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Effects of age and stimulus frequency on gap discrimination

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Effects of age and stimulus frequency on gap discrimination
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Carlton, Alan
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frequency disparity
frequency region
age
temporal resolution
Dissertations, Academic -- Audiology -- Doctoral -- USF   ( lcsh )
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bibliography   ( marcgt )
theses   ( marcgt )
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Summary:
ABSTRACT: Abstract Objective: Deficits in temporal resolution may underlie the speech understanding difficulties experienced by older listeners in degraded acoustic environments. In real listening environments, important temporal cues are surrounded by stimuli of varying frequency. This study was designed to assess temporal resolution as a function of frequency region, frequency-disparity, and age in listeners with normal hearing. Design: Gap duration difference limens (GDDLs) were measured using leading and trailing markers that were fixed at the same frequency (fixed-frequency) or at frequencies 1/2 octave apart (frequency-disparate) for two groups of listeners with normal hearing: (1) 18-22 years and (2) 55-66 years. Two distinct frequency regions were represented, 500 Hz and 4000 Hz. Results: The results indicated significant effects of age, frequency region, and frequency disparity on GDDLs. Poorer overall performance was observed for the older listeners, the lower frequency region, and the frequency-disparate condition. Conclusions: Gap discrimination is negatively affected by advanced age, lower marker frequencies, and larger marker frequency disparities.
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Includes bibliographical references.
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by Alan Carlton.
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Professional research project (Au.D.)--University of SoutFlorida, 2004.
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Title from PDF of title page.
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ABSTRACT: Abstract Objective: Deficits in temporal resolution may underlie the speech understanding difficulties experienced by older listeners in degraded acoustic environments. In real listening environments, important temporal cues are surrounded by stimuli of varying frequency. This study was designed to assess temporal resolution as a function of frequency region, frequency-disparity, and age in listeners with normal hearing. Design: Gap duration difference limens (GDDLs) were measured using leading and trailing markers that were fixed at the same frequency (fixed-frequency) or at frequencies 1/2 octave apart (frequency-disparate) for two groups of listeners with normal hearing: (1) 18-22 years and (2) 55-66 years. Two distinct frequency regions were represented, 500 Hz and 4000 Hz. Results: The results indicated significant effects of age, frequency region, and frequency disparity on GDDLs. Poorer overall performance was observed for the older listeners, the lower frequency region, and the frequency-disparate condition. Conclusions: Gap discrimination is negatively affected by advanced age, lower marker frequencies, and larger marker frequency disparities.
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Effects of age and stimulus frequency on gap discrimination 1 Effects of age and stimulus frequency on gap discrimination Alan Carlton Audiology Doctoral Project Jennifer Lister, Ph.D., Chair Richard A. Roberts, Ph.D. Judith L. Reese, Ph.D. April 9, 2004 Tampa, Florida Keywords: Temporal Resolu tion, Age, Frequency Region, Frequency Disparity

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Effects of age and stimulus frequency on gap discrimination 2 Abstract Objective: Deficits in temporal resolution may be one element underl ying the speech understanding difficulties experienced by older listeners in degraded acoustic environments In real listening environments, important temporal cues are surrounded by stimuli of varying frequenc ies This study was designed to a ssess temporal resolution as a function of frequency region, frequency disparity, and age in listeners with normal hearing. Desig n: Gap duration difference limens (GDDLs) were measured using leading and trailing markers that were fixed at the same frequency (fixed frequency) or at frequencies one half octave apart (frequency disparate) for two groups of listeners with normal heari ng: (1) 18 22 years and (2) 55 66 years. Two distinct frequency regions were represented, 500 Hz and 4000 Hz. Results: The results indicated significant effects of age, frequency region, and frequency disparity on GDDLs. Poorer overall performance was obs erved for the older listeners, the lower frequency region, and the frequency disparate condition. Conclusions: Gap discrimination is negatively affected by advanced age, lower marker frequencies, and larger marker frequency disparities.

