Behavioral measures of frequency selectivity in the chinchilla.

A simultaneous masking procedure was used to derive four measures of frequency selectivity in the chinchilla. The first experiment measured critical masking ratios (CRs) at various signal frequencies. Estimates of the chinchillas' critical bandwidths derived from the CRs were much broader than comparable human estimates, indicating that the chinchilla may have inferior frequency selectivity. The second experiment measured critical bandwidths at 1, 2, and 4 kHz in a band-narrowing experiment. This technique yielded narrower estimates of critical bandwidth; however, chinchillas continued to exhibit poor frequency selectivity compared to man. The third experiment measured auditory-filter shape at 0.5, 1, and 2 kHz via rippled noise masking. Results of the rippled noise masking experiment indicate that auditory filters of humans and chinchillas are similar in terms of shape and bandwidth with chinchillas showing only slightly poorer frequency selectivity. The final experiment measured auditory filter shape at 0.5, 1, 2, and 4 kHz using notched noise masking. This experiment yielded auditory filter shapes and bandwidths similar to those derived from man. The discrepancy between the indirect estimates of frequency selectivity derived from CR and band-narrowing techniques and the direct estimates derived from rippled noise and notched noise masking are explained by taking into account the processing efficiency of the subjects.     

Auditory filter shapes at low center frequencies

Auditory-filter shapes were estimated in normally hearing subjects for signal frequencies (fs) of 100, 200, 400, and 800 Hz using the notched-noise method [R. D. Patterson and I. Nimmo-Smith, J. Acoust. Soc. Am. 67, 229-245 (1980)]. Two noise bands, each 0.4fs wide, were used; they were placed both symmetrically and asymmetrically about the signal frequency to allow the measurement of filter shape and asymmetry. Two overall noise levels were used: 77 and 87 dB SPL. In deriving the shapes of the auditory filters, account was taken of the nonflat frequency response of the Sennheiser HD424 earphone, and also of the frequency-dependent attenuation produced by the middle ear. The auditory filters were asymmetric; the upper skirt was steeper than the lower skirt. The asymmetry tended to be greater at the higher noise level. The equivalent rectangular bandwidths (ERBs) of the filters at the lower noise level had average values of 36, 47, 87, and 147 Hz for values of fs of 100, 200, 400, and 800 Hz, respectively. The standard deviations of the ERBs across subjects were typically about 10% of the ERB values. The signal-to-masker ratio at the output of the auditory filter required to achieve threshold increased markedly with decreasing fs.     

Dynamic range and asymmetry of the auditory filter

This experiment was designed to measure the shape and asymmetry of the auditory filter over a wider dynamic range than has been measured previously. Thresholds were measured for 2-kHz sinusoidal signals in the presence of two 800-Hz-wide noise bands, one above and one below the signal frequency. The spectrum level of the noise was 45 dB (re: 20 muPa), and the noise bands were placed both symmetrically and asymmetrically about the signal frequency. The deviation of the signal frequency from the nearer edge of each noise band varied from 0 to 0.8 times the signal frequency. Each ear of six subjects was tested, and the subjects' ages ranged from 22 to 74 years. The auditory filters derived from the data were somewhat asymmetric, with steeper slopes on the high-frequency side; the degree of asymmetry varied across subjects. The asymmetry could be characterized as a uniform stretching of the (linear) frequency scale on one side of the filter. The dynamic range of the auditory filter exceeded 60 dB in the younger listeners, but the dynamic range and sharpness of the filter tended to decrease with increasing age.     

High-frequency auditory filter shape for the Atlantic bottlenose dolphin

High-frequency auditory filter shapes of an Atlantic bottlenose dolphin (Tursiops truncatus) were measured using a notched noise masking source centered on pure tone signals at frequencies of 40, 60, 80 and 100?kHz. A dolphin was trained to swim into a hoop station facing the noise/signal transducer located at a distance of 2?m. The dolphin's masked threshold was determined using an up-down staircase method as the width of the notched noise was randomly varied from 0, 0.2, 04, 0.6, and 0.8 times the test tone frequency. The masked threshold decreased as the width of the notched increased and less noise fell within the auditory filter associated with the test tone. The auditory filter shapes were approximated by fitting a roex (p,r(r)) function to the masked threshold results. A constant-Q value of 8.4 modeled the results within the frequency range of 40 to 100 kHz relatively well. However, between 60 and 100?kHz, the 3?dB bandwidth was relatively similar between 9.5 and 10?kHz, indicating a constant-bandwidth system in this frequency range The mean equivalent rectangular bandwidth calculated from the filter shape was approximately 16.0%, 17.0%, 13.6% and 11.3% of the tone frequencies of 40, 60, 80, and 100?kHz.     

