Masked and unmasked pure-tone thresholds of infants and adults: development of auditory frequency selectivity and sensitivity

Detection thresholds for pure tones (1000 Hz and 4000 Hz) in noise and in quiet were estimated for infants at 6 months and 12 months of age and for adults. A visually reinforced head-turn procedure under control of a PDP-11/03 minicomputer was used. An adaptive protocol with a 5-dB step size was employed for the threshold estimates. Infant thresholds were poorer than adult thresholds in each condition. In noise, infant-adult differences were 8 dB (6-month-old infants) and 6 dB (12-month-old infants) at each frequency. In quiet, infant-adult differences were 14 dB (6-month-old infants) and 12 dB (12-month-old infants) at 1000 Hz but were only 7 dB (6-month-old infants) and 5 dB (12-month-old infants) at 4000 Hz. The masking data suggest that infants are at only a slight disadvantage in detecting a target in a background of noise and are consistent with a frequency selectivity mechanism that is proportional to that of adults. The detection-in-quiet data, with greater correspondence among the groups at 4000 Hz than at 1000 Hz, support the notion that hearing sensitivity varies with frequency in a different way in infants than in adults. Data on task performance reveal significant age effects, and the possibility that such effects have biased the observed differences in the estimates of sensory function among the groups is considered.     

Temporal summation in hearing-impaired listeners

A study was conducted to explore variations in auditory temporal summation in listeners with normal hearing, and impairment due to otosclerosis, sensori-neural hearing loss and acoustic neuroma. Using a two-interval forced-choice procedure the detection threshold was measured for one-third octave noise bands centered at either 1000 or 4000 Hz, in combination with eight signal durations (2.5, 5, 10, 20, 40, 80, 160, and 640 ms). The results indicated that for normal listeners: (1) the slope of the function relating the detection threshold and the signal duration varied inversely with the frequency tested, and (2) the variability in the detection threshold was greater for 4000 Hz than for 1000 Hz. A comparison of performance across groups showed that the magnitude of the slope of the temporal integration function decreased as the site of lesion moved from middle ear to eighth nerve. For listeners with normal hearing and those with otosclerosis, temporal integration appeared to be incomplete at 640 ms.     

Forward Masking Additivity and Auditory Compression at Low and High Frequencies

The additivity of nonsimultaneous masking can be used as a measure of nonlinearity in the auditory system. For example, two equally effective forward maskers should produce 3 dB of additional masking when they are combined, assuming linearity with respect to intensity. A combined effect greater than this (excess masking) indicates compression. In the present experiments, the signal was a 10-ms pure tone presented 20 ms after a 200-ms narrowband noise masker and/or immediately after a 20-ms narrowband noise masker. The signal frequency was 250, 500, or 4000 Hz. The signal threshold produced by combining two equally effective maskers was measured. At all three frequencies, little excess masking was observed for a signal 10 dB above absolute threshold, indicating linear additivity (no compression). At signal levels 30 dB above absolute threshold, excess masking was observed at all three frequencies. The estimated compression exponents were 0.29 at 250 Hz, 0.34 at 500 Hz, and 0.17 at 4000 Hz. In contrast with physiological studies on other mammals, the present results provide evidence for substantial compression at low frequencies in humans.     

Effects of activation of the efferent system on psychophysical tuning curves as a function of signal frequency

It has been shown that electrical stimulation of the efferent auditory system can influence neural tuning curves in animals. Here, we examined a psychophysical analog of this effect in humans. All of the 19 normally hearing subjects showed a reduction in the amplitude of otoacoustic emissions in one ear when contralateral broadband noise was presented, indicating a functioning efferent system. Psychophysical tuning curves (PTCs) were measured in simultaneous masking in the absence and presence of contralateral stimulation (CS). The CS was a continuous narrowband noise centered at the signal frequency and presented at a level of 50 or 60 dB SL. The CS had no consistent effect on the masker level at the tips of the PTCs. For the two highest signal frequencies (2000 and 4000 Hz), the CS reduced the masker level required for threshold on both the low- and high-frequency sides of the PTCs, and the sharpness of tuning, as measured by Q10, decreased significantly. For the two lowest signal frequencies (500 and 1000 Hz), the masker level required for threshold on the low-frequency sides of the PTCs increased with CS, and the Q10 values increased significantly. The general pattern of the results was consistent with that observed for electrical stimulation of the efferent system in animals.     

