John H. Mills

Longitudinal study of pure-tone thresholds in older persons.

Objective: Pure-tone thresholds for conventional and extended high frequencies were analyzed for 188 older adult human subjects (91 females, 97 males). The objectives were to study longitudinal changes in thresholds as well as the effects of initial threshold levels, age, gender, and noise history on these longitudinal changes. Design: At the time of entry into the study, subjects’ ages ranged from 60 to 81 years, with a mean age of 68 years. Subjects had between 2 and 21 visits (mean = 9.81 visits) over a period of 3 to 11.5 years (mean = 6.40 years). Conventional pure-tone thresholds at 0.25 to 8 kHz were measured during most visits. Extended high-frequency (EHF) thresholds at 9 to 18 kHz were measured every 2 to 3 years. The slope of a linear regression was used to estimate the rate of change in pure-tone thresholds at 0.25 to 18 kHz for each ear. A questionnaire was used to identify those subjects with a positive noise history. Results: The average rate of change in thresholds was 0.7 dB per year at 0.25 kHz, increasing gradually to 1.2 dB per year at 8 kHz and 1.23 dB per year at 12 kHz. The rate of change for thresholds increased significantly with age, at 0.25 to 3, 10, and 11 kHz for females and at 6 kHz for males. After adjusting for age, females had a significantly slower rate of change at 1 kHz but a significantly faster rate of change at 6 to 12 kHz than males. For 0.25 and 1 kHz, subjects with more hearing loss at higher frequencies had a faster rate of change at these frequencies, whereas for 6 and 8 kHz, subjects with more hearing loss at mid and high frequencies had a slower rate of change at these frequencies. The rates of threshold change for subjects with a positive noise history were not statistically different from those with a negative noise history. Conclusions: On average, hearing threshold increased approximately 1 dB per year for subjects age 60 and over. Age, gender, and initial threshold levels can affect the rate of change in thresholds. Older female subjects (≥70 years) had faster rate of change at 0.25 to 3, 10, and 11 kHz than younger female subjects (60 to 69 years). Older male subjects had faster rate of change at 6 kHz than younger male subjects. Females had a slower rate of change at 1 kHz and a faster rate of change at 6 to 12 kHz than males. Subjects with higher initial thresholds at low and mid frequencies tended to have faster rate of threshold change at 0.25 to 2 kHz in the following years. Subjects with higher initial thresholds at mid and higher frequencies tended to have slower rate of change at 6 to 8 kHz in the following years. Noise history did not have a significant effect on the rate of threshold changes.

Age-related changes in auditory potentials of Mongolian gerbil

The Mongolian gerbil is being evaluated as an animal model of age-related hearing loss (presbyacusis). Part of this evaluation involves estimating auditory thresholds from evoked potentials arising from the auditory nerve and brainstem. The gerbils are born and reared in an environment where the ambient noise level is less than 40 dBA. Some animals are followed longitudinally (8, 19, 23.5 and 36 months), others are studied at 6-8 months (controls), or at 36 months (cross-sectional). Physiological responses are obtained with the animals anesthetized with ketamine and xylazine and transdermal electrodes attached to the head. Auditory signals are tone pips with center frequencies from 1 to 16 kHz in octave steps. Signal levels are varied from 10 to 80 dB SPL in 10 dB steps. For animals (N = 48) in the age range of 6-8 months, mean auditory thresholds were about 20 dB SPL between 2.0 and 8.0 kHz, 25 dB at 16 kHz and 30 dB at 1.0 kHz. By age 22-24 months (N = 15) thresholds had increased by about 10 dB at nearly all frequencies. By age 36 months (N = 37 ears, 32 animals) threshold increases were about 30-35 dB at 8 and 16 kHz, were 25 dB at 4 kHz and 2 kHz, and were 19 dB at 1 kHz. These hearing losses in 36-month gerbil are qualitatively similar to human data for 60-65-year-old males and 70-year-old females. Individual differences in hearing loss were large with the range exceeding 65 dB. While some animals (26/37) had a high-frequency sloping loss, others (11/37) had a bimodal audiometric shape where the hearing loss was smallest at 4 kHz and increased by at least 10 dB at adjacent frequencies.

Frequency and intensity discrimination measured in a maximum-likelihood procedure from young and aged normal-hearing subjects

A maximum-likelihood method was applied in measurements of frequency and intensity discrimination for aged and young normal-hearing subjects with closely matched audiograms. This method was preferred over other psychophysical procedures because it is efficient and controls experimental variance, features that are highly desirable for testing aged subjects. In order to implement the method, psychometric functions for each task were also measured from young subjects using a constant-stimuli procedure. For the young subjects, the differential thresholds obtained from these two procedures were generally comparable. Further, both sets of data were consistent with previous literature, indicating that the maximum-likelihood method was successfully applied for frequency and intensity discrimination. A frequency-dependent difference between young and aged subjects in both frequency and intensity discrimination was observed. Even with closely matched audiograms, aged subjects demonstrated poorer discrimination abilities than young subjects. The age-related difference was always largest at 500 Hz and decreased as frequency increased.

