Increasing Masker Level Research Articles

Extensions of a neural-fluctuation perspective on simultaneous notched-noise masking

A previous paper [Maxwell et al., J. Acoust. Soc. Am. 3523, 3537 (2020)] suggested that listeners may identify the target interval in a simultaneous notched-noise task using neural fluctuations, which change with the addition of a tone to the masker. Neural fluctuations are changes in the instantaneous firing rate of auditory-nerve fibers, generally at a longer (slower) time scale than temporal fine structure, that are shaped by peripheral nonlinearities. Differences in neural-fluctuation amplitudes across the peripheral tonotopic axis are proposed to lead to differences in rates across the population of auditory midbrain neurons, due to the sensitivity of these neurons to amplitude modulation. Here, using similar methods to Maxwell et al. (2020) and stimuli matching those of Baker and Rosen (J.Ascoust. Soc. Am. 454, 462 (2006)], we extend previous work by demonstrating that neural fluctuations may account for additional features of simultaneous notched-noise masking results. These features include (1) increase in filter bandwidth at higher stimulus frequencies, (2) higher bandwidth with increasing masker level, and (3) filter asymmetry. These results, in conjunction with the previous conclusion that auditory-nerve excitation patterns cannot explain notched-noise thresholds in all cases, support the interpretation that psychophysical auditory filters may be based on the output of the subcortical auditory system. [Work supported by NIDCD-R01-010813.]

Effect of improving audibility on better-ear glimpsing using non-linear amplification.

Better-ear glimpsing (BEG) utilizes interaural level differences (ILDs) to improve speech intelligibility in noise. This spatial benefit is reduced in most hearing-impaired (HI) listeners due to their increased hearing loss at high frequencies. Even though this benefit can be improved by providing increased amplification, the improvement is limited by loudness discomfort. An alternative solution therefore extends ILDs to low frequencies, which has been shown to provide a substantial benefit from BEG. In contrast to previous studies, which only applied linear stimulus manipulations, wide dynamic range compression was applied here to improve the audibility of soft sounds while ensuring loudness comfort for loud sounds. Performance in both speech intelligibility and BEG was measured in 13 HI listeners at three different masker levels and for different interaural stimulus manipulations. The results revealed that at low signal levels, performance substantially improved with increasing masker level, but this improvement was reduced by the compressive behaviour at higher levels. Moreover, artificially extending ILDs by applying infinite (broadband) ILDs provided an extra spatial benefit in speech reception thresholds of up to 5 dB on top of that already provided by natural ILDs and interaural time differences, which increased with increasing signal level.

Nonuniform impacts of forward suppression on neural responses to preferred stimuli and nonpreferred stimuli in the rat auditory cortex.

In natural conditions, human and animals need to extract target sound information from noisy acoustic environments for communication and survival. However, how the contextual environmental sounds impact the tuning of central auditory neurons to target sound source azimuth over a wide range of sound levels is not fully understood. Here, we determined the azimuth-level response areas (ALRAs) of rat auditory cortex neurons by recording their responses to probe tones varying with levels and sound source azimuths under both quiet (probe alone) and forward masking conditions (preceding noise + probe). In quiet, cortical neurons responded stronger to their preferred stimuli than to their nonpreferred stimuli. In forward masking conditions, an effective preceding noise reduced the extents of the ALRAs and suppressed the neural responses across the ALRAs by decreasing the response strength and lengthening the first-spike latency. The forward suppressive effect on neural response strength was increased with increasing masker level and decreased with prolonging the time interval between masker and probe. For a portion of cortical neurons studied, the effects of forward suppression on the response strength to preferred stimuli was weaker than those to nonpreferred stimuli, and the recovery from forward suppression of the response strength to preferred stimuli was earlier than that to nonpreferred stimuli. We suggest that this nonuniform forward suppression of neural responses to preferred stimuli and to nonpreferred stimuli is important for cortical neurons to maintain their relative stable preferences for target sound source azimuth and level in noisy acoustic environments.

Masking release by combined spatial and masker-fluctuation effects in the open sound field.