The relationship between MOC reflex and masked threshold

Otoacoustic emission (OAE) amplitude can be reduced by acoustic stimulation. This effect is produced by the medial olivocochlear (MOC) reflex. Past studies have shown that the MOC reflex is related to listening in noise and attention. In the present study, the relationship between strength of the contralateral MOC reflex and masked threshold was investigated in 19 adults. Detection thresholds were determined for 1000-Hz, 300-ms tone presented simultaneously with one repetition of a 300-ms masker in an ongoing train of masker bursts. Three masking conditions were tested: 1) broadband noise 2) a fixed-frequency 4-tone complex masker and 3) a random-frequency 4-tone complex masker. Broadband noise was expected to produce energetic masking and the tonal maskers were expected to produce informational masking in some listeners. DPOAEs were recorded at fine frequency intervals from 500 to 4000 Hz, with and without contralateral acoustic stimulation. MOC reflex strength was estimated as a reduction in baseline level and a shift in frequency of DPOAE fine-structure maxima near 1000-Hz. MOC reflex and psychophysical testing were completed in separate sessions. Individuals with poorer thresholds in broadband noise and in random-frequency maskers were found to have stronger MOC reflexes.     

Modulation transfer functions: a comparison of the results of three methods

Modulation transfer functions (MTFs) were measured with three different psychoacoustical paradigms in the same normal-hearing subjects. In the temporal-probe method, the threshold of a 4-ms probe tone (frequencies of 1000 and 4000 Hz) was measured at various envelope phases within a 100% sinusoidally amplitude-modulated (SAM) noise at modulation frequencies from 2 to 256 Hz. For the derived-MTF method, the threshold of a 500-ms tone at 1000 and 4000 Hz was measured in the same noise at the same modulation frequencies. For the modulation-detection paradigm, modulation thresholds were measured as a function of modulation frequency for bandpass filtered SAM noise centered at 1000 and 4000 Hz. MTFs with lowpass shapes were observed with all three methods. Differences were observed in the cutoff frequencies and/or attenuation rates when the data were fitted with lowpass filter transfer functions. Factors influencing those differences are discussed.     

Growth of masking in sensorineural hearing loss.

The purpose of the present study was to measure the growth of masking in both normal-hearing and sensorineural hearing-impaired subjects. The masker was a narrow band of noise centered at 1,000 Hz, and masked thresholds for signal frequencies both above and below the masker frequency were obtained for a range of masker levels. For signal frequencies above the masker frequency, the slopes of the growth of masking functions were greater than 1 dB/dB for the normal-hearing group, while for the hearing-impaired subjects the slopes were less than those of the normal subjects, and in many cases slopes were less than 1. The slope of masking was inversely related to the threshold at the signal frequency. These data support the concept that a loss of nonlinearity at the signal place is responsible for the slower growth of masking in hearing-impaired subjects for signal frequencies greater than the masker. In addition, the slower-than-normal growth of masking of the hearing-impaired subjects suggests that some hearing aid signal-processing strategies which provide greater amounts of high-frequency emphasis at high input levels may not be appropriate.     

Compression estimates using behavioral and otoacoustic emission measures

Cochlear compression in normal-hearing listeners was estimated at octave frequencies from 250 to 4000 Hz using a forward-masking paradigm. Temporal masking curves (TMCs) for a 10-dB SL signal were obtained with two maskers -- one equal in frequency to the signal and another an octave below the signal. The ratio of the slope of the off-frequency function to that of the mid-level portion of the on-frequency function was computed as an estimate of the amount of compression at each frequency. Compression was less frequency selective at low frequencies, so an average of the off-frequency slopes at high frequencies (1000, 2000, and 4000 Hz) was used in computing the ratio for each signal frequency. Results indicated strong compression (approximately 0.15-0.30) at all frequencies using the averaged off-frequency slopes, indicating little difference in compression across frequencies. Distortion product otoacoustic emission (DPOAE) input-output (I-O) functions were obtained for each subject at 1000, 2000, and 4000 Hz. The slopes of the DPOAE I-O functions and the psychophysical growth rates were similar to one another, reinforcing the assumption that the forward-masking procedure is providing an estimate of cochlear compression, at least at frequencies from 1000 to 4000 Hz.     

