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Although animal models show a clear link between noise exposure and damage to afferent cochlear synapses, the relationship between noise exposure and efferent function appears to be more complex. Animal studies indicate that high intensity noise exposure reduces efferent medial olivocochlear (MOC) reflex strength, whereas chronic moderate noise exposure is associated with a conditioning effect that enhances the MOC reflex. The MOC reflex is predicted to improve speech-in-noise perception and protects against noise-induced auditory damage by reducing cochlear gain. In humans, MOC reflex strength can be estimated by measuring contralateral inhibition of distortion product otoacoustic emissions (DPOAEs). The objective of this study was to determine the impact of military noise exposure on efferent auditory function by measuring DPOAE contralateral inhibition in young Veterans and non-Veterans with normal audiograms. Compared with non-Veteran controls, Veterans with high levels of reported noise exposure demonstrated a trend of reduced contralateral inhibition across a broad frequency range, suggesting efferent damage. Veterans with moderate noise exposure showed trends of reduced inhibition from 3 to 4 kHz but greater inhibition from 1 to 1.5 kHz, consistent with conditioning. These findings suggest that, in humans, the impact of noise exposure on the MOC reflex differs depending on the noise intensity and duration.

For listeners with bilateral cochlear implants (BiCIs), patient-specific differences in the interface between cochlear implant (CI) electrodes and the auditory nerve can lead to degraded temporal envelope information, compromising the ability to distinguish between targets of interest and background noise. It is unclear how comparisons of degraded temporal envelope information across spectral channels (i.e., electrodes) affect the ability to detect differences in the temporal envelope, specifically amplitude modulation (AM) rate. In this study, two pulse trains were presented simultaneously via pairs of electrodes in different places of stimulation, within and/or across ears, with identical or differing AM rates. Results from 11 adults with BiCIs indicated that sensitivity to differences in AM rate was greatest when stimuli were paired between different places of stimulation in the same ear. Sensitivity from pairs of electrodes was predicted by the poorer electrode in the pair or the difference in fidelity between both electrodes in the pair. These findings suggest that electrodes yielding poorer temporal fidelity act as a bottleneck to comparisons of temporal information across frequency and ears, limiting access to the cues used to segregate sounds, which has important implications for device programming and optimizing patient outcomes with CIs.

The ability of older adults (48 to 72) with relatively intact low-frequency hearing to detect the motion of an acoustic source was investigated using dynamically varying interaural delays. Thresholds were measured using a single-interval two-alternative forced-choice task in which listeners determined if the sound source was moving or stationary. Motion thresholds were significantly larger than stationary localization thresholds. No correlation was observed between age and motion-detection ability for the age range tested. An interesting finding was that there were similar thresholds for older and younger adults. Results suggest reliance on dominant low-frequency binaural timing cues unaffected by high-frequency hearing loss in older adults.

Infrared laser stimulation has been studied as an alternative approach to auditory prostheses. This study evaluated the feasibility of infrared laser stimulation of the cochlea from the outer ear, bypassing the middle ear function. An optic fiber was inserted into the ear canal, and a laser was used to irradiate the cochlea through the tympanic membrane in Mongolian gerbils. A pulsed infrared laser (6.9 mJ/cm2) and clicking sound (70 peak-to-peak equivalent sound pressure level) were presented to the animals. The amplitude of the laser-evoked cochlear response was systematically decreased following insertion of a filter between the tympanic membrane and cochlea; however, the auditory-evoked cochlear response did not decrease. The filter was removed, and the laser-evoked response returned to around the original level. The amplitude ratio and the relative change in response amplitude before and during filter insertion significantly decreased as the absorbance of the infrared filter increased. These results indicate that laser irradiation could bypass the function of the middle ear and directly activate the cochlea. Therefore, laser irradiation from the outer ear is a possible alternative for stimulating the cochlea, circumventing the middle ear.

Distortion product otoacoustic emissions (DPOAEs) provide a window into active cochlear processes and have become a popular clinical and research tool. DPOAEs are commonly recorded using stimulus with fixed presentation levels and frequency ratio irrespective of the test frequency. However, this is inconsistent with the changing mechanical properties of the cochlear partition from the base to the apex that lend specific frequency-dependent spatial properties to the cochlear traveling wave. Therefore, the frequency and level characteristics between the stimulus tones should also need to be adjusted as a function of frequency to maintain optimal interaction between them. The goal of this investigation was to establish a frequency-specific measurement protocol guided by local cochlear mechanics. A broad stimulus parameter space extending up to 20 kHz was explored in a group of normal-hearing individuals. The stimulus frequency ratio yielding the largest 2 f1- f2 DPOAE level changed as a function of frequency and stimulus level. Specifically, for a constant stimulus level, the frequency ratio producing the largest DPOAE level decreased with increasing frequency. Similarly, at a given f2 frequency, the stimulus frequency ratio producing the largest DPOAE level became wider as stimulus level increased. These results confirm and strengthen our current understanding of DPOAE generation in the normally functioning cochlea and expand our understanding to previously unexamined higher frequencies. These data support the use of frequency- and level-specific stimulus frequency ratios to maximize DPOAE generation.