In a complex auditory scene, signals of interest can be distinguished from masking sounds by differences in source location [spatial release from masking (SRM)] and by differences between masker-alone and masker-plus-signal envelopes. This study investigated interactions between those factors in release of masking of 700-Hz tones in an open sound field. Signal and masker sources were colocated in front of the listener, or the signal source was shifted 90° to the side. In Experiment 1, the masker contained a 25-Hz-wide on-signal band plus flanking bands having envelopes that were either mutually uncorrelated or were comodulated. Comodulation masking release (CMR) was largely independent of signal location at a higher masker sound level, but at a lower level CMR was reduced for the lateral signal location. In Experiment 2, a brief signal was positioned at the envelope maximum (peak) or minimum (dip) of a 50-Hz-wide on-signal masker. Masking was released in dip more than in peak conditions only for the 90° signal. Overall, open-field SRM was greater in magnitude than binaural masking release reported in comparable closed-field studies, and envelope-related release was somewhat weaker. Mutual enhancement of masking release by spatial and envelope-related effects tended to increase with increasing masker level.

Nonadditivity of forward and simultaneous masking

The current study measured the additional masking obtained for combinations of forward and simultaneous maskers as a function of forward masker bandwidth, signal delay, and simultaneous masker level. The effects of the two individual maskers were equated in all conditions. Additional masking increased with increasing masker level, increasing signal delay, and decreasing masker bandwidth. The portion of the simultaneous masker that made the greater contribution to additional masking was the part that overlapped with the signal, not with the forward masker. The changes in additional masking observed as a function of forward masker bandwidth and the interaction between the effects of forward and simultaneous maskers call into question the use of additional masking as a measure of basilar membrane compression and present problems for the use of simultaneous noise to simulate hearing loss.

Distortion products and their influence on representation of pitch-relevant information in the human brainstem for unresolved harmonic complex tones

Pitch experiments aimed at evaluating temporal pitch mechanism(s) often utilize complex sounds with only unresolved harmonic components, and a low-pass noise masker to eliminate the potential contribution of audible distortion products to the pitch percept. Herein we examine how: (i) masker induced reduction of neural distortion products (difference tone: DT; and cubic difference tone: CDT) alters the representation of pitch relevant information in the brainstem; and (ii) the pitch salience is altered when distortion products are reduced and/or eliminated. Scalp recorded brainstem frequency following responses (FFR) were recorded in normal hearing individuals using a complex tone with only unresolved harmonics presented in quiet, and in the presence of a low-pass masker at SNRs of +15, +5, and −5 dB. Difference limen for F0 discrimination (F0 DL) was obtained in quiet and in the presence of low-pass noise. Magnitude of DT components (with the exception of components at F0 and 2F0), and the CDT components decreased with increasing masker level. Neural pitch strength decreased with increasing masker level for both the envelope-related (FFRENV) and spectral-related (FFRSPEC) phase-locked activity. Finally, F0 DLs increased with decreasing SNRs suggesting poorer F0 discrimination with reduction of the distortion products. Collectively, these findings support the notion that both DT and CDT, as reflected in the FFRENV and FFRSPEC, respectively, influence both the brainstem representation of pitch relevant information and the pitch salience of the complex sounds.

Neural correlates of binaural masking level difference in the inferior colliculus of the barn owl (Tyto alba)

Humans and animals are able to detect signals in noisy environments. Detection improves when the noise and the signal have different interaural phase relationships. The resulting improvement in detection threshold is called the binaural masking level difference. We investigated neural mechanisms underlying the release from masking in the inferior colliculus of barn owls in low-frequency and high-frequency neurons. A tone (signal) was presented either with the same interaural time difference as the noise (masker) or at a 180 degrees phase shift as compared with the interaural time difference of the noise. The changes in firing rates induced by the addition of a signal of increasing level while masker level was kept constant was well predicted by the relative responses to the masker and signal alone. In many cases, the response at the highest signal levels was dominated by the response to the signal alone, in spite of a significant response to the masker at low signal levels, suggesting the presence of occlusion. Detection thresholds and binaural masking level differences were widely distributed. The amount of release from masking increased with increasing masker level. Narrowly tuned neurons in the central nucleus of the inferior colliculus had detection thresholds that were lower than or similar to those of broadly tuned neurons in the external nucleus of the inferior colliculus. Broadly tuned neurons exhibited higher masking level differences than narrowband neurons. These data suggest that detection has different spectral requirements from localization.

