High Masker Levels Research Articles

Masking release at 4 kHz in harbor porpoises (Phocoena phocoena) associated with sinusoidal amplitude-modulated masking noise

Acoustic masking reduces the efficiency of communication, prey detection, and predator avoidance in marine mammals. Most underwater sounds fluctuate in amplitude. The ability of harbor porpoises (Phocoena phocoena) to detect sounds in amplitude-varying masking noise was examined. A psychophysical technique evaluated hearing thresholds of three harbor porpoises for 500-2000 ms tonal sweeps (3.9-4.1 kHz), presented concurrently with sinusoidal amplitude-modulated (SAM) or unmodulated Gaussian noise bands centered at 4 kHz. Masking was assessed in relation to signal duration and masker level, amplitude modulation rate (1, 2, 5, 10, 20, 40, 80, and 90 Hz), modulation depth (50%, 75%, and 100%) and bandwidth (1/3 or 1 octave). Masking release (MR) due to SAM was assessed by comparing thresholds in modulated and unmodulated maskers. Masked thresholds were affected by SAM rate with the lowest thresholds (i.e., largest MR was 14.5 dB) being observed for SAM rates between 1 and 5 Hz at higher masker levels. Increasing the signal duration from 500-2000 ms increased MR by 3.3 dB. Masker bandwidth and depth of modulation had no substantial effect on MR. The results are discussed with respect to MR resulting from envelope variation and the impact of noise in the environment.

Facilitation and refractoriness of the electrically evoked compound action potential

In this study we aim to resolve the contributions of facilitation and refractoriness at very short pulse intervals. Measurements of the refractory properties of the electrically evoked compound action potential (ECAP) of the auditory nerve in cochlear implant (CI) users at inter pulse intervals below 300 μs are influenced by facilitation and recovery effects. ECAPs were recorded using masker pulses with a wide range of current levels relative to the probe pulse levels, for three suprathreshold probe levels and pulse intervals from 13 to 200 μs. Evoked potentials were measured for 21 CI patients by using the masked response extraction artifact cancellation procedure.During analysis of the measurements the stimulation current was not used as absolute value, but in relation to the patient's individual ECAP threshold. This enabled a more general approach to describe facilitation as a probe level independent effect. Maximum facilitation was found for all tested inter pulse intervals at masker levels near patient's individual ECAP threshold, independent from probe level. For short inter pulse intervals an increased N1P1 amplitude was measured for subthreshold masker levels down to 120 CL below patient's individual ECAP threshold in contrast to the recreated state.ECAPs recorded with inter pulse intervals up to 200 μs are influenced by facilitation and recovery. Facilitation effects are most pronounced for masker levels at or below ECAP threshold, while recovery effects increase with higher masker levels above ECAP threshold. The local maximum of the ECAP amplitude for masker levels around ECAP threshold can be explained by the mutual influence of maximum facilitation and minimal refractoriness.

Effects of stimulus level and rate on psychophysical thresholds for interleaved pulse trains in cochlear implants.

This study examined channel interactions using interleaved pulse trains to assess masking and potential facilitative effects in cochlear-implant recipients using clinically relevant stimuli. Psychophysical thresholds were measured for two adjacent mid-array electrodes; one served as the masker and the other as the probe. Two rates representative of those found in present-day strategies were tested: 1700 and 3400 pulses per second per channel. Four masker levels ranging from sub-threshold to loud-but-comfortable were tested. It was hypothesized that low-level maskers would produce facilitative effects, shifting to masking effects at high levels, and that faster rates would yield smaller masking effects due to greater stochastic neural firing patterns. Twenty-nine ears with Cochlear or Advanced Bionics devices were tested. High-level maskers produced more masking than low-level maskers, as expected. Facilitation was not observed for sub-threshold or threshold-level maskers in most cases. High masker levels yielded reduced probe thresholds for two Advanced Bionics subjects. This was partly eliminated with a longer temporal offset between each masker-probe pulse pair, as was used with Cochlear subjects. These findings support the use of temporal gaps between stimulation of subsequent electrodes to reduce channel interactions.

The potential role of the medial olivocochlear reflex in the estimation of cochlear input-output functions.

