Testing coherent reflection in chinchilla: Auditory-nerve responses predict stimulus-frequency emissions

Coherent-reflection theory explains the generation of stimulus-frequency and transient-evoked otoacoustic emissions by showing how they emerge from the coherent "backscattering" of forward-traveling waves by mechanical irregularities in the cochlear partition. Recent published measurements of stimulus-frequency otoacoustic emissions (SFOAEs) and estimates of near-threshold basilar-membrane (BM) responses derived from Wiener-kernel analysis of auditory-nerve responses allow for comprehensive tests of the theory in chinchilla. Model predictions are based on (1) an approximate analytic expression for the SFOAE signal in terms of the BM traveling wave and its complex wave number, (2) an inversion procedure that derives the wave number from BM traveling waves, and (3) estimates of BM traveling waves obtained from the Wiener-kernel data and local scaling assumptions. At frequencies above 4 kHz, predicted median SFOAE phase-gradient delays and the general shapes of SFOAE magnitude-versus-frequency curves are in excellent agreement with the measurements. At frequencies below 4 kHz, both the magnitude and the phase of chinchilla SFOAEs show strong evidence of interference between short- and long-latency components. Approximate unmixing of these components, and association of the long-latency component with the predicted SFOAE, yields close agreement throughout the cochlea. Possible candidates for the short-latency SFOAE component, including wave-fixed distortion, are considered. Both empirical and predicted delay ratios (long-latency SFOAE delay/BM delay) are significantly less than 2 but greater than 1. Although these delay ratios contradict models in which SFOAE generators couple primarily into cochlear compression waves, they are consistent with the notion that forward and reverse energy propagation in the cochlea occurs predominantly by means of traveling pressure-difference waves. The compelling overall agreement between measured and predicted delays suggests that the coherent-reflection model captures the dominant mechanisms responsible for the generation of reflection-source otoacoustic emissions.     

Near equivalence of human click-evoked and stimulus-frequency otoacoustic emissions

Otoacoustic emissions (OAEs) evoked by broadband clicks and by single tones are widely regarded as originating via different mechanisms within the cochlea. Whereas the properties of stimulus-frequency OAEs (SFOAEs) evoked by tones are consistent with an origin via linear mechanisms involving coherent wave scattering by preexisting perturbations in the mechanics, OAEs evoked by broadband clicks (CEOAEs) have been suggested to originate via nonlinear interactions among the different frequency components of the stimulus (e.g., intermodulation distortion). The experiments reported here test for bandwidth-dependent differences in mechanisms of OAE generation. Click-evoked and stimulus-frequency OAE input/output transfer functions were obtained and compared as a function of stimulus frequency and intensity. At low and moderate intensities human CEOAE and SFOAE transfer functions are nearly identical. When stimulus intensity is measured in "bandwidth-compensated" sound-pressure level (cSPL), CEOAE and SFOAE transfer functions have equivalent growth functions at fixed frequency and equivalent spectral characteristics at fixed intensity. This equivalence suggests that CEOAEs and SFOAEs are generated by the same mechanism. Although CEOAEs and SFOAEs are known by different names because of the different stimuli used to evoke them, the two OAE "types" are evidently best understood as members of the same emission family.     

Two-tone suppression of stimulus frequency otoacoustic emissions

Stimulus frequency otoacoustic emissions (SFOAEs) measured using a suppressor tone in human ears are analogous to two-tone suppression responses measured mechanically and neurally in mammalian cochleae. SFOAE suppression was measured in 24 normal-hearing adults at octave frequencies (f(p)=0.5-8.0 kHz) over a 40 dB range of probe levels (L(p)). Suppressor frequencies (f(s)) ranged from -2.0 to 0.7 octaves re: f(p), and suppressor levels ranged from just detectable suppression to full suppression. The lowest suppression thresholds occurred for "best" f(s) slightly higher than f(p). SFOAE growth of suppression (GOS) had slopes close to one at frequencies much lower than best f(s), and shallow slopes near best f(s), which indicated compressive growth close to 0.3 dBdB. Suppression tuning curves constructed from GOS functions were well defined at 1, 2, and 4 kHz, but less so at 0.5 and 8.0 kHz. Tuning was sharper at lower L(p) with an equivalent rectangular bandwidth similar to that reported behaviorally for simultaneous masking. The tip-to-tail difference assessed cochlear gain, increasing with decreasing L(p) and increasing f(p) at the lowest L(p) from 32 to 45 dB for f(p) from 1 to 4 kHz. SFOAE suppression provides a noninvasive measure of the saturating nonlinearities associated with cochlear amplification on the basilar membrane.     

