Response dynamics of goldfish saccular fibers: effects of stimulus frequency and intensity on fibers with different tuning, sensitivity, and spontaneous activity

The effects of stimulus frequency and intensity on response patterns (PST histograms) to tone burst stimulation were examined in differently tuned saccular fibers of the goldfish. In addition, the sensitivity of these fibers to amplitude-modulated (AM) signals of different carrier frequencies was measured. The response patterns evoked by unmodulated signals were a complex function of tuning, spontaneous activity and sensitivity of the fiber, and the frequency and intensity of the signal. Frequency-dependent response patterns were found in low-frequency fibers with best frequencies (BF) below 200 Hz. Responses in these fibers ranged from tonic to phasic in nonspontaneous fibers and included more complex patterns in spontaneously active fibers, such as suppression of evoked activity below spontaneous levels. Midfrequency fibers (BF = 500-600 Hz) showed responses similar to those in low-frequency fibers, but with less dependence on frequency. In contrast, both high-frequency (BF = 800-1000 Hz) and wideband, untuned fibers showed frequency-invariant patterns of adaptation. High-frequency fibers were equally sensitive to AM signals at all frequencies tested. The sensitivity of low-frequency fibers to AM, however, increased as a function of carrier frequency and corresponded to the degree of adaptation in response to unmodulated tones. In general, the AM sensitivity of a fiber could be predicted more by its pattern of response to unmodulated signals than by its tuning characteristics.     

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.     

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.     

Masked auditory thresholds in sciaenid fishes: a comparative study

Western Atlantic sciaenids comprise a taxonomically diverse teleost family with significant variations in the relationship between the swim bladder and the otic capsule. In this study, the auditory brainstem response (ABR) was used to test the hypothesis that fishes with different peripheral auditory structures (black drum, Pogonias chromis and Atlantic croaker, Micropogonias undulatus) show differences in frequency selectivity. In a black drum the swim bladder is relatively distant from the otic capsule while the swim bladder in Atlantic croaker possesses anteriorly-directed diverticulas that terminate relatively near the otic capsule. Signals were pure tones in the frequency range, 100 Hz to 1.5 kHz, and thresholds were determined both with and without the presence of simultaneous white noise at two intensity levels (124 dB and 136 dB, re: 1 microPa). At the 124 dB level of white noise background, both the black drum and Atlantic croaker showed similar changes in auditory sensitivity. However, in the presence of the 136 dB white noise masker, black drum showed significantly greater shifts in auditory thresholds between 300 and 600 Hz. The results indicate that the two species differ in frequency selectivity since the Atlantic croaker was less susceptible to auditory threshold shifts, particularly at the higher level of masking. This difference may be linked to peripheral auditory mechanisms.     

Rate responses of auditory nerve fibers to tones in noise near masked threshold

The rate responses of auditory nerve fibers were measured for best frequency (BF) tone bursts in the presence of continuous background noise. Rate functions for BF tones were constructed over a 32-dB range of levels, centered on the behavioral masked thresholds of cats. The tone level at which noticeable rate changes are evoked by the tones corresponds closely to behavioral masked threshold at all noise levels used (-10- to 30-dB spectrum level). As the noise level increases, the response rate to the background noise approaches saturation, and the incremental rate response to tones decreases. At high noise levels, the rate responses to tones of low and medium spontaneous rate fibers are larger than those of high spontaneous rate fibers. Empirical statistics of auditory nerve fiber spike counts are reported; these differ from those expected of a Poisson process in that the variance is smaller than the mean. A new measure of discharge rate is described that allows rate changes to be expressed in units of a standard deviation. This measure allows tone-evoked responses to be interpreted in terms of their detectability in a signal detection task. Rate responses of low and medium spontaneous rate fibers are more detectable than those of high spontaneous rate fibers, especially at high noise levels. There appears to be sufficient information in the rate response of a small number of auditory nerve fibers to support behaviorally observed levels of detection performance.     

