory nerve: representation in terms of discharge rate.

Responses of large populations of auditory-nerve fibers to synthesized steady-state vowels were recorded in anesthetized cats. Driven discharge rate to vowels, normalized by dividing by saturation rate (estimated from the driven rate to CF tones 50 dB above threshold), was plotted versus fiber CF for a number of vowel levels. For the vowels /I/ and /e/, such rate profiles showed a peak in the region of the first formant and another in the region of the second and third formants, for sound levels below about 70 dB SPL. For /a/ at levels below about 40 dB SPL there are peaks in the region of the first and second formants. At higher levels these peaks disappear for all the vowels because of a combination of rate saturation and two-tone suppression. This must be qualified by saying that rate profiles plotted separately for units with spontaneous rates less than one spike per second may retain peaks at higher levels. Rate versus level functions for units with CFs above the first formant can saturate at rates less than the saturation rate to CF to-es or they can be nonmonotonic; these effects are most likely produced by the same mechanism as that involved in two-tone suppression.

Single-tone intensity discrimination based on auditory-nerve rate responses in backgrounds of quiet, noise, and with stimulation of the crossed olivocochlear bundle

We use simple statistical models of the firing patterns of high, medium, and low spontaneous rate auditory-nerve fibers to study mechanisms which determine the overall dynamic range of the auditory periphery. The models relate experimentally measured rate response properties of fibers with best frequency (BF) near 8.0 kHz to their ability to encode changes in BF tone level by changes in discharge rate in backgrounds of quiet and noise, with and without electrical stimulation of the crossed olivocochlear bundle (COCB). Application of the models to the BF tone rate responses of auditory-nerve fibers in backgrounds of quiet shows that optimum processing of the rate responses of fibers with BF near 8.0 kHz yields performance in the intensity discrimination task meeting or exceeding that of human subjects over an 80 dB range of levels. By defining a statistical measure of dynamic range, we confirm the results of Costalupes et al. (1984) demonstrating that masking noise shifts the dynamic range of auditory-nerve fibers to higher stimulus levels, thus preventing rate saturation. However, model analysis shows that masking noise also produces large reductions of dynamic range as well as large increases in the minimum intensity difference that can be encoded by the rate responses of single and ensembles of fibers. Electrical stimulation of the COCB can restore auditory-nerve fiber dynamic range and sensitivity to changes in BF tone level in noise backgrounds, in some cases to roughly that observed in backgrounds of quiet.

Effects of nonlinearities on speech encoding in the auditory nerve

The representation of steady‐state vowels in terms of both average rate and temporal aspects of the discharge patterns of populations of auditory‐nerve fibers is discussed. The effects of auditory‐nerve nonlinearities on this representation is emphasized. Aspects of two rate‐related nonlinearities, rate saturation and two‐tone suppression, are reviewed. At low sound levels, profiles of discharge rate versus characteristic frequency in populations of auditory‐nerve fibers show well‐defined peaks at frequencies corresponding to the formants of a vowel stimulus. At levels above about 60 dB SPL, these peaks are not seen because of a combination of rate saturation and two‐tone suppression. Units with spontaneous rates less than 1/s show the effects of suppression more dramatically than do units with higher spontaneous rates; nonetheless, this population can retain formant peaks in its rate profiles up to levels at least 20 dB higher than does the higher spontaneous rate population. Aspects of phase‐locking to ...

Discharge patterns of single fibers in the pigeon auditory nerve

Summary and Conclusions (1) The discharge patterns of single fibers in the auditory nerve of ketamine anesthetized pigeons were studied and compared with unit responses in cats studied under similar conditions. (2) All fibers show spontaneous activity; rates of spontaneous activity were generally higher than those in mammalian auditory nerve fibers. (3) Temporal patterns of responses to tones were similar to those of mammalian fibers. Phase-locking of discharges was observed for tone frequencies at least as high as 4 kHz. For frequencies below 100 Hz, two or more spikes could occur per cycle. (4) Tuning curves were similar in shape and sharpness to those of mammalian fibers. Characteristic frequencies were in the range 0.1–6.0 kHz. For CFs below 3.5 kHz, fiber thresholds followed behavioral thresholds quite closely; for higher CFs fiber thresholds were about 10 dB higher than behavioral thresholds. (5) Maximum response rates to single tones were higher in pigeon than in cat. (6) Two-tone inhibition was demonstrated. Spontaneous activity was not inhibited by acoustic stimuli. (7) Click-response histograms showed multiple peaks separated by the reciprocal of fiber CF. Ten or more peaks could be observed in response to a single click. Click latencies were less than 1.4 msec for fibers with CFs greater than 1.8 kHz. For lower CFs, latency increased with decreasing CF. (8) Iso-rate contours for single fibers showed evidence of non-linearity similar to that observed in the vibration patterns of the basilar membrane 24 . (9) We conclude, therefore, that the structurally different avian and mammalian cochleae lead to auditory nerve discharge patterns which are similar in almost all details except average rates of activity.

Two‐tone suppression in auditory‐nerve fibers: Extension of a stimulus‐response relationship

Average discharge rate of single auditory‐nerve fibers in cats was measured in response to one‐ and two‐tone stimuli. One component (the ’’suppressor tone’’) of each two‐tone stimulus was at a frequency (f2) which produced two‐tone suppression at some stimulus levels. The other component (excitor tone) produced an increase in rate above the spontaneous rate when presented alone. Fractional response was defined as the driven rate to the two‐tone stimulus divided by the driven rate to the excitor alone. Fractional response is thus a quantitative measure of the amount of suppression produced by a suppressor tone. A number of qualitative differences were found in the dependence of fractional response for f2≳CF and f2<CF. For suppressor tone frequencies greater than CF, fractional response depends only on the ratio of suppressor to excitor levels (P2/P1) for a range of excitor levels (P1). For P1 large enough to drive a unit into saturation, fractional response increases with P1. For f2<CF, however, fractional...