Transient-evoked stimulus-frequency and distortion-product otoacoustic emissions in normal and impaired ears

Transient-evoked stimulus-frequency otoacoustic emissions (SFOAEs), recorded using a nonlinear differential technique, and distortion-product otoacoustic emissions (DPOAEs) were measured in 17 normal-hearing and 10 hearing-impaired subjects using pairs of tone pips (pp), gated tones (gg), and for DPOAEs, continuous and gated tones (cg). Temporal envelopes of stimulus and OAE waveforms were obtained by narrow-band filtering at the stimulus or DP frequency. Mean SFOAE latencies in normal ears at 2.7 and 4.0 kHz decreased with increasing stimulus level and were larger at 4.0 kHz than latencies in impaired ears. Equivalent auditory filter bandwidths were calculated as a function of stimulus level from SFOAE latencies by assuming that cochlear transmission is minimum phase. DPOAE latencies varied less with level than SFOAE latencies. The ppDPOAEs often had two (or more) peaks separated in time with latencies consistent with model predictions for distortion and reflection components. Changes in ppDPOAE latency with level were sometimes explained by a shift in relative amplitudes of distortion and reflection components. The pp SFOAE SPL within the main spectral lobe of the pip stimulus was higher for normal ears in the higher-frequency half of the pip than the lower-frequency half, which is likely an effect of basilar membrane two-tone suppression.     

Input-output functions for stimulus-frequency otoacoustic emissions in normal-hearing adult ears

Input-output (I/O) functions for stimulus-frequency (SFOAE) and distortion-product (DPOAE) otoacoustic emissions were recorded in 30 normal-hearing adult ears using a nonlinear residual method. SFOAEs were recorded at half octaves from 500-8000 Hz in an L1=L2 paradigm with L2=0 to 85 dB SPL, and in a paradigm with L1 fixed and L2 varied. DPOAEs were elicited with primary levels of Kummer et al. [J. Acoust. Soc. Am. 103, 3431-3444 (1998)] at f2 frequencies of 2000 and 4000 Hz. Interpretable SFOAE responses were obtained from 1000-6000 Hz in the equal-level paradigm. SFOAE levels were larger than DPOAEs levels, signal-to-noise ratios were smaller, and I/O functions were less compressive. A two-slope model of SFOAE I/O functions predicted the low-level round-trip attenuation, the breakpoint between linearity and compression, and compressive slope. In ear but not coupler recordings, the noise at the SFOAE frequency increased with increasing level (above 60 dB SPL), whereas noise at adjacent frequencies did not. This suggests the existence of a source of signal-dependent noise producing cochlear variability, which is predicted to influence basilar-membrane motion and neural responses. A repeatable pattern of notched SFOAE I/O functions was present in some ears, and explained using a two-source mechanism of SFOAE generation.     

Medial efferent effects on auditory-nerve responses to tail-frequency tones II: alteration of phase

It is often assumed that at frequencies in the tuning-curve tail there is a passive, constant coupling of basilar-membrane motion to inner hair cell (IHC) stereocilia. This paper shows changes in the phase of auditory-nerve-fiber (ANF) responses to tail-frequency tones and calls into question whether basilar-membrane-to-IHC coupling is constant. In cat ANFs with characteristic frequencies > or = 10 kHz, efferent effects on the phase of ANF responses to tail-frequency tones were measured. Efferent stimulation caused substantial changes in ANF phase (deltaphi) (range -80 degrees to +60 degrees, average -15 degrees, a phase lag) with the largest changes at sound levels near threshold and 3-4 octaves below characteristic frequency (CF). At these tail frequencies, efferent stimulation had much less effect on the phase of the cochlear microphonic (CM) than on ANF phase. Thus, since CM is synchronous with basilar-membrane motion for low-frequency stimuli in the cochlear base, the efferent-induced change in ANF phase is unlikely to be due entirely to a change in basilar-membrane phase. At tail frequencies, ANF phase changed with sound level (often by 90 degrees-180 degrees) and the deltaphi from a fiber was positively correlated with the slope of its phase-versus-sound-level function at the same frequency, as if deltaphi were caused by a 2-4 dB increase in sound level. This correlation suggests that the processes that produce the change in ANF phase with sound level at tail frequencies are also involved in producing deltaphi. It is hypothesized that both efferent stimulation and increases in sound level produce similar phase changes because they both produce a similar mix of cochlear vibrational modes.     