Frequency glides in the impulse responses of auditory-nerve fibers

Previous reports of frequency modulations, or glides, in the impulse responses of the auditory periphery have been limited to analyses of basilar-membrane measurements and responses of auditory-nerve (AN) fibers with best frequencies (BFs) greater than 1.7 kHz. These glides increased in frequency as a function of time. In this study, the instantaneous frequency as a function of time was measured for impulse responses of AN fibers in the cat with a range of BFs (250-4500 Hz). Impulse responses were estimated from responses to wideband noise using the reverse-correlation technique. The impulse responses had increasing frequency glides for fibers with BFs greater than 1500 Hz, nearly constant frequency as a function of time of BFs between 750 and 1500 Hz, and decreasing frequency glides for BFs below 750 Hz. Over the levels tested, the glides for fibers at all BFs were nearly independent of stimulus level, consistent with previous reports of impulse responses of the basilar membrane and AN fibers. Implications of the different glide directions observed for different BFs are discussed, specifically in relation to models for the auditory periphery as well as for the derivation of impulse responses for the human auditory periphery based on psychophysical measurements.     

The relationship between frequency selectivity and pitch discrimination: sensorineural hearing loss

This study tested the relationship between frequency selectivity and the minimum spacing between harmonics necessary for accurate fo discrimination. Fundamental frequency difference limens (fo DLs) were measured for ten listeners with moderate sensorineural hearing loss (SNHL) and three normal-hearing listeners for sine- and random-phase harmonic complexes, bandpass filtered between 1500 and 3500 Hz, with fo's ranging from 75 to 500 Hz (or higher). All listeners showed a transition between small (good) fo DLs at high fo's and large (poor) fo DLs at low fo's, although the fo at which this transition occurred (fo,tr) varied across listeners. Three measures thought to reflect frequency selectivity were significantly correlated to both the fo,tr and the minimum fo DL achieved at high fo's: (1) the maximum fo for which fo DLs were phase dependent, (2) the maximum modulation frequency for which amplitude modulation and quasi-frequency modulation were discriminable, and (3) the equivalent rectangular bandwidth of the auditory filter, estimated using the notched-noise method. These results provide evidence of a relationship between fo discrimination performance and frequency selectivity in listeners with SNHL, supporting "spectral" and "spectro-temporal" theories of pitch perception that rely on sharp tuning in the auditory periphery to accurately extract fo information.     

Low-frequency and high-frequency distortion product otoacoustic emission suppression in humans

Distortion product otoacoustic emission suppression (quantified as decrements) was measured for f(2)=500 and 4000 Hz, for a range of primary levels (L(2)), suppressor frequencies (f(3)), and suppressor levels (L(3)) in 19 normal-hearing subjects. Slopes of decrement-versus-L(3) functions were similar at both f(2) frequencies, and decreased as f(3) increased. Suppression tuning curves, constructed from decrement functions, were used to estimate (1) suppression for on- and low-frequency suppressors, (2) tip-to-tail differences, (3) Q(ERB), and (4) best frequency. Compression, estimated from the slope of functions relating suppression "threshold" to L(2) for off-frequency suppressors, was similar for 500 and 4000 Hz. Tip-to-tail differences, Q(ERB), and best frequency decreased as L(2) increased for both frequencies. However, tip-to-tail difference (an estimate of cochlear-amplifier gain) was 20 dB greater at 4000 Hz, compared to 500 Hz. Q(ERB) decreased to a greater extent with L(2) when f(2)=4000 Hz, but, on an octave scale, best frequency shifted more with level when f(2)=500 Hz. These data indicate that, at both frequencies, cochlear processing is nonlinear. Response growth and compression are similar at the two frequencies, but gain is greater at 4000 Hz and spread of excitation is greater at 500 Hz.     

A nonlinear filter-bank model of the guinea-pig cochlear nerve: rate responses

The aim of this study is to produce a functional model of the auditory nerve (AN) response of the guinea-pig that reproduces a wide range of important responses to auditory stimulation. The model is intended for use as an input to larger scale models of auditory processing in the brain-stem. A dual-resonance nonlinear filter architecture is used to reproduce the mechanical tuning of the cochlea. Transduction to the activity on the AN is accomplished with a recently proposed model of the inner-hair-cell. Together, these models have been shown to be able to reproduce the response of high-, medium-, and low-spontaneous rate fibers from the guinea-pig AN at high best frequencies (BFs). In this study we generate parameters that allow us to fit the AN model to data from a wide range of BFs. By varying the characteristics of the mechanical filtering as a function of the BF it was possible to reproduce the BF dependence of frequency-threshold tuning curves, AN rate-intensity functions at and away from BF, compression of the basilar membrane at BF as inferred from AN responses, and AN iso-intensity functions. The model is a convenient computational tool for the simulation of the range of nonlinear tuning and rate-responses found across the length of the guinea-pig cochlear nerve.     

