Effects of cochlear-implant pulse rate and inter-channel timing on channel interactions and thresholds

Interactions among the multiple channels of a cochlear prosthesis limit the number of channels of information that can be transmitted to the brain. This study explored the influence on channel interactions of electrical pulse rates and temporal offsets between channels. Anesthetized guinea pigs were implanted with 2-channel scala-tympani electrode arrays, and spike activity was recorded from the auditory cortex. Channel interactions were quantified as the reduction of the threshold for pulse-train stimulation of the apical channel by sub-threshold stimulation of the basal channel. Pulse rates were 254 or 4069 pulses per second (pps) per channel. Maximum threshold reductions averaged 9.6 dB when channels were stimulated simultaneously. Among nonsimultaneous conditions, threshold reductions at the 254-pps rate were entirely eliminated by a 1966-micros inter-channel offset. When offsets were only 41 to 123 micros, however, maximum threshold shifts averaged 3.1 dB, which was comparable to the dynamic ranges of cortical neurons in this experimental preparation. Threshold reductions at 4069 pps averaged up to 1.3 dB greater than at 254 pps, which raises some concern in regard to high-pulse-rate speech processors. Thresholds for various paired-pulse stimuli, pulse rates, and pulse-train durations were measured to test possible mechanisms of temporal integration.     

Repetition rate and signal level effects on neuronal responses to brief tone pulses in cat auditory cortex

This study describes the effects on the spike count, spike timing, and entrainment of cat auditory cortex neurons of parametric variations in the repetition rate and amplitude of a brief, characteristic frequency tone pulse. Data were obtained from single neurons in barbiturate-anesthetized cats to which signals were presented monaurally to the ear contralateral to the recording electrode. All neurons showed low-pass sensitivity to tone repetition rate. In cells with a monotonic rate response, the effect of an increasing stimulus level was to elevate the response rate and to extend performance to higher repetition rates. In nonmonotonic cells, cutoff frequencies (for repetition rate) varied with overall spike count. Latent periods increased with increases in repetition rate. This effect developed over the first few stimulus trials at any given repetition rate. Spike entrainment to the tone pulses varied with both repetition rate and signal level. Increases in signal level improved entrainment for responses to stimuli presented at low repetition rates, but entrainment at high repetition rates always saturated at significantly imperfect levels.     

Coding of spectral fine structure in the auditory nerve. II: Level-dependent nonlinear responses

Phase-locked discharge patterns of single cat auditory-nerve fibers were analyzed in response to complex tones centered at fiber characteristic frequency (CF). Signals were octave-bandwidth harmonic complexes defined by a center frequency F and an intercomponent spacing factor N, such that F/N was the fundamental frequency. Parameters that were manipulated included the phase spectrum, the number of components, and the intensity of the center component. Analyses employed Fourier transforms of period histograms to assess the degree to which responses were synchronized to the frequencies present in the acoustic stimulus. Several nonlinearities were observed in the response as intensity was varied between threshold and 80-90 dB SPL. Response nonlinearities were strong for all signals except those with random phase spectra. The most commonly observed nonlinearity was an emphasis of one or more stimulus components in the response. The degree of nonlinearity usually increased with intensity and signal complexity and decreased with fiber frequency selectivity. Half-wave rectification introduced synchronization to the missing fundamental. The strength of the response at the fundamental was related to stimulus crest factor. Signals with low center frequencies and high crest factors often elicited instantaneous discharge rates at the theoretical maximum of pi CF. This suggests that the probability of spike generation approaches one during high-amplitude waveform segments. Response nonlinearity was interpreted as arising from three sources, namely, cochlear mechanics, compression of instantaneous discharge rate, and saturation of average discharge rate. At near-threshold intensities, fibers with high spontaneous rates exhibited responses that were linear functions of stimulus waveshape, whereas fibers with low spontaneous spike rates produced responses that were best described in terms of an expansive nonlinearity.     

