Modeling the Intelligibility Benefit of Active Noise Cancelation in Hearing Devices That Improve Signal-to-Noise Ratio.

The extent to which active noise cancelation (ANC), when combined with hearing assistance, can improve speech intelligibility in noise is not well understood. One possible source of benefit is ANC's ability to reduce the sound level of the direct (i.e., vent-transmitted) path. This reduction lowers the "floor" imposed by the direct path, thereby allowing any increases to the signal-to-noise ratio (SNR) created in the amplified path to be "realized" at the eardrum. Here we used a modeling approach to estimate this benefit. We compared pairs of simulated hearing aids that differ only in terms of their ability to provide ANC and computed intelligibility metrics on their outputs. The difference in metric scores between simulated devices is termed the "ANC Benefit." These simulations show that ANC Benefit increases as (1) the environmental sound level increases, (2) the ability of the hearing aid to improve SNR increases, (3) the strength of the ANC increases, and (4) the hearing loss severity decreases. The predicted size of the ANC Benefit can be substantial. For a moderate hearing loss, the model predicts improvement in intelligibility metrics of >30% when environments are moderately loud (>70 dB SPL) and devices are moderately capable of increasing SNR (by >4 dB). It appears that ANC can be a critical ingredient in hearing devices that attempt to improve SNR in loud environments. ANC will become more and more important as advanced SNR-improving algorithms (e.g., artificial intelligence speech enhancement) are included in hearing devices.

Perceptual learning evidence for tuning to spectrotemporal modulation in the human auditory system.

Natural sounds are characterized by complex patterns of sound intensity distributed across both frequency (spectral modulation) and time (temporal modulation). Perception of these patterns has been proposed to depend on a bank of modulation filters, each tuned to a unique combination of a spectral and a temporal modulation frequency. There is considerable physiological evidence for such combined spectrotemporal tuning. However, direct behavioral evidence is lacking. Here we examined the processing of spectrotemporal modulation behaviorally using a perceptual-learning paradigm. We trained human listeners for ∼1 h/d for 7 d to discriminate the depth of spectral (0.5 cyc/oct; 0 Hz), temporal (0 cyc/oct; 32 Hz), or upward spectrotemporal (0.5 cyc/oct; 32 Hz) modulation. Each trained group learned more on their respective trained condition than did controls who received no training. Critically, this depth-discrimination learning did not generalize to the trained stimuli of the other groups or to downward spectrotemporal (0.5 cyc/oct; -32 Hz) modulation. Learning on discrimination also led to worsening on modulation detection, but only when the same spectrotemporal modulation was used for both tasks. Thus, these influences of training were specific to the trained combination of spectral and temporal modulation frequencies, even when the trained and untrained stimuli had one modulation frequency in common. This specificity indicates that training modified circuitry that had combined spectrotemporal tuning, and therefore that circuits with such tuning can influence perception. These results are consistent with the possibility that the auditory system analyzes sounds through filters tuned to combined spectrotemporal modulation.

Acoustical correlates of performance on a dynamic range compression discrimination task.

Dynamic range compression is widely used to reduce the difference between the most and least intense portions of a signal. Such compression distorts the shape of the amplitude envelope of a signal, but it is unclear to what extent such distortions are actually perceivable by listeners. Here, the ability to distinguish between compressed and uncompressed versions of a noise vocoded sentence was initially measured in listeners with normal hearing while varying the threshold, ratio, attack, and release parameters. This narrow condition was selected in order to characterize perception under the most favorable listening conditions. The average behavioral sensitivity to compression was highly correlated to several acoustical indices of modulation depth. In particular, performance was highly correlated to the Euclidean distance between the modulation spectra of the uncompressed and compressed signals. Suggesting that this relationship is not restricted to the initial test conditions, the correlation remained largely unchanged both (1) when listeners with normal hearing were tested using a time-compressed version of the original signal, and (2) when listeners with impaired hearing were tested using the original signal. If this relationship generalizes to more ecologically valid conditions, it will provide a straightforward method for predicting the detectability of compression-induced distortions.

Perceptual learning of auditory spectral modulation detection.