Cochlear-implant (CI) users have previously demonstrated perceptual restoration, or successful repair of noise-interrupted speech, using the interrupted sentences paradigm [Bhargava, Gaudrain, and Başkent (2014). “Top-down restoration of speech in cochlear-implant users,” Hear. Res. 309, 113–123]. The perceptual restoration effect was defined experimentally as higher speech understanding scores with noise-burst interrupted sentences compared to silent-gap interrupted sentences. For the perceptual restoration illusion to occur, it is often necessary for the masking or interrupting noise bursts to have a higher intensity than the adjacent speech signal to be perceived as a plausible masker. Thus, signal processing factors like noise reduction algorithms and automatic gain control could have a negative impact on speech repair in this population. Surprisingly, evidence that participants with cochlear implants experienced the perceptual restoration illusion was not observed across the two planned experiments. A separate experiment, which aimed to provide a close replication of previous work on perceptual restoration in CI users, also found no consistent evidence of perceptual restoration, contrasting the original study's previously reported findings. Typical speech repair of interrupted sentences was not observed in the present work's sample of CI users, and signal-processing factors did not appear to affect speech repair.

Bionic camouflage covert underwater acoustic communication has recently attracted great attention. However, we have not found relevant methods or literature to recognize these bionic camouflage communication signals (BCCSs) in the area of anti-reconnaissance. Focused on recognizing the BCCSs, this article proposes a recognition method based on the statistics of inter-click intervals to recognize the camouflaged click communication train (CCCT), which is modulated by time delay difference (TDD). We first analyze the characteristics of TDD distributions of CCCT and real click train (RCT). According to the coding principle, the TDDs of CCCTs present a ladder-like distribution with a fixed time step, and the TDDs are equal to the integral multiple of the fixed time step. On the contrary, the TDDs of RCTs are approximately random distribution within a certain time range. Therefore, based on the different TDD distributions, this article classifies CCCTs and RCTs by utilizing the statistical property of TDD distributions. To measure the TDDs of diverse cetacean clicks accurately, a new click location scheme based on the dynamic window energy ratio is proposed. Next, based on the statistics of TDD distribution, the influences of the TDDs that are caused by multipath interferences are eliminated by iteration. Simulations demonstrate the accuracy of the recognition method under different conditions.

This paper investigates the influence of visual cues in the perception of the /r/-/w/ contrast in Anglo-English. Audio-visual perception of Anglo-English /r/ warrants attention because productions are increasingly non-lingual, labiodental (e.g., [ʋ]), possibly involving visual prominence of the lips for the post-alveolar approximant [ɹ]. Forty native speakers identified [ɹ] and [w] stimuli in four presentation modalities: auditory-only, visual-only, congruous audio-visual, and incongruous audio-visual. Auditory stimuli were presented in noise. The results indicate that native Anglo-English speakers can identify [ɹ] and [w] from visual information alone with almost perfect accuracy. Furthermore, visual cues dominate the perception of the /r/-/w/ contrast when auditory and visual cues are mismatched. However, auditory perception is ambiguous because participants tend to perceive both [ɹ] and [w] as /r/. Auditory ambiguity is related to Anglo-English listeners' exposure to acoustic variation for /r/, especially to [ʋ], which is often confused with [w]. It is suggested that a specific labial configuration for Anglo-English /r/ encodes the contrast with /w/ visually, compensating for the ambiguous auditory contrast. An audio-visual enhancement hypothesis is proposed, and the findings are discussed with regard to sound change.

Manipulation of cochlear implant (CI) place pitch was carried out with current steering by stimulating two CI electrodes sequentially. The objective was to investigate whether shifts in activated neural populations could be achieved to produce salient pitch differences and to determine which stimulation parameters would be more effective in steering of current. These were the pulse rate and pulse width of electrical stimuli and the distance between the two current-steering electrodes. Nine CI users participated, and ten ears were tested. The pattern of pitch changes was not consistent across listeners, but the data suggest that individualized selection of stimulation parameters may be used to effect place pitch changes with sequential current steering. Individual analyses showed that pulse width generally had little influence on the effectiveness of current steering with sequential stimuli, while more salient place pitch shifts were often achieved at wider electrode spacing or when the stimulation pulse rate was the same as that indicated on the clinical MAP (the set of stimulation parameters) of the listener. Results imply that current steering may be used in CIs that allow only sequential stimulation to achieve place pitch manipulation.

Application of a kurtosis correction to frequency-weighted sound exposure level (SEL) improved predictions of risk of hearing damage in humans and terrestrial mammals for sound exposures with different degrees of impulsiveness. To assess whether kurtosis corrections may lead to improved predictions for marine mammals, corrections were applied to temporary threshold shift (TTS) growth measurements for harbor porpoises ( Phocoena phocoena) exposed to different sounds. Kurtosis-corrected frequency-weighted SEL predicted accurately the growth of low levels of TTS (TTS1-4 < 10 dB) for intermittent sounds with short (1–13 s) silence intervals but was not consistent with frequency-weighted SEL data for continuous sound exposures.

