Population Of Auditory Nerve Fibers Research Articles

A phenomenological model for the responses of auditory-nerve fibers: I. Nonlinear tuning with compression and suppression.

A phenomenological model was developed to describe responses of high-spontaneous-rate auditory-nerve (AN) fibers, including several nonlinear response properties. Level-dependent gain (compression), bandwidth, and phase properties were implemented with a control path that varied the gain and bandwidth of tuning in the signal-path filter. By making the bandwidth of the control path broad with respect to the signal path, the wide frequency range of two-tone suppression was included. By making the control-path filter level dependent and tuned to a frequency slightly higher than the signal-path filter, other properties of two-tone suppression were also included. These properties included the asymmetrical growth of suppression above and below the characteristic frequency and the frequency offset of the suppression tuning curve with respect to the excitatory tuning curve. The implementation of this model represents a relatively simple phenomenological description of a single mechanism that underlies several important nonlinear response properties of AN fibers. The model provides a tool for studying the roles of these nonlinearities in the encoding of simple and complex sounds in the responses of populations of AN fibers.

Hyperpolarization-activated, mixed-cation current (I(h)) in octopus cells of the mammalian cochlear nucleus.

Octopus cells in the posteroventral cochlear nucleus of mammals detect the coincidence of synchronous firing in populations of auditory nerve fibers and convey the timing of that coincidence with great temporal precision. Earlier recordings in current clamp have shown that two conductances contribute to the low input resistance and therefore to the ability of octopus cells to encode timing precisely, a low-threshold K(+) conductance and a hyperpolarization-activated mixed-cation conductance, g(h). The present experiments describe the properties of g(h) in octopus cells as they are revealed under voltage clamp with whole-cell, patch recordings. The hyperpolarization-activated current, I(h), was blocked by extracellular Cs(+) (5 mM) and 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride (50-100 nM) but not by extracellular Ba(2+) (2 mM). The reversal potential for I(h) in octopus cells under normal physiological conditions was -38 mV. Increasing the extracellular potassium concentration from 3 to 12 mM shifted the reversal potential to -26 mV; lowering extracellular sodium concentration from 138 to 10 mM shifted the reversal potential to -77 mV. These pharmacological and ion substitution experiments show that I(h) in octopus cells is a mixed-cation current that resembles I(h) in other neurons and in heart muscle cells. Under control conditions when cells were perfused intracellularly with ATP and GTP, I(h) had an activation threshold between about -35 to -40 mV and became fully activated at -110 mV. The maximum conductance associated with hyperpolarizing voltage steps to -112 mV ranged from 87 to 212 nS [150 +/- 30 (SD) nS, n = 36]. The voltage dependence of g(h) obtained from peak tail currents is fit by a Boltzmann function with a half-activation potential of -65 +/- 3 mV and a slope factor of 7. 7 +/- 0.7. This relationship reveals that g(h) was activated 41% at the mean resting potential of octopus cells, -62 mV, and that at rest I(h) contributes a steady inward current of between 0.9 and 2.1 nA. The voltage dependence of g(h) was unaffected by the extracellular application of dibutyryl cAMP but was shifted in hyperpolarizing direction, independent of the presence or absence of dibutyryl cAMP, by the removal of intracellular ATP and GTP.

Detection of small across-channel timing differences by cochlear implantees

Five post-lingually deafened users of the LAURA cochlear implant were presented with two trains of biphasic pulses applied concurrently to two widely separated channels. They could all discriminate between stimuli where pulses on the two channels were nearly synchronous (inter-channel delay=0.1 ms) and those where there was a longer delay applied to one channel. All showed an asymmetry, being more sensitive when the longer delay was on either the more basal or, depending on the listener, the more apical channel. For four out of the five listeners this asymmetry could be at least partly attributed to one stimulus, with a 0.1-ms delay in either the apical (three listeners) or basal (one listener) channel, sounding markedly different from all other stimuli used in the experiment. Both the overall sensitivity of listeners and the general pattern of results survived the presentation of maskers on intermediate channels, and did not vary markedly with changes in the polarity of the pulses applied to one channel. Although the results varied substantially across listeners, it is concluded that they demonstrate a genuine sensitivity to the relative timing of stimulation applied to discrete populations of auditory nerve fibers.