Sensitivity to binaural timing in bilateral cochlear implant users

Various measures of binaural timing sensitivity were made in three bilateral cochlear implant users, who had demonstrated moderate-to-good interaural time delay (ITD) sensitivity at 100 pulses-per-second (pps). Overall, ITD thresholds increased at higher pulse rates, lower levels, and shorter durations, although intersubject differences were evident. Monaural rate-discrimination thresholds, using the same stimulation parameters, showed more substantial elevation than ITDs with increased rate. ITD sensitivity with 6000 pps stimuli, amplitude-modulated at 100 Hz, was similar to that with unmodulated pulse trains at 100 pps, but at 200 and 300 Hz performance was poorer than with unmodulated signals. Measures of sensitivity to binaural beats with unmodulated pulse-trains showed that all three subjects could use time-varying ITD cues at 100 pps, but not 300 pps, even though static ITD sensitivity was relatively unaffected over that range. The difference between static and dynamic ITD thresholds is discussed in terms of relative contributions from initial and later arriving cues, which was further examined in an experiment using two-pulse stimuli as a function of interpulse separation. In agreement with the binaural-beat data, findings from that experiment showed poor discrimination of ITDs on the second pulse when the interval between pulses was reduced to a few milliseconds.     

Sound intensity processing by the goldfish

Capacities of the goldfish for intensity discrimination were studied using classical respiratory conditioning and a staircase psychophysical procedure. Physiological studies on single saccular (auditory) nerve fibers under similar stimulus conditions helped characterize the dimensions of neural activity used in intensity discrimination. Incremental intensity difference limens (IDLs in dB) for 160-ms increments in continuous noise, 500-ms noise bursts, and 500-ms, 800-Hz tone bursts are 2 to 3 dB, are independent of overall level, and vary with signal duration according to a power function with a slope averaging - 0.33. Noise decrements are relatively poorly detected and the silent gap detection threshold is about 35 ms. The IDLs for increments and decrements in an 800-Hz continuous tone are about 0.13 dB, are independent of duration, and are level dependent. Unlike mammalian auditory nerve fibers, some goldfish saccular fibers show variation in recovery time to tonal increments and decrements, and adaptation to a zero rate. Unit responses to tone increments and decrements show rate effects generally in accord with previous observations on intracellular epsp's in goldfish saccular fibers. Neurophysiological correlates of psychophysical intensity discrimination data suggest the following: (1) noise gap detection may be based on spike rate increments which follow gap offset; (2) detection of increments and decrements in continuous tones may be determined by steep low-pass filtering in peripheral neural channels which enhance the effects of spectral "splatter" toward the lower frequencies; (3) IDLs for pulsed signals of different duration can be predicted from the slopes of rate-intensity functions and spike rate variability in individual auditory nerve fibers; and (4) at different sound pressure levels, different populations of peripheral fibers provide the information used in intensity discrimination.     

Pulse-rate discrimination by cochlear-implant and normal-hearing listeners with and without binaural cues

Experiment 1 measured rate discrimination of electric pulse trains by bilateral cochlear implant (CI) users, for standard rates of 100, 200, and 300 pps. In the diotic condition the pulses were presented simultaneously to the two ears. Consistent with previous results with unilateral stimulation, performance deteriorated at higher standard rates. In the signal interval of each trial in the dichotic condition, the standard rate was presented to the left ear and the (higher) signal rate was presented to the right ear; the non-signal intervals were the same as in the diotic condition. Performance in the dichotic condition was better for some listeners than in the diotic condition for standard rates of 100 and 200 pps, but not at 300 pps. It is concluded that the deterioration in rate discrimination observed for CI users at high rates cannot be alleviated by the introduction of a binaural cue, and is unlikely to be limited solely by central pitch processes. Experiment 2 performed an analogous experiment in which 300-pps acoustic pulse trains were bandpass filtered (3900-5400 Hz) and presented in a noise background to normal-hearing listeners. Unlike the results of experiment 1, performance was superior in the dichotic than in the diotic condition.     

