High Presentation Levels Research Articles

The influence of the stimulus level used to prescribe nonlinear frequency compression on speech perception.

Nonlinear frequency compression (NFC) is a signal processing technique designed to lower high frequency inaudible sounds for a listener to a lower frequency that is audible. Because the maximum frequency that is audible to a listener with hearing loss will vary with the input speech level, the input level used to set nonlinear frequency compression could impact speech recognition. The purpose of this study was to determine the influence of the input level used to set nonlinear frequency compression on nonsense syllable recognition. Nonsense syllable recognition was measured for three NFC fitting condition (i.e., with nonlinear frequency compression set based on speech input levels of 50-, 60-, and 70-dB SPL, respectively), as well as without nonlinear frequency compression (restricted bandwidth condition). Twenty-three adults (ages 42-80 years old) with hearing loss. Data were collected, monaurally, using a hearing aid simulator. The start frequency and frequency compression ratios were set based on the SoundRecover Fitting Assistant. Speech stimuli were 657 consonant-vowel-consonant nonwords presented at 50, 60, and 70 dB SPL and mixed with steady noise (6 dB SNR) and scored based on entire word, initial consonant, vowel, and final consonant. Linear mixed effects examined the effects of NFC fitting condition , presentation level, and scoring method on percent correct recognition. Additional predictor variables of start frequency and frequency-compression ratio were examined. Nonsense syllable recognition increased as presentation level increased. Nonsense syllable recognition for all presentation levels was highest when nonlinear frequency compression was set based on the 70 dB SPL input level and decreased significantly when set based on the 60- and 50-dB SPL inputs. Relative to consonant recognition, there was a greater reduction in vowel recognition. Nonsense syllable recognition between NFC fitting conditions improved with increases in the start frequency, where higher start frequencies led to better nonsense word recognition. Nonsense syllable recognition was highest when setting nonlinear frequency compression based on a 70 dB SPL presentation level and suggest that a high presentation level should be used to determine nonlinear frequency compression parameters for an individual patient.

Speech Perception in Ménière Disease.

Introduction Ménière disease (MD) affects the inner ear, comprising the cochlea and semicircular canals. Symptoms include severe incapacitating vertigo, nausea, vomit, aural fullness, and sensorineural hearing loss - in which speech discrimination and intelligibility are impaired and can be quantified with speech audiometry. Objective To investigate the influence of the stimuli presentation level in speech audiometry and the quality of life in adults with and without a diagnosis of MD. Method Two groups were formed with nine individuals each - one with and the other without MD. The Speech Recognition Percentage Index was researched with stimuli presented above the self-reported comfort level or 5 dB below the discomfort level. Dizziness Handicap and Tinnitus Handicap Inventories were administered to individuals with tinnitus and vertigo complaints. Results Speech recognition was better in the study group with higher presentation levels, as 75% of the sample improved their performance. The presence of vertigo significantly impacted the quality of life of individuals in the study group. Conclusion Speech recognition improves with higher presentation levels. Also, MD impacts the quality of life, especially regarding limitations caused by vertigo.

Factors Influencing the Korean Version of the Digit-in-Noise Test.

The digits-in-noise (DIN) test was developed as a simple and time-efficient hearing-in-noise test worldwide. The Korean version of the DIN (K-DIN) test was previously validated for both normal-hearing and hearing-impaired listeners. This study aimed to explore the factors influencing the outcomes of the K-DIN test further by analyzing the threshold (representing detection ability) and slope (representing test difficulty) parameters for the psychometric curve fit. Subjects and. In total, 35 young adults with normal hearing participated in the K-DIN test under the following four experimental conditions: 1) background noise (digit-shaped vs. pink noise); 2) gender of the speaker (male vs. female); 3) ear side (right vs. left); and 4) digit presentation levels (55, 65, 75, and 85 dB). The digits were presented using the method of constant stimuli procedure. Participant responses to the stimulus trials were used to fit a psychometric function, and the threshold and slope parameters were estimated according to pre-determined criteria. The accuracy of fit performance was determined using the root-mean-square error calculation. The listener's digit detection ability (threshold) was slightly better with pink noise than with digit-shaped noise, with similar test difficulties (slopes) across the digits. Gender and the tested ear side influenced neither the detection ability nor the task difficulty. Additionally, lower presentation levels (55 and 65 dB) elicited better thresholds than the higher presentation levels (75 and 85 dB); however, the test difficulty varied slightly across the presentation levels. The K-DIN test can be influenced by stimulus factors. Continued research is warranted to understand the accuracy and reliability of the test better, especially for its use as a promising clinical measure.

