Auditory mismatch negativity: detecting spectral contrasts in a modulated noise

The ability to detect sinusoidal spectral envelope from an unmodulated (flat) spectrum has shown a strong correlation with speech perception in quiet and noisy listening conditions in normal hearing, hearing-impaired, and cochlear implant users. In this study, the mismatch negativity (MMN) was collected from 10 normal-hearing adult listeners and used to determine if modulation detection thresholds derived electrophysiologically correlate with the psychophysically measured spectral modulation detection thresholds. MMNs were present for most participants with spectral contrasts of 10 and 20 dB, suggesting that the MMN may be a promising neurophysiologic measure of spectral envelope perception when nonbehavioral measures are desired.     

Effect of masker level on infants’ detection of tones in noise

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from ?5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants’ performance was more adultlike for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N₀ at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants’ thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants’ thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.     

Effect of masker level on infants' detection of tones in noise

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from -5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants' performance was more adult-like for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N0 at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants' thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants' thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.     

A comparison of three adaptive psychophysical procedures using inexperienced listeners

Thresholds for a masked 1-kHz tone were obtained in single sessions from 60 inexperienced listeners. A two-alternative forced-choice procedure was used in conjunction with one of three adaptive psychophysical techniques. These techniques comprised two staircase techniques targeting either 70.7% or 79.4% correct detection (Staircase-71 and Staircase-79), and parameter estimation by sequential testing (PEST), targeting 80% correct detection. Listeners were provided with a rationale for maintaining concentration at weak signal levels. Similar threshold values were obtained from the Staircase-79 and PEST groups in equal numbers of trials. The degree of oscillation in the level of the signal around the value finally chosen as the threshold was comparable for both staircase techniques. Subsequent fixed-level testing did not provide a true indication of the subjects' capabilities. The amount by which percent correct in fixed-level testing differed from expectations based on adaptive testing varied among the techniques. Additional thresholds were obtained in a second session from 30 of the original subjects. Thresholds with both staircase techniques improved by about 1 dB on retest, while thresholds with PEST were constant across sessions. The variability of the data compared well with that from studies involving experienced listeners.     

Effects of rate and word boundary ambiguity on recall by normal and impaired listeners

Twenty normal-hearing and 20 hearing-impaired persons heard experimental sentences that included two syllables with an ambiguous word boundary and control sentences that did not include an ambiguous word boundary. The sentences were produced at two rates. Difficulty in generating a meaning for the ambiguous word boundary that made sense within the sentence appeared to reduce recall. For the normal-hearing persons, the difficulty with the ambiguous word boundary reduced recall of experimental sentence words when the speech was fast but not when it was slow. For the hearing-impaired persons, the difficulty reduced the recall of the experimental sentence words when the speech was slow but not when it was fast. Two weeks later, the same listeners heard a set of two syllables having ambiguous word boundaries that were isolated from their sentences. Varying speed of speaking affected the segmentation of the two syllables by the normal-hearing more than it affected the segmentation by the hearing-impaired.This work was supported by NINDS Grants No. NS10468 and 05362.     

Lateralization of narrow-band noise by blind and sighted listeners

The effects of varying interaural time delay (ITD) and interaural intensity difference (IID) were measured in normal-hearing sighted and congenitally blind subjects as a function of eleven frequencies and at sound pressure levels of 70 and 90 dB, and at a sensation level of 25 dB (sensation level refers to the pressure level of the sound above its threshold for the individual subject). Using an 'acoustic' pointing paradigm, the subject varied the IID of a 500 Hz narrow-band (100 Hz) noise (the 'pointer') to coincide with the apparent lateral position of a 'target' ITD stimulus. ITDs of 0, +/-200, and +/-400 micros were obtained through total waveform delays of narrow-band noise, including envelope and fine structure. For both groups, the results of this experiment confirm the traditional view of binaural hearing for like stimuli: non-zero ITDs produce little perceived lateral displacement away from 0 IID at frequencies above 1250 Hz. To the extent that greater magnitude of lateralization for a given ITD, presentation level, and center frequency can be equated with superior localization abilities, blind listeners appear at least comparable and even somewhat better than sighted subjects, especially when attending to signals in the periphery. The present findings suggest that blind listeners are fully able to utilize the cues for spatial hearing, and that vision is not a mandatory prerequisite for the calibration of human spatial hearing.     