The detection of temporal gaps as a function of frequency region and absolute noise bandwidth.

Temporal gap detection was measured as a function of absolute signal bandwidth at a low-, a mid-, and a high-frequency region in six listeners with normal hearing sensitivity. Gap detection threshold decreased monotonically with increasing stimulus bandwidth at each of the three frequency regions. Given conditions of equivalent absolute bandwidth, gap detection thresholds were not significantly different for upper cutoff frequencies ranging from 600 to 4400 Hz. A second experiment investigated gap detection thresholds at two pressure-spectrum levels, conditions typically resulting in substantially different estimates of frequency selectivity. Estimates of frequency selectivity were collected at the two levels using a notched-noise masker technique. The gap threshold-signal bandwidth functions were almost identical at pressure-spectrum levels of 70 dB and 40 dB for the two subjects in experiment II, while estimates of frequency selectivity showed poorer frequency selectivity at the 70-dB level than at 40 dB. Data from both experiments indicated that gap detection in bandlimited noise was inversely related to signal bandwidth and that gap detection did not vary significantly with changes in signal frequency over the range of 600 to 4400 Hz. Over the range of frequencies investigated, the results indicated no clear relation between gap detection for noise stimuli and peripheral auditory filtering.     

Masking patterns in the bullfrog (Rana catesbeiana). II: Physiological effects

Responses of individual eighth-nerve fibers in the bullfrog (Rana catesbeiana) were measured to tone bursts at best frequency against a background of continuous, broadband masking noise. These data were used to calculate critical masking ratios to describe the fibers' responses to tones embedded in noise. In the frequency response range of the amphibian papilla (100-1000 Hz), critical ratios increase with tone frequency. Critical ratios of basilar papilla fibers (1000-2000 Hz) are generally higher than those of amphibian papilla fibers. Critical ratios are also significantly related to fiber threshold such that fibers with high thresholds, regardless of their best frequencies, have higher critical ratios and are thus less selective to signals embedded in noise. Critical ratios based on neural responses show a somewhat different frequency-dependent trend than do critical ratios based on psychophysical data presented previously for this species [A. M. Simmons, J. Acoust. Soc. Am. 83, 1087-1092 (1988a)]. In addition, these neural critical ratios do not appear to be level independent, as are psychophysical critical ratios. The data suggest that frequency selectivity of hearing in the bullfrog as measured behaviorally is probably not mediated solely by spectral filtering in the auditory periphery.     

Coding of spectral fine structure in the auditory nerve. I. Fourier analysis of period and interspike interval histograms

The temporal fine structure of discharge patterns of single auditory-nerve fibers in adult cats was analyzed in response to signals consisting of a variable number of equal-intensity, in-phase harmonics of a common low-frequency fundamental. Two analytic methods were employed. The first method considered Fourier spectra of period histograms based on the period of the fundamental, and the second method considered Fourier spectra of interspike interval histograms (ISIH's). Both analyses provide information about fiber tuning properties, but Fourier spectra of ISIH's also allow estimates to be made of the degree of resolution of individual stimulus components. At low intensities (within 20-40 dB of threshold), indices of synchronization to individual components of complex tones were similar to those obtained for pure tones. This was true even when fibers were capable of responding to several signal components simultaneously. Response spectra obtained at low intensities resembled fibers' tuning curves, and fibers with low spontaneous discharge rates tended to provide better resolution of stimulus components than fibers with high spontaneous rates. Strongly nonlinear behavior existed at higher stimulus intensities. In this, information was transmitted about progressively fewer signal components and about frequencies not present in the acoustic stimulus, and the component eliciting the largest response shifted away from the fiber's characteristic frequency and toward the edges of the stimulus spectrum. This high-intensity "edge enhancement" can result from the combined effects of a compressive input-output nonlinearity, suppression, and the fortuitous addition of internally generated combination tones. The data indicate that sufficient information exists for the auditory system to determine the frequencies of narrowly spaced stimulus components from the temporal fine structure of nerve fiber's responses.     

