Peter J. Fitzgibbons

Gap detection in normal and hearing‐impaired listeners

Temporal resolution, estimated by measuring the minimum detectable gap (delta t ms) separating two successive signals, was assessed in five normal-hearing and five cochlear-impaired listeners. The signals were octave-band noises (400-800 Hz, 800-1600 Hz, and 2000-4000 Hz) presented in a background of continuous, broadband notched noise that was applied to eliminate unwanted spectral cues. Temporal resolution in all listeners showed systematic improvement with an increase in octave-band center frequency. Resolution in the hearing-impaired subjects was significantly poorer than normal regardless of whether the comparisons were made at equal sound pressure level or at equal sensation level.

Temporal gap detection in noise as a function of frequency, bandwidth, and level

Temporal gap resolution is measured with Békésy tracking procedure and filtered noise stimuli in the frequency range below 6000 Hz. Stimulus parameters include high-pass and low-pass cutoff frequency, band center frequency, bandwidth in a 2-oct range, and signal level in the low-to-moderate intensity range. The pattern of results indicates that gap resolution improves with an increase in stimulus frequency in a manner that can be described by a linear function with a slope of about 2 ms/oct. This relationship applies to signal levels greater than 25--30 dB SL. A linear trend also describes gap threshold as a function of the empirical critical bandwidth within the same frequency range. Implications of the results for simple functional models of temporal processing are examined.

AGE EFFECTS ON DURATION DISCRIMINATION WITH SIMPLE AND COMPLEX STIMULI

This study examined age-related changes in temporal processing by measuring DLs for signal duration using simple and complex stimuli. Previous research has shown that elderly listeners exhibit difficulty discriminating duration changes in simple sounds, suggesting the possibility of age-related changes in central timing mechanisms. The present experiments examined the interactive effects of aging, hearing loss, and stimulus complexity on duration discrimination. Four groups participated: young and elderly listeners with normal hearing, and young and elderly listeners with hearing loss. Duration DLs were measured for 250-ms tone bursts and for silent gaps between tone bursts that were presented either in isolation or embedded as target stimuli within tonal sequences. The tone sequences were composed of five sequential 250-ms components. Stimulus complexity was varied by changing the sequential order of tone frequencies and the location of an embedded target component across listening conditions. Analyses of results revealed the following: Elderly listeners performed more poorly than younger listeners in nearly all stimulus conditions, the effects of stimulus complexity on discrimination were greatest among elderly listeners, and hearing loss had no systematic effect on discrimination performance.

Age-related differences in identification and discrimination of temporal cues in speech segments

This study investigated age-related differences in sensitivity to temporal cues in modified natural speech sounds. Listeners included young noise-masked subjects, elderly normal-hearing subjects, and elderly hearing-impaired subjects. Four speech continua were presented to listeners, with stimuli from each continuum varying in a single temporal dimension. The acoustic cues varied in separate continua were voice-onset time, vowel duration, silence duration, and transition duration. In separate conditions, the listeners identified the word stimuli, discriminated two stimuli in a same-different paradigm, and discriminated two stimuli in a 3-interval, 2-alternative forced-choice procedure. Results showed age-related differences in the identification function crossover points for the continua that varied in silence duration and transition duration. All listeners demonstrated shorter difference limens (DLs) for the three-interval paradigm than the two-interval paradigm, with older hearing-impaired listeners showing larger DLs than the other listener groups for the silence duration cue. The findings support the general hypothesis that aging can influence the processing of specific temporal cues that are related to consonant manner distinctions.

Effects of stimulus and noise rate variability on speech perception by younger and older adults

The present experiments examine the effects of listener age and hearing sensitivity on the ability to understand temporally altered speech in quiet when the proportion of a sentence processed by time compression is varied. Additional conditions in noise investigate whether or not listeners are affected by alterations in the presentation rate of background speech babble, relative to the presentation rate of the target speech signal. Younger and older adults with normal hearing and with mild-to-moderate sensorineural hearing losses served as listeners. Speech stimuli included sentences, syntactic sets, and random-order words. Presentation rate was altered via time compression applied to the entire stimulus or to selected phrases within the stimulus. Older listeners performed more poorly than younger listeners in most conditions involving time compression, and their performance decreased progressively with the proportion of the stimulus that was processed with time compression. Older listeners also performed more poorly than younger listeners in all noise conditions, but both age groups demonstrated better performance in conditions incorporating a mismatch in the presentation rate between target signal and background babble compared to conditions with matched rates. The age effects in quiet are consistent with the generalized slowing hypothesis of aging. Performance patterns in noise tentatively support the notion that altered rates of speech signal and background babble may provide a cue to enhance auditory figure-ground perception by both younger and older listeners.