Psychophysical assessment of the level-dependent representation of high-frequency spectral notches in the peripheral auditory system

To discriminate between broadband noises with and without a high-frequency spectral notch is more difficult at 70-80 dB sound pressure level than at lower or higher levels [Alves-Pinto, A. and Lopez-Poveda, E. A. (2005). "Detection of high-frequency spectral notches as a function of level," J. Acoust. Soc. Am. 118, 2458-2469]. One possible explanation is that the notch is less clearly represented internally at 70-80 dB SPL than at any other level. To test this hypothesis, forward-masking patterns were measured for flat-spectrum and notched noise maskers for masker levels of 50, 70, 80, and 90 dB SPL. Masking patterns were measured in two conditions: (1) fixing the masker-probe time interval at 2 ms and (2) varying the interval to achieve similar masked thresholds for different masker levels. The depth of the spectral notch remained approximately constant in the fixed-interval masking patterns and gradually decreased with increasing masker level in the variable-interval masking patterns. This difference probably reflects the effects of peripheral compression. These results are inconsistent with the nonmonotonic level-dependent performance in spectral discrimination. Assuming that a forward-masking pattern is a reasonable psychoacoustical correlate of the auditory-nerve rate-profile representation of the stimulus spectrum, these results undermine the common view that high-frequency spectral notches must be encoded in the rate-profile of auditory-nerve fibers.

Visual–auditory interaction in saccadic reaction time: Effects of auditory masker level

Saccadic reaction time (SRT) to a visual target tends to be shorter when auditory stimuli are presented in close temporal and spatial proximity, even when subjects are instructed to ignore the auditory non-target (focused attention paradigm). Observed SRT reductions typically range between 10 and 50 ms and decrease as spatial disparity between the stimuli increases. Previous studies using pairs of visual and auditory stimuli differing in both azimuth and vertical position suggest that the amount of SRT facilitation decreases not with the physical but with the perceivable distance between visual target and auditory accessory. Here we probe this hypothesis by presenting an additional white-noise masker background of 3 s duration. Increasing the masker level had a diametrical effect on SRTs in spatially coincident vs. disparate stimulus configurations: saccadic responses to coincident visual–auditory stimuli are slowed down, whereas saccadic responses to disparate stimuli are speeded up. As verified in a separate auditory localization task, localizability of the auditory accessory decreases with masker level. The SRT results are accounted for by a conceptual model positing that increasing masker level enlarges the area of possible auditory stimulus locations: it implies that perceivable distances decrease for disparate stimulus configurations and increase for coincident stimulus pairs.

Word recognition in noise at higher-than-normal levels: Decreases in scores and increases in masking

Under certain conditions, speech recognition in noise decreases above conversational levels when signal-to-noise ratio is held constant. The current study was undertaken to determine if nonlinear growth of masking and the subsequent reduction in "effective" signal-to-noise ratio accounts for this decline. Nine young adults with normal hearing listened to monosyllabic words at three levels in each of three levels of a masker shaped to match the speech spectrum. An additional low-level noise equated audibility by producing equivalent masked thresholds for all subjects. If word recognition was determined entirely by signal-to-noise ratio and was independent of overall speech and masker levels, scores at a given signal-to-noise ratio should remain constant with increasing level. Masked pure-tone thresholds measured in the speech-shaped maskers increased linearly with increasing masker level at lower frequencies but nonlinearly at higher frequencies, consistent with nonlinear growth of upward spread of masking that followed the peaks in the spectrum of the speech-shaped masker. Word recognition declined significantly with increasing level when signal-to-noise ratio was held constant which was attributed to nonlinear growth of masking and reduced "effective" signal-to-noise ratio at high speech-shaped masker levels, as indicated by audibility estimates based on the Articulation Index.