Pseudospontaneous activity: stochastic independence of auditory nerve fibers with electrical stimulation

We describe a novel signal processing strategy for cochlear implants designed to emphasize stochastic independence across the excited neural population. The strategy is based on the observation that high rate pulse trains may produce random spike patterns in auditory nerve fibers that are statistically similar to those produced by spontaneous activity in the normal cochlea. We call this activity `pseudospontaneous'. A supercomputer-based computational model of a population of auditory nerve fibers suggests that different average rates of pseudospontaneous activity can be created by varying the stimulus current of a fixed-amplitude, high-rate pulse train, e.g. 5000 pps. Electrically-evoked compound action potentials recorded in a human cochlear implant subject are consistent with the hypothesis that such a stimulus can desynchronize the fiber population. This desynchronization may enhance neural representation of temporal detail and dynamic range with a cochlear implant and eliminate a major difference between acoustic and electric hearing.

Spatiotemporal Tuning of Low-Frequency Cells in the Anteroventral Cochlear Nucleus

Low-frequency cells in the anteroventral cochlear nucleus (AVCN) can be sensitive to changes in the spatiotemporal pattern of discharges across their auditory nerve (AN) inputs (). This sensitivity suggests that these cells may be tuned to particular spatiotemporal patterns, or features, in the discharge patterns of populations of AN fibers. To evaluate and characterize this sensitivity, we developed a technique whereby the physiological responses of AVCN cells to wide-band noise were analyzed using the simulated response of a population of AN fibers to the same noise stimulus. By averaging the simulated two-dimensional spatiotemporal pattern of AN activity that preceded each AVCN discharge, it was possible to derive a two-dimensional reverse-correlation function that characterized the spatiotemporal tuning of each AVCN cell. The derived spatiotemporal tuning pattern represented a feature in the AN population response that was most likely to precede discharges of the AVCN cell. To test the spatiotemporal tuning characterizations, we used these patterns to predict the responses of cells to noise stimuli statistically independent from the stimuli used to characterize the cells. This technique provides a general tool for the study of any neural system that involves the analysis of spatiotemporal input patterns.

Neural correlates of the pitch of complex tones. II. Pitch shift, pitch ambiguity, phase invariance, pitch circularity, rate pitch, and the dominance region for pitch.

1. The neural correlates of low pitches produced by complex tones were studied by analyzing temporal discharge patterns of auditory nerve fibers in Dial-anesthetized cats. In the previous paper it was observed that, for harmonic stimuli, the most frequent interspike interval present in the population of auditory nerve fibers always corresponded to the perceived pitch (predominant interval hypothesis). The fraction of these most frequent intervals relative to the total number of intervals qualitatively corresponded to strength (salience) of the low pitches that are heard. 2. This paper addresses the neural correlates of stimuli that produce more complex patterns of pitch judgments, such as shifts in pitch and multiple pitches. Correlates of pitch shift and pitch ambiguity were investigated with the use of harmonic and inharmonic amplitude-modulated (AM) tones varying either in carrier frequency or modulation frequency. Pitches estimated from the pooled interval distributions showed shifts corresponding to "the first effect of pitch shift" (de Boer's rule) that is observed psychophysically. Pooled interval distributions in response to inharmonic stimulus segments showed multiple maxima corresponding to the multiple pitches heard by human listeners (pitch ambiguity). 3. AM and quasi-frequency-modulated tones with low carrier frequencies produce very similar patterns of pitch judgments, despite great differences in their phase spectra and waveform envelopes. Pitches estimated from pooled interval distributions were remarkably similar for the two kinds of stimuli, consistent with the psychophysically observed phase invariance of pitches produced by sets of low-frequency components. 4. Trains of clicks having uniform and alternating polarities were used to investigate the relation between pitches associated with periodicity and those associated with click rate. For unipolar click trains, where periodicity and rate coincide, physiologically estimated pitches closely follow the fundamental period. This corresponds to the pitch at the fundamental frequency (F0) that is heard. For alternating click trains, where rate and periodicity do not coincide, physiologically estimated pitches always closely followed the fundamental period. Although these pitch estimates corresponded to periodicity pitches that are heard for F0s > 150 Hz, they did not correspond to the rate pitches that are heard for F0s < 150 Hz. The predominant interval hypothesis thus failed to predict rate pitch. 5. When alternating-polarity click trains are high-pass filtered, rate pitches are strengthened and can also be heard at F0s > 150 Hz. Pitches for high-pass-filtered alternating click trains were estimated from pooled responses of fibers with characteristic frequencies (CFs) > 2 kHz. Roughly equal numbers of intervals at 1/rate and 1/F0 were found for all F0s studied, from 80 to 160 Hz, producing pitch estimates consistent with the rate pitches that are heard after high-pass filtering. The existence region for rate pitch also coincided with the presence of clear periodicities related to the click rate in pooled peristimulus time histograms. These periodicities were strongest for ensembles of fibers with CFs > 2 kHz, where there is widespread synchrony of discharges across many fibers. 6. The "dominance region for pitch" was studied with the use of two harmonic complexes consisting of harmonics 3-5 of one F0 and harmonics 6-12 of another fundamental 20% higher in frequency. When the complexes were presented individually, pitch estimates were always close to the fundamental of the complex. When the complexes were presented concurrently, pitch estimates always followed the fundamental of harmonics 3-5 for F0s of 150-480 Hz. For F0s of 125-150 Hz, pitch estimates followed one or the other fundamental, and for F0s < 125 Hz, pitch estimates followed the fundamental of harmonics 6-12. (ABSTRACT TRUNCATED)