Age-related rollover for spectrally degraded temporal speech contrasts

Some speech sounds, or phonemes, are primarily distinguished acoustically from other phonemes by temporal cues. A decline in temporal-processing abilities is a common consequence of aging that might cause a listener to have difficulty differentiating between sounds, especially when the speech is degraded. One type of signal degradation occurs in a cochlear-implant vocoder simulation, which eliminates most of the spectral information in speech while maintaining the temporal envelope. In addition, temporal processing abilities may be challenged at higher presentation levels causing a decrease in performance (i.e., rollover), particularly in older listeners. This study measures the phoneme classification of younger and older listeners of four spectro-temporal contrasts when presented in quiet with and without vocoding at a range of presentation levels. We hypothesized that, due to declines in temporal processing abilities, older listeners would be more susceptible to rollover, measured by a decrease in phoneme classification performance for degraded temporal contrasts as presentation levels increase. We further hypothesized that performance rollover would occur more often for vocoded speech than for unprocessed speech, especially in older listeners. Preliminary data show interactions between speech contrasts, presentation levels, and vocoding. Our results will help facilitate understanding speech perception difficulties in older cochlear-implant users.

Integrating IoT-Based Devices for Monitoring The Humidity and Temperature In the Boarding House Space

Boarding houses provide the service of renting out living quarters. A good boarding house maintains a pleasant temperature and humidity level. High humidity may damage the interior and the people who live in it; therefore, it must be monitored more closely. Simpang Kost is the boarding house the author used as an object of research on monitoring temperature and humidity using an application called Smart Life. The goal of this study is to use IoT-based sensors to monitor and automatically solve temperature and humidity concerns in various rooms. Smart temperature and humidity sensors are IoT-based. This device measures the temperature and humidity in the room. According to the collected data, three categories of rooms have issues with high humidity presentation levels, so the authors offer several solutions to address these issues. The dehumidifier functions as an actuator, starting when the humidity in the room rises and stopping when the humidity in the room reaches the standards. An exhaust fan is also linked to an IoT-based smart breaker and serves as an actuator. When the room temperature rises above a certain threshold, this tool will activate and turn off automatically.

Factors Underlying Individual Differences in Speech-Recognition Threshold (SRT) in Noise Among Older Adults.

Many older adults have difficulty understanding speech in noisy backgrounds. In this study, we examined peripheral auditory, higher-level auditory, and cognitive factors that may contribute to such difficulties. A convenience sample of 137 volunteer older adults, 90 women, and 47 men, ranging in age from 47 to 94 years (M = 69.2 and SD = 10.1 years) completed a large battery of tests. Auditory tests included measures of pure-tone threshold, clinical and psychophysical, as well as two measures of gap-detection threshold and four measures of temporal-order identification. The latter included two monaural and two dichotic listening conditions. In addition, cognition was assessed using the complete Wechsler Adult Intelligence Scale-3rd Edition (WAIS-III). Two monaural measures of speech-recognition threshold (SRT) in noise, the QuickSIN, and the WIN, were obtained from each ear at relatively high presentation levels of 93 or 103 dB SPL to minimize audibility concerns. Group data, both aggregate and by age decade, were evaluated initially to allow comparison to data in the literature. Next, following the application of principal-components factor analysis for data reduction, individual differences in speech-recognition-in-noise performance were examined using multiple-linear-regression analyses. Excellent fits were obtained, accounting for 60–77% of the total variance, with most accounted for by the audibility of the speech and noise stimuli and the severity of hearing loss with the balance primarily associated with cognitive function.