Auditory brainstem responses to chirps delivered by different insert earphones

The frequency response and sensitivity of the ER-3A and ER-2 insert earphones are measured in the occluded-ear simulator using three ear canal extensions. Compared to the other two extensions, the DB 0370 (Bru?el & Kj?r), which is recommended by the international standards, introduces a significant resonance peak around 4500 Hz. The ER-3A has an amplitude response like a band-pass filter (1400 Hz, 6 dB/octave -4000 Hz, -36 dB/octave), and a group delay with "ripples" of up to ±0.5 ms, while the ER-2 has an amplitude response, and a group delay which are flat and smooth up to above 10000 Hz. Both earphones are used to record auditory brainstem responses, ABRs, from 22 normal-hearing ears in response to two chirps and a click at levels from 20 to 80 dB nHL. While the click-ABRs are slightly larger for ER-2 than for ER-3A, the chirp-ABRs are much larger for ER-2 than for ER-3A at levels below 60 dB nHL. With a simulated amplitude response of the ER-3A and the smooth group delay of the ER-2 it is shown that the increased chirp-ABR amplitude with the ER-2 is caused by its broader amplitude response and not by its smoother group delay.     

Physiological responses to the pulsation threshold paradigm. II: Representations of high-pass noise in average rate measures of auditory-nerve fiber discharge

In a companion article [L. I. Hellstrom, J. Acoust. Soc. Am. 85, 230-242 (1989)], it was shown that psychophysical pulsation threshold masking patterns (PTPs) for high-pass noise maskers are not a simple transformation of the profile of activity evoked in the auditory nerve by the masker. In this article, PTPs are compared with neural representations in which interactions of masker and probe are considered. It is hypothesized that, at pulsation threshold, some criterion value of rate change occurs when the stimulus switches from masker to probe. The iso-rate probe level, defined for single auditory-nerve fibers, is the probe level at which this rate change is zero. Iso-rate probe levels are lowest when probe frequency equals best frequency (BF) of the fiber. Profiles of iso-rate probe level versus BF (equal to probe frequency) are qualitatively similar to PTPs but differ quantitatively, e.g., in the rate of growth of probe level with masker level (1.2 dB/dB for PTPs, 0.54 dB/dB for iso-rate profiles). Quantitative differences can be further reduced by requiring a positive rate criterion. These results suggest that PTPs are not solely a reflection of the internal representation of the masker, but reflect responses to the probe tone as well.     

Basilar membrane mechanics at the base of the chinchilla cochlea. I. Input-output functions, tuning curves, and response phases

Basilar membrane (BM) velocity was measured at a site 3.5 mm from the basal end of the chinchilla cochlea using the M?ssbauer technique. The threshold of the compound action potential recorded at the round window in response to tone bursts was used as an indicator of the physiological state of the cochlea. The BM input-output functions display a compressive nonlinearity for frequencies around the characteristic frequency (CF, 8 to 8.75 kHz), but are linear for frequencies below 7 and above 10.5 kHz. In preparations with little surgical damage, isovelocity tuning curves at 0.1 mm/s are sharply tuned, have Q10's of about 6, minima as low as 13 dB SPL, tip-to-tail ratios (at 1 kHz) of 56 to 76 dB, and high-frequency slopes of about 300 dB/oct. These mechanical responses are as sharply tuned as frequency-threshold curves of chinchilla auditory nerve fibers with corresponding CF. There is a progressive loss of sensitivity of the mechanical response with time for the frequencies around CF, but not for frequencies on the tail of the tuning curve. In some experiments the nonlinearity was maintained for several hours, in spite of a considerable loss of sensitivity of the BM response. High-frequency plateaus were observed in both isovelocity tuning curves and phase-frequency curves.     

Diversity of adaptation patterns in responses of eighth nerve fibers in the bullfrog, Rana catesbeiana

The firing patterns of eighth nerve fibers in the bullfrog, Rana catesbeiana, were analyzed for responses to long duration tone bursts at best excitatory frequency ( BEF ) and at frequencies along the upper and lower boundaries of the excitatory tuning curve of each fiber. These firing patterns were used as an index of the degree of short-term adaptation of each fiber. Amphibian papilla fibers (with BEFs 100-1000 Hz) exhibited marked diversity in their firing patterns to BEF tones, ranging from very flat or tonic (sustained responses throughout the duration of the stimulus) to very peaked or phasic (responding primarily or exclusively to stimulus onset). Moreover, the degree of short-term adaptation shown by an individual fiber varied with stimulating frequency. The firing patterns of amphibian papilla fibers tended to become more tonic as stimulus frequency was lowered below BEF ; conversely, as stimulus frequency was increased above BEF , firing patterns either showed little change from that at BEF , or became more phasic. A similar frequency dependence of adaptation has not been reported in responses of mammalian eighth nerve fibers with comparable BEFs . The firing patterns of basilar papilla fibers ( BEFs greater than 1000 Hz) remained similar in response to both BEF and non- BEF tones. These data reveal that the firing patterns and degrees of short-term adaptation of amphibian papilla fibers vary considerably across the tuning curve, whereas those of basilar papilla fibers remain relatively more constant with changes in stimulating frequency.(ABSTRACT TRUNCATED AT 250 WORDS)     

