Primary Auditory Nerve Fibers Research Articles

Spectral characteristics of the responses of primary auditory-nerve fibers to frequency-modulated signals

The spectral responses of cat single primary auditory nerve fibers to sinusoidal frequency-modulated acoustic signals applied to the ear are examined. Period histograms were constructed from the neural spike-train data, and the frequency spectrum was determined by Fourier transforming these histograms. Several clusters of spectral components were present. The lowest-frequency cluster consists of components at DC, at the modulation frequency, and at its harmonics. In the next cluster, components surround the carrier frequency and are separated from it by the modulation frequency and its harmonics. Higher-frequency clusters surround frequencies that are twice and three times the carrier frequency. The components in each cluster are separated from the multiples of the carrier frequency by the modulation frequency and its harmonics. The magnitudes of the spectral components were investigated for carrier frequencies located below, at, and above the unit characteristic frequency, and for different signal levels, modulation frequencies, and modulation indices. The components at the modulation frequency and its harmonics were strong and present over a wide range of signal levels, carrier frequencies, modulation frequencies, and nerve-fiber characteristics. The presence of components at the modulation frequency indicates that a demodulation process is occurring. This process may be significant for speech recognition.

Spectral characteristics of the responses of primary auditory-nerve fibers to amplitude-modulated signals

The spectral responses of cat single primary auditory nerve fibers to sinusoidal amplitude-modulated (AM) and double-sideband (DSB) acoustic signals applied to the ear were examined. DSB is an amplitude-modulated signal with a suppressed carrier. Period histograms were compiled from the neural spike-train data, and the frequency spectrum was determined by Fourier transforming these histograms. For DSB signals, spectral components were found to be present at the frequencies of the stimulus as well as at certain combination frequencies. For AM signals, several clusters of spectral components were present. The lowest-frequency cluster consisted of components at DC, at the modulation frequency, and at its harmonics. A higher frequency cluster occurs around a component with the frequency of the carrier. The components of cluster are separated from the carrier by the modulation frequency and its harmonics. Yet higher-frequency clusters appear around multiples of the carrier frequency with components at frequencies separated from these multiples by the modulation frequency and its harmonics. The magnitudes of these spectral components were determined for carrier frequencies located below, at, and above the characteristic frequency of the units, and for different stimulus levels, modulation frequencies, and modulation depths. The low-frequency components present in the neural spike train appear to be the result of demodulation taking place in the inner ear. The demodulated components are strong and are present over a wide range of sound levels, carrier frequencies, modulation frequencies, and nerve-fiber characteristics. This demodulation may be significant for speech recognition.

Functional differentiation of sensory cells in the avian auditory periphery

Mammals and birds have independently developed different populations of sensory cells grouped across the width of their auditory papillae. Although in mammals there is clear evidence for disparate functions for the two hair-cell populations, the different anatomical pattern in birds has made comparisons difficult. In two species of birds, we have used single-fibre staining techniques to trace physiologically-characterized primary auditory nerve fibres to their peripheral synapses. As in mammals, acoustically-active afferent fibres of these birds innervate exclusively the neurally-lying group of hair cells in a 1∶1 relationship, suggesting important parallels in the functional organization of the auditory papillae in these two vertebrate classes. In addition, we found a strong trend of the threshold to acoustic stimuli at the characteristic frequency across the width of the avian papilla.

Spectral characteristics of the responses of primary auditory-nerve fibers to frequency-modulated signals

The spectral responses of cat single primary auditory nerve fibers to sinusoidal frequency-modulated (FM) acoustic signals applied to the ear are reported. Period histograms were constructed from the neural spike-train data, and the frequency spectrum was determined by Fourier transforming these histograms. Several clusters of spectral components were present. The lowest-frequency cluster consists of components at dc, at the modulation frequency, and at its harmonics. In the next cluster, components surround the carrier frequency and are separated from it by the modulation frequency and its harmonics. Higher-frequency clusters surround frequencies that are twice and three times the carrier frequency. The components in each cluster are separated from the multiples of the carrier frequency by the modulation frequency and its harmonics. The magnitudes of the spectral components were investigated for carrier frequencies located below, at, and above the unit characteristic frequency (CF), and for different signal levels, modulation frequencies, and modulation indices. The components at the modulation frequency and its harmonics were strong and present over a wide range of signal levels, carrier frequencies, modulation frequencies, and nerve-fiber characteristics. The presence of components at the modulation frequency indicates that a demodulation process is occurring. This process may be significant for speech recognition. [Work supported by NIH and NSF.]