Overshoot and the ‘‘severe departure’’ from Weber’s law

The link between the two phenomena of overshoot and the ‘‘severe departure’’ from Weber’s law in intensity discrimination was investigated. Masked thresholds for a brief 6.5-kHz signal were measured in the presence of a notched-noise masker over a range of masker levels. The masker was either gated with the signal (overshoot condition) or was of long duration with the signal temporally centered in it. At mid masker levels, thresholds in the gated condition were substantially higher than in the long-masker condition (overshoot). The overall amount of overshoot and the variation in overshoot as a function of masker level support the view that, even at high frequencies, the overshoot effect is dominated by off-frequency masker components. Pure-tone intensity discrimination tasks were then performed in quiet, and in the presence of the same notched noise in both gated and long-duration noise conditions. It was found that the temporal characteristics of the notched noise and not, as previously thought, the pedestal determined whether or not a severe departure from Weber’s law in intensity discrimination was observed. The severe departure from Weber’s law was found only in an overshoot condition and then only when the pedestal was presented near masked threshold. This is not in accordance with a previous explanation of the severe departure in terms of two populations of auditory nerve fibers (high- and low-threshold fibers). A further experiment measured psychometric functions for the detection of the 6.5-kHz signal in both gated and long-duration notched noise. It was found that the psychometric function was markedly shallower in an overshoot condition (mid-level gated masker) than in the other conditions. These findings are consistent with a new theory, explaining the severe departure from Weber’s law in terms of both the variance in the internal representation of stimuli in an overshoot condition, and the loudness of partially masked signals.

A model of selective processing of auditory-nerve inputs by stellate cells of the antero-ventral cochlear nucleus.

Stellate cells in the cat antero-ventral cochlear nucleus (AVCN) maintain a robust rate-place representation of vowel spectra over a wide range of stimulus levels. This rate-place representation resembles that of low threshold, high spontaneous rate (SR) auditory nerve fibers (ANFs) at low stimulus levels, and that of high threshold, low-medium SR ANFs at high stimulus levels. One hypothesis accounting for this phenomenon is that AVCN stellate cells selectively process inputs from different SR population of ANFs in a level-dependent fashion. In this paper, we investigate a neural mechanism that can support selective processing of ANF inputs by stellate cells. We study a physiologically detailed compartmental model of stellate cells. The model reproduces PST histograms and rate-versus-level functions measured in real cells. These results indicate that simple and plausible distribution patterns of excitatory and inhibitory inputs within the stellate cell dendritic tree can support level dependent selective processing. Factors affecting selective processing are identified. This study thus represents a first step towards the development of a computational model of the AVCN stellate cell receptive field.