Normal sensory perception requires the ability to detect and identify patterns of activity distributed across the receptor surface. In the visual system, the ability to perceive these patterns across the retina improves with training. This learning differs in magnitude for different trained stimuli and does not generalize to untrained spatial frequencies or retinal locations. Here, we asked whether training to detect patterns of activity across the cochlea yields learning with similar characteristics. Differences in learning between the visual and auditory systems would be inconsistent with the suggestion that the ability to detect these patterns is limited by similar constraints in these two systems. We trained three groups of normal-hearing listeners to detect spectral envelopes with a sinusoidal shape (spectral modulation) at 0.5, 1, or 2 cycles/octave and compared the performance of each group to that of a separate group that received no training. On average, as the trained spectral modulation frequency increased, the magnitude of training-induced improvement and the time to reach asymptotic performance decreased, while the tendency for performance to worsen within a training session increased. The training-induced improvements did not generalize to untrained spectral modulation frequencies or untrained carrier spectra. Thus, for both visual-spatial and auditory spectral modulation detection, learning depended upon and was specific to analogous features of the trained stimulus. Such similarities in learning could arise if, as has been suggested, similar constraints limit the ability to detect patterns across the receptor surface between the auditory and visual systems.

Generalization lags behind learning on an auditory perceptual task.

The generalization of learning from trained to untrained conditions is of great potential value because it markedly increases the efficacy of practice. In principle, generalization and the learning itself could arise from either the same or distinct neural changes. Here, we assessed these two possibilities in the realm of human perceptual learning by comparing the time course of improvement on a trained condition (learning) to that on an untrained condition (generalization) for an auditory temporal-interval discrimination task. While significant improvement on the trained condition occurred within 2 d, generalization to the untrained condition lagged behind, only emerging after 4 d. The different time courses for learning and generalization suggest that these two types of perceptual improvement can arise from at least partially distinct neural changes. The notably longer time course for generalization than learning demonstrates that increasing the duration of training can be an effective means to increase the number of conditions to which learning generalizes on perceptual tasks.

Enhancing perceptual learning by combining practice with periods of additional sensory stimulation.

Perceptual skills can be improved even in adulthood, but this learning seldom occurs by stimulus exposure alone. Instead, it requires considerable practice performing a perceptual task with relevant stimuli. It is thought that task performance permits the stimuli to drive learning. A corresponding assumption is that the same stimuli do not contribute to improvement when encountered separately from relevant task performance because of the absence of this permissive signal. However, these ideas are based on only two types of studies, in which the task was either always performed or not performed at all. Here we demonstrate enhanced perceptual learning on an auditory frequency-discrimination task in human listeners when practice on that target task was combined with additional stimulation. Learning was enhanced regardless of whether the periods of additional stimulation were interleaved with or provided exclusively before or after target-task performance, and even though that stimulation occurred during the performance of an irrelevant (auditory or written) task. The additional exposures were only beneficial when they shared the same frequency with, though they did not need to be identical to, those used during target-task performance. Their effectiveness also was diminished when they were presented 15 min after practice on the target task and was eliminated when that separation was increased to 4 h. These data show that exposure to an acoustic stimulus can facilitate learning when encountered outside of the time of practice on a perceptual task. By properly using additional stimulation one may markedly improve the efficiency of perceptual training regimens.

Weighting function-based mapping of descriptors to frequency-gain curves in listeners with hearing loss.

OBJECTIVES: The frequency-gain curve (FGC) is among the most important parameters to consider when fitting a hearing aid. In practice, a prescriptive FGC, derived from the audiogram, is initially applied. In the subsequent fine-tuning stage, the patient often communicates their concerns about the sound quality using descriptors (e.g., "it sounds hollow") and the clinician modifies the FGC accordingly. In this study, we present and evaluate a method that could enhance this process by rapidly mapping descriptors to FGC shapes. In addition, we begin to use this method to examine the extent to which there is across-individual agreement in how descriptors map to FGC shapes. DESIGN: Ten listeners with hearing loss rated the extent to which each of a series of FGCs captured the meaning of a particular descriptor. Regression analyses were conducted to determine the degree to which these ratings were correlated with the gain values associated with each of 25 frequency bands. The array of slopes of these regression lines across frequency bands is termed the weighting function and was interpreted as the FGC shape that corresponded to the descriptor. We used this procedure to determine the FGC shapes associated with four of the most common descriptors used to describe hearing aid sound quality problems ("tinny," "sharp," "hollow," and "in a barrel, tunnel, or well"). RESULTS: The weighting function shape was highly replicable despite variable listener responses, reached asymptotic performance quickly (<20 ratings), and was predictive of listener responses. On the global level, there was some agreement across individuals about how common descriptors mapped to weighting function shape. However, considerable differences were apparent between individuals in terms of the specifics of that mapping. CONCLUSIONS: The current approach for descriptor-to-FGC mapping is a quick, reliable method for determining individualized changes to the FGC. Given the range of individual differences in the specifics of the descriptor-to-FGC mappings observed, this approach could be useful in a clinical setting to easily quantify these acoustic parameters. Implementation of such procedures could lead to more personalized fine-tuning of amplification devices.