Multinomial pulse-number distributions for neural spikes in primary auditory fibers: theory.

We previously reported experimental short- and long-counting-time pulse-number distributions (PND's) for the neural spike train in cat primary auditory nerve fibers. Data were obtained for spontaneous activity, pure-tone stimuli with a wide range of frequencies and intensity levels, and Gaussian noise. The irregular shapes of the PND's are an indication of the presence of spike clusters of various sizes in the neural impulse train. We develop a family of theoretical cluster counting distributions and examine their suitability for describing the experimental PND's. The reduced-quintinomial distribution provides theoretical results that describe the characteristics of the PND's quite well, accounting for the smooth or scalloped behavior of short-counting-time data, the jagged nature of long-counting-time data, and the Poisson-like character of very-short-counting-time data. This family of distributions admits values for the spike-number mean-to-variance ratio that are independent of stimulus level, in agreement with experimental observation. A number of procedures for fitting the theoretical distributions to the experimental PND's are studied. These include the use of a minimum mean-square error criterion, the factorial moments of the data, and the discrete Fourier transform of the PND. The first of these techniques appears to be the most useful.

Auditory enhancement of changes in spectral amplitude.

An auditory enhancement effect occurs when one component of a harmonic series is omitted for a few hundred milliseconds and then reintroduced: The reintroduced harmonic stands out perceptually. Three experiments are reported that studied a version of this effect in which several components of a harmonic series are enhanced to define the formants of a vowel. Using the accuracy of vowel identification to measure the prominence of the formant peaks in the effective auditory representation, forms of the effect were identified that are qualitatively similar to the incremental and decremental responses seen in primary auditory-nerve fibers. These results are compatible with an origin for the enhancement effect in peripheral auditory adaptation. However, an additional mechanism is required to account for the demonstration [Viemeister and Bacon, J. Acoust. Soc. Am. 71, 1502-1507 (1982)] that enhancement can involve a true gain in the frequency region of the reintroduced component. These effects demonstrate one way in which the auditory system may attenuate the prominence of background noises while preserving the ability to represent changes in spectral amplitude produced by newly arriving signals.

Postnatal development of auditory nerve and cochlear nucleus neuronal responses in kittens

Neurons located within the auditory periphery of kittens (i.e., primary auditory nerve fibers and neurons of the cochlear nucleus (CN)) exhibit similar response properties throughout the early stages of postnatal development. Neural thresholds to acoustic stimuli are uniformly high, spontaneous and acoustically-evoked discharge rates are low, input/output slopes are shallow, and temporal discharge patterns are markedly immature. Phase-locking abilities are poor in developing mammals and all neurons exhibit broad bandpass tuning curves, with cen0ter frequencies clustering near 1.5 kHz. Throughout the first week, response thresholds, maximum discharge rates, rate-intensity slopes, dynamic ranges and other response indices remain essentially unchanged. Thereafter, between the 7th and 20th postnatal days, peripheral auditory development proceeds rapidly, such that thresholds, tuning properties, temporal discharge patterns, and input/output functions achieve maturity. The role of synaptogenesis in the development of adult response properties has been studied through microionophoresis of neuroactive molecules onto the surface of neurons in the caudal divisions of the cochlear nuclei of developing kittens. Results of preliminary experiments suggest that inhibitory postsynaptic receptor function precedes intrinsic excitatory neurotransmission. Furthermore, during the first two weeks of postnatal development in kittens, GABA microionophoresis onto immature caudal CN neurons, exhibiting sustained responses to acoustic stimuli, converts response patterns to the onset type in the majority of neurons encountered.

Hearing of modulation in sounds.