Effects of noise bands on contralateral pure tone adaptation.

The purpose of this study was to investigate the changes in threshold tone decay which result from contralaterally presented stimuli at an intensity less than interaural crossover. Five tone decay tests were administered at both 1,000 Hz and 2,000 Hz to 25 normal hearing adults. At each test frequency five contralateral conditions (no competing noise, white noise, narrow band noise centered at the test frequency, narrow band noise centered below the test frequency, and narrow band noise centered above the test frequency), were presented at 40 db above detection threshold to the nontest ear. Results showed a significant shift in tone decay scores when the conditions of white noise and narrow band noise centered around the test frequency were compared to the condition of no competing noise. A shift was seen at both frequencies but that occuring at 2,000 Hz was significantly greater than the shift at 1,000 Hz. No significant increase in tone decay was found for contralateral noise bands above or below the test frequencies.     

Auditory Brain Stem Potentials Evoked by Clicks in Notch-Filtered Masking Noise: Audiological Relevance

Auditory brainstem-evoked potentials (ABEP) to clicks in notch-filtered masking noise were recorded from 10 adults with normal audiograms and from 10 adults with hearing losses. Masking noise had 0.5-octave-wide notches with central frequencies of 8 000, 2 000, 1 000, 500 and 250 Hz. For each notch frequency, decreasing-intensity series in 10-dB steps (for both the click and the noise) were derived until the detection threshold of ABEP was reached. From the detection thresholds of normal subjects, the normative detection threshold for each frequency was calculated. Deviations from the normative detection thresholds were considered ABEP estimates of hearing loss. The results of this study showed that while ABEP to clicks in notched noise were sensitive to hearing losses, correlations between audiometric hearing loss and ABEP estimates of hearing losses at specific frequencies were nonsignificant. At the present stage of the procedure, ABEP to clicks in notched noise cannot estimate hearing thresholds at specific frequencies.     

Frequency selectivity and comodulation masking release in adults and in 6-year-old children

Frequency selectivity and comodulation masking release (CMR) for a 1000-Hz signal frequency were examined in 6-year-old children and adults. An abbreviated measure of frequency selectivity was also conducted for a 500-Hz signal. Frequency selectivity was measured using a notched-noise masking method, and CMR was measured using narrow bands of noise whose amplitude envelopes were either uncorrelated or correlated. There were 6 listeners in each age group. No differences were observed between the adults and children for either auditory measure. Similarly, no differences were observed in the ability to detect a pure-tone signal in a relatively wideband noise masker. When the masking noise was narrowband, however, the masked thresholds of the children were higher than those of the adults. Two characteristics that distinguish narrowband noise from wideband noise are: (1) narrowband noise has a pitch quality corresponding to its center frequency, whereas wideband noise does not have definite pitch; (2) the intensity fluctuations are relatively greater in narrowband noise than in wideband noise. This may suggest that 6-year-old children have a reduced ability to detect signals in noise backgrounds where the signal has perceptual qualities similar to the noise, or in noise backgrounds having a high degree of fluctuation.     

Frequency characteristics of contralateral sound suppression of 40-Hz auditory steady-state response

Sound presented to the contralateral ear suppresses the amplitude of the 40-Hz auditory steady-state response (ASSR). The frequency characteristics of this suppression of the 40-Hz ASSR for amplitude modulated (AM) tones at 1,000 Hz (79-dB SPL) were examined in 12 healthy volunteers (10 males and 2 females, mean age 32.3 years) using contralateral AM tones (500, 1,000, 2,000, and 4,000 Hz) and 1/3 octave-band noise (500, 1,000, 2,000, and 4,000 Hz). The 40-Hz ASSR at 1,000 Hz was suppressed by a relatively wide frequency range of contralateral sound than expected from the known characteristics of psychophysical central masking by contralateral sound: the greatest suppression was obtained with 500- and 1,000-Hz sounds, but considerable suppression was also obtained with 2,000- and 4,000-Hz sounds. Substantial differences in the suppression pattern were not observed between two types of contra-suppressors; i.e., AM tones and 1/3 octave-band noise. Therefore, any sound presented to the contralateral ear, regardless of the frequency, can suppress the 40-Hz ASSR. Moreover, the different frequency characteristics of the contralateral sound effects between the psychophysical central masking and the 40-Hz ASSR would support the idea that the 40-Hz ASSR has an additive role in the processing of auditory signals to simple threshold judgment. Investigation of the type of psychophysical measurement using the AM signal showing similar suppression patterns by the presentation of contralateral sound would be helpful to reveal the functional relevance of ASSRs.     