Spatiotemporal encoding of sound level: Models for normal encoding and recruitment of loudness

This study explores the hypothesis that sound level is encoded in the spatiotemporal response patterns of auditory nerve (AN) fibers. The temporal properties of AN fiber responses depend upon sound level due to nonlinearities in the auditory periphery. In particular, the compressive nonlinearity of the inner ear introduces systematic changes in the timing of the responses of AN fibers as a function of level. Changes in single fiber responses that depend upon both sound level and characteristic frequency (CF) result in systematic changes in the spatiotemporal response patterns across populations of AN fibers. This study investigates the changes in the spatiotemporal response patterns as a function of level using a computational model for responses of low-frequency AN fibers. A mechanism that could extract information encoded in this form is coincidence detection across AN fibers of different CFs. This study shows that this mechanism could play a role in encoding of sound level for simple and complex stimuli. The model demonstrates that this encoding scheme would be influenced by auditory pathology that affects the peripheral compressive nonlinearity in a way that is consistent with the phenomenon of recruitment of loudness, which often accompanies sensorineural hearing loss.

Neural encoding of single-formant stimuli in the cat. I. Responses of auditory nerve fibers.

1. We have studied auditory responses to a set of speech-related narrowband sounds, single-formant stimuli (SFSs), in populations of auditory nerve fibers (ANFs). An analytic method was developed to extract the envelope of temporal discharge patterns of the ANF responses to nonsinusoidally modulated stimuli, whose spectra have multiple clusters of components. Such responses are often encountered in the auditory system when complex stimuli are used and have traditionally been studied by analyzing the fundamental component of the responses. 2. The envelope modulation in the SFSs is shown to be represented by the response patterns of ANFs. When the whole ANF population is considered, the information on modulation in stimulus envelope does not disappear at the highest sound level tested at all best frequencies (BFs) we studied (1-10 kHz). The representation is the best at medium sound levels and degrades at high sound levels. Low/medium-spontaneous rate (SR) ANFs showed greater envelope modulation in their responses at high sound levels than do high-SR ANFs. The quality of the representation at high sound levels is, on average, proportional to BF threshold of an ANF. On the basis of populations of ANFs with all SRs, the envelope modulation in the SFSs is represented over a wide range of sound levels. 3. We found that low-BF ANFs differ from high-BF ANFs in representing envelope modulation in the SFSs. For ANFs with BFs less than approximately 6 kHz, information on stimulus envelope is not only contained in spectral components near direct current but also in components at the vicinities of frequencies equal to BF and its multiples. In fact, for ANFs with BFs < 3 kHz, the contribution from spectral components centered at BF to overall response modulation is greater than that from spectral components near direct current. These findings indicate that, by using measures solely based on the fundamental component, the amount of modulation in the responses to narrowband stimuli is underestimated for low-BF ANFs. 4. Off-BF stimulation of ANFs with SFSs was found to result in increased envelope modulation in responses at high sound levels. The further away the stimulus is centered relative to unit BF, the greater the modulation it induces, provided that the stimulus is capable of exciting the unit. An SFS centered as close as 15% off unit BF can produce a significant increase in the modulation of responses at very high sound levels.(ABSTRACT TRUNCATED AT 400 WORDS)

A nonlinear filter bank model for the auditory periphery

The intrinsically nonlinear character of auditory processing has long been the bane of linear models for the auditory periphery. A previous computational model [Carney, J. Acoust. Soc. Am. 93, 401–417 (1993)] for single auditory nerve fibers contained a feedback mechanism to vary the bandwidth of a narrow-band gammatone filter as a function of sound pressure level. That model simulated the compressive nonlinearity, including aspects of temporal and average discharge rate characteristics of low-frequency auditory nerve fiber responses. The proposed model extends the previous work; its architecture is motivated by the most recent empirical evidence on outer hair cell (OHC) physiology. The nonlinear narrow-band gammatone filter is replaced with a broad second-order bandpass filter [G. von Békésy, J. Acoust. Soc. Am. 21, 245–254 (1949)] inside a feedback loop. The feedback path consists of a nonlinearity representing OHC transduction, followed by a lowpass filter for the cell membrane, and finally a nonlinearity representing OHC motility. A bank of such models allows representation of spatio-temporal discharge patterns across populations of auditory nerve fibers. [Work supported by The Whitaker Foundation and NIDCD.]

