The effect of temporal placement on gap detectability

The detectability of a masked sinusoid increases as its onset approaches the temporal center of a masker. This study was designed to determine whether a similar change in detectability would occur for a silent gap as it was parametrically displaced from the onset of a noise burst. Gap thresholds were obtained for 13 subjects who completed five replications of each condition in 3 to 13 days. Six subjects were inexperienced listeners who ranged in age from 18 to 25 years; seven subjects were highly experienced and ranged in age from 20 to 78 years. The gaps were placed in 150-ms, 6-kHz, low-passed noise bursts presented at an overall level of 75 dB SPL; the bursts were digitally shaped at onset and offset with 10-ms cosine-squared rise-fall envelopes. The gated noise bursts were presented in a continuous, unfiltered, white noise floor attenuated to an overall level of 45 dB SPL. Gap onsets were parametrically delayed from the onset of the noise burst (defined as the first nonzero point on the waveform envelope) by 10, 11, 13, 15, 20, 40, 60, 110, 120, and 130 ms. Results of ANOVAs indicated that the mean gap thresholds were longer when the gaps were proximal to signal onset or offset and shorter when the gaps approached the temporal center of the noise burst. Also, the thresholds of the younger, highly experienced subjects were significantly shorter than those of the younger, inexperienced subjects, especially at placements close to signal onset or offset. The effect of replication (short-term practice) was not significant nor was the interaction between gap placement and replication. Post hoc comparisons indicated that the effect of gap placement resulted from significant decreases in gap detectability when the gap was placed close to stimulus onset and offset.     

Gap detection by early-deafened cochlear-implant subjects

Two studies investigating gap-detection thresholds were conducted with cochlear-implant subjects whose onset of profound hearing loss was very early in life. The Cochlear Limited multiple-electrode prosthesis was used. The first study investigated the effects of pulse rate (200, 500, and 1000 pulses/s) and stimulus duration (500 and 1000 ms) on gap thresholds in 15 subjects. Average gap thresholds were 1.8 to 32.1 ms. There was essentially no effect of pulse rate and for almost all subjects, no effect of stimulus duration. For two subjects, performance was poorer for the 1000-ms stimulus duration. The second study investigated the relationships between gap thresholds, subject variables, and speech-perception scores. Data from the first study were combined with those from previous studies [Busby et al., Audiology 31, 95-111 (1992); Tong et al., J. Acoust. Soc. Am. 84, 951-962 (1988)], providing data from 27 subjects. A significant negative correlation was found between age at onset of deafness and gap thresholds and most variability in gap thresholds was for the congenitally deaf subjects. Significant negative correlations were found between gap thresholds and word scores for open-set Bamford-Kowal-Bench (BKB) sentences in the auditory-visual condition and lipreading enhancement scores for the same test.     

Detection of partially filled gaps in noise and the temporal modulation transfer function

Results of experiments on the detection of silent intervals, or gaps, in broadband noise are reported for normal-hearing listeners. In some preliminary experiments, a gap threshold of about 2 ms was measured. This value was independent of the duration of the noise burst, variation of the noise level on each presentation, or the temporal position of the gap within the noise burst. In the main experiments, the thresholds for partial decrements in the noise waveform as well as brief increments were determined. As predicted by a model that assumes a single fixed peak-to-valley detection ratio, thresholds for increments are slightly higher than thresholds for decrements when the signal is measured as the change in rms noise level. A first-order model describes the temporal properties of the auditory system as a low-pass filter with a 7- to 8-ms time constant. Temporal modulation transfer functions were determined for the same subjects, and the estimated temporal parameters agreed well with those estimated from the gap detection data. More detailed modeling was carried out by simulating Viemeister's three-stage temporal model. Simulations, using an initial stage bandwidth of 4000 Hz and a 3-ms time constant for the low-pass filter, generate data that are very similar to those obtained from human subjects in both modulation and gap detection.     

Word recognition in competing babble and the effects of age,temporal processing,and absolute sensitivity

This study was designed to clarify whether speech understanding in a fluctuating background is related to temporal processing as measured by the detection of gaps in noise bursts. Fifty adults with normal hearing or mild high-frequency hearing loss served as subjects. Gap detection thresholds were obtained using a three-interval, forced-choice paradigm. A 150-ms noise burst was used as the gap carrier with the gap placed close to carrier onset. A high-frequency masker without a temporal gap was gated on and off with the noise bursts. A continuous white-noise floor was present in the background. Word scores for the subjects were obtained at a presentation level of 55 dB HL in competing babble levels of 50, 55, and 60 dB HL. A repeated measures analysis of covariance of the word scores examined the effects of age, absolute sensitivity, and temporal sensitivity. The results of the analysis indicated that word scores in competing babble decreased significantly with increases in babble level, age, and gap detection thresholds. The effects of absolute sensitivity on word scores in competing babble were not significant. These results suggest that age and temporal processing influence speech understanding in fluctuating backgrounds in adults with normal hearing or mild high-frequency hearing loss.     