Hearing of modulation in sounds.R H KayR H KayPublished Online:01 Jul 1982https://doi.org/10.1152/physrev.1982.62.3.894MoreSectionsPDF (13 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformationCited ByFormant-frequency discrimination of synthesized vowels in budgerigars ( Melopsittacus undulatus ) and humansThe Journal of the Acoustical Society of America, Vol. 142, No. 4Encoding frequency contrast in primate auditory cortexBrian J. Malone, Brian H. Scott, and Malcolm N. Semple1 June 2014 | Journal of Neurophysiology, Vol. 111, No. 11Dynamic Amplitude Coding in the Auditory Cortex of Awake Rhesus MacaquesBrian J. Malone, Brian H. Scott, and Malcolm N. Semple1 September 2007 | Journal of Neurophysiology, Vol. 98, No. 3Neural Rate and Timing Cues for Detection and Discrimination of Amplitude-Modulated Tones in the Awake Rabbit Inferior ColliculusPaul C. Nelson, and Laurel H. Carney1 January 2007 | Journal of Neurophysiology, Vol. 97, No. 1Pulse Rise Time But Not Duty Cycle Affects the Temporal Selectivity of Neurons in the Anuran Midbrain That Prefer Slow AM RatesChristofer J. Edwards, Todd B. Alder, and Gary J. Rose1 March 2005 | Journal of Neurophysiology, Vol. 93, No. 3Neural Processing of Amplitude-Modulated SoundsP. X. JORIS, C. E. SCHREINER, and A. REES1 April 2004 | Physiological Reviews, Vol. 84, No. 2Impaired sensitivity to dynamic stimuli in poor readers of a regular orthographyBrain and Language, Vol. 87, No. 2The Sensory Basis of Reading ProblemsDevelopmental Neuropsychology, Vol. 20, No. 2Latency of the Auditory Evoked Neuromagnetic Field Components: Stimulus Dependence and Insights Toward PerceptionJournal of Clinical Neurophysiology, Vol. 17, No. 2Effects of Similarity in Bandwidth on the Auditory Sequential Streaming of Two-Tone Complexes25 June 2016 | Perception, Vol. 28, No. 10Hemispheric differences in processing tone frequency and amplitude modulationsNeuroReport, Vol. 10, No. 14Can sensitivity to auditory frequency modulation predict childrenʼs phonological and reading skills?NeuroReport, Vol. 10, No. 10An auditory cortical theory of primitive auditory grouping9 July 2009 | Network: Computation in Neural Systems, Vol. 7, No. 2Temporal coding of 200% amplitude modulated signals in the ventral cochlear nucleus of catHearing Research, Vol. 77, No. 1-2Temporal integration and oscillatory responses of the human auditory cortex revealed by evoked magnetic fields to click trainsHearing Research, Vol. 68, No. 1Periodicity coding in the auditory systemHearing Research, Vol. 60, No. 2Across-channel Masking of Changes in Modulation Depth for Amplitude- and Frequency-modulated Signals1 January 2018 | The Quarterly Journal of Experimental Psychology Section A, Vol. 43, No. 3Spectral characteristics of the responses of primary auditory-nerve fibers to frequency-modulated signalsHearing Research, Vol. 39, No. 1-2Topographic representation of periodicities in the forebrain of the mynah bird: one map for pitch and rhythm?Brain Research, Vol. 422, No. 2Pavlov Institute research in speech perception: Finding phonetic messages in modulationsSpeech Communication, Vol. 4, No. 1-3Delineation of FM rate channels in man by detectability of a three-component modulation waveformHearing Research, Vol. 18, No. 3Coding of amplitude-modulated tones in the central auditory system of catfishHearing Research, Vol. 17, No. 3 More from this issue > Volume 62Issue 3July 1982Pages 894-975 Copyright & PermissionsCopyright © 1982 by American Physiological Societyhttps://doi.org/10.1152/physrev.1982.62.3.894PubMed7045902History Published online 1 July 1982 Published in print 1 July 1982 Metrics

