Effect of age on detection of gaps in speech and nonspeech markers varying in duration and spectral symmetry

Gap detection thresholds for speech and analogous nonspeech stimuli were determined in younger and older adults with clinically normal hearing in the speech range. Gap detection thresholds were larger for older than for younger listeners in all conditions, with the size of the age difference increasing with stimulus complexity. For both ages, gap detection thresholds were far smaller when the markers before and after the gap were the same (spectrally symmetrical) compared to when they were different (spectrally asymmetrical) for both speech and nonspeech stimuli. Moreover, gap detection thresholds were smaller for nonspeech than for speech stimuli when the markers were spectrally symmetrical but the opposite was observed when the markers were spectrally asymmetrical. This pattern of results may reflect the benefit of activating well-learned gap-dependent phonemic contrasts. The stimulus-dependent age effects were interpreted as reflecting the differential effects of age-dependent losses in temporal processing ability on within- and between-channel gap detection.     

Detection of silent temporal gaps in sinusoidal markers

Gap detection thresholds were measured by forced-choice procedure for conditions where the duration of a silent gap was varied adaptively between pairs of sinusoidal markers of the same or different frequency. Frequencies of the first sinusoid in a pair of markers ranged from F1 = 500 to 4000 Hz. Second-sinusoid marker frequencies F2 included F1 = F2, and usually frequencies 2%, 5%, 24%, and 50% higher than F1. In preliminary studies the role of presentation level (E/N0) on gap detection was considered. Preliminary data revealed confounding extraneous factors arising from gating transients and from overall stimulus (i.e., markers + gap) and/or masker duration cues. In the main experiments, the contributions of these extraneous cues were evaluated with experimental designs aimed at identifying and minimizing the confounding roles of these cues in gap detection. For conditions where extraneous gating transient cues were minimized (by presenting the sinusoidal markers in a continuous noise masker with random onset phase for the second sinusoid in every pair of markers) and overall stimulus duration cues were diminished (by randomizing the duration of each marker independently), gap detection thresholds increased from 5 to 90 ms as the frequency separation between F1 and F2 was increased by half an octave. When the gap detection thresholds were treated as filter attenuation values by normalizing and converting the data into decibels, the data were closely fit by the roex filter model. On average, the listeners' performances were modeled well by a constant-percentage (7%) bandwidth filter centered on F1.     

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.     

Gap detection thresholds as a function of tonal duration for younger and older listeners.

Twenty normal hearing younger and twenty older adults in the early stages of presbycusis, but with relatively normal hearing at 2 kHz, were asked to discriminate between the presence versus absence of a gap between two equal-duration tonal markers. The duration of each marker was constant within a block of trials but varied between 0.83 and 500 ms across blocks. Notched-noise, centered at 2 kHz, was used to mask on- and off-transients. Gap detection thresholds of older adults were markedly higher than those of younger adults for marker durations of less than 250 ms but converged on those of younger adults at 500 ms. For both age groups, gap detection thresholds were independent of audiometric thresholds. These results indicate that older adults have more difficulty detecting a gap than younger adults when short marker durations (i.e., durations characteristic of speech sounds) are employed. It is shown that these results cannot be explained by linear models of temporal processing but are consistent with differential adaptation effects in younger and older adults.     

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.     

Gap detection for similar and dissimilar gap markers

Detection thresholds for temporal gaps between markers of dissimilar frequency are usually elevated with respect to thresholds for gaps between markers of similar frequency. Because gaps between markers of dissimilar frequency represent both a spectrally based perceptual discontinuity as well as a temporal discontinuity, it is not clear what factors underlie the threshold elevation. This study sought to examine the effects of perceptual dissimilarities on gap detection. The first experiment measured gap detection for configurations of narrow-band gap markers comprised of pure tones, frequency-modulated tones, and amplitude-modulated tones. The results showed that gap thresholds for frequency-disparate pure-tone markers were elevated with respect to isofrequency tonal markers, but that perceptual discontinuities between markers restricted to the same frequency region did not uniformly elevate threshold. The second experiment measured gap detection for configurations of markers where the leading and trailing markers could differ along the dimensions of bandwidth, duration, and pitch. The results showed that, in most cases, gap detection deteriorated when the bandwidth of the two markers differed, even when the spectral content of the narrower-band marker was completely subsumed by the spectral content of the wider-band marker. This finding suggests that gap detection is sensitive to spectral dissimilarity between markers in addition to spectral discontinuity. The effects of marker duration depended on the marker bandwidth. Pitch differences across spectrally similar markers had no effect.     