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Effects of age and stimulus frequency on gap discrimination 3 INTRODUCTION Many auditory perceptual abilities decline w ith increasing age and hearing loss, (W illott 1991). Most critical is that listeners with presbycusis have difficulty understanding speech, particularly when that speech is presented in reverberation and noise (e.g. Koehnke & Besing, 1996; Kramer et al., 1998 ; Nabelek & Mason, 1981 ). One factor known to be essential to speech p erception in such everyday environments is temporal resolution (i.e., the ability to follow rapid temporal fluctuations and integrate acoustic stimuli over time ) Often, t he background noise found in everyday listening situations is characterized by fluctuations in intensity over time. It has been suggested that temporal resolution enables a listener to use brief dips in the intensity of interfering noise to understand spee ch in these situations ( Dubno et al., 2003; Oxenham, 2002; Peters et al., 1998 ). In fact, several studies have shown links between temporal resolution and the understanding of acoustically degraded speech ( Gordon Salant & Fitzgibbons, 1993; Irwin & McAuley, 1987; Snell et al., 2002; Tyler et al., 1982 ). Recent literature describes two primary measures of temporal resolution: (1) gap detection, a measure of temporal acuity typically described as a gap detection threshold (GDT) and (2) gap discrimination, a measure of temporal discrimination described here as a gap duration difference limen (GDDL). A GDT is a traditional measure representing the sm allest silent interval in a stimulus that a listener can detect, and GDDL represents the smallest change in the duration of a silent interval that a listener can discriminate. In the traditional GDT task, the standard interval consists of a continuous sig nal or two contiguous signals, and the target interval consists of a signal interrupted by a silent temporal gap of varying duration. Divenyi and Danner (1977) hypothesized that this type

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Effects of age and stimulus frequency on gap discrimination 4 of temporal task may rely on detection of gating transients that are present in the target interval and absent in the standard interval. Therefore, an advantage of using discrimination tasks to measure temporal resolution is that similar gating transients are present in all stimuli (i.e., all stimulus choices are interrupt ed by a silent gap, one of which is longer than the others). Studies of gap discrimination and gap detection suggest that reduced temporal resolution in older listeners may occur independently of peripheral hearing loss (Fitzgibbons & Gordon Salant, 1994; Grose et al., 2001; Lister et al., 2000; Roberts & Lister, 2004). This effect is often attributed to age related changes within the central auditory system and slowed auditory processing ( Fitzgibbons & Gordon Sal ant, 1994; Gordon Salant & Fitzgibbons, 1999 ; Salthouse, 1985 ). Fitzgibbons and Gordon Salant (1994) measured gap discrimination using fixed frequency and frequency disparate tone burst markers centered at 500 and 4000 Hz for four groups of listeners (you ng/older, with/without hearing loss). Gap discrimination was poorer for the older listeners and for the frequency disparate markers. The differences between age groups were larger for the frequency disparate markers than for the fixed frequency markers. Ho wever, no effect of frequency region was observed. Some evidence also exists for hearing loss related deficits in temporal resolution ( Fitzgibbons & Wightman, 1982 ; Florentine & Buus, 1984; Glasberg et al., 1987; Grose & Hall, 1996 ; Tyler et al., 1982). Tyler et al. (1982) measured gap detection and gap discrimination in listeners with and without hearing loss using 500 and 4000 Hz noise burst markers. Listeners with heari ng loss showed significantly poorer performance than listeners with normal hearing. Performance was significantly better for the higher

