High-pass Noise Research Articles

Enhanced Place Specificity of the Parallel Auditory Brainstem Response: An Electrophysiological Study

PurposeThis study investigates the effect of parallel stimulus presentation on the place specificity of the auditory brainstem response (ABR) in human listeners. Frequency-specific stimuli do not guarantee a response from the place on the cochlea corresponding only to that characteristic frequency — especially for brief and high-level stimuli. Adding masking noise yields responses that are more place specific, and our prior modeling study has suggested similar effects when multiple frequency-specific stimuli are presented in parallel. We tested this hypothesis experimentally here, comparing the place specificity of responses to serial and parallel stimuli at two stimulus frequencies and three stimulus rates.MethodsParallel ABR (pABR) stimuli were presented alongside high-pass filtered noise with a varied cutoff frequency. Serial presentation was also tested by isolating and presenting single-frequency stimulus trains from the pABR ensemble. Latencies of the ABRs were examined to assess place specificity of responses. Response bands were derived by subtracting responses from different high-pass noise conditions. The response amplitude from each derived response band was then used to determine how much individual frequency regions of the auditory system were contributing to the overall response.ResultsWe found that parallel presentation improves place specificity of ABRs for the lower stimulus frequency and at higher stimulus rates. At a higher stimulus frequency, serial and parallel presentations were equally place specific.ConclusionParallel presentation can provide more place-specific responses than serial for lower stimulus frequencies. The improvement increases with higher stimulus rates and is in addition to the pABR’s primary benefit of faster test times.

Contralateral Routing of Signal Disrupts Monaural Sound Localization.

In the absence of binaural hearing, individuals with single-sided deafness can adapt to use monaural level and spectral cues to improve their spatial hearing abilities. Contralateral routing of signal is the most common form of rehabilitation for individuals with single-sided deafness. However, little is known about how these devices affect monaural localization cues, which single-sided deafness listeners may become reliant on. This study aimed to investigate the effects of contralateral routing of signal hearing aids on localization performance in azimuth and elevation under monaural listening conditions. Localization was assessed in 10 normal hearing adults under three listening conditions: (1) normal hearing (NH), (2) unilateral plug (NH-plug), and (3) unilateral plug and CROS aided (NH-plug + CROS). Monaural hearing simulation was achieved by plugging the ear with E-A-Rsoft™ FX™ foam earplugs. Stimuli consisted of 150 ms high-pass noise bursts (3-20 kHz), presented in a random order from fifty locations spanning ±70° in the horizontal and ±30° in the vertical plane at 45, 55, and 65 dBA. In the unilateral plugged listening condition, participants demonstrated good localization in elevation and a response bias in azimuth for signals directed at the open ear. A significant decrease in performance in elevation occurs with the contralateral routing of signal hearing device on, evidenced by significant reductions in response gain and low r2 value. Additionally, performance in azimuth is further reduced for contralateral routing of signal aided localization compared to the simulated unilateral hearing loss condition. Use of the contralateral routing of signal device also results in a reduction in promptness of the listener response and an increase in response variability. Results suggest contralateral routing of signal hearing aids disrupt monaural spectral and level cues, which leads to detriments in localization performance in both the horizontal and vertical dimensions. Increased reaction time and increasing variability in responses suggests localization is more effortful when wearing the contralateral rerouting of signal device.

Derived-band auditory brainstem responses: cochlear contributions determined by narrowband maskers

Objective The present study sought to determine the cochlear frequency regions represented by Auditory Brainstem Responses (ABRs) obtained using the high-pass noise/derived response (HP/DR) technique. Design Broadband noise sufficient to mask the ABR to 50 dB nHL clicks was HP filtered (96 dB/oct) at 8000, 4000, 2000, 1000 and 500 Hz. Mixed with the clicks and HP noise masker was narrowband noise. Three derived response bands, denoted by the upper and lower high-pass noise frequencies, were obtained: DR4000-2000, DR2000-1000, and DR1000-500. Study sample Ten adults with normal hearing, aged 19–27 years (mean age: 22.4 years), were recruited from the community. Results Frequencies contributing to each DR were determined from the wave V percent amplitude (or latency shift) vs narrowband masker frequency profiles (relative to a no-narrowband-noise condition). Overall, results indicate derived band centre frequencies were closer to the lower HP cut-off frequencies for DR4000-2000 and DR2000-1000, and approximately halfway between the lower HP cut-off and the geometric mean of the two HP frequencies for DR1000-500, with bandwidths of 0.5-1 octave in width. Conclusions These results confirm the validity of the HP/DR technique for assessing narrow cochlear regions (≤1.0 octave wide), with centre frequencies within ½-octave of the lower HP frequency.