Gap duration discrimination for frequency-asymmetric gap markers: psychophysical and electrophysiological findings

This study investigated gap duration discrimination (GDD) for frequency-asymmetric gap markers, where one marker was a two-tone complex consisting of a primary tone and a secondary tone, and the other marker was the primary tone alone. Three experiments were undertaken to examine the order effect wherein performance is better when the two-tone marker is the leading marker than when it is the trailing marker. Experiment 1 demonstrated that GDD for frequency-asymmetric markers is intermediate between the boundaries of within-frequency-channel versus across-frequency-channel processing. Experiment 2 compared psychophysical performance with auditory brainstem responses (ABRs) elicited by the same stimuli. Whereas GDD thresholds were elevated for a complex trailing marker relative to a within-frequency-channel baseline, ABRs elicited by the complex marker were more robust. Experiment 3 tested the hypothesis that poor GDD performance with frequency-asymmetric markers is due to some form of nonenergetic, or informational, masking. The results did not support a role for informational masking conferred by synthetic listening; however, informational masking conferred by the occurrence of novel spectral events provided a parsimonious account. One possible interpretation is that the capacity to accurately encode a gap is undermined by the occurrence of novel spectral events that engage limited attentional resources.     

Gap detection as a measure of electrode interaction in cochlear implants.

Gap detection thresholds were measured as an indication of the amount of interaction between electrodes in a cochlear implant. The hypothesis in this study was as follows: when the two stimuli that bound the gap stimulate the same electrode, and thus the same neural population, the gap detection threshold will be short. As two stimuli are presented to two electrodes that are more widely separated, the amount of neural overlap of the two stimuli decreases, the stimuli sound more dissimilar, and the gap thresholds increase. Gap detection thresholds can thus be used to infer the amount of overlap in neural populations stimulated by two electrodes. Three users of the Nucleus cochlear implant participated in this study. Gap detection thresholds were measured as a function of the distance between the two electrode pairs and as a function of the spacing between the two electrodes of a bipolar pair (i.e., using different modes of stimulation). The results indicate that measuring gap detection thresholds may provide an estimate of the amount of electrode interaction. Gap detection thresholds were a function of the physical separation of the electrode pairs used for the two stimuli that bound the gap. Lower gap thresholds were observed when the two electrode pairs were closely spaced, and gap thresholds increased as the separation increased, resulting in a "psychophysical tuning curve" as a function of electrode separation. The sharpness of tuning varied across subjects, and for the three subjects in this study, the tuning was generally sharper for the subjects with better speech recognition. The data also indicate that increasing the separation between active and reference electrodes has limited effect on spatial selectivity (or tuning) as measured perceptually.     

Influence of spatial and temporal coding on auditory gap detection

This study investigated the effect on gap detection of perceptual channels, hypothesized to be tuned to spatial location or fundamental frequency (f0). Thresholds were measured for the detection of a silent temporal gap between two markers. In the first experiment, the markers were broadband noise, presented either binaurally or monaurally. In the binaural conditions, the markers were either diotic, or had a 640-micros interaural time difference (ITD) or a 12-dB interaural level difference (ILD). Reversing the ITD across the two markers had no effect on gap detection relative to the diotic condition. Reversing the ILD across the two markers produced a marked deterioration in performance. However, the same deterioration was observed in the monaural conditions when a 12-dB level difference was introduced between the two markers. The results provide no evidence for the role of spatially tuned neural channels in gap detection. In the second experiment, the markers were harmonic tone complexes, filtered to contain only high, unresolved harmonics. Using complexes with a fixed spectral envelope, where the f0 (of 140 or 350 Hz) was different for the two markers, produced a deterioration in performance, relative to conditions where the f0 remained the same. A larger deterioration was observed when the two markers occupied different spectral regions but had the same f0. This supports the idea that peripheral coding is dominant in determining gap-detection thresholds when the two markers differ along any physical dimension. Higher-order neural coding mechanisms of f0 and spatial location seem to play a smaller role and no role, respectively.     

Relationships among age-related differences in gap detection and word recognition

The relationships among age-related differences in gap detection and word recognition in subjects with normal hearing or mild sensorineural hearing loss were explored in two studies. In the first study, gap thresholds were obtained for 40 younger and 40 older subjects. The gaps were carried by 150-ms, modulated, low-pass noise bursts with cutoff frequencies of 1 or 6 kHz. The noise bursts were presented at an overall level of 80 dB SPL in three background conditions. Mean gap thresholds ranged between 2.6 and 7.8 ms for the younger age group and between 3.4 and 10.0 ms for the older group. Mean gap thresholds were significantly larger for the older group in all six conditions. Gap thresholds were not significantly correlated with audiometric thresholds in either age group but the 1-kHz gap thresholds increased with age in the younger group. In the second study, the relationships among gap thresholds, spondee-in-babble thresholds, and audiometric thresholds of 66 subjects were examined. Compared with the older subjects, the younger group recognized the spondees at significantly lower (more difficult) spondee-to-babble ratios. In the younger group, spondee-in-babble thresholds were significantly correlated with gap thresholds in conditions of high-frequency masking. In the older group, spondee-in-babble thresholds, gap thresholds, and audiometric thresholds were not significantly correlated, but the spondee-in-babble thresholds and two audiometric thresholds increased significantly with age. These results demonstrate that significant age-related changes in auditory processing occur throughout adulthood. Specifically, age-related changes in temporal acuity may begin decades earlier than age-related changes in word recognition.     