Lateralization discrimination of interaural time delays in four-pulse sequences in electric and acoustic hearing

This study examined the sensitivity of four cochlear implant (CI) listeners to interaural time difference (ITD) in different portions of four-pulse sequences in lateralization discrimination. ITD was present either in all the pulses (referred to as condition Wave), the two middle pulses (Ongoing), the first pulse (Onset), the last pulse (Offset), or both the first and last pulse (Gating). All ITD conditions were tested at different pulse rates (100, 200, 400, and 800 pulses/s pps). Also, five normal hearing (NH) subjects were tested, listening to an acoustic simulation of CI stimulation. All CI and NH listeners were sensitive in condition Gating at all pulse rates for which they showed sensitivity in condition Wave. The sensitivity in condition Onset increased with the pulse rate for three CI listeners as well as for all NH listeners. The performance in condition Ongoing varied over the subjects. One CI listener showed sensitivity up to 800 pps, two up to 400 pps, and one at 100 pps only. The group of NH listeners showed sensitivity up to 200 pps. The result that CI listeners detect ITD from the middle pulses of short trains indicates the relevance of fine timing of stimulation pulses in lateralization and therefore in CI stimulation strategies.     

Effect of stimulus level and place of stimulation on temporal pitch perception by cochlear implant users

Three experiments studied the effect of pulse rate on temporal pitch perception by cochlear implant users. Experiment 1 measured rate discrimination for pulse trains presented in bipolar mode to either an apical, middle, or basal electrode and for standard rates of 100 and 200 pps. In each block of trials the signals could have a level of -0.35, 0, or +0.35 dB re the standard, and performance for each signal level was recorded separately. Signal level affected performance for just over half of the combinations of subject, electrode, and standard rate studied. Performance was usually, but not always, better at the higher signal level. Experiment 2 showed that, for a given subject and condition, the direction of the effect was similar in monopolar and bipolar mode. Experiment 3 employed a pitch comparison procedure without feedback, and showed that the signal levels in experiment 1 that produced the best performance for a given subject and condition also led to the signal having a higher pitch. It is concluded that small level differences can have a robust and substantial effect on pitch judgments and argue that these effects are not entirely due to response biases or to co-variation of place-of-excitation with level.     

Difference thresholds for vibration of the foot: Dependence on frequency and magnitude of vibration

The smallest change in vibration intensity for the change to be perceptible (i.e. intensity difference threshold) has not previously been reported for vibration of the foot. This study investigated the influence of vibration magnitude and vibration frequency on intensity difference thresholds for the perception of vertical sinusoidal vibration of the foot. It was hypothesised that relative intensity difference thresholds (i.e. Weber fractions) for 16-Hz vibration mediated by the non-Pacinian I (NPI) channel would differ from relative intensity difference thresholds for 125-Hz vibration mediated by the Pacinian (P) channel. Absolute thresholds, difference thresholds, and the locations of vibration sensation caused by vertical vibration of the right foot were determined for 12 subjects using the up-down-transformed-response method together with the three-down-one-up rule. The difference thresholds and locations of sensation were obtained at six reference magnitudes (at 6, 9, 12, 18, 24, 30 dB above absolute threshold—i.e. sensation levels, SL). For 16-Hz vibration, the median relative difference thresholds were not significantly dependent on vibration magnitude and were in the range 0.19 (at 30 dB SL) to 0.27 (at 9 dB SL). For 125-Hz vibration, the median relative difference thresholds varied between 0.17 (at 9 dB SL) and 0.34 (at 30 dB SL), with difference thresholds from 6 to 12 dB SL significantly less than those from 18 to 30 dB SL. At vibration magnitudes slightly in excess of absolute thresholds (i.e. 6-12 dB SL) there were no significant differences between Weber fractions obtained from the P channel (at 125 Hz) and the NPI channel (at 16 Hz). At 24 and 30 dB SL, the 125-Hz Weber fractions were significantly greater than the 16-Hz Weber fractions. Differences in the 125-Hz Weber fractions may have been caused by a reduction in the discriminability of the P channel at high levels of excitation, resulting in one or more NP channel mediating the difference thresholds at magnitudes greater than 18 dB SL. At high magnitudes, a change of channel mediating the Weber fractions may have been responsible for different Weber fractions with 16- and 125-Hz vibration.     