An auditory-periphery model of the effects of acoustic trauma on auditory nerve responses

Acoustic trauma degrades the auditory nerves tonotopic representation of acoustic stimuli. Recent physiological studies have quantified the degradation in responses to the vowel /E/ and have investigated amplification schemes designed to restore a more correct tonotopic representation than is achieved with conventional hearing aids. However, it is difficult from the data to quantify how much different aspects of the cochlear pathology contribute to the impaired responses. Furthermore, extensive experimental testing of potential hearing aids is infeasible. Here, both of these concerns are addressed by developing models of the normal and impaired auditory peripheries that are tested against a wide range of physiological data. The effects of both outer and inner hair cell status on model predictions of the vowel data were investigated. The modeling results indicate that impairment of both outer and inner hair cells contribute to degradation in the tonotopic representation of the formant frequencies in the auditory nerve. Additionally, the model is able to predict the effects of frequency-shaping amplification on auditory nerve responses, indicating the models potential suitability for more rapid development and testing of hearing aid schemes.

Representation of stop consonants in the discharge patterns of auditory‐nerve fibers

The representation of the speech syllables /da/ and /ba/ in populations of auditory-nerve fibers was studied. Post-stimulus-time histograms were computed from 20-ms segments of fiber spike trains occurring in response to the stimulus. Discrete Fourier transforms with a resolution of 50 Hz were computed from each histogram. As a measure of the response of the population of fibers to each harmonic of the 50-Hz resolution frequency of the transform, the magnitude of the response to that frequency was averaged across all fibers whose characteristic frequencies were within one-fourth octave of that harmonic. We have previously called this measure the average localized synchronized rate (ALSR). Response profiles for the 20-ms segments of the stimulus were generated by plotting the ALSR versus frequency. Time-varying spectral features of the /da/ and /ba/ stimuli are well preserved by such profiles. For example, the onset spectrum and formant transitions of the consonant-vowel syllable are well represented. Furthermore, the fine structure in the speech spectrum related to the pitch of the excitation source is maintained in these ALSR plots. Average discharge rate profiles were generated in a manner similar to that for the ALSR; in this case average rate replaces Fourier transform components as response measure. Such average rate profiles can represent the transitions of at least formants two and three. However, such average rate profiles do not represent the steady-state formants or the voice pitch.

Nonlinearities in auditory‐nerve fiber responses to bandlimited noise

Discharge rate was measured as a function of spectral level for noise bursts of one bandwidth and center frequency. Such rate-level functions were measured for a number of bandwidths; either the low- or high-cutoff frequencies were set at fiber characteristic frequency (CF). Rate-level functions were also measured, simultaneously, for single tones at CF. We define dynamic range as the range in descibels over which rate increases from 10% to 80% of the maximum driven rate to CF tones. When pooling data across CF in single cats, dynamic range is an increasing function of fiber threshold for CF tones and noise stimuli. Narrow bands of noise produce rate-level functions that are similar to those for CF tones. For noise bands centered above CF, rate-level functions become less steep as bandwidth is increased, and are always monotonic. For wide bands of noise centered below CF, rate-level functions can be nonmonotonic or appear to plateau at rates less than the saturation rate to CF tones. Thus, wide bands of noise centered above or below CF can produce lower discharge rates than do narrow bands at the same spectral level. This rate reduction has properties similar to those for two-tone suppression. The suppressive effects observed for bandlimited noise are most pronounced on low spontaneous units and least pronounced on high spontaneous units.

Response Properties of Neurons in the Avian Auditory System: Comparisons with Mammalian Homologues and Consideration of the Neural Encoding of Complex Stimuli

The neural encoding of so-called “biologically relevant” sounds has been one focus for the efforts of auditory neurophysiologists in recent years (e.g., Woorden and Galambos 1972). Studies on amphibians have shown that the peripheral auditory systems of these animals are highly specialized for the processing of species-specific vocalizations (Frishkopf, Capranica, and Goldstein 1968). Cells have been described in the auditory cortex of squirrel monkeys that respond only to a very limited set of the vocalizations produced by these species (Newman and Wollberg 1973); the responses of such cells to these vocalizations are not easily explained in terms of their response to “simple” stimuli such as tones. Similarly, Leppelsack and Vogt (1976) and Scheich, Langner, and Koch (1977) have found cells in the avian field L and nucleus magnocellularis lateralis pars dorsalis whose selective responsiveness to vocalizations are not easily explained in terms of a relationship between those single frequencies that excite the neuron and the spectral content of the vocalizations. Suga (1978) has described a neural organization in the auditory cortex of bats that is highly specialized for the echolocation functions of these animals. Knudsen and Konishi (1978) have also demonstrated an exquisite neural substrate of sound localizations in the MLD of the barn owl, Tyco alba. As a result of these various lines of research, it now appears that at least a portion of the auditory system is a hierarchically organized analyzing network in which is found a progressive degree of abstraction of the acoustic signal as one proceeds centrally along the auditory pathway (Bullock 1977, p. 300).