Known errors exist in loudspeaker array processing techniques, often degrading source localization and timbre. The goal of the present study was to use virtual loudspeaker arrays to investigate how treatment of the interaural time delay (ITD) cue from each loudspeaker impacts these errors. Virtual loudspeaker arrays rendered over headphones using head-related impulse responses (HRIRs) allow flexible control of array size. Here, three HRIR delay treatment strategies were evaluated using minimum-phase loudspeaker HRIRs: reapplying the original HRIR delays, applying the relative ITD to the contralateral ear, or separately applying the HRIR delays prior to virtual array processing. Seven array sizes were simulated, and panning techniques were used to estimate HRIRs from 3000 directions using higher-order Ambisonics, vector-base amplitude panning, and the closest loudspeaker technique. Compared to a traditional, physical array, the prior HRIR delay treatment strategy produced similar errors with a 95% reduction in the required array size. When compared to direct spherical harmonic (SH) fitting of head-related transfer functions (HRTFs), the prior delays strategy reduced errors in reconstruction accuracy of timbral and directional psychoacoustic cues. This result suggests that delay optimization can greatly reduce the number of virtual loudspeakers required for accurate rendering of acoustic scenes without SH-based HRTF representation.

The best cochlear–neural delay model for designing a chirp that can produce the largest auditory brainstem response (ABR) has not been established. This study comprised two experiments. Experiment I aimed to estimate the delay model by measuring derived-band ABR latencies at different levels. The results demonstrated that, as the level decreased, the delay between the center frequencies of 0.7 and 5.7 kHz increased. The aim of experiment II was to compare ABRs generated by three stimuli: (1) a level-dependent derived-band (DB)-Chirp, designed based on the model in experiment I; (2) a level-dependent level specific (LS)-Chirp from Kristensen and Elberling [(2012). J. Am. Acad. Audiol. 23, 712–721]; and (3) a click. The results demonstrated that the DB-Chirp produced significantly larger wave V than the LS-Chirp at 45 dB normal hearing level (nHL); however, no differences were observed at other levels. The wave I generated by the DB-Chirp and LS-Chirp were significantly larger than those evoked by the click at 45 and 60 dB nHL and at 30 and 45 dB nHL, respectively; however, at all levels, no differences between these two chirps were observed. The DB-Chirp may be a valuable stimulus for producing ABRs for clinical applications such as assessing cochlear synaptopathy and estimating hearing sensitivity.

Natural compensation of speech production in challenging listening environments is referred to as the Lombard effect (LE). The resulting acoustic differences between neutral and Lombard speech have been shown to provide intelligibility benefits for normal hearing (NH) and cochlear implant (CI) listeners alike. Motivated by this outcome, three LE perturbation approaches consisting of pitch, duration, formant, intensity, and spectral contour modifications were designed specifically for CI listeners to combat speech-in-noise performance deficits. Experiment 1 analyzed the effects of loudness, quality, and distortion of approaches on speech intelligibility with and without formant-shifting. Significant improvements of +9.4% were observed in CI listeners without the formant-shifting approach at +5 dB signal-to-noise ratio (SNR) large-crowd-noise (LCN) when loudness was controlled, however, performance was found to be significantly lower for NH listeners. Experiment 2 evaluated the non-formant-shifting approach with additional spectral contour and high pass filtering to reduce spectral smearing and decrease distortion observed in Experiment 1. This resulted in significant intelligibility benefits of +30.2% for NH and +21.2% for CI listeners at 0 and +5 dB SNR LCN, respectively. These results suggest that LE perturbation may be useful as front-end speech modification approaches to improve intelligibility for CI users in noise.

Recent work has suggested that there may be specialized mechanisms in the auditory system for coding spectrotemporal modulations (STMs), tuned to different combinations of spectral modulation frequency, temporal modulation frequency, and STM sweep direction. The current study sought evidence of such mechanisms using a psychophysical forward masking paradigm. The detectability of a target comprising upward sweeping STMs was measured following the presentation of modulated maskers applied to the same carrier. Four maskers were tested, which had either (1) the same spectral modulation frequency as the target but a flat temporal envelope, (2) the same temporal modulation frequency as the target but a flat spectral envelope, (3) the same spectral and temporal modulation frequencies as the target but the opposite sweep direction (downward sweeping STMs), or (4) the same spectral and temporal modulation frequencies as the target and the same sweep direction (upward sweeping STMs). Forward masking was greatest for the masker fully matched to the target (4), intermediate for the masker with the opposite sweep direction (3), and negligible for the other two (1, 2). These findings are consistent with the suggestion that the detectability of the target was mediated by an STM-specific coding mechanism with sweep-direction selectivity.