Simulations of cochlear implant hearing using filtered harmonic complexes: implications for concurrent sound segregation

Two experiments used simulations of cochlear implant hearing to investigate the use of temporal codes in speech segregation. Sentences were filtered into six bands, and their envelopes used to modulate filtered alternating-phase harmonic complexes with rates of 80 or 140 pps. Experiment 1 showed that identification of single sentences was better for the higher rate. In experiment 2, maskers (time-reversed concatenated sentences) were scaled by -9 dB relative to a target sentence, which was added with an offset of 1.2 s. When the target and masker were each processed on all six channels, and then summed, processing the masker on a different rate to the target improved performance only when the target rate was 140 pps. When the target sentence was processed on the odd-numbered channels and the masker on the even-numbered channels, or vice versa, performance was worse overall, but showed similar effects of pulse rate. The results, combined with recent psychophysical evidence, suggest that differences in pulse rate are unlikely to prove useful for concurrent sound segregation.     

Statistical and receiver operating characteristic analysis of empirical spike-count distributions: quantifying the ability of cochlear nucleus units to signal intensity changes

Analytical methods from signal detection theory were applied in an effort to quantify the ability of cochlear nucleus (CN) units to signal changes in intensity. Of particular interest was the relation between this ability and the different patterns of discharge that characterize auditory neurons. Single-unit responses to best-frequency (BF) tone bursts were recorded from neurons in the gerbil cochlear nucleus, and empirical spike-count distributions were generated. The mean-to-variance ratios for regular units were generally larger than those of irregular units. Receiver operating characteristic (ROC) curves were generated from empirical spike-count distributions. The area under the ROC curve [P(A)] was computed and used to define the performance of an observer detecting whether or not a change in firing rate has occurred, thus signaling a change in intensity. For a given change in mean spike count, units characterized by regular interspike-interval (ISI) histograms typically gave larger P(A) values than did units characterized by irregular ISI histograms. In addition, onset units gave larger values of P(A) than did irregular units for a given change in mean spike count. These results suggest that regular and onset units are better able to signal intensity changes than are irregular units.     

Effects of carrier pulse rate and stimulation site on modulation detection by subjects with cochlear implants

Most modern cochlear-implant speech processors convey speech-envelope information using amplitude-modulated pulse trains. The use of higher-rate carrier pulse trains allows more envelope detail in the signal. However, neural response properties could limit the efficacy of high-rate carriers. This study examined effects of carrier rate and stimulation site, on psychophysical modulation detection thresholds (MDTs). Both of these variables could affect the neural representation of the carrier and thus affect perception of the modulation. Twelve human subjects with cochlear implants were tested. Phase duration of symmetric biphasic pulses was modulated sinusoidally at 40 Hz. MDTs were determined for monopolar stimulation at two carrier rates [250 and 4000 pulses/s (pps)], three stimulation sites (basal, middle, and apical), and five stimulus levels (10%, 30%, 50%, 70%, and 90% of the dynamic range). MDTs were lower for 250 pps carriers than for 4000 pps carriers in 71% of the 180 cases studied. Effects of carrier rate were greatest at the apical stimulation site and effects of stimulation site on MDTs depended on carrier rate. The data suggest a distinct disadvantage to using carrier pulse rates as high as 4000 pps. Stimulation site should be considered in evaluating modulation detection ability.     

The effect of parametric variations of cochlear implant processors on speech understanding

This study investigated the effect of five speech processing parameters, currently employed in cochlear implant processors, on speech understanding. Experiment 1 examined speech recognition as a function of stimulation rate in six Med-E1/CIS-Link cochlear implant listeners. Results showed that higher stimulation rates (2100 pulses/s) produced a significantly higher performance on word and consonant recognition than lower stimulation rates (<800 pulses/s). The effect of stimulation rate on consonant recognition was highly dependent on the vowel context. The largest benefit was noted for consonants in the /uCu/ and /iCi/ contexts, while the smallest benefit was noted for consonants in the /aCa/ context. This finding suggests that the /aCa/ consonant test, which is widely used today, is not sensitive enough to parametric variations of implant processors. Experiment 2 examined vowel and consonant recognition as a function of pulse width for low-rate (400 and 800 pps) implementations of the CIS strategy. For the 400-pps condition, wider pulse widths (208 micros/phase) produced significantly higher performance on consonant recognition than shorter pulse widths (40 micros/phase). Experiments 3-5 examined vowel and consonant recognition as a function of the filter overlap in the analysis filters, shape of the amplitude mapping function, and signal bandwidth. Results showed that the amount of filter overlap (ranging from -20 to -60 dB/oct) and the signal bandwidth (ranging from 6.7 to 9.9 kHz) had no effect on phoneme recognition. The shape of the amplitude mapping functions (ranging from strongly compressive to weakly compressive) had only a minor effect on performance, with the lowest performance obtained for nearly linear mapping functions. Of the five speech processing parameters examined in this study, the pulse rate and the pulse width had the largest (positive) effect on speech recognition. For a fixed pulse width, higher rates (2100 pps) of stimulation provided a significantly better performance on word recognition than lower rates (<800 pps) of stimulation. High performance was also achieved by jointly varying the pulse rate and pulse width. The above results indicate that audiologists can optimize the implant listener's performance either by increasing the pulse rate or by jointly varying the pulse rate and pulse width.     