Detection and identification thresholds for consonant-vowel syllables

Detection and identification thresholds were obtained for 6- and 10-year-old normal children and normal adults using five-formant synthesized consonant-vowel ([baHdflrfea]) stimuli. Sixyear-old children were found to have poorer detection than adults, just as they do for pure tones. For the identification task, the slopes of the performance-intensity functions were more shallow for 6-year-old children than for 10-year-olds and adults. Consequently, compared with 10-yearolds and adults, 6-year-old listeners require a greater increase in stimulus intensity above detection threshold to identify these stimuli at a high performance level. The influence of acoustic characteristics of the stimuli on all listeners is also discussed.     

The effect of vibrato on the recognition of masked vowels

Five experiments on the identifiability of synthetic vowels masked by wideband sounds are reported. In each experiment, identification thresholds (signal/masker ratios, in decibels) were measured for two versions of four vowels: a vibrated version, in which FO varied sinusoidally around 100 Hz; and a steady version, in which F0 was fixed at 100 Hz. The first three experiments were performed on naive subjects. Experiment 1 showed that for maskers consisting of bursts of pink noise, vibrato had no effect on thresholds. In Experiment 2, where the maskers were periodic pulse trains with an F0 randomly varied between 120 and 140 Hz from trial to trial, vibrato slightly improved thresholds when the sound pressure level of the maskers was 40 dB, but had no effect for 65-dB maskers. In Experiment 3, vibrated rather than steady pulse trains were used as maskers; when these maskers were at 40 dB, the vibrated versions of the vowels were slightly less identifiable than their steady versions; but, as in Experiment 2, vibrato had no effect when the maskers were at 65 dB. Experiment 4 showed that the unmasking effect of vibrato found in Experiment 2 disappeared in subjects trained in the identification task. Finally, Experiment 5 indicated that in trained listeners, vibrato had no influence on identification performance even when the maskers and the vowels had synchronous onsets and offsets. We conclude that vibrating a vowel masked by a wideband sound can affect its identification threshold, but only for tonal maskers and in untrained listeners. This effect of vibrato should probably be considered as a Gestalt phenomenon originating from central auditory mechanisms.     

Effects of Sign Language Instruction on Social Interaction Between Hearing-Impaired and Normal-Hearing Children

This experiment investigated the efficacy of a practical intervention designed to improve the social integration of hearing-impaired students within the school environment. The study involved the development, implementation, and empirical evaluation of a sign language course offered to 16 normal-hearing students in a public elementary school in which 6 mainstreamed hearing-impaired students were enrolled. Sixteen additional normal-hearing students served as controls. A measure of acceptance attitudes on the part of the hearing subjects toward their hearing-impaired peers, as well as a sociometric index of social interaction, were administered pre-mid-, and post-intervention to all subjects. The results revealed no significant difference between the experimental and control subjects on the measure of acceptance attitudes toward the hearing-impaired students as a group. However, for both experimental and control groups, there were significant improvements over time in the scores of some subscales of the acceptance measure and significant increases in aspects of the sociometrically measured peer interaction. In addition, one hearing-impaired student was involved in significantly more social interaction with experimental subjects than with control subjects.     

Frequency selectivity using tonal-temporal masking: Diotic vs. dichotic masking

A combined forward-backward masking procedure was used to investigate the threshold of a 30-msec, 500-Hz signal as a function of masker frequency. The signal thresholds were obtained in two signal conditions, diotic (So) and dichotic (S_π), and for two different temporal separations of the maskers. The maskers were 500 msec in duration and were presented at 75 dB SPL. The function relating masked signal threshold to masker frequency was used to describe frequency selectivity in the four conditions. There were no differences in frequency selectivity measured between the diotic and dichotic signal conditions and only a small difference measured between the two intermasker interval conditions. The S_π conditions yielded lower thresholds than did the So conditions. The change in intermasker interval from 10 to 50 msec lowered the threshold maximally 18 dB for the So condition and 13 dB for the S_π condition. The results indicate that in this tonal temporal masking procedure there are no differences between the diotic and dichotic critical bands.     

The impact of aging and hearing status on verbal short-term memory

The aim of this study is to assess the impact of hearing status on age-related decrease in verbal short-term memory (STM) performance. This was done by administering a battery of verbal STM tasks to elderly and young adult participants matched for hearing thresholds, as well as to young normal-hearing control participants. The matching procedure allowed us to assess the importance of hearing loss as an explanatory factor of age-related STM decline. We observed that elderly participants and hearing-matched young participants showed equal levels of performance in all verbal STM tasks, and performed overall lower than the normal-hearing young control participants. This study provides evidence for recent theoretical accounts considering reduced hearing level as an important explanatory factor of poor auditory-verbal STM performance in older adults.     

Differential thresholds for limb movement measured using adaptive techniques.