Across-frequency binaural interference occurs when the sensitivity to changes in interaural differences in a target sound is decreased by a spectrally remote diotic interfering sound. For interaural time differences (ITDs), low-frequency (e.g., 0.5 kHz) interferers cause more interference on high-frequency (e.g., 4 kHz) targets than vice versa. For interaural level differences (ILDs), however, it is unclear if a frequency dependence exists. Therefore, ILD discrimination thresholds and across-frequency binaural interference were measured for target and interferer frequencies between 0.5 and 8 kHz (for tones) or 12 kHz (for narrowband noises). For tones, 8-kHz targets experienced the least interference and 8-kHz interferers produced the most interference, suggesting that higher-frequency ILDs are a more heavily weighted localization cue than lower-frequency ILDs. For narrowband noises, the frequency-dependent interference patterns increased in complexity in comparison to tones. Low-frequency ITD dominance (from randomly varying onset ITDs) and grouping cues (e.g., envelope modulations) might explain some of the complexity in the interference patterns for the noises. These data contribute to a better understanding of across-frequency ILD processing, which remains poorly understood.

This study describes data on auditory-visual integration and visually-guided adaptation of auditory distance perception using the ventriloquism effect (VE) and ventriloquism aftereffect (VAE). In an experiment, participants judged egocentric distance of interleaved auditory or auditory-visual stimuli with the auditory component located from 0.7 to 2.04 m in front of listeners in a real reverberant environment. The visual component of auditory-visual stimuli was displaced 30% closer (V-closer), 30% farther (V-farther), or aligned (V-aligned) with respect to the auditory component. The VE and VAE were measured in auditory and auditory-visual trials, respectively. Both effects were approximately independent of target distance when expressed in logarithmic units. The VE strength, defined as a difference of V-misaligned and V-aligned response bias, was approximately 72% of the auditory-visual disparity regardless of the visual-displacement direction, while the VAE was stronger in the V-farther (44%) than the V-closer (31%) condition. The VAE persisted to post-adaptation auditory-only blocks of trials, although it was diminished. The rates of build-up/break-down of the VAE were asymmetrical, with slower adaptation in the V-closer condition. These results suggest that auditory-visual distance integration is independent of the direction of induced shift, while the re-calibration is stronger and faster when evoked by more distant visual stimuli.

This study investigated whether visual attention affects the reliability (i.e., repeatability) of transiently evoked otoacoustic emission (TEOAE) magnitudes or of medial olivocochlear reflex (MOCR) estimates. TEOAEs were measured during three visual attentional conditions: control (subject were seated with eyes closed); passive (subjects looked at a pattern of squares on a computer screen); and active (subjects silently counted an occasionally inverted pattern). To estimate reliability, the whole recording session was repeated the next day. The results showed that visual attention does not significantly affect TEOAE or MOCR magnitudes—or their reliability. It is therefore possible to employ visual stimuli (e.g., watching a silent movie) during TEOAE experiments, a procedure sometimes used during testing to prevent subjects from falling asleep or to keep children still and quiet.

The medial olivocochlear reflex (MOCR), usually assessed by the inhibition of transiently evoked otoacoustic emissions (TEOAEs) with contralateral noise, is a very small effect. In understanding the origin of the MOCR, it is crucial to obtain data of the highest accuracy, i.e., with a high signal-to-noise ratio (SNR), which in turn largely depends on the number of signal averages. This study investigates how the reliability of MOCR measures is affected by the number of averages. At the same time, the effect of the presence of synchronized spontaneous otoacoustic emissions (SSOAEs) is taken into account, as it is known that this factor significantly affects TEOAE amplitudes and SNRs. Each recording session consisted of two series of four measurements, allowing comparison of MOCR magnitude based on 250, 500, 750, and 1000 averages. Reliability was based on comparing the two series. The results show that, for a good quality MOCR measure (i.e., intraclass correlation above 0.9), the required number of averages is at least double that obtainable from a standard TEOAE test (i.e., 500 compared to 250). Ears without SSOAEs needed a higher number of averages to reach a correlation of 0.9 than ears with SSOAEs.

There are psychoacoustic methods thought to measure gain reduction, which may be from the medial olivocochlear reflex (MOCR), a bilateral feedback loop that adjusts cochlear gain. Although studies have used ipsilateral and contralateral elicitors and have examined strength at different signal frequencies, these factors have not been examined within a single study. Therefore, basic questions about gain reduction, such as the relative strength of ipsilateral vs contralateral elicitation and the relative strength across signal frequency, are not known. In the current study, gain reduction from ipsilateral, contralateral, and bilateral elicitors was measured at 1-, 2-, and 4-kHz signal frequencies using forward masking paradigms at a range of elicitor levels in a repeated measures design. Ipsilateral and bilateral strengths were similar and significantly larger than contralateral strength across signal frequencies. Growth of gain reduction with precursor level tended to differ with signal frequency, although not significantly. Data from previous studies are considered in light of the results of this study. Behavioral results are also considered relative to anatomical and physiological data on the MOCR. These results indicate that, in humans, cochlear gain reduction is broad across frequencies and is robust for ipsilateral and bilateral elicitation but small for contralateral elicitation.