Simultaneous recording of stimulus-frequency and distortion-product otoacoustic emission input-output functions in human ears

Stimulus frequency otoacoustic emission (SFOAE) input-output (I/O) functions were elicited in normal-hearing adults using unequal-frequency primaries in equal-level and fixed-suppressor level (Ls) conditions. Responses were repeatable and similar across a range of primary frequency ratios in the fixed-Ls condition. In comparison to equal-frequency primary conditions [Schairer, Fitzpatrick, and Keefe, J. Acoust. Soc. Am. 114, 944-966 (2003)], the unequal-frequency, fixed-Ls condition appears to be more useful for characterizing SFOAE response growth and relating it to basilar-membrane response growth, and for testing the ability to predict audiometric thresholds. Simultaneously recorded distortion-product OAE (DPOAE) I/O functions had higher thresholds than SFOAE I/O functions, and they identified the onset of the nonlinear-distortion mechanism in SFOAEs. DPOAE threshold often corresponded to nonmonotonicities in SFOAE I/O functions. This suggests that the level-dependent nonmonotonicities and associated phase shifts in SFOAE I/O functions were due to varying degrees of cancellation of two sources of SFOAE, such as coherent reflection and distortion mechanisms. Level-dependent noise was observed on-band (at the frequencies of the stimuli) but not off-band, or in the DPOAEs. The variability was observed in ears with normal hearing and ears with cochlear implants. In general, these results indicate the source of the variability is biological, possibly from within the middle ear.     

Use of stimulus-frequency otoacoustic emission latency and level to investigate cochlear mechanics in human ears

Stimulus frequency otoacoustic emission (SFOAE) sound pressure level (SPL) and latency were measured at probe frequencies from 500 to 4000 Hz and probe levels from 40 to 70 dB SPL in 16 normal-hearing adult ears. The main goal was to use SFOAE latency estimates to better understand possible source mechanisms such as linear coherent reflection, nonlinear distortion, and reverse transmission via the cochlear fluid, and how those sources might change as a function of stimulus level. Another goal was to use SFOAE latencies to noninvasively estimate cochlear tuning. SFOAEs were dominated by the reflection source at low stimulus levels, consistent with previous research, but neither nonlinear distortion nor fluid compression become the dominant source even at the highest stimulus level. At each stimulus level, the SFOAE latency was an approximately constant number of periods from 1000 to 4000 Hz, consistent with cochlear scaling symmetry. SFOAE latency decreased with increasing stimulus level in an approximately frequency-independent manner. Tuning estimates were constant above 1000 Hz, consistent with simultaneous masking data, but in contrast to previous estimates from SFOAEs.     

Sources of DPOAEs revealed by suppression experiments,inverse fast Fourier transforms,and SFOAEs in impaired ears

DPOAE sources are modeled by intermodulation distortion generated near the f2 place and a reflection of this distortion near the DP place. In a previous paper, inverse fast Fourier transforms (IFFTs) of DPOAE filter functions in normal ears were consistent with this model [Konrad-Martin et al., J. Acoust. Soc. Am. 109, 2862-2879 (2001)]. In the present article, similar measurements were made in ears with specific hearing-loss configurations. It was hypothesized that hearing loss at f2 or DP frequencies would influence the relative contributions to the DPOAE from the corresponding basilar membrane places, and would affect the relative magnitudes of SFOAEs at frequencies equal to f2 and fDP. DPOAEs were measured with f2 = 4 kHz, f1 varied, and a suppressor near fDP. L2 was 25-55 dB SPL (L1 = L2 + 10 dB). SFOAEs were measured at f2 and at 2.7 kHz (the average fDP produced by the f1 sweep) for stimulus levels of 20-60 dB SPL. SFOAE results supported predictions of the pattern of amplitude differences between SFOAEs at 4 and 2.7 kHz for sloping losses, but did not support predictions for the rising- and flat-loss categories. Unsuppressed IFFTs for rising losses typically had one peak. IFFTs for flat or sloping losses typically have two or more peaks; later peaks were more prominent in ears with sloping losses compared to normal ears. Specific predictions were unambiguously supported by the results for only four of ten cases, and were generally supported in two additional cases. Therefore, the relative contributions of the two DPOAE sources often were abnormal in impaired ears, but not always in the predicted manner.     