The role of frequency selectivity in measures of auditory and vibrotactile temporal resolution.

The purpose of this study was to compare the role of frequency selectivity in measures of auditory and vibrotactile temporal resolution. In the first experiment, temporal modulation transfer functions for a sinusoidally amplitude modulated (SAM) 250-Hz carrier revealed auditory modulation thresholds significantly lower than corresponding vibrotactile modulation thresholds at SAM frequencies greater than or equal to 100 Hz. In the second experiment, auditory and vibrotactile gap detection thresholds were measured by presenting silent gaps bounded by markers of the same or different frequency. The marker frequency F1 = 250 Hz preceded the silent gap and marker frequencies after the silent gap included F2 = 250, 255, 263, 310, and 325 Hz. Auditory gap detection thresholds were lower than corresponding vibrotactile thresholds for F2 markers less than or equal to 263 Hz, but were greater than the corresponding vibrotactile gap detection thresholds for F2 markers greater than or equal to 310 Hz. When the auditory gap detection thresholds were transformed into filter attenuation values, the results were modeled well by a constant-percentage (10%) bandwidth filter centered on F1. The vibrotactile gap detection thresholds, however, were independent of marker frequency separation. In a third experiment, auditory and vibrotactile rate difference limens (RDLs) were measured for a 250-Hz carrier at SAM rates less than or equal to 100 Hz. Auditory RDLs were lower than corresponding vibrotactile RDLs for standard rates greater than 10 Hz. Combination tones may have confounded auditory performance for standard rates of 80 and 100 Hz. The results from these experiments revealed that frequency selectivity influences auditory measures of temporal resolution, but there was no evidence of frequency selectivity affecting vibrotactile temporal resolution.     

Psychometric functions for gap detection in noise measured from young and aged subjects.

Psychometric functions for gap detection of temporal gaps in wideband noise were measured in a "yes/no" paradigm from normal-hearing young and aged subjects with closely matched audiograms. The effects of noise-burst duration, gap location, and uncertainty of gap location were tested. A typical psychometric function obtained in this study featured a steep slope, which was independent of most experimental conditions as well as age. However, gap thresholds were generally improved with increasing duration of the noise burst for both young and aged subjects. Gap location and uncertainty had no significant effects on the thresholds for the young subjects. For the aged subjects, whenever the gap was sufficiently away from the onset or offset of the noise burst, detectability was robust despite uncertainty about the gap location. Significant differences between young and aged subjects could be observed only when the gap was very close to the signal onset and offset.     

Modulation and gap detection for broadband and filtered noise signals

Modulation detection thresholds (as a function of sinusoidal amplitude modulation frequency) and temporal gap detection thresholds were measured for three low-pass-filtered noise signals (fc = 1000, 2000, and 4000 Hz), a high-pass-filtered noise signal (fc = 4000 Hz), and a broadband signal. The two latter noise signals were effectively low-pass filtered (fc = 6500 Hz) by the earphone. Each of the filtered signals was presented with a complementary filtered noise masker. Modulation and gap detection thresholds were lowest for the broadband and high-pass signals. Thresholds were significantly higher for the low-pass signals than for the broadband and high-pass signals. For these tasks and conditions, the high-frequency content of the noise signal was more important than was the signal bandwidth. Sensitivity (s) and time constant (tau) indices were derived from functions fitted to the modulation detection data. These indices were compared with gap detection thresholds for corresponding signals. The gap detection thresholds were correlated inversely (rho = -1.0, p less than 0.05) with s (i.e., smaller gap detection thresholds were correlated with greater sensitivity to modulation), but were not correlated significantly with tau, which was relatively invariant across signal conditions.     

Temporal resolution and temporal masking properties of transient stimuli: data and an auditory model.