Research in our laboratory suggests that forward masking may partially be a result of reduced cochlear gain via the medial olivocochlear reflex (MOCR). The MOCR is not activated for approximately 20 ms following elicitor onset (in this case the masker). Growth of masking functions (GOM) and temporal masking curves (TMCs) are forward masking techniques that have been used to estimate the cochlear input-output (I-O) function. Both techniques assume that there is a linear response to a masker approximately an octave below the signal frequency. In the GOM technique, masker threshold is measured as a function of signal level, while in the TMC technique, masker threshold is measured as a function of signal delay from the masker. A short (20-ms) masker was used to estimate I-O functions using the two techniques. Gain should not be reduced in the GOM condition, but could be reduced with delay in the TMC condition. I-O functions estimated using the GOM and TMC techniques diverge at high masker levels. These points correspond to long delays in the TMC technique, where the MOCR may be playing a role. The gain reduction hypothesis and other hypotheses will be explored. [Research Supported by a Grant from NIH(NIDCD) R01 DC008327.]

Spatial and Temporal Effects of Interleaved Masking in Cochlear Implants

Modern cochlear implants utilize interleaved presentation of pulses on different electrodes to avoid physical interference among multiple current fields, yet neural interaction still exists. In the present study, masking was examined with four Nucleus24 users with the banded electrode array in an interleaved masking paradigm, where a probe stimulus was interleaved with a masker stimulus. Spatial and temporal aspects of masking were addressed by fixing the masker at the middle of the electrode array and changing the location of the probe and by testing various stimulation rates: 125, 500, 2,000, and 6,410 Hz. In addition, growth of masking (GOM) was assessed by changing the masker level in six steps. Results indicated that masking patterns were generally much wider, regardless of stimulation rate, than those in acoustic hearing. The amount of masking decreased from the peak at the rate of approximately 0.5 dB/mm even at the highest masker level. The pattern of GOM with the rates higher than 500 Hz was different from that observed in previous masking studies, characterized by markedly shallow growth at low masker levels or overall shallow growth. A facilitating effect of the masker (lowering the threshold) was suspected, except for the 125-Hz condition, due to the fibers that were subliminally excited, but not discharged, by the masker with local perturbations of membrane potentials, and were subsequently discharged easily by a lower level probe when the temporal gap between masker and probe was sufficiently short. These results suggest that both refractory characteristics of neurons and neural summation be considered in interleaved stimulation of pulses at high, but clinically relevant, stimulation rates. Overall, the present masking study might provide a basis for models in psychophysics and speech understanding in current cochlear implant systems utilizing high-rate interleaved stimulation.

Wide-dynamic-range forward suppression in marmoset inferior colliculus neurons is generated centrally and accounts for perceptual masking.

An organism's ability to detect and discriminate sensory inputs depends on the recent stimulus history. For example, perceptual detection thresholds for a brief tone can be elevated by as much as 50 dB when following a masking stimulus. Previous work suggests that such forward masking is not a direct result of peripheral neural adaptation; the central pathway apparently modifies the representation in a way that further attenuates the input's response to short probe signals. Here, we show that much of this transformation is complete by the level of the inferior colliculus (IC). Single-neuron extracellular responses were recorded in the central nucleus of the awake marmoset IC. The threshold for a 20 ms probe tone presented at best frequency was determined for various masker-probe delays, over a range of masker sound pressure levels (SPLs) and frequencies. The most striking aspect of the data was the increased potency of forward maskers as their SPL was increased, despite the fact that the excitatory response to the masker was often saturating or nonmonotonic over the same range of levels. This led to probe thresholds at high masker levels that were almost always higher than those observed in the auditory nerve. Probe threshold shifts were not usually caused by a persistent excitatory response to the masker; instead we propose a wide-dynamic-range inhibitory mechanism locked to sound offset as an explanation for several key aspects of the data. These findings further delineate the role of subcortical auditory processing in the generation of a context-dependent representation of ongoing acoustic scenes.

Speech Recognition in Noise: Estimating Effects of Compressive Nonlinearities in the Basilar-Membrane Response