Transmission of Binaural Cues by Bilateral Cochlear Implants: Examining the Impacts of Bilaterally Independent Spectral Peak-Picking, Pulse Timing, and Compression.

Acoustic hearing listeners use binaural cues—interaural time differences (ITDs) and interaural level differences (ILDs)—for localization and segregation of sound sources in the horizontal plane. Cochlear implant users now often receive two implants (bilateral cochlear implants [BiCIs]) rather than one, with the goal to provide access to these cues. However, BiCI listeners often experience difficulty with binaural tasks. Most BiCIs use independent sound processors at each ear; it has often been suggested that such independence may degrade the transmission of binaural cues, particularly ITDs. Here, we report empirical measurements of binaural cue transmission via BiCIs implementing a common “n-of-m” spectral peak-picking stimulation strategy. Measurements were completed for speech and nonspeech stimuli presented to an acoustic manikin “fitted” with BiCI sound processors. Electric outputs from the BiCIs and acoustic outputs from the manikin’s in-ear microphones were recorded simultaneously, enabling comparison of electric and acoustic binaural cues. For source locations away from the midline, BiCI binaural cues, particularly envelope ITD cues, were found to be degraded by asymmetric spectral peak-picking. In addition, pulse amplitude saturation due to nonlinear level mapping yielded smaller ILDs at higher presentation levels. Finally, while individual pulses conveyed a spurious “drifting” ITD, consistent with independent left and right processor clocks, such variation was not evident in transmitted envelope ITDs. Results point to avenues for improvement of BiCI technology and may prove useful in the interpretation of BiCI spatial hearing outcomes reported in prior and future studies.

Associations between speech recognition at high levels, the middle ear muscle reflex and noise exposure in individuals with normal audiograms

It has been hypothesized that noise-induced cochlear synaptopathy in humans may result in functional deficits such as a weakened middle ear muscle reflex (MEMR) and degraded speech perception in complex environments. Although relationships between noise-induced synaptic loss and the MEMR have been demonstrated in animals, effects of noise exposure on the MEMR have not been observed in humans. The hypothesized relationship between noise exposure and speech perception has also been difficult to demonstrate conclusively. Given that the MEMR is engaged at high sound levels, relationships between speech recognition in complex listening environments and noise exposure might be more evident at high speech presentation levels. In this exploratory study with 41 audiometrically normal listeners, a combination of behavioral and physiologic measures thought to be sensitive to synaptopathy were used to determine potential links with speech recognition at high presentation levels. We found decreasing speech recognition as a function of presentation level (from 74 to 104 dBA), which was associated with reduced MEMR magnitude. We also found that reduced MEMR magnitude was associated with higher estimated lifetime noise exposure. Together, these results suggest that the MEMR may be sensitive to noise-induced synaptopathy in humans, and this may underlie functional speech recognition deficits at high sound levels.

Dynamic Current Focusing: A Novel Approach to Loudness Coding in Cochlear Implants.