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Effects of age and stimulus frequency on gap discrimination 5 frequency (4000 Hz) stimuli than for the lower frequency (500 Hz) stimuli, across groups. Because the listeners with hea ring loss (mean age = 53) were older than the listeners with normal hearing (mean age = 23), the results were confounded by listener age. Studies showing normal gap resolution by listeners with sensorineural hearing loss ( Grose et al., 2001; Lister et al., 2000 ) and impaired gap resolution by listeners with normal hearing (Fitzgibbons & Gordon Salant, 1994; Lister et al., 2002) seem to suggest that hearing sensitivity alone does not determine temporal resolution. In addition, the eff ects of hearing loss may be confounded somewhat by the effects of stimulus frequency region on temporal resolution. Snell et al. (1994) suggested that a region of dominant temporal sensitivity exists around 4 kHz. Others have also suggested that gap d etection for broad band stimuli is influenced by the high frequency components of the stimulus ( Fitzgibbons, 1983; Formby & Muir, 1988; Snell et al., 1994). As older listeners often have reduced hearing sensitivity in this frequency range, it is poss ible that reduced high frequency hearing contributes to poor temporal resolution. Other literature (Fitzgibbons & Gordon Salant, 1987) suggests that as long as the markers are presented at 25 30 dB sensation level, gap perception across frequency should be optimal. Resolution of silent gaps is also highly dependent upon the frequency disparity of the signals (markers) that bound the gap, a dependence that has been explained using the perceptual channel hypothesis ( Formby et al., 1998; Grose et al., 2001; Oxenham, 2000; Phillips et al., 1997 ). According to this hypothesis, the discrimination of gaps between markers differing in frequency by more than half an octave requires across channel processin g for example, across two or more perceptual channels. The

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Effects of age and stimulus frequency on gap discrimination 6 discrimination of gaps between markers that are close in frequency (less than half an octave apart) utilizes within channel processing for example, within a single perceptual channel. For acros s channel processing, the listener must discriminate a gap that exists between the offset of a marker in one channel and the onset of a marker in another channel. In the within channel case, the listener need only monitor the activity in a single chann el. Experimental results obtained for listeners with normal and impaired hearing support this hypothesis; measures requiring within channel processing result in better gap detection and gap discrimination than those that require across channel processing ( Lister et al., 2002; Roberts & Lister, 2004) Within channel and across channel gap detection and discrimination has been widely used to document age and hearing loss related deficits of temporal resolution ( Lister et al., 2002; Moore et al., 1992; Schneider et al., 1994; Snell, 1997; Strouse et al., 1998 ). The purpose of th is study was to assess temporal resolution using a gap discrimination task in two age groups of listeners with normal hearing, one younger and one older. Silent gap discrimination was measured using markers of the same frequency (fixed frequency) and markers that differed in frequency before and after the gap (frequency disparate). In addition, gap discrimination was measured in two frequency regions : 500 Hz and 4000 Hz. Specifically, we hypothesized that: 1) Older listeners would have poorer overall GDDLs than younger listeners; 2) GDDLs for frequency disparate markers would be poorer than GDDLs for fixed frequency markers; 3) GDDLs for higher frequ ency markers (4000 Hz) would be better than GDDLs for lower frequency markers (500 Hz);

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Effects of age and stimulus frequency on gap discrimination 7 and 4) The effects of frequency disparity would be greater for older listeners than for young listeners with normal hearing. METHOD Participants Two groups of listeners were recruited from current subject pools, from USF faculty, staff, and students, and from the Tampa Bay community: (1) 6 listeners aged 18 22 years (mean age = 20; s.d. = 1.41) with normal hearing (YNH) and (2) 6 listeners aged 55 66 years (mean age = 6 0; s.d. = 3.87) with normal hearing (ONH). Normal hearing was defined as pure tone thresholds of 20 dB HL or better at frequencies from 250 through 8000 Hz in both ears. Each subject participated in two test sessions (1 2 hours each). Informed consent was requested and received from all listeners. Average and individual pure tone thresholds are presented in Table 1. A three way mixed analysis of variance (ANOVA) with one between subjects factor (listener group) and two within subjects factors (ear and frequ ency) revealed that the pure tone thresholds of the YNH listeners were significantly better than those of the ONH listeners F(1,10)=32.49, p=0.0002. The thresholds did not differ significantly across frequency F(5,50)=0.186, p=0.966 or between the two ears F(1,10)=0.026, p=0.874; therefore, pure tone thresholds presented in Table 1 are averaged across ear. It is noted that the thresholds of the ONH listeners are within the range of what is considered normal audiometric hearing sensitivity, especially given their ages (Brant & Fozard, 1990).