Minimum audible movement angle: marking the end points of the path traveled by a moving sound source

Four experienced subjects were tested on their ability to discriminate the direction of motion or the order of events in a single-interval, two-alternative, forced-choice adaptive paradigm. Two conditions, employing a broadband "pink" noise (500-8000 Hz), were examined: (1) A continuous noise was available from the moving sound source during the entire period of travel; and (2) 10-ms noise pulses were presented from the moving source at the beginning and end of the arc traveled (during the interpulse interval the source was inactive). Minimum audible movement angle (MAMA) thresholds were significantly lower when the moving source was active throughout the period of travel (0.914 degrees) than when only the end points of the arc of travel were "marked" (1.604 degrees). These results do not support the notion that the discrimination of motion can be reduced to a simple comparison of the location of the source at signal onset and the position of the source at signal offset. The MAMA thresholds obtained with broadband noise in the current experiment are considerably lower than the thresholds previously observed with tonal targets.     

A diminished rate of "physiological decay" at noise offset contributes to age-related changes in temporal acuity in the CBA mouse model of presbycusis

The persistence of afferent activity at stimulus offset was examined in mice as a function of age (experiment 1) and noise level (experiment 2), using a procedure devised by von Bekesy to study the physiological decay of afferent activity. Noise offset was presented from 1 to 10 ms (the temporal gap) prior to an acoustic startle stimulus, with an abrupt edge or a linear ramp having the duration of the gap. Noise offset inhibited the startle reflex, this effect declining with age. For young (2-3 months old) and "young-old" mice (18-19 months old), the abrupt offset was always more inhibitory than the ramp, even for the one millisecond gap. In "old-old" mice (24-26 months old) abrupt and ramped offsets were not different until the gap exceeded 4 ms. The behavioral difference between abrupt and ramped decay times did not depend on noise level in young mice (4-5 months old), though overall inhibition increased with level. These data indicate that temporal acuity as measured by this gap detection method is limited in very old mice by their reduced ability to follow the abrupt change in noise level at the beginning of the gap.     

The effects of real and illusory glides on pure-tone frequency discrimination

Experiment 1 measured pure-tone frequency difference limens (DLs) at 1 and 4 kHz. The stimuli had two steady-state portions, which differed in frequency for the target. These portions were separated by a middle section of varying length, which consisted of a silent gap, a frequency glide, or a noise burst (conditions: gap, glide, and noise, respectively). The noise burst created an illusion of the tone continuing through the gap. In the first condition, the stimuli had an overall duration of 500 ms. In the second condition, stimuli had a fixed 50-ms middle section, and the overall duration was varied. DLs were lower for the glide than for the gap condition, consistent with the idea that the auditory system contains a mechanism specific for the detection of dynamic changes. DLs were generally lower for the noise than for the gap condition, suggesting that this mechanism extracts information from an illusory glide. In a second experiment, pure-tone frequency direction-discrimination thresholds were measured using similar stimuli as for the first experiment. For this task, the type of the middle section hardly affected the thresholds, suggesting that the frequency-change detection mechanism does not facilitate the identification of the direction of frequency changes.     

A binaural analog of gap detection.

The temporal resolution of the binaural auditory system was measured using a binaural analog of gap detection. A binaural "gap" was defined as a burst of interaurally uncorrelated noise (Nu) placed between two bursts of interaurally correlated noise (N0). The Nu burst creates a dip in the output of a binaural temporal window integrating interaural correlation, analogous to the dip created by a silent gap in the output of a monaural temporal window integrating intensity. The equivalent rectangular duration (ERD) of the binaural window was used as an index of binaural temporal resolution. In order to derive the ERD, both the shortest-detectable binaural gap and the jnd for a reduction in interaural correlation from unity were measured. In experiment 1, binaural-gap thresholds were measured using narrow-band noise carriers as a function of center frequency from 250 to 2000 Hz (fixed 100-Hz bandwidth) and a function of lower-cutoff frequency from 100 to 400 Hz (fixed 500-Hz upper-cutoff frequency). Binaural-gap thresholds (1) increased significantly with increasing frequency in both tasks, and (2) at frequencies below 500 Hz, were shorter than corresponding silent-gap thresholds measured with the same N0 noises. In experiment 2, interaural-correlation jnd's were measured for the same conditions. The jnd's also increased significantly with increasing frequency. The results were analyzed using a temporal window integrating the output of a computational model of binaural processing. The ERD of the window varied widely across listeners, with a mean value of 140 ms, and did not significantly depend on frequency. This duration is about an order of magnitude longer than the ERD of the monaural temporal window and is, therefore, consistent with "binaural sluggishness."     