The effects of noise exposure and aging on the acoustic reflex in normal-hearing people

Noise exposure and aging can cause subclinical hearing problems that are not identified with conventional audiometry. Nerve damage among the normal hearing could reduce afferent auditory-nerve activity consequently reducing the middle ear muscle reflex (MEMR). It was hypothesized that normal-hearing individuals reporting more noise exposure would have slower growth of MEMR with increasing levels. Data were analyzed from sixty-one young, normal-hearing people. Each completed the four MEMR assessments and the Life-Time Exposure to Noise and Solvents Questionnaire. Reflex amplitudes were measured as a function of sound level using clinical equipment. High-pass noise stimuli (1.5-4.0 kHz) and a 1-KHz tone were used as elicitors with a 226-Hz probe tone. Data were analyzed using a linear mixed effects model. Individuals reporting higher noise exposure with the LENS-Q had significantly reduced growth of MEMR with level, supporting the hypothesis. Independent effects of age and gender were also observed. The effect of the pure-tone average on MEMR growth functions did not reach significance at the 5% level (p = 0.052). No significant relationships were observed between the MEMR measures and extended high-frequency thresholds nor with word recognition in noise. The results suggest that the MEMR could indicate subclinical nerve damage.

The offset auditory brainstem response in bottlenose dolphins (Tursiops truncatus): Evidence for multiple underlying processes.

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.

Band-Limited Chirp-Evoked Compound Action Potential in Guinea Pig: Comprehensive Neural Measure for Cochlear Implantation Monitoring.

Patients with severely impaired high-frequency hearing and sufficient residual low-frequency hearing can be provided with a cochlear implant (CI), thereby facilitating ipsilateral electric and acoustic stimulation with established advantages over electric stimulation alone. However, partial or complete hearing loss often occurred after implantation due to, inter alia, acute mechanical trauma to cochlear structures during electrode insertion. Possibilities of intraoperative monitoring using electrocochleography (ECochG) have recently been studied in CI patients, primarily using the ongoing response to low-frequency tone bursts consisting of the cochlear microphonic (CM) and the auditory nerve neurophonic. By contrast, the transient neural response to tone bursts, that is, compound action potential (CAP), was generally less detectable or less sensitive as a monitoring measure, thus falling short of providing useful contribution to electrocochleography analysis. In this study, we investigate using chirps to evoke more robust CAP responses in a limited frequency band by synchronizing neural firing, and thereby improving CAP sensitivity to mechanical trauma in a guinea pig model of cochlear implantation. Stimuli were band-limited between 100 Hz and 10 kHz to investigate their frequency range selectivity as a preliminary model for low-frequency hearing. They were constructed by adding a harmonic series either with zero phase delay (click) or by adjusting the phase delay at a rate that is inversely related to a traveling wave delay model (chirp), with three different parameters to examine level-dependent delay compression. The amplitude spectrum was thus identical between stimuli with differences only in phase. In Experiment 1, we compared input-output functions recorded at the round window in normal-hearing guinea pigs and implemented a high-pass noise masking paradigm to infer neural contribution to the CAP. In Experiment 2, guinea pigs were implanted with a custom-built CI electrode using a motorized micromanipulator. Acute mechanical trauma was simulated during the electrode insertion. At each insertion step, CAP and CM responses were measured at the round window for the following stimuli: broad-band click, band-limited click, and band-limited chirps (3 parameters), and tone bursts at frequencies 1, 2, 4, and 8 kHz. Chirps compared with the equal-band click showed significantly lower thresholds and steeper slopes of sigmoid-fitted input-output functions. The shorter chirp evoked significantly larger amplitudes than click when compared at equal sensation level. However, the click evoked larger amplitudes than chirps at higher levels and correspondingly achieved larger saturation amplitudes. The results of the high-pass noise masking paradigm suggest that chirps could efficiently synchronize neural firing in their targeted frequency band, while the click recruited more basal fibers outside its limited band. Finally, monitoring sensitivity during electrode insertion, defined as relative amplitude change per unit distance, was higher for chirp-evoked CAP and tone burst-evoked CM, but smaller for CAP responses evoked by clicks or tone bursts. The chirp was shown to be an efficient stimulus in synchronizing neural firing for a limited frequency band in the guinea pig model. This study provides a proof of principle for using chirp-evoked CAP as a comprehensive neural measure in CI patients with residual hearing.

