Intermodulation distortion in the cochlea as shown by offset action potential (AP) masking curves

In their recent article "Offset AP masker tuning curve and the FFT of the stimulus" [J. Acoust. Soc. Am. 84, 1354-1362 (1988)], Henry and Lewis demonstrated that the tuning curve obtained by the simultaneous masking of the whole nerve action potential (AP) could have two tips when the AP is generated at the offset of the envelope of a high-level probe. The primary tip falls below the probe frequency, whereas the secondary tip falls above the probe frequency. Curves obtained for the onset response with either forward or simultaneous masking did not show the secondary peak, nor did curves obtained for the offset response with forward masking. Henry and Lewis discussed various reasons for the secondary tip, but came to no conclusion as to the underlying mechanisms. Here, it is reasoned that the secondary tip of the offset curve can be simply explained by the generation within the cochlea of intermodulation distortion (IMD), which acts as a forward masker to the offset response. The IMD is dominated by the cubic component (2f1-f2) and arises from the interaction of the probe tone and the simultaneous masker. Finally, it is reasoned that the lower sideband of the frequency splatter present at probe offset is the primary stimulus for the evoked neural response under probe offset conditions. Thus the offset curve will always have a primary tip that is lower in frequency than that of the respective onset curve. These hypotheses are supported by single-fiber data.     

Cochlear action potential tuning curves recorded with a derived response technique

Previous action potential (AP) tuning curve methods have used a reduction in amplitude of the probe-elicited AP as an indication of tone-induced masking. The reduction criterion used in different studies has varied from 25% to 100%. For low level probe stimuli, which elicit a low-amplitude AP, this is a sensitive indicator. In contrast, for high-amplitude AP responses elicited by high-level stimuli, the required reduction in absolute terms is large, making it an insensitive indicator. AP tuning curves have been recorded using a sensitive method for detecting masker/probe interaction with a fixed criterion, unrelated to the unmasked AP amplitude. For each masking condition, a derived response was obtained by digitally subtracting the tone-masked AP waveform from the unmasked response. Derived responses are generated if there are ANY changes in the AP waveform induced by the masker, including amplitude changes, latency changes, or even changes in AP morphology not necessarily associated with the major peaks. A fixed criterion (10 microV) of tone-derived (TD) response was used as an indication of interaction of the responses to the masker and probe. Tuning curves generated by this method were compared with those generated by conventional amplitude reduction (AR) methods. TD tuning curves show different characteristics, especially with respect to increasing probe levels. They appear to give a good representation of the array of afferent fibers responding to a probe stimulus. In addition, frequency regions making minor contributions to the AP are better represented in TD tuning curves.     

The effects of cochlear hypothermia on compound action potential tuning

The effects of lowered cochlear temperature on eighth-nerve tuning were assessed by using forward masking of whole nerve action potential (AP) responses to generate AP tuning curves (APTCs) at cochlear temperatures ranging from 38.5 degrees to 30 degrees C for probe frequencies from 8 to 36 kHz. The data indicate that subnormal cochlear temperatures result in: broadened APTCs for probe frequencies above 10 kHz which are interpreted as resulting from reduced hair-cell frequency selectivity, lowered or more sensitive APTC tips where tone-burst thresholds are unchanged, and raised or less sensitive tips where thresholds to tone bursts were elevated. Increased tip sensitivity is explained in terms of enhanced eighth-nerve adaptation which occurred during hypothermia. Experiments directly addressing adaptation were performed, in which the masker-probe interval (delta t) was systematically lengthened. The normalized AP decrement versus delta t functions indicate an enhancement of both the amount and duration of adaptation during hypothermia. Functions relating the growth of response to the masker (AP decrement versus masker intensity functions) were reduced at low temperatures.     