In an attempt to improve spectral resolution and speech intelligibility, several current focusing methods have been proposed to increase spatial selectivity by decreasing intracochlear current spread. For example, tripolar stimulation administers current to a central electrode and uses the two flanking electrodes as the return pathway, creating a narrower intracochlear electrical field and hence increases spectral resolution when compared with monopolar (MP) stimulation. However, more current is required, and in some patients, specifically the ones with high electrode impedances, full loudness growth cannot be supported because of compliance limits. The present study describes and analyses a new loudness encoding approach that uses tripolar stimulation near threshold and gradually broadens the excitation (by decreasing compensation coefficient σ) to increase loudness without the need to increase overall current. It is hypothesized that this dynamic current focusing (DCF) strategy increases spatial selectivity, especially at lower loudness levels, while maintaining maximum selectivity at higher loudness levels, without reaching compliance limits. Eleven adult cochlear implant recipients with postlingual hearing loss, with at least 9 months of experience with their HiRes90K implant, were selected to participate in this study. Baseline performance regarding speech intelligibility in noise (Dutch matrix sentence test), spectral ripple discrimination at 45 and 65 dB, and temporal modulation detection thresholds were assessed using their own clinical program, fitted on a Harmony processor. Subsequently, the DCF strategy was fitted on a research Harmony processor. Threshold levels were determined with σ = 0.8, which means 80% of current is returned to the flanking electrodes and the remaining 20% to the extracochlear ground electrode. Instead of increasing overall pulse magnitude, σ was decreased to determine most comfortable loudness. After 2 to 3 hr of adaptation to the research strategy, the same psychophysical measures were taken. At 45 dB, average spectral ripple scores improved significantly from 2.4 ripples per octave with their clinical program to 3.74 ripples per octave with the DCF strategy (p = 0.016). Eight out of 11 participants had an improved spectral resolution at 65 dB. Nevertheless, no significant difference between DCF and MP was observed at higher presentation levels. Both speech-in-noise and temporal modulation detection thresholds were equal for MP and DCF strategies. Subjectively, 2 participants preferred the DCF strategy over their own clinical program, 2 preferred their own strategy, while the majority of the participants had no preference. Battery life was decreased and ranged from 1.5 to 4 hr. The DCF strategy gives better spectral resolution, at lower loudness levels, but equal performance on speech tests. These outcomes warrant for a longer adaptation period to study long-term outcomes and evaluate if the outcomes in the ripple tests transfer to the speech scores. Further research, for example, with respect to fitting rules and reduction of power consumption, is necessary to make the DCF strategy suitable for routine clinical application.

Age-Related Temporal Processing Deficits in Word Segments in Adult Cochlear-Implant Users.

Aging may limit speech understanding outcomes in cochlear-implant (CI) users. Here, we examined age-related declines in auditory temporal processing as a potential mechanism that underlies speech understanding deficits associated with aging in CI users. Auditory temporal processing was assessed with a categorization task for the words dish and ditch (i.e., identify each token as the word dish or ditch) on a continuum of speech tokens with varying silence duration (0 to 60 ms) prior to the final fricative. In Experiments 1 and 2, younger CI (YCI), middle-aged CI (MCI), and older CI (OCI) users participated in the categorization task across a range of presentation levels (25 to 85 dB). Relative to YCI, OCI required longer silence durations to identify ditch and exhibited reduced ability to distinguish the words dish and ditch (shallower slopes in the categorization function). Critically, we observed age-related performance differences only at higher presentation levels. This contrasted with findings from normal-hearing listeners in Experiment 3 that demonstrated age-related performance differences independent of presentation level. In summary, aging in CI users appears to degrade the ability to utilize brief temporal cues in word identification, particularly at high levels. Age-specific CI programming may potentially improve clinical outcomes for speech understanding performance by older CI listeners.

Complex Consisting of β-Glucan and Antigenic Peptides with Cleavage Site for Glutathione and Aminopeptidases Induces Potent Cytotoxic T Lymphocytes.

The efficient induction of antigen-specific immune responses requires not only promotion of the uptake of antigens and adjuvant molecules into antigen-presenting cells but also control of their intracellular behavior. We previously demonstrated that the β-glucan schizophyllan (SPG) can form complexes with CpG oligonucleotides with attached dA40 (CpG-dA/SPG), which can accumulate in macrophages in the draining inguinal lymph nodes and induce strong immune responses. In this study, we prepared various conjugates composed of antigenic peptide (OVA257-264) and dA40 and made complexes with SPG. The conjugates with a disulfide bond between OVA257-264 and dA40 were easily cleaved by glutathione. The resultant peptides with a hydrophobic amino acid at the C-terminal end was recognized by puromycin-insensitive leucine aminopeptidase (PILS-AP), which trims antigenic peptide precursors and prepares peptides of eight or nine amino acids in length, which is the optimal length for binding to major histocompatibility complex (MHC)-I. The conjugate exposed to such enzymes induced a high antigen presentation level. The antigen presentation level was almost the same before and after the complexation with SPG. Immunization with a mixture of dA-OVA257-264/SPG and CpG-dA/SPG induced high antigen-specific cytotoxic T-lymphocyte activity at a much lower peptide dose than in previous studies. These results can be strongly ascribed to not only the cell-specific delivery by SPG but also the control of the intracellular behavior by the introduction of cleavage sites. Therefore, peptide-dA/SPG complexes could be used as potent vaccine antigens for the treatment of cancers and infectious diseases.