Reliability of electric response audiometry using 80 Hz auditory steady-state responses

The reliability of the Auditory Steady State Response (ASSR) has not been thoroughly evaluated despite its recent application as a clinical tool for threshold estimation. The purpose of this study was to examine test-retest (TR) reliability of ASSR threshold estimates in an empirical research design. The ASSR, tested using modulation frequencies approximately 80 Hz and above, was evaluated against pure tone audiometry (PTA), and the slow vertex potential (SVP, N₁-P₂). Sixteen normal-hearing young female adults were tested twice, one week apart. Varying degrees of sensorineural hearing loss of a notched configuration were simulated with filtered masking noise. Test-retest reliability was assessed using Pearson-product moment correlation analysis, supplemented by other post-hoc analyses. Results demonstrated moderately strong TR reliability for ASSR at 1000, 2000 and 4000 Hz (r?=?0.83–0.93); however, the reliability of ASSR at 500 Hz was weaker (r?=?0.75). Results suggest that ASSR-ERA is a reliable test at mid–high frequencies, at least with the configuration and degrees of simulated sensorineural hearing loss examined in this study.     

Multiple auditory steady-state response thresholds to bone-conduction stimuli in young infants with normal hearing

OBJECTIVE: Multiple auditory steady-state responses (ASSRs) probably will be incorporated into the diagnostic test battery for estimating hearing thresholds in young infants in the near future. Limiting this, however, is the fact that there are no published bone-conduction ASSR threshold data for infants with normal or impaired hearing. The objective of this study was to investigate bone-conduction ASSR thresholds in infants from a Neonatal Intensive Care Unit (NICU) and in young infants with normal hearing and to compare these with adult ASSR thresholds. DESIGN: ASSR thresholds to multiple bone-conduction stimuli (carrier frequencies: 500 to 4000 Hz; 77 to 101-Hz modulation rates; amplitude/frequency modulated; single-polarity stimulus) were obtained in two infant groups [N = 29 preterm (32 to 43 wk PCA), tested in NICU; N = 14 postterm (0 to 8 mo), tested in sound booth]. All infants had passed a hearing screening test. ASSR thresholds, amplitudes, and phase delays for preterm and postterm infants were compared with previously collected adult data. RESULTS: Mean (+/-1 SD) ASSR thresholds were 16 (11), 16 (10), 37 (10), and 33 (13) dB HL for the preterm infants and 14 (13), 2 (7), 26 (6), and 22 (8) dB HL for the postterm infants at 500, 1000, 2000, and 4000 Hz, respectively. Both infant groups had significantly better thresholds for 500 and 1000 Hz compared with 2000 and 4000 Hz, in contrast to adults who have similar thresholds across frequency (22, 26, 18, and 18 dB HL). When 500- and 1000-Hz thresholds were pooled, pre- and postterm infants had better low-frequency thresholds than adults. When 2000- and 4000-Hz thresholds were pooled, pre- and postterm infants had poorer thresholds than adults. ASSR amplitudes were significantly larger for low frequencies compared with high frequencies for both infant groups, in contrast to adults, who show little difference across frequency. ASSR phase delays were later for lower frequencies compared with higher frequencies for infants and adults, except for 500 Hz in the preterm group. ASSR phase delays were later for infants compared with adults across frequency. CONCLUSIONS: Infant bone-conduction ASSR thresholds are very different from those of adults. Overall, these results indicate that low-frequency bone-conduction thresholds worsen and high-frequency bone-conduction thresholds improve with maturation. Bone-conduction ASSR threshold differences between the postterm infants and adults probably are due to skull maturation. Differences between preterm and older infants may be explained both by skull changes and a masking effect of high ambient noise levels in the NICU (and possibly to other issues due to prematurity).     