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Effects of age and stimulus frequency on gap discrimination 8 Table 1. Pure tone thresholds (dB HL) for individual older normal hearing (ONH) and mean thresholds for the young normal hearing (YNH) group are shown. Standard errors are shown in parentheses. Pure Tone Thre sholds (dB HL) Group Age 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz YNH 20 2.9 4.2 5.4 5.0 2.1 3.8 n = 6 (0.77) (1.5) (0.8) (2.5) (1.0) (1.8) ON1 59 20 10 5 10 15 15 ON2 62 15 10 20 15 15 15 ON3 55 10 20 20 10 20 20 ON4 66 15 20 10 15 20 20 ON5 57 10 2 0 5 5 0 0 ON6 61 10 15 10 10 5 20 ONH 60 13.8 13.8 12.1 13.3 13.8 15.0 n = 6 (1.7) (1.5) (2.8) (1.5) (3.1) (3.1) Stimuli Noise band stimuli (markers) for the gap discrimination tasks, were computer generated, 300 ms in duration, 1/4 octave wide, and geometrically centered on one of the 6 frequencies listed in Table 2. Table 2. Gap Discrimination Frequency Conditions Marker Frequency Condition Center Frequency of Lead (Hz) Center Frequency of Trail (Hz) Disparity Fixed Frequency 4000 4000 None Fixed Frequency 500 500 None Frequency Disparate 421 595 1/2 octave Frequency Disparate 3364 4760 1/2 octave The frequencies and durations were chosen to facilitate comparisons with the results of Tyler et al. (1982) and Fitzgibbons and Gordon Salant (1994) All stimuli were presented at a fixed, audible level of 75 dB SPL. Instrumentation All noise band markers were generated digitally (20 kHz sampling rate) using a Tucker Davis Technologies (TDT) Psychoacoustics System consisting of a 32 bit

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Effects of age and stimulus frequency on gap discrimination 9 digital to an alog (D/A) converter, anti aliasing filters (9 kHz cutoff), attenuators, a headphone buffer, and a laboratory computer. Stimulus presentation and recording of listener responses was controlled by locally developed software All stimuli were presented bina urally via Sennheiser earphones. Listeners were tested individually in a sound treated room where they used a computer mouse to make selections on a computer screen relative to their perception of the auditory stimuli. Procedures A three interval, three alternative, forced choice (3I/3AFC) procedure targeting 70.7% correct discrimination (Levitt, 1971) was employed to reduce cognitive task demands. In this type of task, listeners must only select the odd stimulus and detailed understanding of the stimulus parameters is not required. This procedure has been recommended (Leek, 2002) for investigations of the auditory perception of aged listeners. Prior to each temporal resolution measure, the listener was familiarized with the task and stimuli by listening p assively to several trials. The noise band markers were paired so that the center frequency of the leading (before the gap) and trailing (after the gap) markers were fixed at the same frequency (fixed frequency condition) or at frequencies octave apart ( frequency disparate condition) for each experimental run. The specific frequency combinations are detailed in Table 2. As a result, the presence/absence of frequency disparity was varied between runs and two distinct frequency regions were represented. Each marker pair was separated by a silent temporal gap, and gap duration difference limens (GDDLs) were measured in random order for the four marker center

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Effects of age and stimulus frequency on gap discrimination 10 frequency combinations (Table 2). The following instructions were given to each listener : Gap discrimination is a test that measures your ability to hear that one sound is the same or different from another. In this test, you will hear three noise bursts, two will be the same and one of them will be different from the other two. Y our job is to pick the one that is different. Depending on your response, the anomalous or different burst may become easier to hear or totally undetectable to you. The compu ter program will automatically track your response and calculate the smallest difference that you can detect among the three tone bursts. The listener chose among two standard intervals in which the markers were separated by a standard gap (100 ms to facilitate comparison with previous studies) and one target interval in which the gap duration was va ried adaptively. Presentation order of the standard and target intervals was randomized across trials. Additional experimental runs were completed if a listener demonstrated inconsistent performance. Within an experimental run, the marker center frequency combination remained constant. Marker duration was roved within a range of 250 to 350 ms to control for extraneous marker duration cues that may aid gap discrimination. Three runs of each condition (four center frequency combinations, four noise condition s) were completed for a total of 48 runs. Each run lasted 3 5 minutes; therefore, total data collection time was approximately 2.5 4 hours. RESULTS The effect of age and frequency region on gap discrimination was measured using fixed frequency and fre quency disparate noise band markers. We hypothesized that GDDLs would be poorest for older listeners, lower frequency markers, and frequency disparate markers.