Speech audiometry findings from HIV+ and HIV− adults in the MACS and WIHS longitudinal cohort studies

The purpose of this study was to compare various speech audiometry measures between HIV+ and HIV− adults and to further evaluate the association between speech audiometry and HIV disease variables in HIV+ adults only. Three hundred ninety-six adults from the Multicenter AIDS Cohort Study (MACS) and Women’s Interagency HIV Study (WIHS) completed speech audiometry testing. There were 262 men, of whom 117 (44.7%) were HIV+, and 134 women, of whom 105 (78.4%) were HIV+. Speech audiometry was conducted as part of the standard clinical audiological evaluation that included otoscopy, tympanometry, and pure-tone air- and bone-conduction thresholds. Specific speech audiometry measures included speech recognition thresholds (SRT) and word recognition scores in quiet presented at 40dB sensation level (SL) in reference to the SRT. SRT data were categorized in 5-dB steps from 0 to 25dB hearing level (HL) with one category as ≥30dB HL while word recognition scores were categorized as <90%, 90–99%, and 100%. A generalized estimating equations model was used to evaluate the association between HIV status and both ordinal outcomes. The SRT distributions across HIV+ and HIV− adults were similar. HIV+ and HIV− adults had a similar percentages of word recognition scores <90%, a lower percentage of HIV− adults had 90–99%, but HIV− adults had a higher percentage of 100%. After adjusting for covariables, HIV+ adults were borderline significantly more likely to have a higher SRT than HIV− adults (odds ratio [OR]=1.45, p=0.06). Among HIV+ adults, HIV-related variables (i.e., CD4+ T-cell counts, HIV viral load, and ever history of clinical AIDS) were not significantly associated with either SRT or word recognition score data. There was, however, a ceiling effect for word recognition scores, probably the result of obtaining this measure in quiet with a relatively high presentation level. A more complex listening task, such as speech-in-noise testing, may be a more clinically informative test to evaluate the effects of HIV on speech communication.

Single- and multi-channel modulation detection in cochlear implant users.

Single-channel modulation detection thresholds (MDTs) have been shown to predict cochlear implant (CI) users’ speech performance. However, little is known about multi-channel modulation sensitivity. Two factors likely contribute to multichannel modulation sensitivity: multichannel loudness summation and the across-site variance in single-channel MDTs. In this study, single- and multi-channel MDTs were measured in 9 CI users at relatively low and high presentation levels and modulation frequencies. Single-channel MDTs were measured at widely spaced electrode locations, and these same channels were used for the multichannel stimuli. Multichannel MDTs were measured twice, with and without adjustment for multichannel loudness summation (i.e., at the same loudness as for the single-channel MDTs or louder). Results showed that the effect of presentation level and modulation frequency were similar for single- and multi-channel MDTs. Multichannel MDTs were significantly poorer than single-channel MDTs when the current levels of the multichannel stimuli were reduced to match the loudness of the single-channel stimuli. This suggests that, at equal loudness, single-channel measures may over-estimate CI users’ multichannel modulation sensitivity. At equal loudness, there was no significant correlation between the amount of multichannel loudness summation and the deficit in multichannel MDTs, relative to the average single-channel MDT. With no loudness compensation, multichannel MDTs were significantly better than the best single-channel MDT. The across-site variance in single-channel MDTs varied substantially across subjects. However, the across-site variance was not correlated with the multichannel advantage over the best single channel. This suggests that CI listeners combined envelope information across channels instead of attending to the best channel.