Validation of a Self-Fitting Method for Over-the-Counter Hearing Aids.

In common practice, hearing aids are fitted by a clinician who measures an audiogram and uses it to generate prescriptive gain and output targets. This report describes an alternative method where users select their own signal processing parameters using an interface consisting of two wheels that optimally map to simultaneous control of gain and compression in each frequency band. The real-world performance of this approach was evaluated via a take-home field trial. Participants with hearing loss were fitted using clinical best practices (audiogram, fit to target, real-ear verification, and subsequent fine tuning). Then, in their everyday lives over the course of a month, participants either selected their own parameters using this new interface (Self group; n = 38) or used the parameters selected by the clinician with limited control (Audiologist Best Practices Group; n = 37). On average, the gain selected by the Self group was within 1.8 dB overall and 5.6 dB per band of that selected by the audiologist. Participants in the Self group reported better sound quality than did those in the Audiologist Best Practices group. In blind sound quality comparisons conducted in the field, participants in the Self group slightly preferred the parameters they selected over those selected by the clinician. Finally, there were no differences between groups in terms of standard clinical measures of hearing aid benefit or speech perception in noise. Overall, the results indicate that it is possible for users to select effective amplification parameters by themselves using a simple interface that maps to key hearing aid signal processing parameters.

Perceptual-learning evidence for inter-onset-interval- and frequency-specific processing of fast rhythms.

Rhythm is fundamental to music and speech, yet little is known about how even simple rhythmic patterns are processed. Here we investigated the processing of isochronous rhythms in the short inter-onset-interval (IOI) range (IOIs < 250-400 ms) using a perceptual-learning paradigm. Trained listeners (n=8) practiced anisochrony detection with a 100-ms IOI marked by 1-kHz tones, 720 trials per day for 7 days. Between pre- and post-training tests, trained listeners improved significantly more than controls (no training; n=8) on the anisochrony-detection condition that the trained listeners practiced. However, the learning on anisochrony detection did not generalize to temporal-interval discrimination with the trained IOI (100 ms) and marker frequency (1 kHz) or to anisochrony detection with an untrained marker frequency (4 kHz or variable frequency vs. 1 kHz), and generalized negatively to anisochrony detection with an untrained IOI (200 ms vs. 100 ms). Further, pre-training thresholds were correlated among nearly all of the conditions with the same IOI (100-ms IOIs), but not between conditions with different IOIs (100-ms vs. 200-ms IOIs). Thus, it appears that some task-, IOI-, and frequency-specific processes are involved in fast-rhythm processing. These outcomes are most consistent with a holistic rhythm-processing model in which a holistic "image" of the stimulus is compared to a stimulus-specific template.

Binaural weighting of monaural spectral cues for sound localization.

For human listeners, cues for vertical-plane localization are provided by direction-dependent pinna filtering. This study quantified listeners' weighting of the spectral cues from each ear as a function of stimulus lateral angle, interaural time difference (ITD), and interaural level difference (ILD). Subjects indicated the apparent position of headphone-presented noise bursts synthesized in virtual auditory space. The synthesis filters for the two ears either corresponded to the same location or to two different locations separated vertically by 20 deg. Weighting of each ear's spectral information was determined by a multiple regression between the elevations to which each ear's spectrum corresponded and the vertical component of listeners' responses. The apparent horizontal source location was controlled either by choosing synthesis filters corresponding to locations on or 30 deg left or right of the median plane or by attenuating or delaying the signal at one ear. For broadband stimuli, spectral weighting and apparent lateral angle were determined primarily by ITD. Only for high-pass stimuli were weighting and lateral angle determined primarily by ILD. The results suggest that the weighting of monaural spectral cues and the perceived lateral angle of a sound source depend similarly on ITD, ILD, and stimulus spectral range.

Human sound localization at near-threshold levels.

Physiological studies of spatial hearing show that the spatial receptive fields of cortical neurons typically are narrow at near-threshold levels, broadening at moderate levels. The apparent loss of neuronal spatial selectivity at increasing sound levels conflicts with the accurate performance of human subjects localizing at moderate sound levels. In the present study, human sound localization was evaluated across a wide range of sensation levels, extending down to the detection threshold. Listeners reported whether they heard each target sound and, if the target was audible, turned their heads to face the apparent source direction. Head orientation was tracked electromagnetically. At near-threshold levels, the lateral (left/right) components of responses were highly variable and slightly biased towards the midline, and front vertical components consistently exhibited a strong bias towards the horizontal plane. Stimulus levels were specified relative to the detection threshold for a front-positioned source, so low-level rear targets often were inaudible. As the sound level increased, first lateral and then vertical localization neared asymptotic levels. The improvement of localization over a range of increasing levels, in which neural spatial receptive fields presumably are broadening, indicates that sound localization does not depend on narrow spatial receptive fields of cortical neurons.