The representations of the steady-state vowel sound /e/ in the discharge patterns of cat anteroventral cochlear nucleus neurons

1. We have recorded the responses of neurons in the anteroventral cochlear nucleus (AVCN) of barbiturate-anesthetized cats to the synthetic, steady-state-vowel sound /e/, presented over a range of stimulus intensities. 2. The responses of (putative) spherical bushy cells [primary-like (Pri) units] to the vowel resemble those of auditory-nerve fibers (ANFs) in terms of both rate and temporal encoding at low and moderate stimulus levels. It was not possible to study the responses of most Pri units at the highest stimulus level because of the large neurophonic component present in recordings from most primarylike units at higher stimulus levels. 3. The responses of many (putative) globular bushy cells [primarylike with notch (PN) units] to the vowel resemble those of ANFs; however, there appears to be greater heterogeneity in the responses of units in the PN population than in the Pri population in terms of both temporal and rate encoding. 4. Populations of stellate cells (chopper units) have degraded representations of the temporal information in ANF population discharge patterns in response to the vowel; this is consistent with the responses of these units to pure tones. Both regular (ChS) and irregular (ChT) chopper subpopulations, however, maintain better rate-place representations of the vowel spectrum than does the population of ANFs as a whole. The rate-place representations of the vowel spectrum by both chopper populations closely resemble those of low and medium spontaneous rate ANFs at most stimulus levels. 5. The data presented in this paper suggest that a functional partition of the AVCN chopper population could yield two distinct rate representations in response to a complex stimulus: one that is graded with stimulus level (over a 30 to 40 dB range) and that, even at rate saturation, maintains a "low contrast" stimulus representation; and a second that maintains a robust, "high contrast" stimulus representation at all levels but that confers less information about stimulus level.

A population study of auditory-nerve fibers in unanesthetized decerebrate cats: Response to pure tones

Population responses of single auditory-nerve (AN) fibers to 1-kHz pure tones were investigated in unanesthetized decerebrate cats. Driven rate (spikes/s), Fourier component amplitude (spikes/s), and d'e = (driven rate)/(standard deviation) were examined. These results extend the presently available information. First, the AN population responses, previously observed only in anesthetized animals, are now observed in unanesthetized animals. The anesthetized and unanesthetized animal responses are qualitatively similar. Second, a detectability measure d'e was applied, à la signal detection theory, to populations of AN fibers. A previously unknown behavior of the low-spontaneous-rate (SR) (less than 15 spikes/s) fibers was observed: the d'e increased to large values at moderate and high stimulus levels, giving rise to a large, sharply tuned peak in the d'e-place profile, even when the driven rate was saturated. This behavior was absent in the high-SR (greater than 15 spikes/s) fibers. This observation suggests that the d'e of the low-SR fibers is the best code for stimulus intensity among the different response measures examined, and that the d'e-place profile may be a more precise spatial code for stimulus frequency than the commonly considered rate-place code.

Physiological responses to the pulsation threshold paradigm. I: Pulsation threshold patterns do not reproduce physiological rate profiles of high-pass and low-pass noise maskers.

This article compares psychophysical measures of human processing of acoustic stimuli with one neurophysiological representation (normalized discharge rate profiles) of those stimuli. Psychophysical pulsation threshold patterns (PTPs) were derived for high-pass and low-pass noise maskers. Spectral features of both maskers are clearly evident in the PTPs. However, while the representation of high-pass noise in the PTPs becomes sharper with increasing masker level, the representation of low-pass noise degenerates as masker level is increased. One assumption that has been used previously to interpret pulsation threshold data is that PTPs reflect the profile of activity in primary neural elements in response to the masking stimulus. To investigate this hypothesis, normalized-rate profiles of responses to both maskers were derived from populations of auditory-nerve fibers in cats. Normalized-rate profiles do not exhibit the same behavior as PTPs for high-pass noise maskers in that the neural representation of the band edge degenerates as sound level increases. Furthermore, the distinction between the passband and the stop band is lost in the neural rate profiles, whereas the distinction improves in the high-pass noise PTPs.