Effects of the use of personal music players on amplitude modulation detection and frequency discrimination

Measures of auditory performance were compared for an experimental group who listened regularly to music via personal music players (PMP) and a control group who did not. Absolute thresholds were similar for the two groups for frequencies up to 2 kHz, but the experimental group had slightly but significantly higher thresholds at higher frequencies. Thresholds for the frequency discrimination of pure tones were measured for a sensation level (SL) of 20 dB and center frequencies of 0.25, 0.5, 1, 2, 3, 4, 5, 6, and 8 kHz. Thresholds were significantly higher (worse) for the experimental than for the control group for frequencies from 3 to 8 kHz, but not for lower frequencies. Thresholds for detecting sinusoidal amplitude modulation (AM) were measured for SLs of 10 and 20 dB, using four carrier frequencies 0.5, 3, 4, and 6 kHz, and three modulation frequencies 4, 16, and 50 Hz. Thresholds were significantly lower (better) for the experimental than for the control group for the 4- and 6-kHz carriers, but not for the other carriers. It is concluded that listening to music via PMP can have subtle effects on frequency discrimination and AM detection.     

Responses to amplitude-modulated tones in the auditory nerve of the cat.

Sinusoidally amplitude-modulated (AM) tones are frequently used in psychophysical and physiological studies, yet a comprehensive study on the coding of AM tones in the auditory nerve is lacking. AM responses of single auditory-nerve fibers of the cat are studied, systematically varying modulation depth, frequency, and sound level. Synchrony-level functions were nonmonotonic with maximum values that were inversely correlated with spontaneous rate (SR). In most fibers, envelope phase-locking showed a positive gain. Modulation transfer functions were uniformly low pass. Their corner frequency increased with characteristic frequency (CF), but changed little for CFs above 10 kHz. The highest modulation frequencies to which phase locking occurred were more than 0.8 oct lower than the highest frequencies to which phase locking to pure tones occurs. Cumulative, or unwrapped, phase increased linearly with modulation frequency: The slope was inversely related to CF, and slightly higher than group delays reported for pure tones. High SR, low CF fibers showed the poorest envelope phase locking. In some low CF fibers, phase locking increased at high levels, associated with "peak-splitting" phenomena. Changes in average rate due to modulation were small, and could be enhancement or suppression.     

Medial efferent effects on auditory-nerve responses to tail-frequency tones. I. Rate reduction.

One way medial efferents are thought to inhibit responses of auditory-nerve fibers (ANFs) is by reducing the gain of the cochlear amplifier thereby reducing motion of the basilar membrane. If this is the only mechanism of medial efferent inhibition, then medial efferents would not be expected to inhibit responses where the cochlear amplifier has little effect, i.e., at sound frequencies in the tails of tuning curves. Inhibition at tail frequencies was tested for by obtaining randomized rate-level functions from cat ANFs with high characteristic frequencies (CF > or = 5 kHz), stimulated with tones two or more octaves below CF. It was found that electrical stimulation of medial efferents can indeed inhibit ANF responses to tail-frequency tones. The amplitude of efferent inhibition depended on both sound level (largest near to threshold) and frequency (largest two to three octaves below CF). On average, inhibition of high-CF ANFs responding to 1 kHz tones was around 5 dB. Although an efferent reduction of basilar-membrane motion cannot be ruled out as the mechanism producing the inhibition of ANF responses to tail frequency tones, it seems more likely that efferents produce this effect by changing the micromechanics of the cochlear partition.     

Stimulus-frequency otoacoustic emission: measurements in humans and simulations with an active cochlear model

An efficient method for measuring stimulus-frequency otoacoustic emissions (SFOAEs) was developed incorporating (1) stimulus with swept frequency or level and (2) the digital heterodyne analysis. SFOAEs were measured for 550-1450 Hz and stimulus levels of 32-62 dB sound pressure level in eight normal human adults. The mean level, number of peaks, frequency spacing between peaks, phase change, and energy-weighted group delays of SFOAEs were determined. Salient features of the human SFOAEs were stimulated with an active cochlear model containing spatially low-pass filtered irregularity in the impedance. An objective fitting procedure yielded an optimal set of model parameters where, with decreasing stimulus level, the amount of cochlear amplification and the base amplitude of the irregularity increased while the spatial low-pass cutoff and the slope of the spatial low-pass filter decreased. The characteristics of the human cochlea were inferred with the model. In the model, an SFOAE consisted of a long-delay component originating from irregularity in a traveling-wave peak region and a short-delay component originating from irregularity in regions remote from the peak. The results of this study should be useful both for understanding cochlear function and for developing a clinical method of assessing cochlear status.     