Some Perceptual Aspects of Timbre

In the study of how we perceive musical timbre, we encounter a number of different representations of the musical information, several of which have appeared in preceding papers in the symposium. The first is the musical score, which in engineering terms is a graph of frequency versus time, with appended annotations about loudness, scoring, timing, etc. The next representation is sound in a concert hall, created from the score by an orchestra or computer: technically, sound pressure as a function of time. The representations of interest in this paper are those produced by the mechanical and neural systems in our heads in response to this sound. The First Auditory Representation Properties of the Ear Experiments involving auditory masking (see Patterson 1976), critical bands (summarized in Tobias 1970), basilar membrane motion (see Johnstone & Boyle 1967; Rhode 1971; & Evans & Wilson 1973) and tuning curves of primary auditory nerve fibers (see Kiang 1965 & 1974), all indicate that the first neural representation of sound in our heads results from a frequency analysis of the incoming sound by a fluid-filled bony structure called the cochlea. The frequency resolution of this system is somewhat less than one-third of an octave (above 400 Hz), as can be seen from Figure 1, the so-called critical bandwidth data derived from psychophysics. A corresponding set of psychophysical experiments indicate that the temporal resolution of the system is at best a few milliseconds (e.g., see Viemeister 1979). The phase sensitivity of the is still a contro versial topic, but most studies indicate that the system is only minimally sensitive to the relative phase of harmonics. Of course, the is exquisitely sensitive to the interaural phase of any sine wave component below 500 Hz. But this gives rise to auditory localization, and not perception of timbre. Hence in the remainder of this paper we will ignore phase. It may at first glance seem quite incongruous that the auditory system, which has excellent pitch discrimination (two or three hertz for 1000 Hz pure tones), should analyze sound with filters as broad as one third of an octave, that is, three or four notes on the chromatic scale. But it is easy to show that if one uses not just one filter, but several overlapping filters, then accurate information concerning pitch is available, limited only by the slope of the filter characteristic, and not the filter bandwidth. The Model To obtain a better idea of this first neural representation of music and speech, we have constructed a model ear with properties approximating those of the human discussed above. Our model, shown in Figure 2, consists basically of a bank of 1/3 octave filters, covering from 125 Hz to 6.3 kHz, followed by envelope detectors. To simulate the roughly constant critical bandwidth below 400 Hz, we added together the detector outputs of the 125- and 160-Hz channels, and also the 200- and 250-Hz channels (see Dockendorff 1978). The detector time constants were chosen to produce fast rise time consistent with low ripple. In filter systems such as this one which have wider band widths at higher frequencies, the rise time of the filters decrease with increasing cent er frequency. Hence we chose detector time constants to correspond, such that the overall rise times of the filter-detector units were inversely proportional to frequency. Specifically, the 1 kHz channel has an overall rise time of 6 milliseconds, the 2 kHz channel, 3 milliseconds, and so forth. As noted in Figure 2, the detectors are connected to a 16-channel CMOS multiplex switch, which samples the output of each channel every 1.6 milliseconds. (This rate is appropriate for the high-frequency channels, but oversamples the low channels.) The multiplexed output is then passed through a logarithmic amplifier to match the logarithmic nature of perceived loudness in the ear. There are several important aspects of the human auditory system that are not modeled by this system, such as two-tone inhibition, the limited dynamic range of the neural system, etc. …

Identification of local and propagating distortion products from cochlear microphonic responses.

Careful measurements show that sound pressures of 40 dB re 20 mu N/m2 are sufficient to reveal two varieties of well-behaved, nonlinear distortion products in the cochlear microphonic (CM). The first variety appears in the primary-tone area and is designated as a local CM distortion product. The second type exists apical to the primary-tone area and is identical to the mechanically propagated distortion seen in the phase-locked responses of primary auditory nerve fibers. The existence of the propagating distortion product forces the conclusion that there must be a mechanical contribution to the local "CM" distortion product as well. The intrusion of nonlinear mechanical responses at such low levels (less than 40 dB SPL) indicates that the effective mechanical input to the hair cells may be nonlinear over most of the audible range. An important but unanswered question is the range over which the transducer characteristic of CM could be effectively linear, for that would determine whether CM could be used to probe mechanical nonlinear effects in the primary tone area.

A model for discharge patterns of primary auditory-nerve fibers

A model for the synapse between an inner hair cell and afferent fiber is presented. The hair cell is assumed to release 25–100 quantized neurotransmitter packets per sec. Each packet causes an exponentially shaped unitary excitatory postsynaptic potential (EPSP). Any particular afferent fiber is assumed to have a discharge threshold either above or below that of the unitary EPSP. Using any reasonable distribution of threshold values, the model produces a bimodal distribution of spontaneous discharge patterns. The group producing high spontaneous rates (greater than 20/sec) all have discharge thresholds below the unitary EPSP level, while the low-spontaneous group (rates less than 20/sec) all have discharge thresholds above that level. Other physiologically realistic results are obtained from the two groups.

Precise extracellular marking in the auditory nerve with high impedance micropipettes

A procedure is described which permitted for the first time accurate marking of the recording sites of primary auditory nerve fibers, peripheral to their entrance into the brainstem. The technique, which involves a modification of the fast green FCF dye marking procedure, does not interfere with the recording quality of the high-impedance micropipettes, requires no special histology, permits routine production of 5–15 μm diameter marks at extracellular recording sites, and employs a dye known to be noncytotoxic.