Cochlear-implant (CI) listeners experience signal degradation, which leads to poorer speech perception than normal-hearing (NH) listeners. In the present study, difficulty with word segmentation, the process of perceptually parsing the speech stream into separate words, is considered as a possible contributor to this decrease in performance. CI listeners were compared to a group of NH listeners (presented with unprocessed speech and eight-channel noise-vocoded speech) in their ability to segment phrases with word segmentation ambiguities (e.g., "an iceman" vs "a nice man"). The results showed that CI listeners and NH listeners were worse at segmenting words when hearing processed speech than NH listeners were when presented with unprocessed speech. When viewed at a broad level, all of the groups used cues to word segmentation in similar ways. Detailed analyses, however, indicated that the two processed speech groups weighted top-down knowledge cues to word boundaries more and weighted acoustic cues to word boundaries less relative to NH listeners presented with unprocessed speech.

Previous studies have shown that for high-rate click trains and low-frequency pure tones, interaural time differences (ITDs) at the onset of stimulus contribute most strongly to the overall lateralization percept (receive the largest perceptual weight). Previous studies have also shown that when these stimuli are modulated, ITDs during the rising portion of the modulation cycle receive increased perceptual weight. Baltzell, Cho, Swaminathan, and Best [(2020). J. Acoust. Soc. Am. 147, 3883–3894] measured perceptual weights for a pair of spoken words (“two” and “eight”), and found that word-initial phonemes receive larger weight than word-final phonemes, suggesting a “word-onset dominance” for speech. Generalizability of this conclusion was limited by a coarse temporal resolution and limited stimulus set. In the present study, temporal weighting functions (TWFs) were measured for four spoken words (“two,” “eight,” “six,” and “nine”). Stimuli were partitioned into 30-ms bins, ITDs were applied independently to each bin, and lateralization judgements were obtained. TWFs were derived using a hierarchical regression model. Results suggest that “word-initial” onset dominance does not generalize across words and that TWFs depend in part on acoustic changes throughout the stimulus. Two model-based predictions were generated to account for observed TWFs, but neither could fully account for the perceptual data.

Perception of speech sounds has a long history of being compared to perception of nonspeech sounds, with rich and enduring debates regarding how closely they share similar underlying processes. In many instances, perception of nonspeech sounds is directly compared to that of speech sounds without a clear explanation of how related these sounds are to the speech they are selected to mirror (or not mirror). While the extreme acoustic variability of speech sounds is well documented, this variability is bounded by the common source of a human vocal tract. Nonspeech sounds do not share a common source, and as such, exhibit even greater acoustic variability than that observed for speech. This increased variability raises important questions about how well perception of a given nonspeech sound might resemble or model perception of speech sounds. Here, we offer a brief review of extremely diverse nonspeech stimuli that have been used in the efforts to better understand perception of speech sounds. The review is organized according to increasing spectrotemporal complexity: random noise, pure tones, multitone complexes, environmental sounds, music, speech excerpts that are not recognized as speech, and sinewave speech. Considerations are offered for stimulus selection in nonspeech perception experiments moving forward.

The basilar membrane in the cochlea can be modeled as an array of fluid coupled segments driven by stapes vibration and by the undamping nonlinear force simulating cochlear amplification. If stimulated with two tones, the model generates additional tones due to nonlinear distortion. These distortion products (DPs) can be transmitted into the ear canal and produce distortion-product otoacoustic emissions (DPOAEs) known to be generated in the healthy ear of various vertebrates. This study presents a solution for DPs in a two-dimensional nonlinear cochlear model with cochlear roughness—small irregularities in the impedance along the basilar membrane, which may produce additional DPs due to coherent reflection. The solution allows for decomposition of various sources of DPs in the model. In addition to the already described nonlinear-distortion and coherent-reflection mechanisms of DP generation, this study identifies a long-latency DPOAE component due to perturbation of nonlinear force. DP wavelets that are coherently reflected due to impedance irregularities travel toward the stapes across the primary generation region of DPs and there evoke perturbation of the nonlinear undamping force. The ensuing DP wavelets have opposite phase to the wavelets arising from coherent reflection, which results in partial cancellation of the coherent-reflection DP wavelets.

Perception of word stress is an important aspect of recognizing speech, guiding the listener toward candidate words based on the perceived stress pattern. Cochlear implant (CI) signal processing is likely to disrupt some of the available cues for word stress, particularly vowel quality and pitch contour changes. In this study, we used a cue weighting paradigm to investigate differences in stress cue weighting patterns between participants listening with CIs and those with normal hearing (NH). We found that participants with CIs gave less weight to frequency-based pitch and vowel quality cues than NH listeners but compensated by upweighting vowel duration and intensity cues. Nonetheless, CI listeners' stress judgments were also significantly influenced by vowel quality and pitch, and they modulated their usage of these cues depending on the specific word pair in a manner similar to NH participants. In a series of separate online experiments with NH listeners, we simulated aspects of bimodal hearing by combining low-pass filtered speech with a vocoded signal. In these conditions, participants upweighted pitch and vowel quality cues relative to a fully vocoded control condition, suggesting that bimodal listening holds promise for restoring the stress cue weighting patterns exhibited by listeners with NH.