Gap detection and masking in hearing-impaired and normal-hearing subjects

Subjects with cochlear impairments often show reduced temporal resolution as measured in gap-detection tasks. The primary goals of these experiments were: to assess the extent to which the enlarged gap thresholds can be explained by elevations in absolute threshold; and to determine whether the large gap thresholds can be explained by the same processes that lead to a slower-than-normal recovery from forward masking. In experiment I gap thresholds were measured for nine unilaterally and eight bilaterally impaired subjects, using bandlimited noise stimuli centered at 0.5, 1.0, and 2.0 kHz. Gap thresholds were usually larger for the impaired ears, even when the comparisons were made at equal sensation levels (SLs). Gap thresholds tended to increase with increasing absolute threshold, but the scatter of gap thresholds was large for a given degree of hearing loss. In experiment II threshold was measured as a function of the delay between the onset of a 210-ms masker and the onset of a 10-ms signal in both simultaneous- and forward-masking conditions. The signal frequency was equal to the center frequency of the bandlimited noise masker, which was 0.5, 1.0, or 2.0 kHz. Five subjects with unilateral cochlear impairments, two subjects with bilateral impairments, and two normal subjects were tested. The rate of recovery from forward masking, particularly the initial rate, was usually slower for the impaired ears, even when the maskers were presented at equal SLs. Large gap thresholds tended to be associated with slow rates of recovery from forward masking.     

Comparing monaural and interaural temporal windows: effects of a temporal fringe on sensitivity to intensity differences

In an effort to provide a unifying framework for understanding monaural and binaural processing of intensity differences, an experiment was performed to assess whether temporal weighting functions estimated in two-interval monaural intensity-discrimination tasks could account for data in single-interval interaural intensity-discrimination tasks. In both tasks, stimuli consisted of a 50-ms burst of noise with a 5-ms probe segment at temporal positions ranging between the onset and offset of the overall stimulus. During the probe segment, one monaural interval or binaural channel of each trial contained an intensity increment and the other contained a decrement. Listeners were instructed to choose the interval/channel containing the increment. The pattern of monaural thresholds was roughly symmetrical (an inverted U) across temporal position of the probe but interaural thresholds were substantially higher for a brief time interval following stimulus onset. A two-sided exponential temporal window fit to the monaural data accounted for the interaural data well when combined with a post-onset-weighting function that described greatest weighting of binaural information at stimulus onset. A second experiment showed that the specific procedure used in measuring fringed interaural-intensity-difference-discrimination thresholds affects thresholds as a function of fringe duration and influences the form of the best-fitting post-onset-weighting function.     

Humans detect gaps in broadband noise according to effective gap duration without additional cues from abrupt envelope changes

Previous studies of behavior and IC single units in the mouse support theoretical expectations that gaps with ramped trailing markers have reduced detectability compared to equivalent gaps with ramped leading markers. In experiment 1, detection probability and response speeds of humans listening for gaps in broadband noise were investigated by independently varying either leading marker fall-time (FT) or trailing marker rise-time (RT). Gaps with silent duration of 1, 4, or 12 ms were presented 2 s into a 3-s noise burst, with either abrupt marker onsets and offsets or linearly ramped RT/FT of 2, 4, or 8 ms durations. Addition of a nonzero RT or FT to the gap silent period increased detectability and also increased reaction speed on trials with "Yes" response, but there was no difference in detectability or response speeds between RT and FT conditions. Experiment 2 extended this finding to gaps having two, one, or no abrupt marker edges. These findings suggest that human listeners do not make use of abrupt onset or offset information to enhance gap detection, but seem to rely on the effective sound level reduction associated with the gap for detection.     