A DROIC Based on PFM ADCs Employing Over-Integration for Error Shaping

This paper presents a digital readout integrated circuit (DROIC) for small pitch medium-wave infrared (MWIR) starring focal plane arrays (FPAs). To overcome the limited in-pixel resolution of such DROICs, the concept of error shaping is introduced in pulse frequency modulation (PFM)-based pixels. Multiple integration operations are performed in a single frame and the residue charge from each integration phase is retained and injected into the next charge packet. This over-integration routine provides a high pass noise transfer function (NTF) equivalent to what is obtained in a first order sigma delta modulator. The concise structure of a PFM analog-to-digital converter (ADC) lends itself well to such an operation inside the pixel, while consuming low power. Along with a theoretical analysis of the technique, a prototype based on a $16\times 16$ array and sinc decimation filters is developed to demonstrate the performance of the proposed DROIC. With an in-pixel resolution of 5 bits and over-integration ratio (OIR) of 128, 395 noise electrons are demonstrated at full-well fill from the test array fabricated in a 90 nm bulk-CMOS process.

Noise edge pitch and models of pitch perception.

Monaural noise edge pitch (NEP) is evoked by a broadband noise with a sharp falling edge in the power spectrum. The pitch is heard near the spectral edge frequency but shifted slightly into the frequency region of the noise. Thus, the pitch of a lowpass (LP) noise is matched by a pure tone typically 2%–5% below the edge, whereas the pitch of highpass (HP) noise is matched a comparable amount above the edge. Musically trained listeners can recognize musical intervals between NEPs. The pitches can be understood from a temporal pattern-matching model of pitch perception based on the peaks of a simplified autocorrelation function. The pitch shifts arise from limits on the autocorrelation window duration. An alternative place-theory approach explains the pitch shifts as the result of lateral inhibition. Psychophysical experiments using edge frequencies of 100 Hz and below find that LP-noise pitches exist but HP-noise pitches do not. The result is consistent with a temporal analysis in tonotopic regions outside the noise band. LP and HP experiments with high-frequency edges find that pitch tends to disappear as the edge frequency approaches 5000 Hz, as expected from a timing theory, though exceptional listeners can go an octave higher.

Monaural edge pitch and models of pitch perception

Noise with a sharp spectral edge produces a definite, perceived pitch. The pitch of lowpass noise lies slightly below the edge and the pitch of highpass noise slightly above it. The pitch shifts away from the edge, expressed in semitones, increase with the decreasing edge frequency. A neural timing model based on positive peaks of the autocorrelation function accounts for the broad features of the shifts. So, does a place model based on the peak of an excitation pattern as enhanced by lateral inhibition. As the edge frequency decreases below 150 Hz, the pitch persists for a lowpass noise but disappears for highpass noise. This result is consistent with the timing model but not with the place model. For high edge frequencies, the pitch is stronger for highpass noise than for lowpass noise—consistent with both timing and place models. As the edge frequency approaches 5000 Hz, the pitch disappears for most listeners, but for some listeners, a pitch persists for edge frequencies beyond 8000 Hz. The latter res...

An analytic approach to identifying the sources of the low-frequency round window cochlear response

The cochlear microphonic, traditionally thought of as an indication of electrical current flow through hair cells, in conjunction with suppressing high-pass noise or tones, is a promising method of assessing the health of outer hair cells at specific locations along the cochlear partition. We propose that the electrical potential recorded from the round window in gerbils in response to low-frequency tones, which we call cochlear response (CR), contains significant responses from multiple cellular sources, which may expand its diagnostic purview. In this study, CR is measured in the gerbil and modeled to identify its contributing sources. CR was recorded via an electrode placed in the round window niche of sixteen Mongolian gerbils and elicited with a 45 Hz tone burst embedded in 18 high-pass filtered noise conditions to target responses from increasing regions along the cochlear partition. Possible sources were modeled using previously-published hair cell and auditory nerve response data, and then weighted and combined using linear regression to produce a model response that fits closely to the mean CR waveform. The significant contributing sources identified by the model are outer hair cells, inner hair cells, and the auditory nerve. We conclude that the low-frequency CR contains contributions from several cellular sources.