This experiment was designed to estimate effects of cochlear nonlinearities on tonal and speech masking for individuals with normal hearing who have a range of quiet thresholds. Physiological and psychophysical evidence indicates that for signals close to the characteristic frequency (CF) of a place on the basilar membrane, the normal growth of response of the basilar membrane is linear at lower stimulus levels and compressed at medium to higher stimulus levels. In contrast, at moderate to high CFs, the basilar membrane responds more linearly to stimuli at frequencies well below the CF regardless of input level. Thus, the hypothesis tested was that masker effectiveness would change as a function of stimulus level consistent with the underlying basilar membrane response. Specifically, with a fixed-level speech signal and a speech-shaped masker that ranges from low to higher levels, the resulting response of the basilar membrane to the masker would be linear at lower levels and compressed at medium to higher levels. This would result in relatively less effective masking at higher masker levels. It was further hypothesized that the transition from linear to compressed responses to both tones and maskers would occur at higher levels for listeners with higher quiet thresholds than for listeners with lower quiet thresholds. Tonal thresholds and speech recognition in noise were measured as a function of masker level. A 10-msec, 2.0-kHz tone was presented in a lower frequency masker ranging from 40 to 85 dB SPL. Moderate-level speech was presented in interrupted noise at six levels ranging from 47 to 77 dB SPL. To minimize differences in speech audibility that could arise during the "off" periods of the interrupted noise, a low-level steady-state "threshold-matching noise" was also present during measurement of speech recognition. Subjects were 30 adults with normal hearing with a 20-dB range of average quiet thresholds. Tonal breakpoints (i.e., the levels corresponding to the transitions from linear to nonlinear responses) were significantly correlated with quiet thresholds, whereas slopes measured above the breakpoints were not. Speech recognition in noise was consistent with the hypothesis that the response of the basilar membrane to the masker was linear at lower levels and compressed at medium to higher levels, resulting in less effective masking at higher masker levels. That is, at lower masker levels, as masker level increased, mean observed speech scores declined as predicted using the articulation index, an audibility-based model. With further increases in masker level, mean scores declined less than predicted. Moreover, for subjects with higher quiet thresholds, masker effectiveness remained constant for a wider range of masker levels than for subjects with lower quiet thresholds, consistent with the hypothesis that the transition from linear to compressed responses occurred at higher levels. Finally, significant negative correlations were obtained between individual subjects' tonal and speech measures. Results from tonal and speech tasks were consistent with basilar membrane nonlinearities and consistent with changes in nonlinearities with minor threshold elevations, providing support for their role in the understanding of speech in noise with increases in noise level.

Temporal masking curves for hearing-impaired listeners

The decay of forward masking was investigated for three subjects with moderate sensorineural hearing loss. For such subjects, compression on the basilar membrane (BM) is thought to be largely absent, enabling one to determine the decay of masking without the influence of compression. Temporal masking curves (TMCs), plots of the masker level at threshold against delay between masker offset and signal onset, were measured for delays of 0, 15, 30, 45, 60, and 75 ms, for signal frequencies, f s, of 500, 1000, 2000, 4000, and 6000 Hz. Masker frequencies were 0.5, 0.8, 1.0, 1.15, and 1.3 times f s. Most of the TMCs were well fitted with single-segment straight lines, which, except for high masker levels, were roughly parallel for each f s, supporting the belief that BM compression was largely absent in these subjects. However, the slopes of the TMCs were greater for f s = 500 and 1000 Hz than for higher frequencies, which may indicate that the decay of forward masking is not the same for all signal frequencies. The results suggest that it may not be valid to infer BM compression at low signal frequencies by using a reference TMC for a high f s.

Temporal Masking in Electric Hearing

Temporal masking can be defined as the detection threshold of a brief signal as a function of the signal delay in a relatively long masker. The temporal masking pattern in normal acoustic hearing reveals temporal edge enhancement in which the signal detection threshold is greater near the masker onset than in the steady-state portion. Both peripheral and central mechanisms appear to underlie temporal edge enhancement, but their relative contributions remain elusive. Cochlear implants bypass cochlear mechanical processing and stimulate the auditory nerve directly, thereby providing a unique opportunity to separate the peripheral mechanisms from the central mechanisms. Here, we systematically measured temporal masking in electric hearing by examining whether a brief signal was harder to detect at the onset than in the steady-state portion of a long masker (the "overshoot" effect). The signal and the masker were presented (1) either to the same electrode or to different electrodes, (2) at the same stimulation or different rates, and (3) in a simultaneous or an interleaved fashion. A consistent pattern of results was observed, depending on the stimulus configuration between the signal and the masker. Simultaneous stimulation at the same rate and with the same electrode produced no difference in sensitivity between the onset and the steady-state conditions, but interleaved stimulation at different rates or with different electrodes produced a significant difference. Unlike acoustic hearing, high masker levels produced an overshoot effect, and low masker levels produced an undershoot effect. Although the present results are consistent with the "on-frequency vs. off-frequency" hypothesis for the overshoot effect, results also suggest a central "same vs. different" mechanism underlying temporal masking. These results have practical implications for improving cochlear implant design.