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Effects of age and stimulus frequency on gap discrimination 11 Figure 1 shows GDDLs for each group and frequency condition. As illustrated by the figure, the YNH listeners had smaller GDDLs than the ONH listeners. Group differences are more apparent for the fixed frequency conditions than for the frequency disparate conditions. Best overall performance was observed for the 4000 Hz fixed frequency condition for both groups. Figure 1. Average Gap Duration Difference Limens (GDDLs) for the two listener groups and the four frequency conditions. Hatched bars represent performance of the young listeners with normal hearing. Solid bars represent performance of the older listeners with normal hearing. Standard error bars are shown for group and frequency condition. A three way mixed ANOVA revealed that the effect of group [F(1,10)=56.25; p=0.00002] and the effect of frequency condition [F(3,30)=101.36; p<0.00001] were statistically significant as was the interaction between group and frequency condition [F(3,30)=8.86; p=0.0002]. Due to the presence of a significant interaction, the data for each group and frequency condition were compared using a one way ANOVA. Th is

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Effects of age and stimulus frequency on gap discrimination 12 analysis indicated a significant difference between the means [F(7,35)=63.62, p<0.00001]. A Tukey HSD post hoc analysis indicated that the YNH listeners had significantly lower GDDLs than the ONH listeners for all frequency conditions (p<0.0007) except the 3364 4760 Hz condition (p=0.8784). For the ONH listeners, significantly lower GDDLs were found for the high fixed frequency condition (4000 4000 Hz) than for the low fixed frequency condition (500 500 Hz) (p=0.00329) but no difference was found between the high and low frequency disparate conditions (p=0.99) for this group. For the YNH listeners, the reverse was found. Their low and high frequency GDDLs differed significantly for the frequency disparate condition (p=0.0022), but not for the fixed freque ncy condition (p=0.99). When fixed frequency and frequency disparate conditions were compared for the same frequency region (i.e., 500 500 vs. 421 595 Hz and 4000 4000 vs. 3364 4760 Hz), significantly lower GDDLs were found for the fixed frequency conditio ns for both listener groups (p<0.0007). DISCUSSION The purpose of this study was to investigate the effects of age and frequency on gap discrimination. The results indicated significant effects of age, frequency region, and frequency disparity on GDDLs. O lder listeners had poorer overall average GDDLs than the younger listeners. As expected based upon the perceptual channel hypothesis ( Formby et al., 1 998; Grose et al., 2001; Oxenham, 2000; Phillips et al., 1997 ) r esolution of silent gaps was highly dependent upon the characteristics of the signals (markers) that bound the gap. Both listener group s demonstrated poorer GDDLs in the frequency disparate conditions as compared to the fixed frequency conditions.

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Effects of age and stimulus frequency on gap discrimination 13 Based on the studies of Snell et al. (1994) and Tyler et al. (1982), GDDLs for high frequency markers (4000 Hz) were expected to be better than GDDLs for lower frequency markers (500 Hz). Overall lower GDDLs were measured for the 4000 Hz fixed frequency condition than for the 500 Hz fixed frequency condition. Examination of the data for six hearing impaired listeners aged 55 80 years from Tyler et al. (1982) revealed average GDDLs of 95.4 and 91.1 ms for the 500 and 4000 Hz markers, respectively. For their 16 young listeners with normal hearing, Tyler et al. measured average GDDLs of 75.7 and 71.2 ms for 500 and 4000 Hz markers. In the present study, we measured GDDLs of 13.7 and 11.4 ms for 500 and 4000 Hz for the YNH listeners. For the ONH listeners, we measured GDDLs of 48.7 and 31.4 ms, respectively This discrepancy may be attributed to Tyler et al. s us e of 500 ms marker durations doubl e those used in the present study. The effects of frequency condition were expected to be greater for the older listeners than for the younger listeners. Fitzgibbons and Gordon Salant (1994) found larger age effects for frequency disparate markers than for fixed frequency markers using tone burst stimuli. In the present study, g roup differences were actually smaller for the frequency disparate conditions than for the fixed frequency conditions. A repeat testing of the subjects was planned to investigate the rep eata bility of this anomalous finding However, we we re unable to do so due to Institutional Review Board (IRB) regulations and t ime constraints Further investigation into the matter using similar subjects is warranted due to the unusual findings. In partial explanation of the anomalous findings, we offer several observations. The ONH listeners had n ormal hearing levels for 250 through 8000 Hz T he excellent