Growth of masking as a measure of response growth in hearing-impaired listeners

Growth-of-masking functions were obtained from 19 normal and 5 hearing-impaired listeners using a simultaneous-masking paradigm. When masker and probe frequency are identical, the slope of masking approximates 1.0 for both normal-hearing and impaired listeners. For masker frequencies less than or greater than probe frequency, the slopes for impaired listeners are shallower than those of normals. These findings are consistent with previously reported physiological data (single-fiber rate versus level and AP masking functions) for animals with induced cochlear lesions. Results are discussed in terms of a potential masking technique to estimate the growth of response in normal and impaired ears.     

Sinusoidal and noise maskers in simultaneous and forward masking

We compare psychophysical tuning curves obtained with sinusoidal and narrow-band (50-Hz wide) noise maskers in both simultaneous and forward masking. In one experiment, we examine the effects of different combinations of duration and intensity of the 1-kHz sinusoidal signal. In a second experiment, we compare tuning curves obtained with a sinusoidal signal to those obtained with a noise signal. In both experiments, a narrow-band noise is a more effective simultaneous masker than a sinusoid for masker frequencies near the signal frequency. We argue that this is probably due to the use of different detection cues in the presence of sinusoidal and noise maskers, and that the greater masking produced by the noise is not simply due to its greater variability. As observed in other studies, tuning curves are narrower in forward masking than in simultaneous masking.     

Effects of periodic rest on physiological measures of auditory sensitivity following exposure to noise

Whole nerve action potential (AP) and single auditory-nerve fiber thresholds were measured in chinchillas exposed to noise. The exposure stimulus was a 500-Hz octave band of noise presented at 95 dB SPL for 15 min/h, for 4 or 40 days. The AP thresholds were elevated by about 40 dB on day 4, between 0.5 kHz and approximately 8 kHz. On day 40, AP thresholds at the same frequencies were lower by 10-25 dB, even though the noise exposure had continued. Single fiber threshold tuning curves exhibited pathologies similar to those previously observed following noise exposure. Tuning curves measured on day 40 were more normal in appearance. These results confirm that similar recovery of threshold observed in psychophysical experiments [Clark et al., J. Acoust. Soc. Am. 82, 1253-1264 (1987)] can be understood in terms of the sensitivity of the peripheral auditory system.     

Psychophysical tuning curves measured in simultaneous and forward masking

The level of a masker necessary to mask a probe fixed in frequency and level was determined as a function of masker frequency using a two-interval forced-choice technique. Both simultaneous- and forward- masking techniques were used. Parameters investigated include the level of the probe tone and the frequency of the probe tone. The general form of the psychophysical tuning curves obtained in this way is quite similar to that of single-neurone tuning curves, when low-level probe tones are used. However, the curves obtained to forward masking generally show sharper tips and steeper slopes than those found in simultaneous masking, and they are also generally sharper than neurophysiological tuning curves. For frequencies of the masker close to that of the probe a simultaneous masker was sometimes less effective than a forward masker. The results are discussed in relation to possible lateral suppression effects in simultaneous masking, and in relation to the observer's use of pitch cues in forward masking. It is concluded that neither the simultaneous-masking curves nor the forward-masking curves are likely to give an accurate representation of human neural tuning curves.     

Properties of distortion product otoacoustic emissions and neural suppression tuning curves attributable to the tectorial membrane resonance

Mechanically coupled cochlear structures are likely to form a resonator with several degrees of freedom. Consequently one can expect complex, frequency-dependent relative movements between these structures, particularly between the tectorial membrane and reticular lamina. Shearing movement between these two structures excites the cochlear receptors. This excitation should be minimal at the frequency of the hypothesized tectorial membrane resonance. In each preparation, simultaneous masking neural tuning curves and distortion product otoacoustic emissions were recorded. The position of the low-frequency minima in the tuning curves, frequency dependence of the emission bandpass structure, and level-dependent phase reversal were compared to determine if they were generated by a common phenomenon, for example the tectorial membrane resonance. The notch in the masking curves and the phase inversion of the emission growth functions at the auditory thresholds are both situated half an octave below the probe frequency and the high-frequency primary, respectively, and show similar frequency dependence. The emission bandpass structure is, however, likely to be generated by a combination of mechanisms with different ones dominating at different stimulus parameters.     