Behavioural measurement of level-dependent shifts in the vibration pattern on the basilar membrane at 1 and 2 kHz

Physiological data suggest that the peak of the travelling wave on the basilar membrane evoked by a high-frequency sinusoid moves towards the base with increasing level. Previously, we used a forward-masking technique to provide evidence for a similar effect in humans at 4 and 6.5 kHz. In the present study, we used a similar technique to determine whether level-dependent shifts occur for mid-range frequencies. The signal was a brief 1-kHz or 2-kHz tone presented at 10 dB SL (approximately 30 dB SPL). For three fixed masker levels (75, 85 and 95 dB SPL), we measured the duration of the gap between the masker and signal required to give 79.4% correct detection of the signal (called the ‘gap threshold’) as a function of masker frequency; the longer the gap threshold, the more effective is the masker. The gap-threshold patterns nearly always showed a single peak close to the signal frequency. The gap-threshold patterns spread markedly towards lower frequencies with increasing masker level, but the frequency at the peak did not change systematically with level. We conclude that, for mid-range frequencies, the peak of the travelling wave does not shift significantly with increasing level over the range 30–95 dB SPL, but the envelope of the travelling wave becomes more shallow on its basal side.

Behavioural measurement of level-dependent shifts in the vibration pattern on the basilar membrane

Physiological data suggest that the travelling wave on the basilar membrane evoked by a sinusoid of fixed frequency moves towards the base with increasing level. We describe two psychoacoustic experiments that attempted to provide evidence for and quantify the extent of such a shift in humans. In experiment 1, masking patterns were measured in forward masking using a fixed 6-kHz tone presented at 65 or 85 dB sound pressure level. The threshold for detecting a brief sinusoidal signal was measured as a function of signal frequency for several time delays of the signal relative to the end of the masker. A background noise was included to reduce ‘off-frequency listening’. As the signal delay was increased, the signal level at the peaks of the masking patterns decreased and the signal frequency at the peak of the patterns moved progressively towards higher frequencies. The pattern of results was consistent with the idea of a basalward shift of the travelling wave with increasing level. The estimated shift corresponds to about 0.25 octaves for a 40-dB change in level. Experiment 2 also used forward masking. The signal was a 4-kHz tone presented at 10 dB sensation level. For three fixed masker levels (65, 85 and 95 dB), we measured the duration of the gap between the masker and signal required to give 79.4% correct detection of the signal (called the ‘gap threshold’) as a function of masker frequency; the longer the gap threshold, the more effective is the masker. The gap threshold patterns sometimes showed two peaks. One occurred just below the signal frequency and the frequency at the peak was hardly affected by masker level. The second peak fell at a lower frequency, and this frequency tended to decrease with increasing masker level. The gap threshold patterns tended to spread markedly towards lower frequencies with increasing masker level. The shift with level provides further evidence for a basalward spread of the travelling wave with increasing level.

Binaural effects in center-frequency modulation detection interference for vowel formants.

The detection of slow (5 Hz) center-frequency modulations of formants (signals) can be impaired by the simultaneous presentation of off-frequency modulated formants (maskers) to the same ear [J. Lyzenga and R. P. Carlyon, J. Acoust. Soc. Am. 105, 2792-2806 (1999)]. In the present study we examine this "formant-frequency modulation detection interference (FMDI)" for various binaural masker presentation schemes. Signals and maskers were formantlike complex tones, centered around 1500 and 3000 Hz, respectively. Fundamentals of 80 and 240 Hz were used. The signals were presented to the right ear. The maskers were presented either to the right, the left, or to both ears, and they were either unmodulated or modulated at a slow rate (10 Hz). They had the same fundamental as the signals. Hardly any interference was found for the unmodulated maskers. For modulated maskers, the amount of FMDI depended strongly on the binaural masker presentation scheme. Substantial interference was found for the ipsilateral maskers. Interference was smaller for the contralateral maskers. In both cases the FMDI increased with increasing masker level. Substantial interference was also found for the binaural maskers. Imposing different interaural time and level differences (ITDs and ILDs) on maskers and signals did not affect FMDI. The same was true for the ITD condition when the maskers had different fundamentals than the signals, though FMDI was slightly smaller here. The amount of interference for the binaural maskers was roughly equal to that of the corresponding monaural masker with the largest effect. The data could not be described accurately using a model based on the loudness of the maskers. On the other hand, they were well described by a model in which the amount of FMDI was predicted from a "weighted combination" of the monaural masker levels.