Enkephalin-like immunoreactivity of olivocochlear nerve fibers in cochlea of guinea pig and cat.

The distribution of enkephalin-like immunoreactivity in the cochlea of the guinea pig and cat was studied. Indirect immunofluorescence immunohistochemistry using antisera generated against a methionine enkephalin-bovine thyroglobulin conjugate was applied to surface preparations of the organ of Corti and cryostat sections of the whole of the cochlea. In the cochlear osseous spiral lamina, immunofluorescence was localized to unmyelinated fibers of the intraganglionic spiral bundle. In the organ of Corti, immunofluorescence was localized to a small number of fibers at inner hair cells, the inner spiral bundle, and tunnel spiral bundle, to tunnel crossing fibers at the level of the tunnel floor, to an occasional spiral outer fiber, and to the synaptic region of outer hair cells in the three rows of the basal turn of the cochlea. Less immunofluorescence was found in this region as one progressed towards the apex, with none seen at the apex. At the most apical region the inner spiral bundle became patchy and the tunnel spiral bundle developed arcades. There was no immunofluorescence found in spiral ganglion cells, in auditory nerve fibers, or in the hair cells of the organ of Corti. The findings were the same in cat as in guinea pig, the latter being studied in more detail. It was concluded that efferent, olivocochlear neurons of the cochlea, synapsing predominantly with primary auditory nerve fibers from the inner sensory cells or with the sensory cells, contain enkephalin-like immunoreactivity. Also, the findings indicate that endings of olivocochlear neurons that synapse predominantly with outer hair cells contain enkephalin-like immunoreactivity. It has previously been shown that olivocochlear neurons are likely to be cholinergic.

Physiology and bioacoustics in reptiles

Hearing in reptiles is of particular interest because of the diversity in the anatomy of the reptilian auditory system. This diversity is best illustrated in the lizard, where there is significant variation in cochlear anatomy across families. This review of the auditory physiology of reptiles will begin with a detailed discussion of the extensive physiological data available for the alligator lizard (Anguid family). These data include measurements of basilar membrane motion, intracellular hair cell responses, and single-unit activity of primary auditory-nerve fibers. Single-unit data from an Iguanid lizard will be compared to that for the alligator lizard and discussed in terms of the differences in cochlear anatomy of these two families. In addition, single-unit data for other lizards and other reptiles will be reviewed. The presentation will conclude with a comparison of auditory processing in the reptile and the mammal and will consider issues such as the tuning of individual fibers and the relationship of anatomical structure to physiological response.

Influence of a low-frequency tone on the sensitivity of primary auditory neurons: Two fiber populations

The influence of biasing low-frequency tone on the sensitivity of primary auditory nerve fibers to tone bursts at the characteristic frequency is determined. In 7% of the investigated fibers the sensitivity varied systematically with the position of the tone burst within the low-frequency tone period. In the remaining 93% of fibers this dependence was not observed. A comparison of the ratio of these numbers with the ratio of fibers from inner and outer hair cells leads to the suggestion that different responses stem from fibers innervating different types of hair cells.

Auditory Mechanisms of the Lower Brainstem

Brainstem centers of the mammalian auditory system have been under intensive investigation over the past few years. Anatomical, electrophysiological, and bio­ chemical techniques have all been brought to bear on the complex mechanisms by which these centers process acoustic information. In this article, we review work on two closely related cell groups in the auditory pathway, the cochlear nuclei and the superior olivary complex of the medulla and pons, which form major links in the synaptic chain between auditory nerve fibers, on the one hand, and higher auditory centers on the other. The cochlear nuclear complex is the first synaptic station in the auditory pathway. It is within this region that all information encoded in primary auditory nerve fibers is received and recoded. Neurons of the superior olivary complex are principal targets for cells located throughout the cochlear nuclei and are among the first cells to receive and process binaural input. Reciprocal connections between these two structures establishes a basis for functional feedback loops, while a pathway from the superior olivary complex to the cochlear hair cells provides a route for possible central control of primary afferent input. Thus, under­ standing the mechanisms by which information is processed within these brainstem areas is critical to understanding the function of the entire auditory system. We have selected material that we believe represents the current state of understanding in this rapidly moving field of mammalian sensory physiology. There are other reviews of some of this work and related studies (Hawkins 1964, Eldredge & Miller 1971, Erulkar 1972, MS'lller 1972, Harrison & Howe 1974a,b, Evans 1975, Goldberg 1975, Webster & Aitkin 1975).