Temporal pitch perception at high rates in cochlear implants

A recent study reported that a group of Med-El COMBI 40+CI (cochlear implant) users could, in a forced-choice task, detect changes in the rate of a pulse train for rates higher than the 300 pps "upper limit" commonly reported in the literature [Kong, Y.-Y., et al. (2009). J. Acoust. Soc. Am. 125, 1649-1657]. The present study further investigated the upper limit of temporal pitch in the same group of CI users on three tasks [pitch ranking, rate discrimination, and multidimensional scaling (MDS)]. The patterns of results were consistent across the three tasks and all subjects could follow rate changes above 300 pps. Two subjects showed exceptional ability to follow temporal pitch change up to about 900 pps. Results from the MDS study indicated that, for the two listeners tested, changes in pulse rate over the range of 500-840 pps were perceived along a perceptual dimension that was orthogonal to the place of excitation. Some subjects showed a temporal pitch reversal at rates beyond their upper limit of pitch and some showed a reversal within a small range of rates below the upper limit. These results are discussed in relation to the possible neural bases for temporal pitch processing at high rates.     

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.     

A new proposal of high repetitive Nd:YAG laser power supply adopted the sequential charge and discharge circuit

The pulsed Nd:YAG laser is the most commonly used type of solid-state laser in many fields. In material processing, the power density control of a laser beam has been considered to be significant, which depends on the flashlamp current pulse width and pulse repetition rate.In this study, we have proposed a new method of sequential charge and discharge circuit (SCADC) to control the laser power density. The power supply of SCADC is composed of low frequency capacitors instead of very expensive high frequency capacitors. We could find the stability of laser output as well as the flashlamp current up to the pulse repetition rate of 150 pps. As increasing a repetition rate from 30 to 150 pps by the step of 30 pps, it is known that the laser outputs increased by 10 W.     

Psychophysical studies relevant to the design of a digital electrotactile speech processor

Psychophysical tests were carried out to investigate the perception of electrocutaneous stimuli delivered to the digital nerve bundles. The tests provided data for defining the operating range of a tactile aid for patients with profound-to-total hearing loss, as well as the individual differences between subjects and the information that could be transmitted. Monopolar biphasic constant current pulses with variable pulse widths were used. Threshold pulse widths varied widely between subjects and between fingers for the same subject. Thresholds were reasonably stable, but maximum comfortable levels increased with time. Perceived intensity was weakly dependent on pulse rate. Absolute identification of stimuli differing in pulse width gave information transmissions from 1.3-2.1 bits, limited by the dynamic ranges of the stimuli (3-17 dB). Stimuli from electrodes placed on either side of each finger were identified easily by all subjects. Absolute identification of stimuli differing in pulse rate gave information transmissions from 0.5-2.0 bits. Difference limens for pulse rate varied between subjects and were generally poor above 100 pps. On the basis of the results, an electrotactile speech processor is proposed, which codes the speech amplitude as pulse width, the fundamental frequency as pulse rate, and the second formant frequency as electrode position. Variable performances on tasks relying on amplitude and fundamental frequency cues are expected to arise from the intersubject differences in dynamic range and pulse rate discrimination. The psychophysical results for electrotactile stimulation are compared with previously published results for electroauditory stimulation with a multiple-channel cochlear implant.     