Temporal coding of resonances by low-frequency auditory nerve fibers: single-fiber responses and a population model.

1. We recorded responses of low-frequency auditory nerve fibers (characteristic frequency (CF) less than 3 kHz) in the cat to resonant stimuli with varied natural frequencies, damping coefficients, and sound pressure levels. Responses to resonances were synchronized to frequencies lying between the peak frequency of the stimulus spectrum and a frequency near the fiber's CF. The frequency of the dominant synchrony in the response varied systematically as a function of the stimulus parameters. 2. More lightly damped resonances, which have sharp spectral peaks, elicited synchrony closer to the peak frequency, whereas the broader peaks of more highly damped resonances elicited synchrony closer to the fiber's CF. Thus as the stimulus was varied from an undamped tone to a highly damped transient, the dominant component of the synchronized response moved from the peak frequency of the stimulus toward the CF of the fiber. The trajectory of the dominant component varied as a function of stimulus level, with higher levels resulting in synchrony biased toward the peak of the stimulus spectrum over a wider range of damping. 3. The frequency tuning and synchronization characteristics of a fiber, along with the stimulus parameters, determined the temporal properties of its response to complex stimuli. Using reverse correlation (revcor) filters to characterize the tuning and synchronization of auditory nerve fibers, we were able to predict the temporal properties of responses to resonant stimuli. 4. A parametric model was fit to measured revcor functions derived from responses of auditory nerve fibers to wideband noise. In this way, a bank of model revcor filters was developed based on our population of measured filters. 5. The filter bank was used to model the response of a population of auditory nerve fibers to resonances. Temporal patterns present in the response of a population of fibers encoded the parameters of resonant stimuli. 6. The model revcor filter bank provided a means of studying temporal response patterns of the population of fibers to other complex sounds. 7. The output of the population model is a representation of the temporal information provided by the auditory periphery to the central nervous system; thus it provides a potentially useful tool for testing hypotheses concerning the processing of temporal information by the central auditory system.(ABSTRACT TRUNCATED AT 400 WORDS)

Intensity coding and the dynamic range problem.

The psychophysical data on intensity discrimination indicate that certain schemes are unlikely as general intensity codes at the level of the auditory nerve and indirectly suggest that the most likely code is one based upon the firing rates of frequency-localized groups of fibers. A detection-theory analysis of a rate-based intensity code indicates that information from very few fibers can, if the information is appropriately combined, account for psychophysical discrimination even at high intensities. This suggests that fibers with similar CFs can code intensity over a wide range and that complex spectra can be represented at the level of the auditory nerve by a rate-CF code over the dynamic range of hearing. The analysis also indicates, however, a substantial discrepancy between the psychophysical data on the dependence of discrimination thresholds on level and the predicted discrimination behavior of a representative population of auditory nerve fibers. Thus, if intensity coding is based on localized firing rate, this fundamental psychophysical behavior does not result solely from peripheral processes.

Rate-place and temporal-place representations of vowels in the auditory nerve and anteroventral cochlear nucleus

This paper considers the representation of stimuli in the peripheral parts of the auditory system. We take as an example the representation of steady-state vowels. The spectra of these stimuli are represented across the best frequency (BF) or “place” dimension of a population of auditory-nerve fibers. We discuss average rate and temporal (phase-locking) measures of the fiber responses as bases for such place representations. Formant frequencies are well represented in terms of either measure. At the level of the anteroventral cochlear nucleus, temporal-place representations are preserved in populations of “primary-like” units and not in “chopper” units. We present preliminary evidence that the chopper units may sharpen a rate-place representation of the vowel spectrum.

Correlation between auditory evoked responses in the thalamus and species-specific call characteristics. I. Rana catesbeiana (Anura: Ranidae).