Comparison of auditory filter shapes derived with three different maskers

Auditory filter shapes were derived for three different masker types, by measuring threshold for a 1-kHz sinusoidal signal masked by: (a) a noise with a spectral notch of variable width; (b) two tones with variable frequency separation; and (c) a noise with a sinusoidally rippled spectrum with variable ripple density. In each case the masker spectrum was symmetric about the signal frequency, the signal level was fixed, and the masker level was varied to determine threshold using an adaptive, two-alternative, forced-choice procedure. Both simultaneous and forward masking were used. The auditory filter shapes derived from the data were broader in simultaneous masking than in forward masking for all three masker types. In simultaneous masking the derived filters were similar for the three masker types, although there was a tendency for the filters derived from the rippled-noise data to be broader than those for the other maskers. In forward masking the auditory filters derived from the data for three masker types differed considerably in bandwidth and the slope of the filter skirts, and in that a portion of the rippled-noise filter was negative valued. The results are consistent with the idea that suppression has the effect of enhancing frequency selectivity, and that this effect is revealed in forward but not in simultaneous masking. However, the degree and nature of the enhancement differs for different masker types.     

Auditory filter shapes in subjects with unilateral and bilateral cochlear impairments

The shape of the auditory filter was estimated at three center frequencies, 0.5, 1.0, and 2.0 kHz, for five subjects with unilateral cochlear impairments. Additional measurements were made at 1.0 kHz using one subject with a unilateral impairment and six subjects with bilateral impairments. Subjects were chosen who had thresholds in the impaired ears which were relatively flat as a function of frequency and ranged from 15 to 70 dB HL. The filter shapes were estimated by measuring thresholds for sinusoidal signals (frequency f) in the presence of two bands of noise, 0.4 f wide, one above and one below f. The spectrum level of the noise was 50 dB (re: 20 mu Pa) and the noise bands were placed both symmetrically and asymmetrically about the signal frequency. The deviation of the nearer edge of each noise band from f varied from 0.0 to 0.8 f. For the normal ears, the filters were markedly asymmetric for center frequencies of 1.0 and 2.0 kHz, the high-frequency branch being steeper. At 0.5 kHz, the filters were more symmetric. For the impaired ears, the filter shapes varied considerably from one subject to another. For most subjects, the lower branch of the filter was much less steep than normal. The upper branch was often less steep than normal, but a few subjects showed a near normal upper branch. For the subjects with unilateral impairments, the equivalent rectangular bandwidth of the filter was always greater for the impaired ear than for the normal ear at each center frequency. For three subjects at 0.5 kHz and one subject at 1.0 kHz, the filter had too little selectivity for its shape to be determined.     

Auditory filters and the benefit measured from spectral enhancement

Algorithms designed to improve speech intelligibility for those with sensorineural hearing loss (SNHL) by enhancing peaks in a spectrum have had limited success. Since testing of such algorithms cannot separate the theory of the design from the implementation itself, the contribution of each of these potentially limiting factors is not clear. Therefore, psychophysical paradigms were used to test subjects with either normal hearing or SNHL in detection tasks using well controlled stimuli to predict and assess the limits in performance gain from a spectrally enhancing algorithm. A group of normal-hearing (NH) and hearing-impaired (HI) subjects listened in two experiments: auditory filter measurements and detection of incremented harmonics in a harmonic spectrum. The results show that NH and HI subjects have an improved ability to detect incremented harmonics when there are spectral decrements surrounding the increment. Various decrement widths and depths were compared against subjects' equivalent rectangular bandwidths (ERBs). NH subjects effectively used the available energy cue in their auditory filters. Some HI subjects, while showing significant improvements, underutilized the energy reduction in their auditory filters.     