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.

Responses of chopper units in the ventral cochlear nucleus of the anaesthetised guinea pig to clicks-in-noise and click trains

Auditory brainstem responses (ABRs) have been measured with clicks, clicks masked by noise, click trains and pseudorandom maximum length sequences (MLS) of clicks. To investigate the neuronal populations contributing to the ABR under these stimulation conditions, we measured the extracellular responses of ventral cochlear nucleus (VCN) units in the urethane-anaesthetised guinea pig. We studied 23 chopper, 7 primary-like and 7 onset units. This report focuses on the responses from chopper units. The probability of discharge for chopper units increased with increasing click level reaching nearly 100% in many units, over a range of about 20–30 dB. Following each response to a click there was a 5–10 ms suppression of the spontaneous or noise evoked activity. As the level of the noise was increased over a range of 20–30 dB, the response to the clicks gradually decreased leading to a complete abolition of the click response at high noise levels. In a few units, low level noise produced a facilitation of the response to single clicks. In response to constant level equally spaced click trains, discharge probability increased with increasing minimum pulse interval (MPI), approaching 100% for MPIs of 4–8 ms in some units. The recovery afforded by the gaps in the MLS train often resulted in higher discharge probability for MLS than click trains with the same MPI, while response probabilities for MLS and click trains were similar when compared at equivalent average click rates. At short MPIs (0.5 and 1.0 ms), peri stimulus time histograms in response to click trains resembled those to best frequency (BF) tones and noisebursts, with chopping peaks unrelated to unit BF. VCN units show highly synchronised and reliable responses to click trains, MLS trains and clicks masked by noise. The decrease in discharge rate and increase in latency of chopper units with decreasing click level, increasing click rate and increasing masker level parallel the peak amplitude and latency changes observed in the auditory brainstem response.

Detection of tones in modulated noise: Effects of masker level and masker depth.

It is possible to estimate temporal resolution at discrete spectral locations by subtracting the masked threshold produced by a modulated masker from that produced by an unmodulated masker (the difference is referred to as the modulated‐unmodulated difference, or MUD). This paradigm may be especially useful for measuring temporal resolution in subjects with hearing loss, provided that the MUD is independent of level. The purpose of the present study was to examine the MUD as a function of masker level at several signal frequencies. In the first experiment, the sinusoidally amplitude‐modulated masker had a depth (m) of 1.0. The MUD increased by as much as 15 dB as the spectrum level of the masker increased from 0 to 40 dB SPL. In the second experiment, the modulated masker had a depth of 0.75 or 1.0. When the masker depth was 1.0, the MUD increased with increasing masker level, as in experiment one. When it was 0.75, however, the MUD — though reduced — was essentially independent of masker level. These results suggest that a masker depth of 0.75 may be used to compare temporal resolution between normal‐hearing and hearing‐impaired subjects without being complicated by effects of masker level. [Work supported by NIDCD.]

The role of distortion products in masking by single bands of noise

Masking experiments with frozen‐noise maskers were conducted to investigate the role of distortion products generated by the interaction between the components of a bandpass noise masker. In the first experiment, thresholds of a 900‐Hz sinusoidal signal with a duration of 50 ms (10‐ms ramps included) were measured in the presence of bandpass noise maskers ranging from 1 to 2 kHz. In all measurements the same 500‐ms noise sample was used (frozen noise), presented at overall sound‐pressure levels of 35, 50, 65, or 80 dB. The signal was temporally centered in the masker. Four subjects participated in the experiment. Threshold variations of more than 10 dB were observed on varying the signal phase. The pattern of threshold versus signal phase resembled a sinusoid; the signal phases at the minimum and maximum thresholds differed by about 180 deg. The phase pattern shifted with increasing masker level. The individual shift for the masker range of 45 dB was between 100 and 200 deg. The direction of this shift agreed with data on the phase of cubic difference tones as a function of the level of sinusoidal primaries. In a second experiment a large number of different frozen‐noise samples were used as maskers in order to evaluate the generalizability of the phase effect. Two types of noise maskers were used: a low‐frequency masker (0–800 Hz) and a high‐frequency masker (1000–2000 Hz). For each noise sample, thresholds of a 900‐Hz signal were measured for two signal phases, 0 and 180 deg. An analysis of variance showed that signal phase played a significant role for the high‐frequency, but not for the low‐frequency, masker.