Spatial differentiation as an auditory "second filter": assessment on a nonlinear model of the basilar membrane.

The mechanical tuning of the basilar membrane does not appear to be sharp enough to account for frequency selectivity of primary auditory-nerve fibers. Various ’’second filters’’ have been proposed to provide the required sharpening. We have studied the properties of one such mechanism, in which the spatial derivative of membrane displacement is taken as the excitatory signal for primary fibers, on a nonlinear computational model of the basilar membrance. Because the wavelength in response to a tone decreases as the wave travels from base to characteristic place, both the slope and curvature of membrane response are enhanced in the neighborhood of the characteristic place. Thus spatial differentiation produces sharpening resembling the difference between mechanical and neural tuning. Phase (as a function of frequency) of the spatial derivative is similar to phase of the displacement, but frequency selectivity (amplitude as a function of frequency) for frequencies below the characteristic frequency is sharpened. In addition, spatial differentiation provides the distinction between mechanical nonlinearity (related in the model to membrane velocity) and neural excitatory signal required to account for suppression of response to a tone at the characteristic frequency f1 by a second tone at a lower frequency f2. Without this distinction, a tone at frequency f2 intense enough to suppress the f1 component of response at the f1 place would itself introduce a large component of response at frequency f2.

Response patterns and peripheral origin of auditory nerve fibers in the monitor lizard,Varanus bengalensis

1. The activity patterns of primary auditory nerve fibers inV. bengalensis were studied with microelectrode recordings from the proximal and distal portions of the nerve. 2. Spontaneous firing rates varied from 0.65 to 52.1 spikes/s, and decay rates were exponential (Fig. 1). 3. A low characteristic frequency (CF) group of units had smooth, sharp tuning curves, whereas a higher CF group had less sharp, more complex tuning curves (Figs. 2 and 3). These tuning curves had no particular asymmetry (Figs. 4 and 5). 4. Recordings from fibers near the basilar membrane allowed for accurate mapping of the peripheral origin of different CF's (Fig. 6). The lower CF group was localized in the dorsal portion of the papilla, with the lowest CF's at the ventral end. The constricted-off ventral portion of the papilla had abnormally high CF's, and gave rise to the high CF group. Only the dorsal portion of the papilla was tonotopically organized. It is suggested that the constriction in the papilla allowed independent evolution of the ventral part of the papilla. 5. Click polarity reversal data (Fig. 8) and pure tone response pattern data (Fig. 7) indicate the ventral third of the dorsal papilla has unidirectional hair cell orientation. It is suggested that bidirectional hair cell orientation is a peripheral specialization for sound onset emphasis.

Filter function of the guinea pig cochlea after degeneration of outer hair cells.

After kanamycin induced degeneration of outer hair cells from guinea pigs the tuning properties of primary auditory nerve fibres are compared with those of normal untreated guinea pigs. The existence of fibres with no alteration of the tuning properties leads to the conclusion that there is no neural interaction between inner and outer hair cells needed to enhance the frequency selectivity.

Comparison of the response of the two-fiber populations in the auditory nerve of the alligator lizard

The basilar papilla of the alligator lizard (Gerrhonotus multicarinatus) contains two anatomically distinct hair cell populations. The apical population has unidirectional orientation and is associated with a tectorial membrane. The dorsal population has bidirectional orientation and free-standing cilia. On the basis of tuning curve properties, the primary auditory nerve fibers have been categorized into low (<0.9 kHz) and high (>0.9 kHz) CF populations. These two populations have been associated with the apical and basal regions of the papilla, respectively [Weiss, Mulroy, Turper, and Pike, J. Acoust. Soc. Am. 55, S84(A) (1974)]. The objective of the present study was to determine if other measures of the physiological response of the fibers could distinguish the two fiber populations. Action potentials were recorded from single, primary auditory nerve fibers in response to click, tone burst, and noise burst stimuli. The data indicate significant differences in the response of the two-fiber populations to the same acoustic stimulus. It is proposed that these physiological differences result primarily from the anatomical differences of the associated hair cell populations. [Work supported, in part, by NIH grants NS07287, NS06459, and NS05475, the Deafness Research Foundation, and Henry Ford Hospital.]