The intelligibility of interrupted speech stimuli has been known to be almost perfect when segment duration is shorter than 80 ms, which means that the interrupted segments are perceptually organized into a coherent stream under this condition. However, why listeners can successfully group the interrupted segments into a coherent stream has been largely unknown. Here, we show that the intelligibility for mosaic speech in which original speech was segmented in frequency and time and noise-vocoded with the average power in each unit was largely reduced by periodical interruption. At the same time, the intelligibility could be recovered by promoting auditory grouping of the interrupted segments by stretching the segments up to 40 ms and reducing the gaps, provided that the number of frequency bands was enough ([Formula: see text]) and the original segment duration was equal to or less than 40 ms. The interruption was devastating for mosaic speech stimuli, very likely because the deprivation of periodicity and temporal fine structure with mosaicking prevented successful auditory grouping for the interrupted segments.

Animals localise sound by making use of acoustical cues resulting from space and frequency dependent filtering of sound by the head and body. Sound arrives at each ear at different times, with different intensities, and with varying spectral content, all of which are affected by the animal's head and the relative sound source position. Location cues in mammals benefit from structures (pinnae) that modify these cues and provide information that helps resolve the cone of confusion and provide cues to sound source elevation. Animals without pinnae must rely on other mechanisms to solve localisation problems. Most non-mammals lack pinna-like structures, but some possess other anatomical features that could influence hearing. One such animal is the frill-necked lizard ( Chlamydosaurus kingii). The species' elaborate neck frill has been speculated to act as an aid to hearing, but no acoustical measurements have been reported. In this study, we characterise the frill's influence on the acoustical information available to the animal. Results suggest that the change in binaural cues is not sufficiently large to impact localisation behavior within the species' likely audiometric range; however, the frill does increase gain for sounds directly in front of the animal similar to a directional microphone.

Increasing complaints about infrasound have generated interest in understanding its perception, including binaural effects. This study investigated the level difference between monaural and binaural presentation required for detection and equal loudness (binaural gain) for pure tones with frequencies of 8, 32, and 400 Hz and an 8 Hz sinusoidally amplitude-modulated tone with diotic 400 Hz carrier. Monaural stimuli were compared to binaural stimuli with interaural phase differences (IPDs) of 0°, 90°, and 180° in two experiments: absolute threshold measurements and loudness matching at 40 phons. The latter was repeated with transposed tones (400 Hz carrier multiplied by a half-wave-rectified 8 Hz sinusoid). When expressed as differences in sound pressure level, similar binaural gain was found across all stimulus types under the diotic condition. Confirming previous studies, the gain was larger at supra-threshold levels (40 phons) than at threshold. However, when the loudness-matching results were expressed as binaural gain with respect to the loudness level, they became 17.5, 11.2, and 5.8 phons for the 8, 32, and 400 Hz stimuli, respectively. Results for the 8 Hz pure tone and the transposed stimulus were IPD dependent.

This study assessed the detection of mistuning of a single harmonic in complex tones (CTs) containing either low-frequency harmonics or very high-frequency harmonics, for which phase locking to the temporal fine structure is weak or absent. CTs had F0s of either 280 or 1400 Hz and contained harmonics 6–10, the 8th of which could be mistuned. Harmonics were presented either diotically or dichotically (odd and even harmonics to different ears). In the diotic condition, mistuning-detection thresholds were very low for both F0s and consistent with detection of temporal interactions (beats) produced by peripheral interactions of components. In the dichotic condition, for which the components in each ear were more widely spaced and beats were not reported, the mistuned component was perceptually segregated from the complex for the low F0, but subjects reported no “popping out” for the high F0 and performance was close to chance. This is consistent with the idea that phase locking is required for perceptual segregation to occur. For diotic presentation, the perceived beat rate corresponded to the amount of mistuning (in Hz). It is argued that the beat percept cannot be explained solely by interactions between the mistuned component and its two closest harmonic neighbours.

Three experiments explored the effects of abrupt changes in stimulus properties on streaming dynamics. Listeners monitored 20-s-long low- and high-frequency (LHL-) tone sequences and reported the number of streams heard throughout. Experiments 1 and 2 used pure tones and examined the effects of changing triplet base frequency and level, respectively. Abrupt changes in base frequency (±3-12 semitones) caused significant magnitude-related falls in segregation (resetting), regardless of transition direction, but an asymmetry occurred for changes in level (±12 dB). Rising-level transitions usually decreased segregation significantly, whereas falling-level transitions had little or no effect. Experiment 3 used pure tones (unmodulated) and narrowly spaced (±25 Hz) tone pairs (dyads); the two evoke similar excitation patterns, but dyads are strongly modulated with a distinctive timbre. Dyad-only sequences induced a strongly segregated percept, limiting scope for further build-up. Alternation between groups of pure tones and dyads produced large, asymmetric changes in streaming. Dyad-to-pure transitions caused substantial resetting, but pure-to-dyad transitions sometimes elicited even greater segregation than for the corresponding interval in dyad-only sequences (overshoot). The results indicate that abrupt changes in timbre can strongly affect the likelihood of stream segregation without introducing significant peripheral-channeling cues. These asymmetric effects of transition direction are reminiscent of subtractive adaptation in vision.