The measurement of critical masking bands.

The characteristics of the critical masking band, that spectral region of a wideband noise that is effective in masking a pure-tone signal, were inferred by measuring detectability (d) for tonal signals as a function of the cutoff frequency of a low-pass or high-pass noise masker. As the cutoff frequency of a low-pass noise was decreased from the wide-band (100-7000 Hz) condition toward the signal frequency, (500, 1000, or 4000 Hz) dectability maintained a constant minimum until a further reduction in cutoff frequency increased detectability, presumably due to a reduction in masker power within the critical band. As cutoff frequency was reduced further, detectability increased monotonically until detection reached 100%. This usually occurred when the cutoff frequency is 0.04 to 0.06 octaves below the signal frequency. The range of cutoff frequencies over which detectability changes occurred defines the critical masking band. These ranges correspond closely to well-known critical ratio data. The dependence of d on noise cutoff frequency did not differ at the two signal levels (15 and 25 dB SL) used in this experiment. The critical band appeared symmetrical about the signal frequency for most subjects and most experimental conditions, although some subjects displayed a marked asymmetry in the high frequency direction for some conditions.     

Midfrequency dysfunction in listeners having high-frequency sensorineural hearing loss

Two-tone unmasking, psychophysical tuning curves and pure-tone masking patterns were measured at 500 and 1000 Hz in 17 listeners having high-frequency sensorineural hearing loss due to noise exposure. Results were compared to similar data obtained from 20 normal-hearing young adults. In addition, measures of word-recognition ability were obtained in quiet and in noise for both groups. The primary findings were as follows: (a) 29% (n = 5) of the hearing-impaired subjects exhibited abnormal results on at least one of the psychoacoustic tasks investigated; (b) the observed abnormalities were reliable; and (c) there appeared to be a relation between the presence of midfrequency dysfunction and degree of difficulty on word-recognition tasks. These results and their implications are discussed.     

Human auditory brainstem response to temporal gaps in noise.

Gap detection is a commonly used measure of temporal resolution, although the mechanisms underlying gap detection are not well understood. To the extent that gap detection depends on processes within, or peripheral to, the auditory brainstem, one would predict that a measure of gap threshold based on the auditory brainstem response (ABR) would be similar to the psychophysical gap detection threshold. Three experiments were performed to examine the relationship between ABR gap threshold and gap detection. Thresholds for gaps in a broadband noise were measured in young adults with normal hearing, using both psychophysical techniques and electrophysiological techniques that use the ABR. The mean gap thresholds obtained with the two methods were very similar, although ABR gap thresholds tended to be lower than psychophysical gap thresholds. There was a modest correlation between psychophysical and ABR gap thresholds across participants. ABR and psychophysical thresholds for noise masked by temporally continuous, high-pass, or spectrally notched noise were measured in adults with normal hearing. Restricting the frequency range with masking led to poorer gap thresholds on both measures. High-pass maskers affected the ABR and psychophysical gap thresholds similarly. Notched-noise-masked ABR and psychophysical gap thresholds were very similar except that low-frequency, notched-noise-masked ABR gap threshold was much poorer at low levels. The ABR gap threshold was more sensitive to changes in signal-to-masker ratio than was the psychophysical gap detection threshold. ABR and psychophysical thresholds for gaps in broadband noise were measured in listeners with sensorineural hearing loss and in infants. On average, both ABR gap thresholds and psychophysical gap detection thresholds of listeners with hearing loss were worse than those of listeners with normal hearing, although individual differences were observed. Psychophysical gap detection thresholds of 3- and 6-month-old infants were an order of magnitude worse than those of adults with normal hearing, as previously reported; however, ABR gap thresholds of 3-month-old infants were no different from those of adults with normal hearing. These results suggest that ABR gap thresholds and psychophysical gap detection depend on at least some of the same mechanisms within the auditory system.     

Age-related improvements in auditory backward and simultaneous masking in 6- to 10-year-old children.