A parametric model of the spectral periodicity of stimulus frequency otoacoustic emissions

A model for estimating the spectral period of stimulus frequency otoacoustic emissions (SFOAEs) is presented. The model characterizes the frequency spectrum of an SFOAE in terms of four parameters which can be directly related to cochlear mechanical quantities featuring in the theory of SFOAE generation proposed by Zweig and Shera [J. Acoust. Soc. Am. 98, 2018-2047 (1995)]. The results of applying the parametric model to SFOAEs generated by cochlear models suggest that it gives a sensitive measure of spectral period. It is concluded that the parametric model may be a useful tool for detecting small changes in cochlear function using SFOAE measurements.     

Stimulus-frequency-emission group delay: a test of coherent reflection filtering and a window on cochlear tuning

This paper tests and applies a key prediction of the theory of coherent reflection filtering for the generation of reflection-source otoacoustic emissions. The theory predicts that reflection-source-emission group delay is determined by the group delay of the basilar-membrane (BM) transfer function at its peak. This prediction is tested over a seven-octave frequency range in cats and guinea pigs using measurements of stimulus-frequency-emission (SFOAE) group delay. A comparison with group delays calculated from published measurements of BM mechanical transfer functions supports the theory at the basal end of the cochlea. A comparison across the whole frequency range based on variations in the sharpness of neural tuning with characteristic frequency (CF) suggests that the predicted relation holds in the basal-most 60% of the cochlea. At the apical end of the cochlea, however, the measurements disagree with neural and mechanical group delays. This disagreement suggests that there are important differences in cochlear mechanics and/or mechanisms of emission generation between the base and apex of the cochlea. Measurements in humans over a four-octave range indicate that human SFOAE group delays are roughly a factor of 3 longer than their counterparts in cat and guinea pig but manifest similar trends across CF. The measurements thus reveal global deviations from scaling whose form appears quantitatively similar in all three species. Interpreted using the theory of coherent reflection filtering, the group delay measurements indicate that the wavelength at the peak of the traveling wave decreases with increasing CF at a rate of roughly 25% per octave in the base of the cochlea. The measurements and analysis reported here illustrate the rich potential inherent in OAE measurements for obtaining valuable information about basic cochlear properties such as tuning.     

Comparing stimulus-frequency otoacoustic emissions measured by compression, suppression, and spectral smoothing

Stimulus-frequency otoacoustic emissions (SFOAEs) have been measured in several different ways, including (1) nonlinear compression, (2) two-tone suppression, and (3) spectral smoothing. Each of the three methods exploits a different cochlear phenomenon or signal-processing technique to extract the emission. The compression method makes use of the compressive growth of emission amplitude relative to the linear growth of the stimulus. The emission is defined as the complex difference between ear-canal pressure measured at one intensity and the rescaled pressure measured at a higher intensity for which the emission is presumed negligible. The suppression method defines the SFOAE as the complex difference between the ear-canal pressure measured with and without a suppressor tone at a nearby frequency. The suppressor tone is presumed to substantially reduce or eliminate the emission. The spectral smoothing method involves convolving the complex ear-canal pressure spectrum with a smoothing function. The analysis exploits the differing latencies of stimulus and emission and is equivalent to windowing in the corresponding latency domain. Although the three methods are generally assumed to yield identical emissions, no equivalence has ever been established. This paper compares human SFOAEs measured with the three methods using procedures that control for temporal drifts, contamination of the calibration by evoked emissions, and other potential confounds. At low stimulus intensities, SFOAEs measured using all three methods are nearly identical. At higher intensities, limitations of the procedures contribute to small differences, although the general spectral shape and phase of the three SFOAEs remain similar. The near equivalence of SFOAEs measured by compression, suppression, and spectral smoothing indicates that SFOAE characteristics are not mere artifacts of measurement methodology.     