Tuning and suppression in auditory nerve fibers of aged gerbils raised in quiet or noise

Mongolian gerbils were reared either in quiet or in a continuous noise field (85 dBA, 500-4000 Hz). The gerbils began the noise exposure at 8 months of age and, after the exposure, spent the remainder of their lives in the quiet vivarium with the quiet-aged group. The duration of the noise exposure was between 365 and 724 days. At the terminal experiment the ages of the animals varied between 24 and 43 months, with a mean age of about 36 months, an age representing the average life span of a gerbil in our colony. During the terminal experiment, tuning curves and boundaries of two-tone rate suppression were obtained from single fibers in the auditory nerve. Threshold shifts occurred in both groups of animals. The shift was largely confined to the tip of the tuning curve; i.e., the region around the characteristic frequency (CF) of the fiber. The CF shifts effectively reduced the tip-to-tail ratios of the tuning curves. Two-tone suppression areas above and below CF were present for all fibers in the quiet-aged animals, but were often absent for fibers in the noise-aged group. The presence of suppression was largely independent of fiber thresholds in both groups of animals. Indeed, fibers were found with clearly-defined suppression boundaries above and below CF despite threshold shifts of up to 60 dB. Moreover, in the noise-aged group suppression below CF was sometimes found without concomitant suppression above CF and vice versa, suggesting an independence between the two suppression areas. For fibers with CFs within the bandwidth of the noise, two-tone suppression above CF was always absent, even though suppression below CF was sometimes present. In sum, two-tone suppression was near normal in ears aged in quiet despite relatively large threshold shifts at the fiber CF. However, suppression, especially that above CF, was vulnerable to the effects of chronic noise. Taken with the results of other studies, our data suggest that the micromechanics of the cochlea are largely responsible for two-tone suppression, especially that above CF, and that different mechanisms may underlie suppression above and below CF.

Age-related changes in auditory evoked potentials of gerbils. I. Response amplitudes.

Auditory brainstem responses (ABRs) were recorded in young (6-10 month) and aged (36 month) Mongolian gerbils. For each subject, ABR thresholds and response amplitudes were measured at octave intervals from 1 through 16 kHz. Data from the young animals served as the baselines for comparison to aged animals which were categorized on the basis of auditory brainstem response (ABR) thresholds. The aged groups included subjects with thresholds (a) at the mean of a pool of 50 aged gerbils, (b) one standard deviation (SD) lower than the mean, (c) one sd higher than the mean, and (d) near normal for young gerbils. The amplitudes of ABR waveforms for the aged gerbils were reduced compared to the young subjects, particularly at high sound pressure levels. This was true even for aged subjects with thresholds similar to those for younger subjects. The slopes of the amplitude-intensity (I/O) functions were shallower in all aged subjects compared to young subjects. The results suggest that ABR amplitudes and I/O slopes decrease as a function of age and that the decreases are not a direct result of loss of auditory sensitivity. The reductions in ABR amplitudes from aged gerbils presumably reflect age-related pathology in the auditory periphery, as previous studies have shown reductions in amplitudes of the compound action potential of aged gerbils.

Temporary threshold shifts in humans exposed to octave bands of noise for 16 to 24 hours.

Groups of human subjects were exposed in a diffuse sound field for 16--24 h to an octave-band noise centered at 4, 2, 1, or 0.5 kHz. Sound-pressure levels were varied on different exposure occasions. At specified times during an exposure, the subject was removed from the noise, auditory sensitivity was measured, and the subject was returned to the noise. Temporary threshold shifts (TTS) increased for about 8 h and then reached a plateau or asymptote. The relation between TTS and exposure duration can be described by a simple exponential function with a time constant of 2.1 h. In the frequency region of greatest loss, threshold shifts at asymptote increased about 1.7 dB for every 1 dB increase in the level of the noise above a critical level. Critical levels were empirically estimated to be 74.0 dB SPL at 4 kHz. 78 dB at 2 kHz, and 82 dB at 1 and 0.5 kHz. Except for the noise centered at 4.0 kHz, threshold shifts were maximal about 1/2 octave above the center frequency of the noise. A smaller second maximum was observed also at 7.0 kHz for the noise centered at 2.0 kHz, at 6.0 kHz for the noise centered at 1.0 kHz, and at 5.5 kHz for the noise centered at 0.5 kHz. After termination of the exposure, recovery to within 5 dB of pre-exposure thresholds was achieved within 24 h or less. Recovery can be described by a simple exponential function with a time constant of 7.1 h. The frequency contour defined by critical levels matches almost exactly the frequency contour defined by the E-weighting network.