Comparison between the auditory lateralization ability of normal hearing elderly and youth with filtered noise

Background and Aim: Aging affects the audi­tory lateralization function that is achieved thr­ough processing binaural cues. One of the most important benefits of this process, along with getting informed of sound location, is increasing signal to noise ratio and improvement of speech comprehension in crowded environments, which is one of the most common hearing complaints in the elderly. This study aimed to compare the performance of the lateralization function under the headphone conditions between the elderly and the youth, with a filtered noise stimulus. Methods: This study was performed between 22 elderly aged 60−80 and 22 young people aged 20−30. The auditory threshold was less than 25 dB HL in 250 to 4000 Hz frequencies in both groups. By applying a time delay of −880 to +880 microsecond and the intensity differe­nce of −10 to +10 dB between the two ears, and with high-pass and low-pass noise stimulus, the lateralization function was examined. For descr­iption of the lateralization function, scatter diag­ram and in order to compare the results, paired t-test and independent t-test were used. Results: Findings showed that the elderly's err­ors were increased in all tests compared to those of the youth group. There was a significant difference (p < 0.05) between the two groups in lateralization by time clues with low-pass and high-pass noise. Conclusion: Results confirm the impairment of the lateralization and processing of binaural cues in the elderly differently.

Masking Differentially Affects Envelope-following Responses in Young and Aged Animals

Age-related hearing decline typically includes threshold shifts as well as reduced wave I auditory brainstem response (ABR) amplitudes due to cochlear synaptopathy/neuropathy, which may compromise precise coding of suprathreshold speech envelopes. This is supported by findings with older listeners, who have difficulties in envelope and speech processing, especially in noise. However, separating the effects of threshold elevation, synaptopathy, and degradation by noise on physiological representations may be difficult. In the present study, the effects of notched, low- and high-pass noise on envelope-following responses (EFRs) in aging were compared when sound levels (aged: 85-dB SPL; young: 60- to 80-dB SPL) were matched between groups peripherally, by matching wave I ABR amplitudes, or centrally by matching EFR amplitudes. Low-level notched noise reduced EFRs to sinusoidally amplitude-modulated (SAM) tones in young animals for notch widths up to 2 octaves. High-pass noise above the carrier frequency reduced EFRs. Young animals showed EFR reductions at lower noise levels. Low-pass noise did not reduce EFRs in either young or aged animals. High-pass noise may affect EFR amplitudes in young animals more than aged by reducing the contributions of high-frequency-sensitive inputs. EFRs to SAM tones in modulated noise (NAM) suggest that neurons of young animals can synchronize to NAM at lower sound levels and maintain dual AM representations better than older animals. The overall results show that EFR amplitudes are strongly influenced by aging and the presence of a competing sound that likely reduces or shifts the pool of responsive neurons.

Spatial hearing ability of the pigmented Guinea pig (Cavia porcellus): Minimum audible angle and spatial release from masking in azimuth

Despite the common use of guinea pigs in investigations of the neural mechanisms of binaural and spatial hearing, their behavioral capabilities in spatial hearing tasks have surprisingly not been thoroughly investigated. To begin to fill this void, we tested the spatial hearing of adult male guinea pigs in several experiments using a paradigm based on the prepulse inhibition (PPI) of the acoustic startle response. In the first experiment, we presented continuous broadband noise from one speaker location and switched to a second speaker location (the “prepulse”) along the azimuth prior to presenting a brief, ∼110 dB SPL startle-eliciting stimulus. We found that the startle response amplitude was systematically reduced for larger changes in speaker swap angle (i.e., greater PPI), indicating that using the speaker “swap” paradigm is sufficient to assess stimulus detection of spatially separated sounds. In a second set of experiments, we swapped low- and high-pass noise across the midline to estimate their ability to utilize interaural time- and level-difference cues, respectively. The results reveal that guinea pigs can utilize both binaural cues to discriminate azimuthal sound sources. A third set of experiments examined spatial release from masking using a continuous broadband noise masker and a broadband chirp signal, both presented concurrently at various speaker locations. In general, animals displayed an increase in startle amplitude (i.e., lower PPI) when the masker was presented at speaker locations near that of the chirp signal, and reduced startle amplitudes (increased PPI) indicating lower detection thresholds when the noise was presented from more distant speaker locations. In summary, these results indicate that guinea pigs can: 1) discriminate changes in source location within a hemifield as well as across the midline, 2) discriminate sources of low- and high-pass sounds, demonstrating that they can effectively utilize both low-frequency interaural time and high-frequency level difference sound localization cues, and 3) utilize spatial release from masking to discriminate sound sources. This report confirms the guinea pig as a suitable spatial hearing model and reinforces prior estimates of guinea pig hearing ability from acoustical and physiological measurements.