Intensity discrimination of pulsed tones by the goldfish (Carassius auratus)

Intensity discrimination thresholds for 500-ms pure-tone bursts were measured as a function of frequency in the goldfish (Carassius auratus) using classical respiratory conditioning. At 55-dB sensation level (SL), thresholds range from 1.44-2.2 dB between 100 and 1600 Hz. There is not important effect of frequency on intensity discrimination. Thresholds at 35-dB SL average 0.7 dB higher than at 55-dB SL. This is a small difference in the context of the threshold variability. In intensity discrimination acuity, the goldfish is quantitatively similar to other vertebrates, including birds and mammals.     

Psychophysical studies for two multiple-channel cochlear implant patients

Psychophysical studies were conducted on two multiple-channel cochlear implant patients to examine the nature of the hearing sensations produced by electrical stimulation of auditory nerve fibers using electrodes at different sites in the scala tympani (one electrode at a time). Both time-invariant stimuli, whose parameter values did not vary in time, and time-varying stimuli, specified by a linear variation in parameter values, were used. A sharpness ranking study using time-invariant signals suggested that the hearing sensations produced by different electrodes varied from dull to sharp in an apical to basal direction in the scala tympani. A categorization study showed that the hearing sensations produced by two adjacent electrodes (1.5 mm apart) were rarely confused for a restricted range of time-invariant pulse rates. Discriminability studies by a same-different procedure for stimuli with pulse rate below 250 pps showed: (1) relative difference limens of 6% to 12% for time-invariant pulse rates, and 9% and 13% for time-varying pulse rates; (2) stimuli with time-varying electrode position differing in the direction of electrode trajectory were readily discriminated; and (3) the discrimination of time-varying pulse rates deteriorated with decreases in the duration of the variation, while the discriminability of single-electrode trajectories was the same for the three durations: 25, 50, and 100 ms. A speech processing strategy was also proposed on the bases of these results.     

Frequency selectivity of single cochlear-nerve fibers based on the temporal response pattern to two-tone signals

The physiological basis of auditory frequency selectivity was investigated by recording the temporal response patterns of single cochlear-nerve fibers in the cat. The characteristic frequency and sharpness of tuning was determined for low-frequency cochlear-nerve fibers with two-tone signals whose frequency components were of equal amplitude and starting phase. The measures were compared with those obtained with sinusoidal signals. The two-tone characteristic frequency (2TCF) is defined as the arithmetic-center frequency at which the fiber is synchronized to both signal frequencies in equal measure. The 2TCF closely corresponds to the characteristic frequency as determined by the frequency threshold curve. Moreover, the 2TCF changes relatively little (2%-12%) over a 60-dB intensity range. The 2TCF generally shifts upward with increasing intensity for cochlear-nerve fibers tuned to frequencies below 1 kHz and shifts downward as a function of intensity for units with characteristic frequencies (CF's) above 1 kHz. The shifts in the 2TCF are considerably smaller than those observed with sinusoidal signals. Filter functions were derived from the synchronization pattern to the two-tone signal by varying the frequency of one of the components over the fiber's response area while maintaining the other component at the 2TCF. The frequency selectivity of the two-tone filter function was determined by dividing the vector strength to the variable frequency signal by the vector strength to the CF tone. The filter function was measured 10 dB down from the peak (2T Q 10 dB) and compared with the Q 10 dB of the frequency threshold curve. The correlation between the two measures of frequency selectivity was 0.72. The 2T Q 10 dB does change as a function of intensity. The magnitude and direction of the change is dependent on the sharpness of tuning at low and moderate sound-pressure levels (SPL's). The selectivity of the more sharply tuned fibers (2T Q 10 dB greater than 3) diminishes at intensities above 60 dB SPL. However, the broadening of selectivity is relatively small in comparison to discharge rate-based measures of selectivity. The selectivity of the more broadly tuned units remains unchanged or improves slightly at similar intensity levels. The present data indicate that the frequency selectivity and tuning of low-frequency cochlear-nerve fibers are relatively stable over a 60-dB range of SPL's when measured in terms of their temporal discharge properties.