Sound Advice for ABR Analysis

Sound Advice for ABR Analysis

The Effect of Automatic Gain Control Structure and Release Time on Cochlear Implant Speech Intelligibility

Nucleus cochlear implant systems incorporate a fast-acting front-end automatic gain control (AGC), sometimes called a compression limiter. The objective of the present study was to determine the effect of replacing the front-end compression limiter with a newly proposed envelope profile limiter. A secondary objective was to investigate the effect of AGC speed on cochlear implant speech intelligibility. The envelope profile limiter was located after the filter bank and reduced the gain when the largest of the filter bank envelopes exceeded the compression threshold. The compression threshold was set equal to the saturation level of the loudness growth function (i.e. the envelope level that mapped to the maximum comfortable current level), ensuring that no envelope clipping occurred. To preserve the spectral profile, the same gain was applied to all channels. Experiment 1 compared sentence recognition with the front-end limiter and with the envelope profile limiter, each with two release times (75 and 625 ms). Six implant recipients were tested in quiet and in four-talker babble noise, at a high presentation level of 89 dB SPL. Overall, release time had a larger effect than the AGC type. With both AGC types, speech intelligibility was lower for the 75 ms release time than for the 625 ms release time. With the shorter release time, the envelope profile limiter provided higher group mean scores than the front-end limiter in quiet, but there was no significant difference in noise. Experiment 2 measured sentence recognition in noise as a function of presentation level, from 55 to 89 dB SPL. The envelope profile limiter with 625 ms release time yielded better scores than the front-end limiter with 75 ms release time. A take-home study showed no clear pattern of preferences. It is concluded that the envelope profile limiter is a feasible alternative to a front-end compression limiter.

Bone-Conducted Hearing Assessment with 80-Hz Multiple Auditory Steady-State Responses to Brief Tones in Adults with Normal Hearing

Objective: To investigate interactions (if any) in the bone-conduction auditory steady-state response (BC ASSR) between multiple brief tones presented simultaneously. Methods: 500-, 1,000-, 2,000-, and 4,000-Hz brief tones, repeated at a rate of 77–101 Hz, were presented using a B-71 vibrator. BC ASSR thresholds and amplitudes at 50 dB nHL were measured in two conditions where the stimulus was either presented alone or together with other stimuli. Results: Significantly larger amplitudes in the single-stimulus condition were found at 50 dB nHL. However, there was no significant threshold difference between single- and multiple-stimulus conditions. The BC ASSR thresholds (means ± SD) at 500, 1,000, 2,000, and 4,000 Hz were 96.7 ± 9.7, 75.3 ± 11.5, 65.6 ± 7.4, and 57.8 ± 7.2 dB re 1 µN ppe, respectively. Conclusion: Interactions occurred in the multiple-stimulus condition at high presentation levels, but not at threshold levels. The results of the present study imply that BC ASSR thresholds to multiple brief-tone stimuli can be assessed at the same time, at least in normal-hearing adults.

The Effect of Presentation Level on Memory Performance

A loss of speech intelligibility at high presentation levels is called rollover. It is a phenomenon that increases in prevalence as people age. Whether the adverse effect of high presentation levels extends to processes subsequent to speech intelligibility, such as memory, is unknown. The present study examined this question on the basis of the previous finding that older but not younger adults showed memory impairment when acoustically distorted words were presented at 50 dB SL compared with an undistorted baseline presented at 65 dB SPL. One question investigated in the present study was whether a presentation level of 50 dB SL put older listeners at the cusp of rollover and whether this subsequently impaired memory. Moreover, we wanted to know whether and at what level it was possible to induce a similar impairment in younger listeners. We used a paired-associate memory paradigm in which five word pairs per list were presented at a rate of 4 secs per word pair. After each list, the first word of one of the pairs was presented again and the listener was asked to recall the second word. Over the course of the experiment, all list positions were tested an equal number of times. The word pairs, which were acoustically distorted using a jittering algorithm, were presented at 40 dB SL to all younger and older participants and just below an uncomfortably loud level for younger listeners only. Intelligibility of the distorted words was equated across age groups for each presentation level. The effect of presentation level on memory performance was investigated and compared with data of a previous study that used the same design but presented the distorted and undistorted words at 50 dB SL to both age groups. A total of 58 younger and 24 older adults were tested in two experiments. The results showed that for older adults, memory performance for distorted words was decreased in all list positions at a presentation level of 50 dB SL compared with 40 dB SL and an undistorted 65 dB SPL baseline. This effect did not occur for younger listeners. However, when younger adults were tested at a very high presentation level, they showed the same memory decrease compared with the baseline as older adults showed for 50 dB SL. A high presentation level of distorted words can adversely affect memory even after intelligibility is equated for. Moreover, older listeners are affected at lower presentation levels. Hence, the choice of sound level, particularly for older listeners, is important and may affect their level of cognitive performance beyond its effects on intelligibility. Higher presentation levels may not always lead to better performance when the task involves recall of words previously heard.

