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 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.

Age-related changes in temporal gap detection.

The purpose of this study was to clarify and extend the results of earlier studies of age-related effects on temporal resolution by precisely matching young and old subjects with normal hearing and measuring gap thresholds in a variety of listening conditions. Younger subjects were between 17 and 40 years of age, older subjects between 64 and 77 years. Signals were noisebursts which varied in upper-cutoff frequency, overall level, and sinusoidal-amplitude-modulation depth. Signals were presented in quiet, in a noise floor, and with a gated-high-frequency masker in a noise floor. Significant main effects were found for signal frequency, intensity, modulation, age, and background condition. Mean gap thresholds ranged between 2.1 and 10.1 ms and were larger for the older subjects in all 24 conditions. In some conditions, introduction of a noise floor increased the gap thresholds of the older subjects relative to those of the younger. Analyses of individual data support the conclusion that the mean differences between groups reflect shifts in the distributions of gap thresholds of the older subjects towards poorer temporal resolution.

The effects of signal frequency and absolute bandwidth on gap detection in noise.

Whether temporal resolution in noisebursts is primarily determined by the highest frequency component in the signal or its absolute bandwidth remains unclear. In this study, the absolute bandwidths and upper cutoff frequencies of signal noisebursts were varied across broad frequency ranges, several times greater than previously jointly studied. The purpose was to determine how each independently affects detection, taking into consideration that bandwidth effects at one signal frequency might be very different from bandwidth effects at another. Gap detection thresholds were obtained for five subjects with normal hearing in a 2 IFC paradigm. Signals were noisebursts whose bandwidths and upper cutoff frequencies varied among 1, 2, 4, 8, and 12 kHz. Their duration was 150 ms and they were presented at an overall level of 75 dB SPL above a 45 dB SPL white noise floor. The largest mean gap detection threshold, 6.98 ms, was obtained for a noiseburst with a bandwidth of 1 kHz and upper cutoff frequency of 12 kHz. The smallest mean gap detection threshold, 2.22 ms, was found with a bandwidth and upper cutoff frequency of 12 kHz. Significant interactions were found to exist between absolute bandwidth and upper cutoff frequency. Although gap detection thresholds generally decreased with increasing signal frequency and bandwidth, the pattern was complex. When the absolute bandwidth was at least one-half the upper cutoff frequency then upper cutoff frequency and not bandwidth determined gap sensitivity; but when the absolute bandwidth was less than one-half of the upper frequency, then both determined gap thresholds.(ABSTRACT TRUNCATED AT 250 WORDS)

Duration discrimination of speech and tonal complex stimuli by normally hearing and hearing-impaired listeners.

The ability to discriminate changes in the length of vowels and tonal complexes (filled intervals) and in the duration of closure in stop consonants and gaps in tonal complexes (unfilled intervals) was studied in three normally hearing and seven severely hearing-impaired listeners. The speech stimuli consisted of the vowels (i, I, u, U, a, A) and the consonants (p, t, k), and the tonal complexes consisted of digitally generated sinusoids at 0.5, 1, and 2 kHz. The signals were presented at conversational levels for each listener group, and a 3IFC adaptive procedure was used to estimate difference limens (DLs). The DLs for speech were similar to those for tonal complex stimuli in both the filled and unfilled conditions. Both normally and impaired-hearing listeners demonstrated greater acuity for changes in the duration of filled than unfilled intervals. Mean thresholds for filled intervals obtained from normally hearing listeners were smaller than those obtained from hearing-impaired listeners. For unfilled intervals, however, the difference between listener groups was not significant. A few hearing-impaired listeners demonstrated temporal acuity comparable to that of normally hearing listeners for several listening conditions. Implications of these results are discussed with regard to speech perception in normally and impaired-hearing individuals.

Efficacy of a distinctive feature model of therapy for hearing-impaired adolescents.

Extensive measures of speech production and perception were secured before and after the individualized training of 75 hearing-impaired postsecondary students to evaluate the efficacy of therapy. Additional measures of hearing, reading, writing, nonverbal intelligence, and manual and simultaneous communication reception were made to explore variables that might influence response to training. Significant improvements in vowel, word and sentence production, and word and sentence perception were observed. Pretherapy measures of speech production and perception were the most powerful factors explaining variability in posttest scores. Other variables predicting the outcome of speech therapy were residual hearing and linguistic competence. Clinical implications are discussed.

Detection of a temporal gap in low-frequency narrow-band signals by normal-hearing and hearing-impaired listeners.

Temporal processing ability in the hearing impaired was investigated in a 2IFC gap-detection paradigm. The stimuli were digitally constructed 50-Hz-wide bands of noise centered at 250, 500, and 1000 Hz. On each trial, two 400-ms noise samples were paired, shaped at onset and offset, filtered, and presented in the quiet with and without a temporal gap. A modified up-down procedure with trial-by-trial feedback was used to establish threshold of detection of the gap. Approximately 4 h of practice preceded data collection; final estimate of threshold was the average of six listening blocks. There were 10 listeners, 19-25 years old. Five had normal hearing; five had a moderate congenital sensorineural hearing loss with relatively flat audiometric configuration. Near threshold (5 dB SL), all listeners performed similarly. At 15 and 25 dB SL, the normal-hearing group performed better than the hearing-impaired group. At 78 dB SPL, equal to the average intensity of the 5-dB SL condition for the hearing impaired, the normal-hearing group continued to improve and demonstrated a frequency effect not seen in the other conditions. Substantial individual differences were found in both groups, though intralistener variability was as small as expected for these narrow-bandwidth signals.