Spectral modulation masking patterns reveal tuning to spectral envelope frequency

Auditory processing appears to include a series of domain-specific filtering operations that include tuning in the audio-frequency domain, followed by tuning in the temporal modulation domain, and perhaps tuning in the spectral modulation domain. To explore the possibility of tuning in the spectral modulation domain, a masking experiment was designed to measure masking patterns in the spectral modulation domain. Spectral modulation transfer functions (SMTFs) were measured for modulation frequencies from 0.25 to 14 cycles/octave superimposed on noise carriers either one octave (800-1600 Hz, 6400-12,800 Hz) or six octaves wide (200-12,800 Hz). The resulting SMTFs showed maximum sensitivity to modulation between 1 and 3 cycles/octave with reduced sensitivity above and below this region. Masked spectral modulation detection thresholds were measured for masker modulation frequencies of 1, 3, and 5 cycles/octave with a fixed modulation depth of 15 dB. The masking patterns obtained for each masker frequency and carrier band revealed tuning (maximum masking) near the masker frequency, which is consistent with the theory that spectral envelope perception is governed by a series of spectral modulation channels tuned to different spectral modulation frequencies.     

Upward spread of masking, hearing loss, and speech recognition in young and elderly listeners

Upward spreading of masking, measured in terms of absolute masked threshold, is greater in hearing-impaired listeners than in listeners with normal hearing. The purpose of this study was to make further observations on upward-masked thresholds and speech recognition in noise in elderly listeners. Two age groups were used: One group consisted of listeners who were more than 60 years old, and the second group consisted of listeners who were less than 36 years old. Both groups had listeners with normal hearing as well as listeners with mild to moderate sensorineural loss. The masking paradigm consisted of a continuous low-pass-filtered (1000-Hz) noise, which was mixed with the output of a self-tracking, sweep-frequency Bekesy audiometer. Thresholds were measured in quiet and with maskers at 70 and 90 dB SPL. The upward-masked thresholds were similar for young and elderly hearing-impaired listeners. A few elderly listeners had lower upward-masked thresholds compared with the young control group; however, their on-frequency masked thresholds were nearly identical to the control group. A significant correlation was found between upward-masked thresholds and the Speech Perception in Noise (SPIN) test in elderly listeners.     

The temporal evolution of masking and frequency selectivity in the goldfish (Carassius auratus)

The temporal evolution of masking and frequency selectivity was studied in the goldfish using classical respiratory conditioning and a tracking psychophysical procedure. The temporal position of a brief tonal signal within a longer duration, tonal masker has little or no effect on signal detectability when the frequency of the masker is less than or equal to that of the signal. For masker frequencies above that of the signal, signal detectability improves as the signal onset is delayed relative to that of the masker. These patterns of tone-on-tone masking are quite similar to those observed for humans. These temporal masking patterns are qualitatively similar in shape to the peristimulus-time histogram profiles of the low-frequency saccular fibers thought to be used in this task. Frequency- and time-dependent changes in signal detectability result in specific changes in the sharpness of psychophysical tuning curves (PTC). In general, PTCs determined for signals occurring at masker onset are the most broadly tuned, and PTCs determined in forward masking are the most sharply tuned. The PTCs for signals temporally centered in the masker are intermediate. These results suggest that temporal tone-on-tone masking patterns and the temporal evolution of psychophysical tuning curves result from the response properties of peripheral auditory-nerve fibers.     