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Effects of age and stimulus frequency on gap discrimination 14 pure tone thresholds of the ONH group may have contributed to the differences found in the present study compared to results of previous research. Also, the ONH listeners were of excellent physical health. There are still many theories add ressing the question of the aging auditory system s inabilit y to understand speech clearly. Research has pointed to a decline in perception of brief acoustic cues, age related declin e in temporal processing ability in general and the age related cha nges in auditory processing. Each of these could contain a possible explanation by itself; however it is often an interaction of these proposed theories that has an effect on an aging auditory system, and more research is called for to narrow the varied p ossibilities. Th e continuing r esearch should utiliz e older listeners with excellent pure tone thresholds and a history of a healthy lifestyle

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Effects of age and stimulus frequency on gap discrimination 15 CONCLUSION The results of this study indicate that age and marker frequency composition negativ ely affect gap discrimination. This effect may influence the speech perception difficulties so often experienced by older listeners in noisy environments. Temporal cues that occur between spectrally dynamic, low frequency stimuli may be particularly diffic ult for older listeners to perceive.

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Effects of age and stimulus frequency on gap discrimination 16 REFERENCES Brant, L. & Fozard, J. (1990). Age changes in pure tone hearing thresholds in a longitudinal study of normal human aging. J. Acoust. Soc. Amer. 88, 813 820 Divenyi, P. & Danner, W. ( 1977 ) Discriminatio n of time intervals marked by brief acoustic pulses of various intensities and spectra Percept. Psychophys. 21 125 142. Dubno, J., Horwitz, A., & Ahlstrom, J. (2003). Recovery from prior stimulation: masking of speech by interrupted noise for younger an d older adults with normal hearing. J. Acoust. Soc. Am. 113, 2083 2094. Fitzgibbons, P. (1983). Temporal gap detection in noise as a function of frequency, bandwidth, and level. J. Acoust. Soc. Am. 74, 67 72. Fitzgibbons, P. & Gordon Salant, S. (1987). Mi nimum stimulus levels for temporal gap resolution in listeners with sensorineural hearing loss. J. Acoust. Soc. Am., 81 1542 1545. Fitzgibbons, P. & Gordon Salant, S. (1994). Age effects on measures of auditory duration discrimination. J. Speech Hear. Res., 37 662 670. Fitzgibbons, P. & Wightman, F. ( 1982 ) Gap detection in normal and hearing impaired listeners J. Acoust. Soc. Am. 72 761 765. Florentine M. & Buus S. (1984). Temporal gap detection in sensorineural and simulated hearing impairments. J Speech Hear Res ., 27 449 55. Formby, C. & Muir, K. (1989). Effects of randomizing signal level and duration on temporal gap detection. Audiology, 28 250 257. Formby, C., Gerber, M. Sherlock, L., & Magder, L. (1998). Evidence for an across