Aging and temporal discrimination in auditory sequences

This study examined age-related changes in temporal sensitivity to increments in the inter-onset intervals (IOI) of successive components in tonal sequences. Temporal discrimination was examined using reference stimulus patterns consisting of five 50-ms, 4000-Hz components with equal tonal IOIs selected from the range 100-600 ms. Discrimination was examined in separate conditions by measuring the relative difference limen (DL) for increments of tonal IOI in comparison sequences. In some conditions, comparison sequences featured equal increments of all tonal lOIs to examined listener sensitivity to uniform changes of sequence rate, or tempo. Other conditions measured the DL for increments of a single target IOI within otherwise uniform-rate comparison sequences. For these measurements, the single target IOI was either fixed in sequence location, or randomized in location across listening trials. Listeners in the study included four groups of young and elderly adults with and without high-frequency hearing loss. The results for all listeners showed the relative DL for rate discrimination to decrease from a maximum at the 100-ms IOI to a smaller stable value across the range of longer sequence IOI. All listeners also exhibited larger relative DLs for discrimination of single target intervals compared to rate discrimination for equivalent reference IOI values. Older listeners showed poorer performance than younger listeners in all conditions, with the largest age differences observed for discrimination of brief single intervals that were varied randomly in sequence location. None of the results revealed significant effects of hearing loss on performance of younger and older listeners.

Age effects on discrimination of timing in auditory sequences

The experiments examined age-related changes in temporal sensitivity to increments in the interonset intervals (IOI) of components in tonal sequences. Discrimination was examined using reference sequences consisting of five 50-ms tones separated by silent intervals; tone frequencies were either fixed at 4 kHz or varied within a 2-4-kHz range to produce spectrally complex patterns. The tonal IOIs within the reference sequences were either equal (200 or 600 ms) or varied individually with an average value of 200 or 600 ms to produce temporally complex patterns. The difference limen (DL) for increments of IOI was measured. Comparison sequences featured either equal increments in all tonal IOIs or increments in a single target IOI, with the sequential location of the target changing randomly across trials. Four groups of younger and older adults with and without sensorineural hearing loss participated. Results indicated that DLs for uniform changes of sequence rate were smaller than DLs for single target intervals, with the largest DLs observed for single targets embedded within temporally complex sequences. Older listeners performed more poorly than younger listeners in all conditions, but the largest age-related differences were observed for temporally complex stimulus conditions. No systematic effects of hearing loss were observed.

Age Effects on Measures of Hearing Disability

This study investigated the effects of age on selfperceived hearing disability among young and elderly people with comparable hearing sensitivity. Subjects were young (18-40 yr) and elderly (65-75 yr) listeners with either normal hearing sensitivity or mild-to-moderate sloping sensorineural hearing loss. The Hearing Handicap Inventory for the Elderly was presented to the older subjects and the Hearing Handicap Inventory for Adults was presented to the younger subjects. Statistical analyses revealed an interaction between age and hearing loss, in which younger subjects with hearing loss reported more handicapping effects of sensitivity loss than the elderly subjects with hearing loss. This age effect was not attributed to differences in hearing sensitivity between the young and elderly subjects with hearing impairment.

Temporal gap resolution in listeners with high-frequency sensorineural hearing loss.

Temporal gap resolution was measured in five normal-hearing listeners and five cochlear-impaired listeners, whose sensitivity losses were restricted to the frequency regions above 1000 Hz. The stimuli included a broadband noise and three octave band noises centered at 0.5, 1.0, and 4.0 kHz. Results for the normal-hearing subjects agree with previous findings and reveal that gap resolution improves progressively with an increase in signal frequency. Gap resolution in the impaired listeners was significantly poorer than normal for all signals including those that stimulated frequency regions with normal pure-tone sensitivity. Smallest gap thresholds for the impaired listeners were observed with the broadband signal at high levels. This result agrees with data from other experiments and confirms the importance of high-frequency signal audibility in gap detection. The octave band data reveal that resolution deficits can be quite large within restricted frequency regions, even those with minimal sensitivity loss.

Age-related differences in discrimination of an interval separating onsets of successive tone bursts as a function of interval duration

The study measured listener sensitivity to increments in the inter-onset interval (IOI) separating pairs of successive 20-ms 4000-Hz tone pulses. A silent interval between the tone pulses was adjusted across conditions to create reference tonal IOI values of 25-600 ms. For each condition, a duration DL for increments of the tonal IOI was measured in listeners comprised of young normal-hearing adults and two groups of older adults with and without high-frequency hearing loss. Discrimination performance of all listeners was poorest for the shorter reference IOIs, and improved to stable levels for longer reference intervals exceeding about 200 ms. Temporal sensitivity of the young listeners was significantly better than that of the elderly listeners in each condition, with the largest age-related differences observed for the shortest reference interval. Age-related differences were also observed for duration DLs measured using single 4000-Hz tone bursts set to three reference durations in the range 50-200 ms. The tone DLs of all listeners were smaller than the corresponding tone-pair IOI DLs, particularly for the shorter reference stimulus durations. There were no significant performance differences observed between the older listeners with and without hearing loss for either discrimination task.