Binaural weighting of monaural spectral cues for sound localization

For human listeners, cues for vertical-plane localization are provided by direction-dependent pinna filtering. This study quantified listeners' weighting of the spectral cues from each ear as a function of stimulus lateral angle, interaural time difference (ITD), and interaural level difference (ILD). Subjects indicated the apparent position of headphone-presented noise bursts synthesized in virtual auditory space. The synthesis filters for the two ears either corresponded to the same location or to two different locations separated vertically by 20 deg. Weighting of each ear's spectral information was determined by a multiple regression between the elevations to which each ear's spectrum corresponded and the vertical component of listeners' responses. The apparent horizontal source location was controlled either by choosing synthesis filters corresponding to locations on or 30 deg left or right of the median plane or by attenuating or delaying the signal at one ear. For broadband stimuli, spectral weighting and apparent lateral angle were determined primarily by ITD. Only for high-pass stimuli were weighting and lateral angle determined primarily by ILD. The results suggest that the weighting of monaural spectral cues and the perceived lateral angle of a sound source depend similarly on ITD, ILD, and stimulus spectral range.     

Effect of spectral envelope smearing on speech reception. I.

The effect of reduced spectral contrast on the speech-reception threshold (SRT) for sentences in noise and on phoneme identification, was investigated with 16 normal-hearing subjects. Signal processing was performed by smoothing the envelope of the squared short-time fast Fourier transform (FFT) by convolving it with a Gaussian-shaped filter, and overlapping additions to reconstruct a continuous signal. Spectral energy in the frequency region from 100 to 8000 Hz was smeared over bandwidths of 1/8, 1/4, 1/3, 1/2, 1, 2, and 4 oct for the SRT experiment. Vowel and consonant identification was studied for smearing bandwidths of 1/8, 1/2, and 2 oct. Results showed the SRT in noise to increase as the spectral energy was smeared over bandwidths exceeding the ear's critical bandwidth. Vowel identification suffered more from this type of processing than consonant identification. Vowels were primarily confused with the back vowels /c,u/, and consonants were confused where place of articulation is concerned.     

Auditory filter shapes at low center frequencies in young and elderly hearing-impaired subjects.

Auditory filter shapes were measured for two groups of hearing-impaired subjects, young and elderly, matched for audiometric loss, for center frequencies (fc) of 100, 200, 400, and 800 Hz using a modified notched-noise method [B. R. Glasberg and B. C. J. Moore, Hear. Res. 47, 103-138 (1990)]. Two noise bands, each 0.4fc wide, were used; they were placed both symmetrically and asymmetrically about the signal frequency to allow the measurement of filter asymmetry. The overall noise level was either 77 or 87 dB SPL. Stimuli were delivered monaurally using Sennheiser HD424 earphones. Although auditory filters for the hearing-impaired subjects were generally broader than for normally hearing subjects [Moore et al., J. Acoust. Soc. Am. 87, 132-140 (1990)], some hearing-impaired subjects with mild losses had normal filters. The filters tended to broaden with increasing hearing loss. There were not any clear differences in filter characteristics between young and elderly hearing-impaired subjects. The signal-to-noise ratios at the outputs of the auditory filters required for threshold (K) tended to be lower than normal for the young hearing-impaired subjects, but were not significantly different from normal for the elderly hearing-impaired subjects. The lower K values for the young hearing-impaired subjects may occur because broadened auditory filters reduce the deleterious effects on signal detection of fluctuations in the noise.     