Speech understanding in noise: Contributions of compressive nonlinearities in the basilar-membrane response

The contribution of compressive nonlinearities in the basilar-membrane response was assessed by measuring speech recognition in noise as a function of noise level and growth of masking for tones. Consonant recognition was measured in interrupted noise at overall masker levels of 47–77 dB SPL. Additionally, thresholds for a 10-ms, 2.0-kHz tone were measured in a masker ranging in level from 40 to 85 dB SPL and centered at 1 kHz. Subjects were younger and older adults with normal hearing. With speech at a constant level and the masker level varying, it was hypothesized that the response of the basilar membrane to the masker would be linear at lower levels and compressed at medium to higher levels, resulting in less effective masking at higher masker levels. Consistent with the hypothesis, differences between observed and predicted consonant-recognition scores, determined using AI calculations, varied as a function of masker level, especially for younger subjects. At lower masker levels, observed scores declined as predicted as masker level increased; with further increases in masker level, scores declined less than predicted. For tonal growth-of-masking functions, breakpoints were correlated with quiet thresholds: both were significantly higher for older than younger subjects. [Work supported by NIH/NIDCD.]

Effects of masker component phase on the forward masking produced by complex tones in normally hearing and hearing-impaired subjects

For normally hearing subjects, harmonic complex tones that give “peaky” waveforms on the basilar membrane (Schroeder-positive phase, sine phase or cosine phase) lead to less forward masking than complex tones that give less peaky waveforms (Schroeder-negative phase or random phase), but have the same power spectrum. This difference has been attributed mainly to the combined effects of peripheral compression and suppression, both of which depend on the operation of the active mechanism in the cochlea. If this explanation is correct, the phase effect should be reduced or absent for subjects with moderate cochlear hearing loss. We measured growth-of-masking functions for forward maskers containing the first 40 harmonics of a 100-Hz fundamental, with components added either in cosine phase or random phase, using both normally hearing subjects and subjects with moderate cochlear hearing loss. The signal frequency was 1 or 2 kHz. For the normally hearing subjects, the mean slopes of the growth-of-masking functions at 1 and 2 kHz, respectively, were 0.53 and 0.44 for the random-phase masker and 0.31 and 0.26 for the cosine-phase masker. For high masker levels, the former produced considerably more masking than the latter. The phase effect was smaller for the hearing-impaired than for the normally hearing subjects, which is consistent with the idea that it is partly caused by peripheral compression and suppression. However, three of the five hearing-impaired subjects showed a significant effect of masker phase for at least one signal frequency. In one case, this occurred when the hearing loss at the signal frequency was 65 dB. The slopes of the growth-of-masking functions were consistently less than one for the hearing-impaired subjects. Further testing suggested that the efferent system was not involved in producing the phase effect.

Phase effects in forward masking for normally hearing and hearing-impaired subjects

The forward masking produced by harmonic complex tones depends on the phases of the masker components; phases giving ‘‘peaky’’ waveforms on the basilar membrane result in less forward masking than phases giving less peaky waveforms. This difference has been attributed to the effects of peripheral compression and suppression, which depend on the active mechanism in the cochlea. Hence, no phase effect would be expected for subjects with moderate cochlear hearing loss. Growth-of-masking functions were measured for forward maskers containing the first 40 harmonics of a 100-Hz fundamental, with components added in cosine (C) or random (R) phase. The signal frequency was 1 or 2 kHz. For three normally hearing subjects, the R masker produced considerably more masking than the C masker, at high masker levels. Of four subjects with moderate cochlear hearing loss, one showed no effect of masker phase, but for the other three there was a significant effect for at least one signal frequency. It is suggested that peripheral compression and suppression play a role in the phase effect, but other factors, possibly depending on the operation of the efferent system, are involved. [Work supported by MRC (UK).]

A new procedure for measuring peripheral compression in normal-hearing and hearing-impaired listeners.