A contralateral "cue" tone presented in continuous broadband noise both lowers the threshold of a signal tone by guiding attention to it and raises its threshold by interference. Here, signal tones were fixed in duration (40 ms, 52 ms with ramps), frequency (1500 Hz), timing, and level, so attention did not need guidance. Interference by contralateral cues was studied in relation to cue-signal proximity, cue-signal temporal overlap, and cue-signal order (cue after: backward interference, BI; or cue first: forward interference, FI). Cues, also ramped, were 12 dB above the signal level. Long cues (300 or 600 ms) raised thresholds by 5.3 dB when the signal and cue overlapped and by 5.1 dB in FI and 3.2 dB in BI when cues and signals were separated by 40 ms. Short cues (40 ms) raised thresholds by 4.5 dB in FI and 4.0 dB in BI for separations of 7 to 40 ms, but by ∼13 dB when simultaneous and in phase. FI and BI are comparable in magnitude and hardly increase when the signal is close in time to abrupt cue transients. These results do not support the notion that masking of the signal is due to the contralateral cue onset/offset transient response. Instead, sluggish attention or temporal integration may explain contralateral proximal interference.

The auditory brainstem response (ABR) to stimulus onset has been extensively used to investigate dolphin hearing. The mechanisms underlying this onset response have been thoroughly studied in mammals. In contrast, the ABR evoked by sound offset has received relatively little attention. To build upon previous observations of the dolphin offset ABR, a series of experiments was conducted to (1) determine the cochlear places responsible for response generation and (2) examine differences in response morphologies when using toneburst versus noiseburst stimuli. Measurements were conducted with seven bottlenose dolphins (Tursiops truncatus) using tonebursts and spectrally "pink" broadband noisebursts, with highpass noise used to limit the cochlear regions involved in response generation. Results for normal-hearing and hearing-impaired dolphins suggest that the offset ABR contains contributions from at least two distinct responses. One type of response (across place) might arise from the activation of neural units that are shifted basally relative to stimulus frequency and shares commonalities with the onset ABR. A second type of response (within place) appears to represent a "true" offset response from afferent centers further up the ascending auditory pathway from the auditory nerve, and likely results from synchronous activity beginning at or above the cochlear nucleus.

Uncertainty regarding the frequency spectrum of a masker can have an adverse effect on the ability to focus selective attention on a target frequency channel, yielding informational masking (IM). This study sought to determine if uncertainty regarding the modulation spectrum of a masker can have an analogous adverse effect on the ability to focus selective attention on a target modulation channel, yielding IM in the modulation domain, or “modulation IM.” A single-interval, two-alternative forced-choice (yes-no) procedure was used. The task was to detect 32-Hz target sinusoidal amplitude modulation (SAM) imposed on a broadband-noise carrier in the presence of masker SAM imposed on the same carrier. Six maskers, spanning the range from 8 to 128 Hz in half-octave steps, were tested, excluding those that fell within a two-octave protected zone surrounding the target. Psychometric functions (d′-vs-target modulation depth) were measured for each masker under two conditions: a fixed (low-uncertainty/low-IM) condition, in which the masker was the same on all trials within a block, and a random (high-uncertainty/high-IM) condition, in which it varied randomly from presentation-to-presentation. Thresholds and slopes extracted from the psychometric functions differed markedly between the conditions. These results are consistent with the idea that IM occurs in the modulation domain.

For abruptly gated sound, interaural time difference (ITD) cues at onset carry greater perceptual weight than those following. This research explored how envelope shape influences such carrier ITD weighting. Experiment 1 assessed the perceived lateralization of a tonal binaural beat that transitioned through ITD (diotic envelope, mean carrier frequency of 500 Hz). Listeners' left/right lateralization judgments were compared to those for static-ITD tones. For an 8 Hz sinusoidally amplitude-modulated envelope, ITD cues 24 ms after onset well-predicted reported sidedness. For an equivalent-duration "abrupt" envelope, which was unmodulated besides 20-ms onset/offset ramps, reported sidedness corresponded to ITDs near onset (e.g., 6 ms). However, unlike for sinusoidal amplitude modulation, ITDs toward offset seemingly also influenced perceived sidedness. Experiment 2 adjusted the duration of the offset ramp (25-75 ms) and found evidence for such offset weighting only for the most abrupt ramp tested. In experiment 3, an ITD was imposed on a brief segment of otherwise diotic filtered noise. Listeners discriminated right- from left-leading ITDs. In sinusoidal amplitude modulation, thresholds were lowest when the ITD segment occurred during rising amplitude. For the abrupt envelope, the lowest thresholds were observed when the segment occurred at either onset or offset. These experiments demonstrate the influence of envelope profile on carrier ITD sensitivity.