Temporal resolution is often measured using the detection of temporal gaps or signals in temporal gaps embedded in long-duration stimuli. In this study, psychoacoustical paradigms are developed for measuring the temporal encoding of transient stimuli. The stimuli consisted of very short pips which, in two experiments, contained a steady state portion. The carrier was high-pass filtered, dynamically compressed noise, refreshed for every stimulus presentation. The first experiment shows that, with these very short stimuli, gap detection thresholds are about the same as obtained in previous investigations. Experiments II and III show that, using the same stimuli, temporal-separation thresholds and duration-discrimination thresholds are better than gap-detection thresholds. Experiment IV investigates the significance of residual spectral cues for the listeners' performance. In experiment V, temporal separation thresholds were measured as a function of the signal-pip sensation level (SL) in both forward- and backward-masking conditions. The separation thresholds show a strong temporal asymmetry with good separation thresholds independent of signal-pip SL in backward-masking conditions and increasing separation thresholds with decreasing signal-pip SL in forward-masking conditions. A model of the auditory periphery is used to stimulate the gap-detection and temporal-separation thresholds quantitatively. By varying parameters like auditory-filter width and transduction time constants, the model provides some insight into how the peripheral auditory system may cope with temporal processing tasks and thus represents a more physiology-related complement to current models of temporal processing.     

Detection and direction-discrimination of diotic and dichotic ramp modulations in amplitude and phase

When the source of a tone moves with respect to a listener's ears, dichotic (or interaural) phase and amplitude modulations (PM and AM) are produced. Two experiments investigated the psychophysical characteristics of dichotic linear ramp modulations in phase and amplitude, and compared them with the psychophysics of diotic PM and AM. In experiment 1, subjects were substantially more sensitive to dichotic PM than diotic PM, but AM sensitivity was equivalent in the dichotic and diotic conditions. Thresholds for discriminating modulation direction were smaller than detection thresholds for dichotic AM, and both diotic AM and PM. Dichotic PM discrimination thresholds were similar to detection thresholds. In experiment 2, the effects of ramp duration were examined. Sensitivity to dichotic AM and PM, and diotic AM increased as duration was increased from 20 ms to 200 ms. The functions relating sensitivity to ramp duration differed across the stimuli; sensitivity to dichotic PM increased more rapidly than sensitivity to dichotic or diotic AM. This was also reflected in shorter time-constants and minimum integration times for dichotic PM detection. These findings support the hypothesis that the analysis of dichotic PM and AM rely on separate mechanisms.     

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)     

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.     

The effects of neural synchronization and peripheral compression on the acoustic-reflex threshold

This study investigates the acoustic reflex threshold (ART) dependency on stimulus phase utilizing low-level reflex audiometry [Neumann et al., Audiol. Neuro-Otol. 1, 359-369 (1996)]. The goal is to obtain optimal broadband stimuli for elicitation of the acoustic reflex and to obtain objective determinations of cochlear hearing loss. Three types of tone complexes with different phase characteristics were investigated: A stimulus that compensates for basilar-membrane dispersion, thus causing a large overall neural synchrony (basilar-membrane tone complex-BMTC), the temporally inversed stimulus (iBMTC), and random-phase tone complexes (rTC). The ARTs were measured in eight normal-hearing and six hearing-impaired subjects. Five different conditions of peak amplitude and stimulus repetition rate were used for each stimulus type. The results of the present study suggest that the ART is influenced by at least two different factors: (a) the degree of synchrony of neural activity across frequency, and (b) the fast-acting compression mechanism in the cochlea that is reduced in the case of a sensorineural hearing loss. The results allow a clear distinction of the two subjects groups based on the different ART for the utilized types and conditions of the stimuli. These differences might be useful for objective recruitment detection in clinical diagnostics.     

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.     

The detection of temporal gaps as a function of frequency region and absolute noise bandwidth.

Temporal gap detection was measured as a function of absolute signal bandwidth at a low-, a mid-, and a high-frequency region in six listeners with normal hearing sensitivity. Gap detection threshold decreased monotonically with increasing stimulus bandwidth at each of the three frequency regions. Given conditions of equivalent absolute bandwidth, gap detection thresholds were not significantly different for upper cutoff frequencies ranging from 600 to 4400 Hz. A second experiment investigated gap detection thresholds at two pressure-spectrum levels, conditions typically resulting in substantially different estimates of frequency selectivity. Estimates of frequency selectivity were collected at the two levels using a notched-noise masker technique. The gap threshold-signal bandwidth functions were almost identical at pressure-spectrum levels of 70 dB and 40 dB for the two subjects in experiment II, while estimates of frequency selectivity showed poorer frequency selectivity at the 70-dB level than at 40 dB. Data from both experiments indicated that gap detection in bandlimited noise was inversely related to signal bandwidth and that gap detection did not vary significantly with changes in signal frequency over the range of 600 to 4400 Hz. Over the range of frequencies investigated, the results indicated no clear relation between gap detection for noise stimuli and peripheral auditory filtering.