This study investigated the development of auditory frequency and temporal resolution using simultaneous and backward masking of a tone by a noise. The participants were 6- to 10-year-old children and adults. On the measure of frequency resolution (the difference in the detection threshold for a tone presented either in a bandpass noise or in a spectrally notched noise), 6-year-old children performed as well as adults. However, for the backward masking task, 6-year-olds had, on average, 34 dB higher thresholds than adults. A negative exponential decay function fitted to the backward masking data for subjects of all ages indicated that adult-like temporal resolution may not be reached until about 11 years of age. These results show that, measured by masking, frequency resolution has reached adult-like performance by 6 years of age, whereas temporal resolution develops beyond 10 years of age. Six-year-old children were also assessed with tests of cognitive ability. Improvements in both frequency and temporal resolution were found with increasing IQ score.     

Recovery from prior stimulation. II: Effects upon intensity discrimination.

We obtained just-noticeable differences (jnds) for the intensity of pure tones following a forward masker. The masker was a 100-ms burst of narrow-band noise centered at 1000 Hz presented at 90 dB SPL; the pure-tone signal was at 1000 Hz and was 25 ms in duration. The masker-signal delay was 100 ms. Under these conditions, there is no threshold shift for the detection of the pure-tone signal following the forward masker. In contrast with the absence of a forward-masker effect upon detection thresholds, unusually large midlevel (40-60 dB SPL) jnds were observed. These large midlevel jnds were measured as a function of signal delay, revealing that they are not completely recovered to the normal (unmasked) values by 400 ms. We interpret these data as a consequence of the slower recovery of low-spontaneous rate, high-threshold neurons following prior stimulation (Relkin and Doucet, 1990). These experiments may therefore provide psychophysical evidence that the low-spontaneous rate, high-threshold neurons are a necessary physiological component in the coding of the large dynamic range for intensity. In addition, the present data provide evidence that the assumption that the effect of forward masking is limited to 100-200 ms is inappropriate, as this recovery time does not necessarily apply to suprathreshold tasks.     

Development of auditory temporal resolution in school-age children revealed by word recognition in continuous and interrupted noise

OBJECTIVE: The purpose of the study was to investigate the development of one aspect of auditory temporal resolution in normal-hearing school-age children with word recognition in quiet and in spectrally identical continuous and interrupted noise. Typically, listeners experience a perceptual advantage (i.e., a "release from masking") in the interrupted noise relative to the continuous noise at equivalent signal-to-noise ratios (S/Ns). Any release from masking observed with children in the interrupted noise compared with the continuous noise at equivalent S/Ns could be interpreted as evidence for acquired temporal resolution ability. Differences in the amount of release from masking in the interrupted noise between children and adults could be interpreted as development of temporal resolution ability, or lack thereof, as revealed by word recognition in noise. It was hypothesized that word recognition performance would be poorer in children than adults; performance differences would be more pronounced with competition; word recognition performance would reach an asymptote to adult levels sooner in quiet than with competing stimuli; children would demonstrate better performance in the interrupted noise relative to the continuous noise (i.e., display a release from masking); and younger children would experience less release from masking compared with older children and adults (i.e., have less developed temporal resolution). DESIGN: Eighty normal-hearing children aged 6 to 15 yr and 16 normal-hearing young adults participated. Word recognition performance with Northwestern University-Children's Perception of Speech (NU-CHIPs) stimuli was evaluated with an open-set response mode in quiet and in backgrounds of competing continuous steady-state and interrupted noise at S/Ns of 10, 0, -10, and -20 dB. Both noises were essentially identical in their spectral content and differed only in their temporal continuity. RESULTS: Performance was better in the interrupted noise at poorer S/Ns, increased with increasing S/N, and improved with increasing age. Younger listeners were more susceptible to noise. They did not experience an equivalent perceptual advantage (i.e., a release from masking) in the interrupted noise at poorer S/Ns (i.e., < 10 dB) and generally required more favorable S/Ns to perform the same as the adult participants. These trends were less pronounced with increasing age. By 8 yr of age, children's performance in quiet equated that of adult levels, but it did not do so in noise until after 11 yr of age. CONCLUSIONS: As revealed by their NU-CHIPs word recognition performance in continuous and interrupted noises, children's temporal resolving abilities improve in their early school years and reach adult performance levels after 11 yr of age. It was speculated that these changes reflect maturation in their central auditory system.