This evoked potential study of the bullfrog's auditory thalamic area (an auditory responsive region in the posterior dorsal thalamus) shows that complex processing, distinct from that reported in lower auditory regions, occurs in this center. An acoustic stimulus consisting of two tones, one which stimulates either the low-frequency or the mid-frequency sensitive population of auditory nerve fibers from the amphibian papilla and the other the high-frequency sensitive population of fibers from the basilar papilla, evoked a maximal response. The amplitude of the response to the simultaneous stimulation of the two auditory organs was, in some locations, much larger than the linear sum of the responses to the individual tones presented separately. Bimodal spectral stimuli that had relatively long rise-times (greater than or equal to 100 ms) evoked much larger responses than similar sounds with short rise-times. The optimal rise-times were close to those occurring in the bullfrog's mating call. The response was dependent on the waveform periodicity and harmonic content, with a fundamental frequency of 200 Hz producing a larger response than those with fundamentals of 50, 100 or 300 Hz. Six of the natural calls in the bullfrog's vocal repertoire were tested and the mating call and warning call were found to evoke the best responses. Each of these calls stimulate the two auditory organs simultaneously. The evoked response had a long refractory period which could not be altered by lesioning the efferent telencephalic pathways. The type of spectral and temporal information extracted by the auditory thalamic area suggests that this center is involved in processing complex sounds and likely plays an important role in the bullfrog's detection of some of its vocal signals.

Single fiber mapping of spatial excitation patterns in the electrically stimulated auditory nerve.

Spatial maps of electrical excitation were constructed by comparing electrical threshold with acoustic CF for large populations of auditory nerve fibers in cats. Thresholds among fibers with the same CF varied by factors of 4 or more. Monopolar electrodes, both intracochlear and extracochlear, excited fibers throughout the cochlea without spatial selectivity. Stimulation with intracochlear bipolar electrodes produced a minimum in the threshold distribution adjacent to the electrodes. With longitudinally oriented pairs, the width, depth, and location of the minimum shifted with stimulus polarity; spread of excitation throughout the cochlea occurred with stimulus intensities 6.2 to 14 dB above the lowest threshold. With radially oriented pairs, minima were sharper and deeper; spread of excitation occurred at intensities 23.7 to 32.8 dB above the minimum threshold.

Acute ultrastructural changes in acoustic trauma: serial-section reconstruction of stereocilia and cuticular plates.

Single-unit recordings were made from populations of auditory-nerve fibers in 12 cats before and after acoustic overstimulation. Cats were killed 4 to 16 h after exposure, and the cochleas were analyzed at the light- and electron-microscopic levels. The exposures were designed to create 40 to 60 dB of acute threshold shift. Physiological changes were similar to those seen in cases of permanent threshold shift: tuning curves with elevation of 'tips' and 'tails' were associated with significant decreases in the mean spontaneous discharge rates; tuning curves with elevated tips but hypersensitive tails were associated with clear elevation of the mean spontaneous rates. At the light-microscopic level, none of the ears showed any significant stereociliary pathology. Some of the ears showed no light-microscopic pathology whatsoever, while others showed signs of swelling and vacuolization in both inner and outer hair cell areas in cochlear regions appropriate to the CF regions showing threshold shifts. The presence or absence of these light-microscopic changes was, to some extent, dependent on the nature of the exposure stimulus. At the electron-microscopic level, in addition to apparent swelling of radial afferent terminals, the inner hair cells themselves were swollen. In two cochlear regions (from two ears) which showed acute threshold shifts of 20 to 40 dB, but no light-microscopic changes, serial-section ultrastructural analysis of stereocilia and cuticular plates was performed. In contrast to the situation in ears with permanent threshold shifts [(1986) Hear Res. 26, 65-88], there was no pathology in the intracuticular portion of the stereocilia rootlets. There were, however, significant changes in the lengths of the supracuticular portion of the rootlets. It is suggested that this attenuation of the supracuticular rootlet could decrease the stiffness of the stereocilia tufts and thereby change the tuning properties and sensitivity of the cochlear partition.