Physiological mechanisms of psychophysical masking: observations from auditory-nerve fibers

Masking might be due either to the spread of the excitation produced by the masker to the place of the tone signal along the cochlea or to the suppression of the response to the signal by the masker. In order to identify the contributions of these two mechanisms to tone-on-tone masking, masked thresholds of auditory-nerve fibers were measured in anesthetized cats using the same stimulus paradigms and detection criteria as in psychophysics. Suppressive masking was identified by comparing thresholds for simultaneous masking with those for a nonsimultaneous masking technique resembling pulsation thresholds. These nonsimultaneous thresholds do not include the contribution of suppression to masking because suppression only occurs for stimuli that overlap in time. For each masker and signal frequency, the fibers with the lowest (or "best") masked thresholds had characteristic frequencies (CF) slightly on the opposite side of the masker frequency with respect to the signal frequency, consistent with the psychophysical phenomenon of off-frequency listening. Patterns of best masked thresholds against signal frequency resembled psychophysical masking patterns in that they showed a maximum for signal frequencies close to the masker, and a skew toward high frequencies. Masking was found to be both excitatory and suppressive, with the relative contribution of the two mechanisms depending on the frequency separation between signal and masker. Suppressive masking was large for signal frequencies well above the masker. For these conditions, simultaneous thresholds grew more rapidly with masker level than did nonsimultaneous thresholds, suggesting that the upward spread of masking is largely due to the growth of suppression rather than to that of excitation.     

Frequency and level dependent masking of the multiple auditory steady-state response in the bottlenose dolphin (Tursiops truncatus)

The potential for interactions between steady-state evoked responses to simultaneous auditory stimuli was investigated in two bottlenose dolphins (Tursiops truncatus). Three experiments were conducted using either a probe stimulus (probe condition) or a probe in the presence of a masker (probe-plus-masker condition). In the first experiment, the probe and masker were sinusoidal amplitude-modulated (SAM) tones. Probe and masker frequencies and masker level were manipulated to provide variable masking conditions. Probe frequencies were 31.7, 63.5, 100.8, and 127.0 kHz. The second experiment was identical to the first except only the 63.5 kHz probe was used and maskers were pure tones. For the third experiment, thresholds were measured for the probe and probe-plus-masker conditions using two techniques, one based on the lowest detectable response and the other based on a regression analysis. Results demonstrated localized masking effects where lower frequency maskers suppressed higher frequency probes and higher amplitude maskers produced a greater masking effect. The pattern of pure tone masking was nearly identical to SAM tone masking. The two threshold estimates were similar in low masking conditions, but in high masking conditions the lowest detectable response tended to overestimate thresholds while the regression-based analysis tended to underestimate thresholds.     

Effects of signal delay on auditory filter shapes derived from psychophysical tuning curves and notched-noise data obtained in simultaneous masking

Psychophysical tuning curves (PTCs) measured in simultaneous masking usually sharpen as a short duration signal is moved from the onset to the temporal center of a longer duration masker. Filter shapes derived from notched-noise maskers have not consistently shown this effect. One possible explanation for this difference is that the signal level is fixed in the PTC paradigm, whereas the masker level is usually fixed in the notched-noise paradigm. In the present study, the signal level was fixed at 10 dB SL in both paradigms. The signal was 20 ms in duration, and presented at the onset or temporal center of the 400-ms masker. The masker was a pure tone presented in quiet (PTC) or in the presence of a pure-tone "restrictor" intended to limit off-frequency listening (PTCr), or it was a noise with a spectral notch placed symmetrically or asymmetrically about the 2-kHz signal frequency. Filter shapes were derived from the PTC, PTCr, and notched-noise data using the roex (p, w, t) model. The effects of signal delay and masking paradigm on filter bandwidth were analyzed with a two-factor repeated-measures ANOVA. There was a significant effect of signal delay (the filters sharpened with time) and masking paradigm (the filters derived from the notched-noise data were significantly wider than those derived from either of the PTC measurements, which did not differ from one another). Although the interaction between delay and paradigm was not significant, the filter derived from the notched-noise data sharpened more with time than did the other filters, and thus the bandwidth of the filters from the three paradigms were more similar at the longer delay than at the shorter delay. It is likely that the tuning-curve and notched-noise paradigms measure the same underlying filtering, but that various other factors contribute differentially to the derived filter shapes.     