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Effects of age and stimulus frequency on gap discrimination 17 frequency, between channel process in asymptotic monaural temporal gap detection. J. Acoust. Soc. Am. 103 3554 3560. Glasberg, B., Moore, B. C. J., & Bacon, S. (1987). Gap detection and masking in hearing impaired and normal hearing subjects. J. Acoust. Soc. Am. 81 1546 1556. Gordon Salant S. & Fitzgibbons P. (1993). Temporal factors and speech recognition performance in young and elderly listeners J Speech Hear Res 36 1276 85. Gordon Sal ant, S. & Fitzgibbons, P. ( 1999 ) Profile of auditory temporal processing in older listeners J. Spch. Hrng. Rsch. 42 300 311. Grose, J., Hall, J., Buss, E., & Hatch, D. (2001). Gap detection for similar and dissimilar gap markers. J. Acoust. Soc. Am. 1 09 1587 1595. Grose J. & Hall J. (1996) Cochlear hearing loss and the processing of modulation: effects of temporal asynchrony. J Acoust Soc Am. 100 519 27. Irwin, R. & McAuley, S. (1987). Relations among temporal acuity, hearing loss, a nd the perception of speech distorted by noise and reverberation. J. Acoust. Soc. Am. 81 1557 1565. Koehnke, J. & Besing, J. (1996). A procedure for testing speech intelligibility in a virtual listening environment. Ear and Hearing, 17 211 217. Kramer, S., Kapteyn, T., & Festen, J. (1998). The self reported handicapping effect of hearing disabilities. Audiology 37 302 312. Leek, M.R. (2001). Adaptive procedures in psychophysical research. Perception and Psychophysics, 63 (8), 1279 129 2.

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Effects of age and stimulus frequency on gap discrimination 18 Levitt, H. (1971). Transformed up down methods in psychoacoustics. J. Acoust. Soc. Am. 49, 467 477. Lister, J., Koehnke, J. & Besing, J. (2000). Binaural gap duration discrimination in listeners with impaired hearing and normal hearing. Ear and Hearing, 21 ,141 150. Lister J Besing J., & Koehnke J. (2002) Effects of age and frequency disparity on gap discrimination. J Acoust Soc Am. 111 2793 800. Moore, B., Peters, R., & Glasberg, B. ( 1992 ) Detection of temporal gaps in sinusoids by elderly subjects with and without hearing loss J. Acoust. Soc. Am. 92 1923 1932. Nabelek, A. & Mason, D. (1981). Effect of noise and reverberation on binaural and monaural word identification by subjects with various audiograms. J. Speech Hear. Res. 24 375 383. Oxenham, A. (2002). Behavioral measures and consequences of cochlear nonlinearity. American Academy of Audiology Presentation, April, 2002, Philadephia, PA. Oxenham, A. (2000). Influence of spatial and temporal coding on au ditory gap Detection. J. Acoust. Soc. Am. 107 2215 2223. Peters, R., Moore, B. C. J., & Baer, T. (1998). Speech reception thresholds in noise with and without spectral and temporal dips for hearing impaired and normally hearing people. J. Acoust. Soc. A m. 103 577 587. Phillips, D., Taylor, T., Hall, S., Carr, M., & Mossop, J. E. (1997). Detection of silent

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Effects of age and stimulus frequency on gap discrimination 19 intervals between noises activating different perceptual channels: some properties of central auditory gap detection. J. Acoust. Soc. Am. 101 369 4 3705. Roberts, R. & Lister, J. (2004). Effects of age and hearing loss on gap detection and the precedence effect: Broad band stimuli. J. Speech. Lang. Hear. Res., in press. Salthouse, T. (1985). A theory of cognitive aging. New York: North Holland. Sch neider B ., Pichora Fuller M ., Kowalchuk D., & Lamb M.(1994). Gap detection and the precedence effect in young and old adults. J Acoust Soc Am. 95 980 91. Snell, K., Ison, J., & Frisina, D. ( 1994 ) The effects of signal frequ ency and absolute bandwidth on gap detection in noise J. Acoust. Soc. Am. 96 1458 1464 Snell, K. (1997), Age related changes in temporal gap detection. J. Acoust. Soc. Am. 101 2214 2220. Strouse, A., Ashmead, D., Ohde, R., & Grantham, W. (1998). Tempo ral processing in the aging auditory system. J. Acoust. Soc. Am., 104 2385 2399. Tyler R Summerfield Q Wood E., & Fernandes M.(1982). Psychoacoustic and phonetic temporal processing in normal and hearing impaired listener s. J Acoust Soc Am. 72 740 52. Snell, K., Mapes, F., Hickman, E., & Frisina, D. (2002). Word recognition in competing babble and the effects of age, temporal processing, and absolute sensitivity. J. Acoust. Soc. Am. 112 720 727. Willott, J. (1991). Aging and the auditor y system: Anatomy, Physiology, and Psychophysics. Singular: San Diego.