Differential thresholds for limb movement were measured in 10 subjects, using the transformed up-down procedure. Subjects were required to indicate which of two random displacement perturbations delivered to their forearms had the larger standard deviation (SD). The SD of the reference signal was fixed for each experimental condition at one of seven values ranging from 0.05 to 3.2 mm. THe SD of the other signal varied depending on the subject's response. Using this procedure, the differential threshold for limb movement was calculated to be 8%, which is very similar to the thresholds estimated previously for changes in limb position (9%) and force (7%). The sensitivity of the human proprioceptive system to changes in limb displacement was much greater than anticipated, with subjects being able to resolve a 5-microns difference between two perturbations delivered to their arms.     

Age-related changes in temporal resolution: envelope and intensity effects.

Gap-detection thresholds were determined for 10 younger and 10 older adults at two sensation levels (40 and 60 dB SL) for tone pips with Gaussian amplitude envelopes whose standard deviations were 0.5, 1, 1.5, and 2 ms. Gap-detection thresholds were larger for the older participants under all conditions. For all participants, gap-detection thresholds increased with the standard deviation of the Gaussian amplitude envelope, were relatively independent of sensation level, and were independent of the degree of hearing loss. Because spectral splatter decreases with increasing standard deviation of the Gaussian amplitude envelope, the age-related differences in gap-detection cannot be attributed to differences between how young and old listeners are affected by off-frequency cues. Furthermore, the consistent age difference in gap-detection at all amplitude envelope standard deviations was shown to be incompatible with the hypothesis that temporal integration time is longer for older listeners.     

Binaural loudness matches in unilaterally impaired listeners

Binaural loudness matching data using a 2IFC adaptive procedure were obtained in high-frequency, unilateral cochlear-impaired listeners. The matches were obtained at frequencies where both ears had similarly normal thresholds, and also at other frequencies where the impaired ear had various degrees of hearing loss. In these listeners, one presumed difference between the ears is the limited or altered spread of excitation in the impaired ear. In agreement with previous studies using other approaches (Hellman, 1974, 1978; Hellman & Meiselman, 1986; Moore, Glasberg, Hess & Birchall, 1985; Schneider & Parker, 1987), the results of the present study suggest that both the range and the slope of loudness growth function are not dependent on the spread of excitation, but instead are related primarily to the degree of threshold elevation at the test frequency. Following this suggestion, a spread-of-excitation-independent model, based upon a group of neurons with the same characteristic frequency (CF) but different thresholds, is proposed to account for loudness growth in both normal and recruitment cases. In particular, it is shown quantitatively that a compressed distribution of thresholds due to threshold elevation may be responsible for loudness recruitment in sensorineural hearing loss.     

Differential thresholds for limb movement measured using adaptive techniques

Differential thresholds for limb movement were measured in 10 subjects, using the transformed up-down procedure. Subjects were required to indicate which of two random displacement perturbations delivered to their forearms had the larger standard deviation (SD). TheSD of the reference signal was fixed for each experimental condition at one of seven values ranging from 0.05 to 3.2 mm. TheSD of the other signal varied depending on the subject’s response. Using this procedure, the differential threshold for limb movement was calculated to be 8%, which is very similar to the thresholds estimated previously for changes in limb position (9%) and force (7%). The sensitivity of the human proprioceptive system to changes in limb displacement was much greater than anticipated, with subjects being able to resolve a 5-μm difference between two perturbations delivered to their arms.     

Size of critical band in infants, children, and adults

Masked thresholds at two signal frequencies (0.8 and 4 kHz) were obtained from listeners aged 6.5 months, 2 years, 5 years, and 20.5 years in the presence of constant spectrum level, narrowband maskers of differing bandwidths. Consistent with the classical results of Fletcher (1940), masked threshold for all age groups increased with bandwidth up to a critical width, beyond which further increases in bandwidth were ineffective in increasing threshold. These critical widths (estimates of critical band size) did not change substantially with age (critical widths for infants were no more than 50% larger than those of adults) despite substantial changes in masked thresholds with age. Thus, contrary to previous claims, changes in auditory filter width cannot account for developmental changes in masked or absolute thresholds.     