Forward-masking growth functions for on-frequency (6-kHz) and off-frequency (3-kHz) sinusoidal maskers were measured in quiet and in a high-pass noise just above the 6-kHz probe frequency. The data show that estimates of response-growth rates obtained from those functions in quiet, which have been used to infer cochlear compression, are strongly dependent on the spread of probe excitation toward higher frequency regions. Therefore, an alternative procedure for measuring response-growth rates was proposed, one that employs a fixed low-level probe and avoids level-dependent spread of probe excitation. Fixed-probe-level temporal masking curves (TMCs) were obtained from normal-hearing listeners at a test frequency of 1 kHz, where the short 1-kHz probe was fixed in level at about 10 dB SL. The level of the preceding forward masker was adjusted to obtain masked threshold as a function of the time delay between masker and probe. The TMCs were obtained for an on-frequency masker (1 kHz) and for other maskers with frequencies both below and above the probe frequency. From these measurements, input/output response-growth curves were derived for individual ears. Response-growth slopes varied from >1.0 at low masker levels to <0.2 at mid masker levels. In three subjects, response growth increased again at high masker levels (>80 dB SPL). For the fixed-level probe, the TMC slopes changed very little in the presence of a high-pass noise masking upward spread of probe excitation. A greater effect on the TMCs was observed when a high-frequency cueing tone was used with the masking tone. In both cases, however, the net effects on the estimated rate of response growth were minimal.

Backward masking in speech perception by children and adults

The current investigation measured discrimination and identification of tone-noise and [b]-[a] stimuli by adults, children with normally developing language, and children with language impairment. The children were aged 4–6 years. These tasks were chosen instead of the more traditional two alternative forced choice (2AFC) technique because it was believed possible that task difficulty influenced previous results (e.g., Wright et al., 1997). The stimuli in the current set of experiments had longer tones (40 ms) than prior studies but used higher masker levels (52 dB spectrum level). However, results showed that all the participants discriminated the stimuli containing the tone or [b] signal significantly above chance, contrary to previous findings. More difficulty was found in the identification task, similar to prior backward masking results. [Supported by an Individual Development Award, NYS/UUP.]

The relative role of beats and combination tones in determining the shapes of masking patterns at 2 kHz: I. Normal-hearing listeners

Masking patterns for a 2-kHz sinusoidal masker at 45, 65 or 85 dB SPL were measured for three normal-hearing subjects, using a 3-AFC method with feedback (condition 1). The patterns showed distinct irregularities, particularly at the highest masker level. In condition 2, a lowpass noise was added to mask combination tones. The noise increased thresholds mainly for the 85 dB masker, for signal frequencies of 2.3–3.0 kHz. In condition 3, a pair of high-frequency tones (‘modulation detection interference (MDI) tones’) was used to introduce beats at the same rate as produced by the interaction of the masker and signal. Thresholds were higher than for condition 1, particularly for signal frequencies adjacent to the masker frequency. In condition 4, the lowpass noise was presented simultaneously with the MDI tones. Thresholds were well predicted as a combination of the effects of the lowpass noise and the MDI tones. In condition 5, a pair of low-frequency MDI tones was added to the masker. The thresholds had the same overall pattern as in condition 4. We conclude that the shapes of masking patterns measured using a 2-kHz masker are influenced by the detection of beats for masker–signal frequency separations up to at least 300 Hz and by the detection of combination tones for separations between 300 and 1000 Hz.

Suppression and the upward spread of masking

The purpose of this study is to clarify the role of suppression in the growth of masking when a signal is well above the masker in frequency (upward spread of masking). Classical psychophysical models assume that masking is primarily due to the spread of masker excitation, and that the nonlinear upward spread of masking reflects a differential growth in excitation between the masker and the signal at the signal frequency. In contrast, recent physiological studies have indicated that upward spread of masking in the auditory nerve is due to the increasing effect of suppression with increasing masker level. This study compares thresholds for signals between 2.4 and 5.6 kHz in simultaneous and nonsimultaneous masking for conditions in which the masker is either at or well below the signal frequency. Maximum differences between simultaneous and nonsimultaneous masking were small (< 6 dB) for the on-frequency conditions but larger for the off-frequency conditions (15-32 dB). The results suggest that suppression plays a major role in determining thresholds at high masker levels, when the masker is well below the signal in frequency. This is consistent with the conclusions of physiological studies. However, for signal levels higher than about 40 dB SPL, the growth of masking for signals above the masker frequency is nonlinear even in the nonsimultaneous-masking conditions, where suppression is not expected. This is consistent with an explanation based on the compressive response of the basilar membrane, and confirms that suppression is not necessary for nonlinear upward spread of masking.

Pure tone masking patterns in adult chickens before and after recovery from acoustic trauma.