Masking and pitch shift of tone bursts and clicks by low-frequency tones

From experiments in animals and investigations in humans it is known that the normally phase-dependent masking of a short stimulus by a low-frequency continuous tone does not occur in the case of endolymphatic hydrops. The recording of the masked threshold of short tone stimuli in a loud tone of 30 Hz is to be evaluated for the clinical diagnostics of Ménière's disease. To this purpose, the main parameters of the measurement (type, frequency, duration of the stimulus, and intensity of the masker) and their effect on phase-dependent masking and pitch-shift are investigated. Stimuli above 2 kHz are masked less than those of lower frequencies. Wide-band stimuli are less useful, since only the low-frequency component of their spectrum is masked. The tone stimuli should be short (1 – 2 ms) in order to make the measurement of the phase dependence more accurate. With increasing masker level the masking at phase 0° corresponds to the increase in level, at phase 270° the amount is twice as much. The pitch shift which is perceived in low-tone masking depends on the phase of the stimulus, and on the levels of the stimulus and the masking tone. The use of brain stem recordings in the investigation of phase-dependent low tone masking is problematic since well-synchronizing stimuli with high frequency spectral components are masked poorly.

Temporal auditory summation in the echolocating bat, Tadarida brasiliensis

Auditory thresholds improve with increasing signal duration within the maximum integration time of the auditory system, a phenomenon called temporal summation. The temporal summation function is a basic characteristic of particular relevance for bat sonar, as it determines the ability to detect targets with short echolocation calls. Temporal summation was studied in 6 Mexican free-tailed bats ( Tadarida brasiliensis) in a forced two-choice behavioural test. Masked auditory thresholds for 40-kHz test tone pulses with durations between 2 ms and 400 ms were determined in broadband noise of two different spectrum levels (−18 dB, + 17 dB). At both masker levels, thresholds decreased by considerably more than 10 dB per decade of duration. The time constants of the summation functions, which are a measure of the maximum integration time, shortened significantly with increasing masker level from 62 ms to 14 ms. The steep summation functions are only partly accounted for by spectral splatter. This suggests that the bats are capable of a neural overintegration of sound intensity. Finally, it is shown that such short time constants arc typical for echolocating animals, and the implications of the found summation functions for echolocation are considered.

Overshoot in normal-hearing and hearing-impaired subjects

Overshoot was measured in both ears of four subjects with normal hearing and in five subjects with permanent, sensorineural hearing loss (two with a unilateral loss). The masker was a 400-ms broadband noise presented at a spectrum level of 20, 30, or 40 dB SPL. The signal was a 10-ms sinusoid presented 1 or 195 ms after the onset of the masker. Signal frequency was 1.0 or 4.0 kHz, which placed the signal in a region of normal (1.0 kHz) or impaired (4.0 kHz) absolute sensitivity for the impaired ears. For the normal-hearing subjects, the effects of signal frequency and masker level were similar to those published previously. In particular, overshoot was larger at 4.0 than at 1.0 kHz, and overshoot at 4.0 kHz tended to decrease with increasing masker level. At 4.0 kHz, overshoot values were significantly larger in the normal ears: Maximum values ranged from about 7-26 dB in the normal ears, but were always less than 5 dB in the impaired ears. The smaller overshoot values resulted from the fact that thresholds in the short-delay condition were considerably better in the hearing-impaired subjects than in the normal-hearing subjects. At 1.0 kHz, overshoot values for the two groups of subjects more or less overlapped. The results suggest that permanent, sensorineural hearing loss disrupts the mechanisms responsible for a large overshoot effect.