Auditory filter shapes in the chinchilla

Auditory filter shapes were determined for the chinchilla using the notched-noise technique [R. D. Patterson, J. Acoust. Soc. Am. 59, 640-654 (1976)]. Here, the derivative of the curve relating threshold to masker gap width outlines the shape of the auditory filter. Three chinchillas were trained, using positive reinforcement techniques, to provide forward masked thresholds at 1.0 and 10.0 kHz, at three masker spectrum levels. Unexpectedly, the threshold curves contained inflection points and regions of constant or nonmonotonic changes in threshold, so that the derived filters contained dips in their central passbands. Nonmonotonic variations in threshold may be discerned in human threshold versus notch width functions of previously published studies, suggesting that the two types of data are qualitatively similar. The filters computed from the chinchilla data widened with increasing masker level and were more broadly tuned than those obtained in humans. The physiological response to each frequency component of any stimulus is likely a combination of excitation and suppression. Hence, one cannot predict masked threshold from the acoustic spectra of the maskers used here since they differ from their internal representations. Thus the threshold versus notch width function probably reflects the operation of both an auditory filter and a nonlinearity.     

Level dependence of auditory filters in nonsimultaneous masking as a function of frequency

Auditory filter bandwidths were measured using nonsimultaneous masking, as a function of signal level between 10 and 35 dB SL for signal frequencies of 1, 2, 4, and 6 kHz. The brief sinusoidal signal was presented in a temporal gap within a spectrally notched noise. Two groups of normal-hearing subjects were tested, one using a fixed masker level and adaptively varying signal level, the other using a fixed signal level and adaptively varying masker level. In both cases, auditory filters were derived by assuming a constant filter shape for a given signal level. The filter parameters derived from the two paradigms were not significantly different. At 1 kHz, the equivalent rectangular bandwidth (ERB) decreased as the signal level increased from 10 to 20 dB SL, after which it remained roughly constant. In contrast, at 6 kHz, the ERB increased consistently with signal levels from 10 to 35 dB SL. The results at 2 and 4 kHz were intermediate, showing no consistent change in ERB with signal level. Overall, the results suggest changes in the level dependence of the auditory filters at frequencies above 1 kHz that are not currently incorporated in models of human auditory filter tuning.     

The temporal course of masking and the auditory filter shape

Recent experiments have shown that frequency selectivity measured in tone-on-tone simultaneous masking improves with increasing delay of a brief signal relative to the onset of a longer duration gated masker. To determine whether a similar improvement occurs for a notched-noise masker, threshold was measured for a 20-ms signal presented at the beginning, the temporal center, or the end of the 400-ms masker (simultaneous masking), or immediately following the masker (forward masking). The notch width was varied systematically and the notch was placed both symmetrically and asymmetrically about the 1-kHz signal frequency. Growth-of-masking functions were determined for each temporal condition, for a noise masker without a spectral notch. These functions were used to express the thresholds from the notched-noise experiment in terms of the level of a flat-spectrum noise which would produce the same threshold. In simultaneous masking the auditory filter shapes derived from the transformed data did not change significantly with signal delay, suggesting that the selectivity of the auditory filter does not develop over time. In forward masking the auditory filter shapes were sharper than those for simultaneous masking, particularly on the high-frequency side, which was attributed to suppression.     

Auditory filter shapes and high-frequency hearing in adults who have impaired speech in noise performance despite clinically normal audiograms

Some individuals complain of hearing difficulties in the presence of background noise even in the absence of clinically significant hearing loss (obscure auditory dysfunction). Previous evidence suggests that these listeners have impaired frequency resolution, but there has been no thorough characterization of auditory filter shapes in this population. Here, the filter shapes of adults (n = 14) who self-reported speech recognition problems in noise and performed poorly on a sentence-in-noise perception test despite having clinically normal audiograms were compared to those of controls (n = 10). The filter shapes were evaluated using a 2-kHz probe with a fixed level of 30, 40, or 50 dB sound pressure level (SPL) and notched-noise simultaneous maskers that were varied in level to determine the masker level necessary to just mask the probe. The filters of the impaired group were significantly wider than those of controls at all probe levels owing to an unusual broadening of the upper slope of the filter. In addition, absolute thresholds were statistically indistinguishable between the groups at the standard audiometric frequencies, but were elevated in the impaired listeners at higher frequencies. These results strengthen the idea that this population has a variety of hearing deficits that go undetected by standard audiometry.     