Three female adult chickens were trained to detect pure tones using a positive reinforcement technique and an adaptive threshold tracking procedure. Masked thresholds for tone bursts were measured in the presence of pure-tone maskers at 500, 1000, and 2000 Hz. Masker levels of 40-, 60-, and 70-dB sound-pressure level (SPL) were employed. Masking was maximal at the frequency of the masker. Tone-on-tone masking patterns were symmetrical at low masker levels, but at higher masker levels, more masking was evident at frequencies above the masker than below the masker. Distinct low threshold notches were frequently observed above and below the peak in the masking pattern. After the normal masking patterns had been measured, the chickens were exposed to a 525-Hz traumatizing tone at 120 dB SPL for 48 h and then allowed to recover. After thresholds had returned to their pre-exposure levels, tone-on-tone masking patterns were remeasured. The post-exposure masking patterns were virtually identical to the pre-exposure masking patterns.

Recovery of forward-masked responses in ventral cochlear nucleus neurons

Single unit responses were obtained from 8 classes of cells in the ventral cochlear nucleus: Primarylike, Primarylike with characteristic frequencies below 1 kHz, Primarylike-Notch, Sustained Chopper, Transient-Chopper, Low-intensity Chopper; Onset with later activity and On-Chopper. Stimuli were paired tonebursts, a masker preceding a probe, separated by time delta t ms. The decrement in discharge rate to the probe was measured as a function of delta t and constituted the forward-masking recovery function. The recovery functions of primarylike units were similar to those reported for auditory nerve fibers, but recovered more slowly than all other classes of units in the ventral cochlear nucleus. Some units, such as onset units, were completely masked at short masker-probe intervals, while others, such as the low-intensity choppers, were less affected by the masker. More masking occurred in the first 2 ms of the response (onset rate) than in the overall response (average rate). Using shorter maskers and measuring the onset rate produced greater differences in masking functions between unit classes. Units with high spontaneous activity were more resistant to the effects of the masker than units with low and medium spontaneous activity. This was especially evident at high masker levels and short masker-probe intervals. Units other than primarylike often showed non-monotonic relationships between the firing rate evoked by the masker and the firing rate decrement in response to the probe, suggesting that both adaptation and inhibition are operating to produce the observed effects.

Effect of masker level on overshoot in running- and frozen-noise maskers.

Masked thresholds were measured with running- and frozen-noise maskers. The 5-kHz signal was 2 ms in duration. The masker was low-pass noise (20 Hz-10 kHz); its total duration was 300 ms. The overall level of the masker was 30, 50, or 70 dB SPL. The onset of the signal was delayed by 0, 3, 8, 18, 198, or 278 ms relative to the onset of the masker. In all frozen-noise measurements, the signal was added to the same fine structure of the noise. Overshoot in frozen noise was measured for two starting phases of the signal that led to a 10-dB difference for large signal-onset delays. In all three configurations (running noise and frozen noise with two different signal phases) masker level had a similar influence on overshoot. At the intermediate masker level (50 dB SPL), a significant amount of overshoot (up to 15 dB) was observed in all three conditions. At the low and the high masker levels, overshoot was very much reduced, and even became negative in most conditions for the 30-dB-SPL masker. For the 50-dB frozen-noise masker, the total variation of thresholds with signal phase was 8 to 11 dB for long signal-onset delays, but only 3 to 6 dB for short delays. For the low- and high-level maskers, where only a small overshoot was observed, the threshold variation with phase for a signal at masker onset was the same as that for the long-delay condition. An explanation for the variation of signal detectability with masker level is proposed that refers explicitly to the compressive input-output characteristic of the basilar membrane at intermediate levels.

Masking period patterns in normal-hearing and hearing-impaired listeners.

Masking period patterns (MPPs), developed by Zwicker [J. Acoust. Soc. Am. 59, 166–175 (1976)] as a psychophysical correlate of the physiological period histogram, were obtained from four normal-hearing and four cochlear hearing-impaired subjects. The just audible level of a brief, relatively high-frequency, test tone was measured as a function of its temporal position within the period of a 40-Hz masker. Zwicker’s findings were replicated in normal listeners. At low masker levels a single maximum (Max 1) was observed between 0 and 90 deg of the masker period. At higher masker levels a second maximum (Max 2) developed between 180 and 270 deg. Zwicker’s model [Biol. Cyb. 23, 49–60 (1976)] suggests that Max 1 reflects level-independent suppression effects and Max 2 reflects some combination of level-dependent excitation and suppression effects, which is consistent with physiological biasing data. For impaired listeners, in regions of hearing loss, the degree of modulation in the MPP was reduced relative to that seen in normal-hearing listeners. The pattern of modulation was usually altered as well. These results are discussed in the context of outer hair cell damage at the place in the cochlea corresponding to the test tone frequency. [Supported by NIDCD.]