Additivity of forward masking

Masked thresholds for a 1000-Hz sinusoidal signal were measured as a function of masker level in both forward and simultaneous masking for two types of maskers: a 1000-Hz sinusoid and a narrowband noise, 60-Hz wide, centered at 1000 Hz. In forward masking, the noise masker produced much steeper growth-of-masking functions than the sinusoid. Presenting a contralateral broadband noise "cue" with the forward masker dramatically reduced the slope of masking for the noise masker but did not influence results for the sinusoidal masker. The noise remained the more effective masker. The amount of masking produced by combinations of equally effective narrowband-noise and sinusoidal maskers was compared to that produced by each masker individually with and without the contralateral cue. No additional masking beyond that predicted by energy summation was measured for forward masking. Additional masking beyond energy-sum predictions was measured for analogous conditions in simultaneous masking. Comparisons of results obtained with and without the contralateral cue suggest that signal thresholds in the presence of narrowband-noise forward maskers can reflect nonperipheral auditory processes.     

Psychophysical correlates of contralateral efferent suppression. I. The role of the medial olivocochlear system in "central masking" in nonhuman primates

An extensive physiological literature, including experimental and clinical studies in humans, demonstrates that activation of the medial olivocochlear (MOC) efferent system, by either contralateral sound or electrical stimulation, can produce significant alterations in cochlear function and suggests a role for the MOC system in influencing the auditory behavior of binaural hearing. The present data are from psychophysical studies in nonhuman primates which seek to determine if the noted physiological changes in response to contralateral acoustic stimulation have a perceptual counterpart. Four juvenile Japanese macaques were trained to respond to the presence of 1-s sinusoids, presented to the test ear, in an operant reinforcement paradigm. Thresholds were compared for frequencies ranging from 1.0 to 4.0 kHz in quiet, with thresholds measured when continuous, two octave-band noise, centered on the test tone frequency, was presented in the contralateral ear. Contralateral noise was presented at levels of 10-60 dB above detection threshold for the test-tone frequency. While some variability was evident across subjects, both in the frequency distribution and magnitude (as a function of contralateral noise level), all subjects exhibited an increase, or suppression of thresholds in the presence of contralateral noise. On average, thresholds increased systematically with contralateral noise level, to a peak of 7 dB. In one subject, the threshold increase seen with contralateral noise was significantly reduced when the MOC was surgically sectioned on the floor of the IVth ventricle. The characteristics of the measured shifts in behavioral thresholds, in the presence of contralateral noise reported here, are qualitatively and quantitatively similar to both efferent physiological suppression effects and psychophysical central masking threshold shifts which have been reported previously. These data suggest that at least some aspects of "central masking" are efferent-mediated peripheral processes, and that the term "central masking" may be incorrect.     

An analysis of psychophysical tuning curves in normal and pathological ears

Simultaneous psychophysical tuning curves were obtained from normal-hearing and hearing-impaired listeners, using probe tones that were either at similar sound pressure levels or at similar sensation levels for the two types of listeners. Tuning curves from the hearing-impaired listeners were flat, erratic, broad, and/or inverted, depending upon the frequency region of the probe tone and the frequency characteristics of the hearing loss. Tuning curves from the normal-hearing listeners at low-SPL's were sharp as expected; tuning curves at high-SPL's were discontinuous. An analysis of high-SPL tuning curves suggests that tuning curves from normal-hearing listeners reflect low-pass filter characteristics instead of the sharp bandpass filter characteristics seen with low-SPL probe tones. Tuning curves from hearing-impaired listeners at high-SPL probe levels appear to reflect similar low-pass filter characteristics, but with much more gradual high-frequency slopes than in the normal ear. This appeared as abnormal downward spread of masking. Relatively good temporal resolution and broader tuning mechanisms were proposed to explain inverted tuning curves in the hearing-impaired listeners.     