Frequency modulation detection: Effects of age, psychophysical method, and modulation waveform

As part of an ongoing study of auditory aging, detection of sinusoidal and quasitrapezoidal frequency modulation (FM) was measured with a 5-Hz modulation frequency and 500- and 4000-Hz carriers in two experiments. In Experiment 1, psychometric functions for FM detection were measured with several modulation waveform time patterns in younger adults with normal hearing. Detection of a three-cycle modulated signal improved when its duration was extended by a preceding unmodulated cycle, an effect similar to adding a modulated cycle. In Experiment 2, FM detection was measured for younger and older adults with normal hearing using two psychophysical methods. Similar to frequency discrimination, FM detection was poorer in older than younger subjects and age-related differences were larger at 500 Hz than at 4000 Hz, suggesting that FM detection with low modulation frequencies and frequency discrimination may share common underlying mechanisms. One mechanism is likely related to temporal information coded by neural phase locking which is strong at low frequencies and decreases with increasing frequency, as observed in animals. The frequency-dependent aging effect suggests that this temporal mechanism may be affected by age. The effect of psychophysical method was sizable and frequency dependent, whereas the effect of modulation waveform was minimal.

Electrophysiologic Correlates of Intensity Discrimination In Cortical Evoked Potentials of Younger and Older Adults

When measured behaviorally, older adults with normal hearing have poorer intensity discrimination thresholds than younger adults, but only at lower frequencies. Poor intensity discrimination at lower but not higher frequencies for older adults can be associated with an age-related decline in temporal processing. The current study was designed to assess age-related effects on intensity discrimination at 500 and 3000 Hz using the cortical auditory evoked potential, N1--P2. Subjects were 10 younger and 10 older adults with normal hearing. The N1--P2 was elicited by an intensity increase in an otherwise continuous pure tone presented at 70 dB SPL. Intensity increments ranged from 0 dB to 5 dB at 500 Hz and from 0 d B to 8 d B at 3000 Hz in 1-dB steps. Intensity discrimination threshold was defined as the smallest intensity change needed to evoke an N1-P2 response. Consistent with behavioral measures, N1-P2 response thresholds were significantly higher for older subjects than younger subjects at 500 Hz but did not differ significantly at 3000 Hz. In addition, N1 and P2 latencies for older subjects were significantly prolonged at 500 Hz, but not at 3000 Hz. As intensity increments increased above threshold, amplitudes tended to be larger in older than in younger subjects, however, these differences were not statistically significant. In older subjects, response latencies and amplitudes were significantly larger at 500 Hz than at 3000 Hz. In younger subjects, response latencies and amplitudes were similar across frequency. Similar intensity discrimination thresholds and age-related differences for behavioral measures and evoked potentials support the notion that the N1-P2 measures reflect the physiological detection of intensity change which in turn relates to intensity discrimination. A possible explanation for the decreased intensity discrimination at low frequencies, and enhanced amplitudes with prolonged latencies in older subjects is an age-related decline in inhibitory control within the central auditory nervous system.

The amplitude-modulation following response in young and aged human subjects

The amplitude-modulation following response (AMFR) is a steady-state auditory response which may be an objective measure of intensity discrimination. Aged subjects with normal hearing have poorer intensity discrimination for low-frequency tones measured behaviorally, which would predict poorer AMFRs for low-frequency carriers. Experiment 1 was designed to assess age-related differences in AMFR characteristics. Response amplitudes were not significantly different among the young and aged groups for either carrier frequency (520 or 4000 Hz) or modulation depth (0--100%). Response phase did not vary systematically among groups. These results suggest that the AMFR may not be directly comparable to behavioral measures of intensity discrimination in aged subjects with normal hearing. To assess the contribution of high-frequency hearing loss on the AMFR in aged subjects, Experiment 2 compared AMFR amplitudes in aged subjects and in young subjects under the condition of high-pass masking. The amplitude of the AMFR was reduced at 520 Hz for both aged subjects and masked young subjects compared to unmasked young subjects, suggesting that reduced amplitudes in aged subjects with high-frequency hearing loss were associated with threshold elevations. Furthermore, the results suggest that the base of the cochlea contributes to the AMFR for low carrier frequencies.

Age-related changes in auditory evoked potentials of gerbils. II. Response latencies

Auditory brainstem responses (ABR) were recorded in young (6-10 month) and aged (36 month) Mongolian gerbils. Data from the young animals served as the baselines for comparison to aged animals which were categorized on the basis of ABR thresholds. Aged gerbils with normal thresholds (re young controls) had wave i and ii latencies of the ABR which were relatively normal at 1-4 kHz and slightly reduced at 8 and 16 kHz. Wave iv latencies in the aged gerbils with normal thresholds were reduced at all frequencies. Aged gerbils with 10-30 dB of hearing loss had wave i, ii, and iv latencies which were prolonged at low sound pressure levels and normal at high stimulus levels. Aged gerbils with 30 dB or greater losses had prolonged wave i, ii, and iv latencies at most levels. Slopes of latency-intensity (L/I) functions were steeper at 1-4 kHz than controls in aged subjects with hearing losses of 10 dB or greater. Slopes of L/I functions for wave iv were normal in aged subjects. The wave i-iv interval was shorter than normal in aged subjects with no hearing loss, normal in aged subjects with 10-30 dB of loss, and prolonged in subjects with greater than 30 dB of loss.