Gap detection with sinusoids and noise in normal, impaired, and electrically stimulated ears

Thresholds for the detection of temporal gaps were measured using two types of signals to mark the gaps: bandpass-filtered noises and sinusoids. The first experiment used seven subjects with relatively flat unilateral moderate cochlear hearing loss. The normal ear of each subject was tested both at the same sound-pressure level (SPL) as the impaired ear, and at the same sensation level (SL). Background noise was used to mask spectral "splatter" associated with the gap. For the noise markers, gap thresholds tended to be larger for the impaired ears than for the normal ears when the comparison was made at equal SPL; the difference was reduced, but not eliminated, when the comparison was made at equal SL. Gap thresholds for both the normal and impaired ears decreased as the center frequency increased from 0.5 to 2.0 kHz. For the sinusoidal markers, gap thresholds were often similar for the normal and impaired ears when tested at equal SPL, and were larger for the normal ears when tested at equal SL. Gap thresholds did not change systematically with frequency. Gap thresholds using sinusoidal markers were smaller than those using noise markers. In the second experiment, three subjects with single-channel cochlear implants were tested. Gap thresholds for noise bands tended to increase with increasing center frequency when the noise bandwidth was fixed, and to decrease with increasing bandwidth when the center frequency was fixed. Gap thresholds for sinusoids did not change with center frequency, but decreased markedly with increasing level. Gap thresholds for sinusoids were considerably smaller than those for noise bands.(ABSTRACT TRUNCATED AT 250 WORDS)     

Discrimination of sound source velocity in human listeners

The ability of six human subjects to discriminate the velocity of moving sound sources was examined using broadband stimuli presented in virtual auditory space. Subjects were presented with two successive stimuli moving in the frontal horizontal plane level with the ears, and were required to judge which moved the fastest. Discrimination thresholds were calculated for reference velocities of 15, 30, and 60 degrees/s under three stimulus conditions. In one condition, stimuli were centered on 0 degrees azimuth and their duration varied randomly to prevent subjects from using displacement as an indicator of velocity. Performance varied between subjects giving median thresholds of 5.5, 9.1, and 14.8 degrees/s for the three reference velocities, respectively. In a second condition, pairs of stimuli were presented for a constant duration and subjects would have been able to use displacement to assist their judgment as faster stimuli traveled further. It was found that thresholds decreased significantly for all velocities (3.8, 7.1, and 9.8 degrees/s), suggesting that the subjects were using the additional displacement cue. The third condition differed from the second in that the stimuli were "anchored" on the same starting location rather than centered on the midline, thus doubling the spatial offset between stimulus endpoints. Subjects showed the lowest thresholds in this condition (2.9, 4.0, and 7.0 degrees/s). The results suggested that the auditory system is sensitive to velocity per se, but velocity comparisons are greatly aided if displacement cues are present.     

Use of context by young and aged adults with normal hearing

Word recognition in sentences with and without context was measured in young and aged subjects with normal but not identical audiograms. Benefit derived from context by older adults has been obscured, in part, by the confounding effect of even mildly elevated thresholds, especially as listening conditions vary in difficulty. This problem was addressed here by precisely controlling signal-to-noise ratio across conditions and by accounting for individual differences in signal-to-noise ratio. Pure-tone thresholds and word recognition were measured in quiet and threshold-shaped maskers that shifted quiet thresholds by 20 and 40 dB. Word recognition was measured at several speech levels in each condition. Threshold was defined as the speech level (or signal-to-noise ratio) corresponding to the 50 rau point on the psychometric function. As expected, thresholds and slopes of psychometric functions were different for sentences with context compared to those for sentences without context. These differences were equivalent for young and aged subjects. Individual differences in word recognition among all subjects, young and aged, were accounted for by individual differences in signal-to-noise ratio. With signal-to-noise ratio held constant, word recognition for all subjects remained constant or decreased only slightly as speech and noise levels increased. These results suggest that, given equivalent speech audibility, older and younger listeners derive equivalent benefit from context.     

Vibrotactile temporal gap detection as a function of age

The ability of subjects to detect temporal gaps between bursts of sinusoids or bursts of bandlimited noise was measured to evaluate the phenomenon of tactile "sensory persistence" in older persons. Vibratory stimuli were delivered to the right thenar eminence of 27 subjects ranging in age from 8-75 years. The subjects' task was to detect the presence of a silent interval or "gap" between flanking 350-ms vibrotactile stimuli. The gap-detection threshold, expressed as the amplitude of vibration relative to the absolute detection threshold, decreased as the gap duration increased and was higher for gaps in noise than for gaps in sinusoids. The threshold for detecting short gaps increased with age for noise stimuli, but not for sinusoidal stimuli. Furthermore, the gap-detection threshold recovered more rapidly in older subjects for noise stimuli, but less rapidly in older subjects for sinusoidal stimuli. Because of these differences, it appears that the effects of age on gap detection cannot be due to a simple increase in sensory persistence, but may be due to multiple processes.     