Frequency and intensity discrimination in human infants and adults

Frequency and intensity DLs were measured in 26 human infants (ages 7-9 months) and six young adults using a repeating standard "yes-no" operant headturning technique and an adaptive staircase (tracking) psychophysical procedure. Subjects were visually reinforced for responding to frequency increments, frequency decrements, intensity increments, or intensity decrements in an ongoing train of 1.0-kHz tone bursts, and stimulus control was monitored using randomly interleaved probe and catch trials. Infants were easily conditioned to respond to both increments and decrements in frequency, and DLs ranged from 11-29 Hz, while adult DLs ranged from 3-5 Hz. Infants also easily discriminated intensity increments, and DLs ranged from 3-12 dB, while adult DLs ranged from 1-2 dB. No infants successfully discriminated intensity decrements, although adults experienced no difficulty with this task and produced DLs similar to those for increments. The apparent inability of infants to discriminate intensity decrements suggests that the infant CNS may not be well adapted to monitor rate decreases in populations of peripheral auditory neurons.     

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.     

Localization and source level estimates of black drum (Pogonias cromis) calls

A four hydrophone linear array was used to localize calling black drum and estimate source levels and signal propagation. A total of 1025 source level estimates averaged 165 dB(RMS) relative (re:) 1 μPa (standard deviation (SD)=1.0). The authors suggest that the diverticulated morphology of the black drum swimbladder increase the bladder's surface area, thus contributing to sound amplitude. Call energy was greatest in the fundamental frequency (94 Hz) followed by the second (188 Hz) and third harmonics (282 Hz). A square root model best described propagation of the entire call, and separately the fundamental frequency and second harmonic. A logarithmic model best described propagation of the third harmonic which was the only component to satisfy the cut-off frequency equation. Peak auditory sensitivity was 300 Hz at a 94 dB re: 1 μPa threshold based on auditory evoked potential measurements of a single black drum. Based on mean RMS source level, signal propagation, background levels, and hearing sensitivity, the communication range of black drum was estimated at 33-108 m and was limited by background levels not auditory sensitivity. This estimate assumed the source and receiver were at approximately 0.5 m above the bottom. Consecutive calls of an individual fish localized over 59 min demonstrated a mean calling period of 3.6 s (SD=0.48), mean swimming speed of 0.5 body lengths/s, and a total distance swam of 1035 m.     

Towards a measure of auditory-filter phase response.

This study investigates how the phase curvature of the auditory filters varies with center frequency (CF) and level. Harmonic tone complex maskers were used, with component phases adjusted using a variant of an equation proposed by Schroeder [IEEE Trans. Inf. Theory 16, 85-89 (1970)]. In experiment 1, the phase curvature of the masker was varied systematically and sinusoidal signal thresholds were measured at frequencies from 125 to 8000 Hz. At all signal frequencies, threshold differences of 20 dB or more were observed between the most effective and least effective masker phase curvature. In experiment 2, the effect of overall masker level on masker phase effects was studied using signal frequencies of 250, 1000, and 4000 Hz. The results were used to estimate the phase curvature of the auditory filters. The estimated relative phase curvature decreases dramatically with decreasing CF below 1000 Hz. At frequencies above 1000 Hz, relative auditory-filter phase curvature increases only slowly with increasing CF, or may remain constant. The phase curvature of the auditory filters seems to be broadly independent of overall level. Most aspects of the data are in qualitative agreement with peripheral physiological findings from other mammals, which suggests that the phase responses observed here are of peripheral origin. However, in contrast to the data reported in a cat auditory-nerve study [Carney et al., J. Acoust. Soc. Am. 105, 2384-2391 (1999)], no reversal in the sign of the phase curvature was observed at very low frequencies. Overall, the results provide a framework for mapping out the phase curvature of the auditory filters and provide constraints on future models of peripheral filtering in the human auditory system.