Quantifying the implications of nonlinear cochlear tuning for auditory-filter estimates

The relation between auditory filters estimated from psychophysical methods and peripheral tuning was evaluated using a computational auditory-nerve (AN) model that included many of the response properties associated with nonlinear cochlear tuning. The phenomenological AN model included the effects of dynamic level-dependent tuning, compression, and suppression on the responses of high-, medium-, and low-spontaneous-rate AN fibers. Signal detection theory was used to evaluate psychophysical performance limits imposed by the random nature of AN discharges and by random-noise stimuli. The power-spectrum model of masking was used to estimate psychophysical auditory filters from predicted AN-model detection thresholds for a tone signal in fixed-level notched-noise maskers. Results demonstrate that the role of suppression in broadening peripheral tuning in response to the noise masker has implications for the interpretation of psychophysical auditory-filter estimates. Specifically, the estimated psychophysical auditory-filter equivalent-rectangular bandwidths (ERBs) that were derived from the nonlinear AN model with suppression always overestimated the ERBs of the low-level peripheral model filters. Further, this effect was larger for an 8-kHz signal than for a 2-kHz signal, suggesting a potential characteristic-frequency (CF) dependent bias in psychophysical estimates of auditory filters due to the increase in strength of cochlear nonlinearity with increases in CF.     

Effect of masker-frequency variability on the detection performance of infants and adults

The effect of masker-frequency variability on the detection performance of 7-9 month-old infants and adults was examined. Listeners detected a 300-ms 1000-Hz pure tone masked by: (1) A random-frequency two-tone complex; (2) a fixed-frequency two-tone complex; or (3) a broadband noise. Maskers repeated at 300-ms intervals throughout testing at 60 dB SPL. The signal was presented simultaneously with one presentation of the masker. Thresholds were determined adaptively using an observer-based method. Infants' thresholds were higher than adults' in all conditions, but infants' and adults' thresholds changed with masker condition in qualitatively similar ways. The fixed two-tone complex produced masking for both age groups, but more masking for infants than for adults. For infants and adults, the random two-tone complex produced more masking than broadband noise, but the difference was greater for infants than for adults. For infants and adults, the random two-tone complex produced more masking than the fixed two-tone complex, and the difference between these conditions was similar for both age groups. These results suggest that infants are more susceptible to informational masking than adults in the absence of spectral variability. Whether infants are more susceptible to the effects of masker-frequency variability than adults remains to be clarified.     

A comparison of threshold estimation methods in children 6-11 years of age

Estimating detection threshold for auditory stimuli in children can be problematic because of lapses in attention and the time limits usually imposed by scheduling restrictions or fatigue. Data reported here were collected to compare the stability of threshold estimation procedures in testing children ages 6 to 11 in a three-alternative, forced-choice paradigm. Stimuli consisted of a 1-kHz tonal signal and a Gaussian noise masker, bandpass filtered between 500-2,000 Hz and presented at 25-dB spectrum level. The signal was either presented for 400 ms in the presence of a continuous masker (simultaneous masking) or for 10 ms just prior to a 400-ms masker (backward masking). For each masking paradigm the 79% correct threshold was assessed via each of three procedures: 3-down, 1-up adaptive staircase (Levitt), maximum likelihood estimation (MLE), and method of constant stimuli. Percent correct was measured at the end of the study for a signal 10 dB above the previously determined threshold in order to estimate the most appropriate psychometric function asymptote for fitting data collected via the method of constant stimuli. Both the MLE and Levitt procedures produced equally stable threshold estimates for both conditions and age groups. This was the case despite considerable variability in backward-masking thresholds.