Effects of Signal Level and Background Noise on Spectral Representations in the Auditory Nerve of the Domestic Cat

Background noise poses a significant obstacle for auditory perception, especially among individuals with hearing loss. To better understand the physiological basis of this perceptual impediment, the present study evaluated the effects of background noise on the auditory nerve representation of head-related transfer functions (HRTFs). These complex spectral shapes describe the directional filtering effects of the head and torso. When a broadband sound passes through the outer ear en route to the tympanic membrane, the HRTF alters its spectrum in a manner that establishes the perceived location of the sound source. HRTF-shaped noise shares many of the acoustic features of human speech, while communicating biologically relevant localization cues that are generalized across mammalian species. Previous studies have used parametric manipulations of random spectral shapes to elucidate HRTF coding principles at various stages of the cat's auditory system. This study extended that body of work by examining the effects of sound level and background noise on the quality of spectral coding in the auditory nerve. When fibers were classified by their spontaneous rates, the coding properties of the more numerous low-threshold, high-spontaneous rate fibers were found to degrade at high presentation levels and in low signal-to-noise ratios. Because cats are known to maintain accurate directional hearing under these challenging listening conditions, behavioral performance may be disproportionally based on the enhanced dynamic range of the less common high-threshold, low-spontaneous rate fibers.

Measures of hearing threshold and temporal processing across the adult lifespan

Psychophysical data on hearing sensitivity and various measures of supra-threshold auditory temporal processing are presented for large groups of young (18–35 y), middle-aged (40–55 y) and older (60–89 y) adults. Hearing thresholds were measured at 500, 1414 and 4000 Hz. Measures of temporal processing included gap-detection thresholds for bands of noise centered at 1000 and 3500 Hz, stimulus onset asynchronies for monaural and dichotic temporal-order identification for brief vowels, and stimulus onset/offset asynchronies for the monaural temporal masking of vowel identification. For all temporal-processing measures, the impact of high-frequency hearing loss in older adults was minimized by a combination of low-pass filtering the stimuli and use of high presentation levels. The performance of the older adults was worse than that of the young adults on all measures except gap-detection threshold at 1000 Hz. Middle-aged adults performed significantly worse than the young adults on measures of threshold sensitivity and three of the four measures of temporal-order identification, but not for any of the measures of temporal masking. Individual differences are also examined among a group of 124 older adults. Cognition and age were found to be significant predictors, although only 10–27% of the variance could be accounted for by these predictors.

The combined effects of distortions on vowel identification.

The effects of intrinsic and extrinsic distortions on vowel perception were evaluated. Potential sources of distortion were high presentation level, background noise, stimulus ambiguity, and hearing status. Vowel identifications were collected for three sets of synthesized vowels that varied in both F1 and F2 (front and back vowels), or F2 and F3 (central vowels). The systematic variation in formant frequencies produced stimulus sets that contained both clear, unambiguous category tokens as well as relatively ambiguous tokens that could be identified as more than one vowel category. As overall presentation level increased from 75–90 dB SPL, identification by normally hearing listeners became less consistent for ambiguous stimuli. Additional reductions in categorization precision occurred in the presence of background noise. The effects were highly dependent on stimulus set, with the largest effects observed for the central vowels. Data from hearing-impaired listeners at high presentation levels and in noise will also be reported. Evaluation of response patterns in the presence of combinations of these sources of distortion will help explore the difficulty people with hearing loss have processing speech in adverse listening conditions. [Work supported by OHSU Medical Research Foundation, Portland, OR.]