Perception of across-frequency asynchrony and the role of cochlear delays

Cochlear filtering results in earlier responses to high than to low frequencies. This study examined potential perceptual correlates of cochlear delays by measuring the perception of relative timing between tones of different frequencies. A brief 250-Hz tone was combined with a brief 1-, 2-, 4-, or 6-kHz tone. Two experiments were performed, one involving subjective judgments of perceived synchrony, the other involving asynchrony detection and discrimination. The functions relating the proportion of "synchronous" responses to the delay between the tones were similar for all tone pairs. Perceived synchrony was maximal when the tones in a pair were gated synchronously. The perceived-synchrony function slopes were asymmetric, being steeper on the low-frequency-leading side. In the second experiment, asynchrony-detection thresholds were lower for low-frequency rather than for high-frequency leading pairs. In contrast with previous studies, but consistent with the first experiment, thresholds did not depend on frequency separation between the tones, perhaps because of the elimination of within-channel cues. The results of the two experiments were related quantitatively using a decision-theoretic model, and were found to be highly correlated. Overall the results suggest that frequency-dependent cochlear group delays are compensated for at higher processing stages, resulting in veridical perception of timing relationships across frequency.     

Multicomponent stimulus interactions observed in basilar-membrane vibration in the basal region of the chinchilla cochlea.

Multicomponent stimuli consisting of two to seven tones were used to study suppression of basilar-membrane vibration at the 3-4-mm region of the chinchilla cochlea with a characteristic frequency between 6.5 and 8.5 kHz. Three-component stimuli were amplitude-modulated sinusoids (AM) with modulation depth varied between 0.25 and 2 and modulation frequency varied between 100 and 2000 Hz. For five-component stimuli of equal amplitude, frequency separation between adjacent components was the same as that used for AM stimuli. An additional manipulation was to position either the first, third, or fifth component at the characteristic frequency (CF). This allowed the study of the basilar-membrane response to off-CF stimuli. CF suppression was as high as 35 dB for two-tone combinations, while for equal-amplitude stimulus components CF suppression never exceeded 20 dB. This latter case occurred for both two-tone stimuli where the suppressor was below CF and for multitone stimuli with the third component=CF. Suppression was least for the AM stimuli, including when the three AM components were equal. Maximum suppression was both level- and frequency dependent, and occurred for component frequency separations of 500 to 600 Hz. Suppression decreased for multicomponent stimuli with component frequency spacing greater than 600 Hz. Mutual suppression occurred whenever stimulus components were within the compressive region of the basilar membrane.     

Dead regions and pitch perception

The perception of pitch for pure tones with frequencies falling inside low- or high-frequency dead regions (DRs) was examined. Subjects adjusted a variable-frequency tone to match the pitch of a fixed tone. Matches within one ear were often erratic for tones falling in a DR, indicating unclear pitch percepts. Matches across ears of subjects with asymmetric hearing loss, and octave matches within ears, indicated that tones falling within a DR were perceived with an unclear pitch and/or a pitch different from "normal" whenever the tones fell more than 0.5 octave within a low- or high-frequency DR. One unilaterally impaired subject, with only a small surviving region between 3 and 4 kHz, matched a fixed 0.5-kHz tone in his impaired ear with, on average, a 3.75-kHz tone in his better ear. When asked to match the 0.5-kHz tone with an amplitude-modulated tone, he adjusted the carrier and modulation frequencies to about 3.8 and 0.5 kHz, respectively, suggesting that some temporal information was still available. Overall, the results indicate that the pitch of low-frequency tones is not conveyed solely by a temporal code. Possibly, there needs to be a correspondence between place and temporal information for a normal pitch to be perceived.     

Basilar-membrane nonlinearity estimated by pulsation threshold

The pulsation threshold technique was used to estimate the basilar-membrane (BM) response to a tone at characteristic frequency (CF). A pure-tone signal was alternated with a pure-tone masker. The frequency of the masker was 0.6 times that of the signal. For signal levels from around 20 dB above absolute threshold to 85 dB SPL, the masker level was varied to find the level at which a transition occurred between the signal being perceived as "pulsed" or "continuous" (the pulsation threshold). The transition is assumed to occur when the masker excitation is somewhat greater than the signal excitation at the place on the BM tuned to the signal. If it is assumed further that the response at this place to the lower-frequency masker is linear, then the shape of the masking function provides an estimate of the BM response to the signal. Signal frequencies of 0.25, 0.5, 1, 2, 4, and 8 kHz were tested. The mean slopes of the masking functions for signal levels between 50 and 80 dB SPL were 0.76, 0.50, 0.34, 0.32, 0.35, and 0.41, respectively. The results suggest that compression on the BM increases between CFs of 0.25 and 1 kHz and is roughly constant for frequencies of 1 kHz and above. Despite requiring a subjective criterion, the pulsation threshold measurements had a reasonably low variability. However, the estimated compression was less than in an earlier study using forward masking. The smaller amount of compression observed here may be due to the effects of off-frequency listening.