Correlations among within-channel and between-channel auditory gap-detection thresholds in normal listeners

We obtained data on within-channel and between-channel auditory temporal gap-detection acuity in the normal population. Ninety-five normal listeners were tested for gap-detection thresholds, for conditions in which the gap was bounded by spectrally identical, and by spectrally different, acoustic markers. Separate thresholds were obtained with the use of an adaptive tracking method, for gaps delimited by narrowband noise bursts centred on 1.0 kHz, noise bursts centred on 4.0 kHz, and for gaps bounded by a leading marker of 4.0 kHz noise and a trailing marker of 1.0 kHz noise. Gap thresholds were lowest for silent periods bounded by identical markers--'within-channel' stimuli. Gap thresholds were significantly longer for the between-channel stimulus--silent periods bounded by unidentical markers (p < 0.0001). Thresholds for the two within-channel tasks were highly correlated (R = 0.76). Thresholds for the between-channel stimulus were weakly correlated with thresholds for the within-channel stimuli (1.0 kHz, R = 0.39; and 4.0 kHz, R = 0.46). The relatively poor predictability of between-channel thresholds from the within-channel thresholds is new evidence on the separability of the mechanisms that mediate performance of the two tasks. The data confirm that the acuity difference for the tasks, which has previously been demonstrated in only small numbers of highly trained listeners, extends to a population of untrained listeners. The acuity of the between-channel mechanism may be relevant to the formation of voice-onset time-category boundaries in speech perception.     

Frequency selectivity in the parakeet (Melopsittacus undulatus) studied with psychophysical tuning curves

Seven parakeets were trained to avoid shock during pure-tone stimulation. A modified method of limits was used to measure detection thresholds of the pure tones. The intensity of masker tones, at numerous frequencies, was varied in order to measure the masked threshold of a probe-tone signal set at a fixed frequency and intensity. Masking curves were obtained for three probe tones (.63, 1.6, and 2.5 kHz) at each of five sensation levels (10, 20, 30, 40, and 50 dB). The masking curves from this procedure are frequently referred to as "psychophysical tuning curves" and provide an indication of frequency selectivity. The results are compared with analogous data in other species and suggest that frequency analysis in the parakeet ear is somewhat less accurate than in the mammalian ear.     

Commonsense beliefs and the identification of familiar voices

Two experiments are reported in which participants attempted to reject the tape‐recorded voice of a stranger and identify by name the voices of three personal associates who differed in their level of familiarity. In Experiment 1 listeners were asked to identify speakers as soon as possible, but were not allowed to change their responses once made. In Experiment 2 listeners were permitted to change their responses over successive presentations of increasing durations of voice segments. Also, in Experiment 2 half of the listeners attempted to identify speakers who spoke in normal‐tone voices, and the remainder attempted to identify the same speakers who spoke in whispers. Separate groups of undergraduate students attempted to predict the performance of the listeners in both experiments. Accuracy of performance depended on the familiarity of speakers and tone of speech. A between‐subjects analysis of rated confidence was diagnostic of accuracy for high familiar and low familiar speakers (Experiment 1), and for moderate familiar and unfamiliar normal‐tone speakers (Experiment 2). A modified between‐subjects analysis assessed across the four levels of familiarity yielded reliable accuracy‐confidence correlations in both experiments. Beliefs about the accuracy of voice identification were inflated relative to the significantly lower actual performance for most of the normal‐tone and whispered‐speech conditions. Forensic significance and generalizations are addressed. Copyright © 2001 John Wiley & Sons, Ltd.     

Signal detectability in the presence of monotic or dichotic noise bands of equal or unequal levels

The application of the power-spectrum model of masking to the detectability of a signal masked by dichotic noise was investigated in three experiments. In each experiment, the signal was a 2-kHz sinusoid of 400-msec duration, masked by either one or two 800-Hz wide bands of noise presented singly or in pairs. In Experiment 1, we compared the detectability of a diotic signal masked by dichotic noise with the detectability of a monaural signal masked by each of the noises separately. The spectrum level of the noise was 35 dB SPL. For dichotic presentations, the signal was sent to both ears while pairs of noise bands, one below and one above the signal frequency, were presented together, one band to each ear. Threshold levels with the dichotic stimuli were lower than or equal to the thresholds with either ear's stimulus on its own. Similar dichotic stimuli were used in Experiment 2, except that the signal frequency was nearer to one or the other of the bands of masking noise, and the noise had a spectrum level of 50 dB SPL. In Experiment 3, thresholds were obtained with two sets of symmetrically and asymmetrically placed notched-noise maskers. For one of these sets, the spectrum level of both noise bands was 35 dB SPL; for the other set, interaural intensity differences were introduced in the form of an inequality in the levels of the noise bands on either side of the signal. In one ear, the spectrum level of the lower frequency noise band was 35 dB SPL and the spectrum level of the higher frequency noise band was 25 dB SPL, whereas in the other ear, the allocation of noise level to noise band was reversed. The dichotic thresholds obtained with the unequal noise maskers could be predicted from the shapes of the auditory filters derived with equal noise maskers. The data from all three experiments suggest that threshold signal levels in the presence of interaural differences in masker intensity depend principally on the ear with the higher signal-to-masker ratio at the output of its auditory filter, a finding consistent with the power-spectrum model of masking.