Vertical-plane sound localization with distorted spectral cues.

For human listeners, the primary cues for localization in the vertical plane are provided by the direction-dependent filtering of the pinnae, head, and upper body. Vertical-plane localization generally is accurate for broadband sounds, but when such sounds are presented at near-threshold levels or at high levels with short durations (<20 ms), the apparent location is biased toward the horizontal plane (i.e., elevation gain <1). We tested the hypothesis that these effects result in part from distorted peripheral representations of sound spectra. Human listeners indicated the apparent position of 100-ms, 50-60 dB SPL, wideband noise-burst targets by orienting their heads. The targets were synthesized in virtual auditory space and presented over headphones. Faithfully synthesized targets were interleaved with targets for which the directional transfer function spectral notches were filled in, peaks were leveled off, or the spectral contrast of the entire profile was reduced or expanded. As notches were filled in progressively or peaks leveled progressively, elevation gain decreased in a graded manner similar to that observed as sensation level is reduced below 30 dB or, for brief sounds, increased above 45 dB. As spectral contrast was reduced, gain dropped only at the most extreme reduction (25% of normal). Spectral contrast expansion had little effect. The results are consistent with the hypothesis that loss of representation of spectral features contributes to reduced elevation gain at low and high sound levels. The results also suggest that perceived location depends on a correlation-like spectral matching process that is sensitive to the relative, rather than absolute, across-frequency shape of the spectral profile.

Initial development of a temporal-envelope-preserving nonlinear hearing aid prescription using a genetic algorithm.

Most hearing aid prescriptions focus on the optimization of a metric derived from the long-term average spectrum of speech, and do not consider how the prescribed values might distort the temporal envelope shape. A growing body of evidence suggests that such distortions can lead to systematic errors in speech perception, and therefore hearing aid prescriptions might benefit by including preservation of the temporal envelope shape in their rationale. To begin to explore this possibility, we designed a genetic algorithm (GA) to find the multiband compression settings that preserve the shape of the original temporal envelope while placing that envelope in the listener's audiometric dynamic range. The resulting prescription had a low compression threshold, short attack and release times, and a combination of compression ratio and gain that placed the output signal within the listener's audiometric dynamic range. Initial behavioral tests of individuals with impaired hearing revealed no difference in speech-in-noise perception between the GA and the NAL-NL2 prescription. However, gap detection performance was superior with the GA in comparison to NAL-NL2. Overall, this work is a proof of concept that consideration of temporal envelope distortions can be incorporated into hearing aid prescriptions.

Sound localization in noise and sensitivity to spectral shape.

Individual differences exist in sound localization performance even for normal-hearing listeners. Some of these differences might be related to acoustical differences in localization cues carried by the head related transfer functions (HRTF). Recent data suggest that individual differences in sound localization performance could also have a perceptual origin. The localization of an auditory target in the up/down and front/back dimensions requires the analysis of the spectral shape of the stimulus. In the present study, we investigated the role of an acoustic factor, the prominence of the spectral shape ("spectral strength") and the role of a perceptual factor, the listener's sensitivity to spectral shape, in individual differences observed in sound localization performance. Spectral strength was computed as the spectral distance between the magnitude spectrum of the HRTFs and a flat spectrum. Sensitivity to spectral shape was evaluated using spectral-modulation thresholds measured with a broadband (0.2-12.8 kHz) or high-frequency (4-16 kHz) carrier and for different spectral modulation frequencies (below 1 cycle/octave, between 1 and 2 cycles/octave, above 2 cycles/octave). Data obtained from 19 young normal-hearing listeners showed that low thresholds for spectral modulation frequency below 1 cycle/octave with a high-frequency carrier were associated with better sound localization performance. No correlation was found between sound localization performance and the spectral strength of the HRTFs. These results suggest that differences in perceptual ability, rather than acoustical differences, contribute to individual differences in sound localization performance in noise.

Hearing outcomes in stapes surgery: a comparison of fat, fascia, and vein tissue seals.