The attention filter for tones in noise has the same shape and effective bandwidth in the elderly as it has in young listeners

Listeners asked to detect tones masked by noise hear frequent signals but miss infrequent probes, suggesting that they attend to spectral regions where they expect the signals to occur. The narrow detection pattern centered on the frequent target approximates that obtained in notched noise, indicating that attention is focused on the auditory filter. We measured attention bands in young and elderly listeners (n=5, 4; 20-25 and 62-82 years of age) for targets (800 or 1200 Hz) and infrequent probe signals (target +/-25-100 Hz) masked in wideband noise. We anticipated that their width would increase with age, as has been reported for auditory filters. A yes-no single-interval procedure provided detection probabilities and detection response speeds. Both measures showed near-linear declines with decreasing signal level, and graded decay functions as probe frequency deviated from the target frequency. Probes deviating from the target by 25 to 50 Hz were equivalent to a 2-dB reduction in signal level for both measures. The equivalent rectangular bandwidth (ERB) for detection approximated 11% of the signal frequency for each age group. Confidence intervals (95%) showed that the elderly ERB could be at most only about 20% larger than that of younger listeners.     

Spectral-peak selection in spectral-shape discrimination by normal-hearing and hearing-impaired listeners

Spectral-shape discrimination thresholds were measured in the presence and absence of noise to determine whether normal-hearing and hearing-impaired listeners rely primarily on spectral peaks in the excitation pattern when discriminating between stimuli with different spectral shapes. Standard stimuli were the sum of 2, 4, 6, 8, 10, 20, or 30 equal-amplitude tones with frequencies fixed between 200 and 4000 Hz. Signal stimuli were generated by increasing and decreasing the levels of every other standard component. The function relating the spectral-shape discrimination threshold to the number of components (N) showed an initial decrease in threshold with increasing N and then an increase in threshold when the number of components reached 10 and 6, for normal-hearing and hearing-impaired listeners, respectively. The presence of a 50-dB SPL/Hz noise led to a 1.7 dB increase in threshold for normal-hearing listeners and a 3.5 dB increase for hearing-impaired listeners. Multichannel modeling and the relatively small influence of noise suggest that both normal-hearing and hearing-impaired listeners rely on the peaks in the excitation pattern for spectral-shape discrimination. The greater influence of noise in the data from hearing-impaired listeners is attributed to a poorer representation of spectral peaks.     

Spectral characteristics and synchrony in primary auditory-nerve fibers in response to pure-tone acoustic stimuli

Under pure-tone stimulation, the spectrum of the period histogram recorded from primary auditory-nerve fibers at low and medium frequencies contains components at DC, at the applied tone frequency (the fundamental), and at a small number of harmonics of the tone frequency. The magnitudes and phases of these spectral components are examined. The spectral magnitudes of the fundamental and various harmonic components generally bear a fixed proportionality to each other over a broad range of signal conditions and nerve-fiber characteristics. This implies that the shape of the underlying rectified wave remains essentially unchanged over a broad range of stimulus intensities. For high-frequency stimuli, the fundamental and harmonic components are substantially attenuated. We provide a theoretical basis for the decrease of the spectralcomponent magnitudes with increasing harmonic number. For low-frequency pure-tone signals, the decrease is caused principally by the uncertainty in the position of neural-event occurrences within the half-wave-rectified period histogram. The lower the stimulus frequency, the greater this time uncertainty and therefore the lower the frequency at which the spectral components begin to diminish. For high-frequency pure-tone signals, on the other hand, the decrease is caused principally by the frequency rolloff associated with nervespike time jitter (it is then called loss of phase locking or loss of synchrony). Since some of this jitter arises from noise in the auditory nerve, it can be minimized by using peak detection rather than level detection. Using a specially designed microcomputer that measures the times at which the peaks of the action potentials occur, we have demonstrated the presence of phase locking to tone frequencies as high as 18 kHz. The traditional view that phase locking is always lost above 6 kHz is clearly not valid. This indicates that the placeversus-periodicity dichotomy in auditory theory requires reexaraination.     