Forward masking among infant and adult listeners

Psychophysical forward-masked thresholds were estimated for 3- and 6-month-old infants and for adults. Listeners detected a repeated 1000-Hz probe, with 16-ms rise time, no steady-state duration, and 16-ms fall time. Unmasked thresholds were determined for one group of listeners who were trained to respond when they heard the probe but not at other times. In the masking conditions, each tone burst was preceded by a 100-ms broadband noise masker at 65 dB SPL. Listeners were trained to respond when they heard the probe and masker, but not when they heard the masker alone. The masker-probe interval, delta t, was either 5, 10, 25, or 200 ms. Four groups of subjects listened in the masked conditions, each at one value of delta t. Each listener attempted to complete a block of 32 trials including four probe levels chosen to span the range of expected thresholds. "Group" thresholds, based on average psychometric functions, as well as thresholds for individual listeners, were estimated. Both group and individual thresholds declined with delta t, as expected, for both infants and adults. Infants' masked thresholds were higher than those of adults, and comparison of masked to unmasked thresholds suggested that infants demonstrate more forward masking than adults, particularly at short delta t. Forward masking appeared to have greater effects on 3-month-olds' detection than on either 6-month-olds' or adults'. Compared to adults, 6-month-olds demonstrated more forward masking only for delta t of 5 ms. Thus, susceptibility to forward masking may be nearly mature by 6 months of age.     

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.     

Effects of age and frequency disparity on gap discrimination

Temporal discrimination was measured using a gap discrimination paradigm for three groups of listeners with normal hearing: (1) ages 18-30, (2) ages 40-52, and (3) ages 62-74 years. Normal hearing was defined as pure-tone thresholds < or = 25 dB HL from 250 to 6000 Hz and < or = 30 dB HL at 8000 Hz. Silent gaps were placed between 1/4-octave bands of noise centered at one of six frequencies. The noise band markers were paired so that the center frequency of the leading marker was fixed at 2000 Hz, and the center frequency of the trailing marker varied randomly across experimental runs. Gap duration discrimination was significantly poorer for older listeners than for young and middle-aged listeners, and the performance of the young and middle-aged listeners did not differ significantly. Age group differences were more apparent for the more frequency-disparate stimuli (2000-Hz leading marker followed by a 500-Hz trailing marker) than for the fixed-frequency stimuli (2000-Hz lead and 2000-Hz trail). The gap duration difference limens of the older listeners increased more rapidly with frequency disparity than those of the other listeners. Because age effects were more apparent for the more frequency-disparate conditions, and gap discrimination was not affected by differences in hearing sensitivity among listeners, it is suggested that gap discrimination depends upon temporal mechanisms that deteriorate with age and stimulus complexity but are unaffected by hearing loss.     

Age differences for stop-consonant and vowel perception in adults

The purpose of this study was to determine the role of static, dynamic, and integrated cues for perception in three adult age groups, and to determine whether age has an effect on both consonant and vowel perception, as predicted by the "age-related deficit hypothesis." Eight adult subjects in each of the age ranges of young (ages 20-26), middle aged (ages 52-59), and old (ages 70-76) listened to synthesized syllables composed of combinations of [b d g] and [i u a]. The synthesis parameters included manipulations of the following stimulus variables: formant transition (moving or straight), noise burst (present or absent), and voicing duration (10, 30, or 46 ms). Vowel perception was high across all conditions and there were no significant differences among age groups. Consonant identification showed a definite effect of age. Young and middle-aged adults were significantly better than older adults at identifying consonants from secondary cues only. Older adults relied on the integration of static and dynamic cues to a greater extent than younger and middle-aged listeners for identification of place of articulation of stop consonants. Duration facilitated correct stop-consonant identification in the young and middle-aged groups for the no-burst conditions, but not in the old group. These findings for the duration of stop-consonant transitions indicate reductions in processing speed with age. In general, the results did not support the age-related deficit hypothesis for adult identification of vowels and consonants from dynamic spectral cues.