OBJECTIVE: To compare short- and long-term hearing results following stapedectomy using 3 different oval window grafting materials with the same stapes prosthesis. STUDY DESIGN: Database review. SETTING: Tertiary referral private practice. SUBJECTS AND METHODS: Subjects were ears that underwent stapedectomy for otosclerosis, with placement of fat, fascia, or vein as an oval window seal and reconstruction with a titanium bucket handle prosthesis. A total of 365 procedures met these inclusion criteria: 98 fat grafts, 135 fascia grafts, and 132 vein grafts. Outcome measures included short-term (<1 year) and long-term follow-up air-bone gap. We compared the preoperative and postoperative amount of change in air-bone gap and preoperative and postoperative amount of change in the high-frequency bone conduction average. RESULTS: Overall median times to short-term and long-term follow-ups were 2.2 months and 36.1 months, respectively. There were no statistically significant differences between the 3 tissue seal groups in the amount of change in air-bone gap. There was no significant difference in amount of change in high-frequency bone conduction (representing sensorineural hearing level) between the 3 tissue seal groups. Most patients in all 3 groups had an air-bone gap at long-term follow-up of ≤ 10 dB (fat, 79.5%; fascia, 78.8%; and vein, 75.6%), with 90.3% of all patients at ≤ 20 dB. CONCLUSIONS: In both the short-term postoperative period and long-term follow-up, there were no significant differences in hearing results among 3 types of tissue seals of the oval window in stapes surgery. Fat, fascia, and vein grafts all provide satisfactory hearing outcomes in stapedectomy.

Different patterns of perceptual learning on spectral modulation detection between older hearing-impaired and younger normal-hearing adults.

Young adults with normal hearing (YNH) can improve their sensitivity to basic acoustic features with practice. However, it is not known to what extent the influence of the same training regimen differs between YNH listeners and older listeners with hearing impairment (OHI)--the largest population seeking treatment in audiology clinics. To examine this issue, we trained OHI listeners on a basic auditory task (spectral modulation detection) using a training regimen previously administered to YNH listeners (≈ 1 h/session for seven sessions on a single condition). For the trained conditions on which pretraining performance was not already at asymptote, the YNH listeners who received training learned more than matched controls who received none, but that learning did not generalize to any untrained spectral modulation frequency. In contrast, the OHI-trained listeners and controls learned similar amounts on the trained condition, implying no effect of the training itself. However, surprisingly the OHI-trained listeners improved over the training phase and on an untrained spectral modulation frequency. These population differences suggest that learning consolidated more slowly, and that training modified an aspect of processing that had broader tuning to spectral modulation frequency, in OHI than YNH listeners. More generally, these results demonstrate that conclusions about perceptual learning that come from examination of one population do not necessarily apply to another.

Separable developmental trajectories for the abilities to detect auditory amplitude and frequency modulation.

Amplitude modulation (AM) and frequency modulation (FM) are inherent components of most natural sounds. The ability to detect these modulations, considered critical for normal auditory and speech perception, improves over the course of development. However, the extent to which the development of AM and FM detection skills follow different trajectories, and therefore can be attributed to the maturation of separate processes, remains unclear. Here we explored the relationship between the developmental trajectories for the detection of sinusoidal AM and FM in a cross-sectional design employing children aged 8-10 and 11-12 years and adults. For FM of tonal carriers, both average performance (mean) and performance consistency (within-listener standard deviation) were adult-like in the 8-10 y/o. In contrast, in the same listeners, average performance for AM of wideband noise carriers was still not adult-like in the 11-12 y/o, though performance consistency was already mature in the 8-10 y/o. Among the children there were no significant correlations for either measure between the degrees of maturity for AM and FM detection. These differences in developmental trajectory between the two modulation cues and between average detection thresholds and performance consistency suggest that at least partially distinct processes may underlie the development of AM and FM detection as well as the abilities to detect modulation and to do so consistently.

Perceptual learning: how much daily training is enough?

The acquisition of many perceptual skills proceeds over a course of days. However, little is known about how much daily training is needed for such learning to occur. Here we investigated this question by examining how varying the number of training trials per day affected learning over multiple days on two auditory discrimination tasks: frequency discrimination and temporal-interval discrimination. For each task, we compared improvements in discrimination thresholds between different groups of listeners who were trained for either 360 or 900 trials per day for 6 days. Improvement on frequency discrimination required >360 trials of training per day while learning on temporal-interval discrimination occurred with 360 training trials per day, and additional daily practice did not increase the amount of improvement. It therefore appears that the accumulation of improvement over days on auditory discrimination tasks may require some critical amount of training per day, that training beyond that critical amount yields no additional learning on the trained condition, and that the critical amount of training needed varies across tasks. These results imply that perceptual skills are transferred from short- to long-term memory (consolidated) daily, but only if a task-specific initiation requirement has been met.