Auditory filter shapes for the bottlenose dolphin (Tursiops truncatus) and the white whale (Delphinapterus leucas) derived with notched noise

Auditory filter shapes were estimated in two bottlenose dolphins (Tursiops truncatus) and one white whale (Delphinapterus leucas) using a behavioral response paradigm and notched noise. Masked thresholds were measured at 20 and 30 kHz. Masking noise was centered at the test tone and had a bandwidth of 1.5 times the tone frequency. Half-notch width to center frequency ratios were 0, 0.125, 0.25, 0.375, and 0.5. Noise spectral density levels were 90 and 105 dB re: 1 microPa2/Hz. Filter shapes were approximated using a roex(p,r) function; the parameters p and r were found by fitting the integral of the roex(p,r) function to the measured threshold data. Mean equivalent rectangular bandwidths (ERBs) calculated from the filter shapes were 11.8 and 17.1% of the center frequency at 20 and 30 kHz, respectively, for the dolphins and 9.1 and 15.3% of the center frequency at 20 and 30 kHz, respectively, for the white whale. Filter shapes were broader at 30 kHz and 105 dB re: 1 microPa2/Hz masking noise. The results are in general agreement with previous estimates of ERBs in Tursiops obtained with a behavioral response paradigm.     

Asymmetry of masking between noise and iterated rippled noise: evidence for time-interval processing in the auditory system.

This study describes the masking asymmetry between noise and iterated rippled noise (IRN) as a function of spectral region and the IRN delay. Masking asymmetry refers to the fact that noise masks IRN much more effectively than IRN masks noise, even when the stimuli occupy the same spectral region. Detection thresholds for IRN masked by noise and for noise masked by IRN were measured with an adaptive two-alternative, forced choice (2AFC) procedure with signal level as the adaptive parameter. Masker level was randomly varied within a 10-dB range in order to reduce the salience of loudness as a cue for detection. The stimuli were filtered into frequency bands, 2.2-kHz wide, with lower cutoff frequencies ranging from 0.8 to 6.4 kHz. IRN was generated with 16 iterations and with varying delays. The reciprocal of the delay was 16, 32, 64, or 128 Hz. When the reciprocal of the IRN delay was within the pitch range, i.e., above 30 Hz, there was a substantial masking asymmetry between IRN and noise for all filter cutoff frequencies; threshold for IRN masked by noise was about 10 dB larger than threshold for noise masked by IRN. For the 16-Hz IRN, the masking asymmetry decreased progressively with increasing filter cutoff frequency, from about 9 dB for the lowest cutoff frequency to less than 1 dB for the highest cutoff frequency. This suggests that masking asymmetry may be determined by different cues for delays within and below the pitch range. The fact that masking asymmetry exists for conditions that combine very long IRN delays with very high filter cutoff frequencies means that it is unlikely that models based on the excitation patterns of the stimuli would be successful in explaining the threshold data. A range of time-domain models of auditory processing that focus on the time intervals in phase-locked neural activity patterns is reviewed. Most of these models were successful in accounting for the basic masking asymmetry between IRN and noise for conditions within the pitch range, and one of the models produced an exceptionally good fit to the data.     

A release from masking by continuous, random, notched noise

Thresholds for 10-ms sinusoids simultaneously masked by bursts of bandpass noise centered on the signal frequency were measured for a wide range of signal frequencies and noise levels. Thresholds were defined as the signal power relative to the masker power at the output of an auditory filter centered on the signal frequency. It was found that the presentation of a continuous random noise, with a spectral notch centered on the signal frequency, produced a reduction in signal thresholds of up to 11 dB. A notched noise spectrum level of 0-5 dB above that of the masker proved most effective in producing a masking release, as measured by a reduction in masked threshold. A release from masking of up to 7 dB could be obtained with a continuous bandpass noise. The most effective spectrum level of this noise was 5 dB below that of the masker. The effect of the continuous notched noise was to reduce signal-to-masker ratios at threshold to about 0 dB, regardless of the threshold in the absence of continuous noise. Thus the greatest release from masking occurred when "unreleased" thresholds were highest. The release from masking is almost complete within 320 ms of notched noise onset, and persists for about 160 ms after notched noise offset, regardless of notched noise level. The phenomenon is similar in many ways to the "overshoot" effect reported by Zwicker [J. Acoust. Soc. Am. 37, 653-663 (1965)]. It is argued that both effects can be largely attributed to peripheral short-term adaptation, a mechanism which is also believed to be involved in forward masking.