Auditory interfaces, such as auditory alarms, are useful tools for human computer interaction. Unfortunately, poor detectability and annoyance inhibit the efficacy of many interface sounds. Here, it is shown in two ways how moving beyond the traditional simplistic temporal structures of normative interface sounds can significantly improve auditory interface efficacy. First, participants rated tones with percussive amplitude envelopes as significantly less annoying than tones with flat amplitude envelopes. Crucially, this annoyance reduction did not come with a detection cost as percussive tones were detected more often than flat tones—particularly, at relatively low listening levels. Second, it was found that reductions in the duration of a tone's harmonics significantly lowered its annoyance without a commensurate reduction in detection. Together, these findings help inform our theoretical understanding of detection and annoyance of sound. In addition, they offer promising original design considerations for auditory interfaces.

Cochlear dispersion causes increasing delays between neural responses from high-frequency regions in the cochlear base and lower-frequency regions toward the apex. For broadband stimuli, this can lead to neural responses that are out-of-phase, decreasing the amplitude of farfield neural response measurements. In the present study, cochlear traveling-wave speed and effects of dispersion on farfield auditory brainstem responses (ABRs) were investigated by first deriving narrowband ABRs in bottlenose dolphins and California sea lions using the high-pass subtractive masking technique. Derived-band ABRs were then temporally aligned and summed to obtain the “stacked ABR” as a means of compensating for the effects of cochlear dispersion. For derived-band responses between 8 and 32 kHz, cochlear traveling-wave speeds were similar for sea lions and dolphins [∼2–8 octaves (oct)/ms for dolphins; ∼3.5–11 oct/ms for sea lions]; above 32 kHz, traveling-wave speed for dolphins increased up to ∼30 oct/ms. Stacked ABRs were larger than unmasked, broadband ABRs in both species. The amplitude enhancement was smaller in dolphins than in sea lions, and enhancement in both species appears to be less than reported in humans. Results suggest that compensating for cochlear dispersion will provide greater benefit for ABR measurements in species with better low-frequency hearing.

The mouse is an important animal model for hearing science. However, our knowledge of the relationship between mouse middle-ear (ME) anatomy and function is limited. The ME not only transmits sound to the cochlea in the forward direction, it also transmits otoacoustic emissions generated in the cochlea to the ear canal (EC) in the reverse direction. Due to experimental limitations, a complete characterization of the mouse ME has not been possible. A fully coupled finite-element model of the mouse EC, ME, and cochlea was developed and calibrated against experimental measurements. Impedances of the EC, ME, and cochlea were calculated, alongside pressure transfer functions for the forward, reverse, and round-trip directions. The effects on sound transmission of anatomical changes such as removing the ME cavity, pars flaccida, and mallear orbicular apophysis were also calculated. Surprisingly, below 10 kHz, the ME cavity, eardrum, and stapes annular ligament were found to significantly affect the cochlear input impedance, which is a result of acoustic coupling through the round window. The orbicular apophysis increases the delay of the transmission line formed by the flexible malleus, incus, and stapes, and improves the forward sound-transmission characteristics in the frequency region of 7–30 kHz.

Traditionally, real-time generation of spectro-temporally modulated noise has been performed on a linear amplitude scale, partially due to computational constraints. Experiments often require modulation that is sinusoidal on a logarithmic amplitude scale as a result of the many perceptual and physiological measures which scale linearly with exponential changes in the signal magnitude. A method is presented for computing exponential spectro-temporal modulation, showing that it can be expressed analytically as a sum over linearly offset sidebands with component amplitudes equal to the values of the modified Bessel function of the first kind. This approach greatly improves the efficiency and precision of stimulus generation over current methods, facilitating real-time generation for a broad range of carrier and envelope signals.

Categorical perception (CP) describes how the human brain categorizes speech despite inherent acoustic variability. We examined neural correlates of CP in both evoked and induced electroencephalogram (EEG) activity to evaluate which mode best describes the process of speech categorization. Listeners labeled sounds from a vowel gradient while we recorded their EEGs. Using a source reconstructed EEG, we used band-specific evoked and induced neural activity to build parameter optimized support vector machine models to assess how well listeners' speech categorization could be decoded via whole-brain and hemisphere-specific responses. We found whole-brain evoked β-band activity decoded prototypical from ambiguous speech sounds with ∼70% accuracy. However, induced γ-band oscillations showed better decoding of speech categories with ∼95% accuracy compared to evoked β-band activity (∼70% accuracy). Induced high frequency (γ-band) oscillations dominated CP decoding in the left hemisphere, whereas lower frequencies (θ-band) dominated the decoding in the right hemisphere. Moreover, feature selection identified 14 brain regions carrying induced activity and 22 regions of evoked activity that were most salient in describing category-level speech representations. Among the areas and neural regimes explored, induced γ-band modulations were most strongly associated with listeners' behavioral CP. The data suggest that the category-level organization of speech is dominated by relatively high frequency induced brain rhythms.