Role of expectancy in front-back confusions during sound source localization

Front-back confusions (FBCs) often occur in laboratory experiments. Stimulus complexity, especially for high-frequency sounds, and head motion significantly reduce FBCs, so that in the everyday world FBCs probably do not often influence sound source localization. There is a small literature suggesting that listeners “expectancies” (based on experience or other sensory inputs) for where sound sources might be also affect FBCs. A low-pass noise that produces a large number of FBCs and a high-pass noise that does not were used. Listeners indicated the location of sound sources on the azimuth plane. The study aimed to investigate FBCs and not sound source localization accuracy. For one condition, listeners were told that the noises would all be coming from in front, that in another condition from in back, and in a third condition from the full 360° azimuth circle. In the first (front) and second (back) conditions, most of the sounds came from the front or back respectfully, but 20% did not. The expectancy that sounds would be in front or in back had a large influence on FBCs as compared to when sounds were presented from the entire azimuth circle. (Work supported by grants from NIDCD and Oculus VR, LLC.)

Early temporary asymmetrical hearing impairs behavioral and neural sensitivity to sound location

Children experiencing asymmetrical hearing early in life often display binaural hearing impairments that persist long after symmetric hearing is restored, suggesting abnormal central auditory development. Here, we test the hypothesis that abnormal neural coding of the interaural level difference (ILD) cue to sound location, resulting from asymmetrical hearing during development, underlies the hearing impairments. Guinea pigs were reared for 8 weeks from birth with a unilateral conductive hearing loss (CHL via earplug). After earplug removal we measured behavioral spatial acuity, the binaural interaction component (BIC) of the auditory brainstem response (ABR), and sensitivity to ILDs in auditory midbrain neurons (inferior colliculus, IC). Animals raised with asymmetric hearing displayed worse spatial acuity (~2x worse) for high-pass noise compared to age-matched controls, suggesting impaired sensitivity to ILDs. Physiologically, animals displayed abnormal BICs of the ABR indicating altered binaural proces...

Neural representation of the self-heard biosonar click in bottlenose dolphins (Tursiops truncatus).

The neural representation of the dolphin broadband biosonar click was investigated by measuring auditory brainstem responses (ABRs) to "self-heard" clicks masked with noise bursts having various high-pass cutoff frequencies. Narrowband ABRs were obtained by sequentially subtracting responses obtained with noise having lower high-pass cutoff frequencies from those obtained with noise having higher cutoff frequencies. For comparison to the biosonar data, ABRs were also measured in a passive listening experiment, where external clicks and masking noise were presented to the dolphins and narrowband ABRs were again derived using the subtractive high-pass noise technique. The results showed little change in the peak latencies of the ABR to the self-heard click from 28 to 113 kHz; i.e., the high-frequency neural responses to the self-heard click were delayed relative to those of an external, spectrally "pink" click. The neural representation of the self-heard click is thus highly synchronous across the echolocation frequencies and does not strongly resemble that of a frequency modulated downsweep (i.e., decreasing-frequency chirp). Longer ABR latencies at higher frequencies are hypothesized to arise from spectral differences between self-heard clicks and external clicks, forward masking from previously emitted biosonar clicks, or neural inhibition accompanying the emission of clicks.

Evaluation of optimal masking levels in place-specific low-frequency chirp-evoked auditory brainstem responses.

The aim of the study is the experimental determination of the optimal required masking level for a given stimulus level when using a band limited "low-frequency chirp" in order to improve frequency and place specificity of auditory brainstem responses (ABRs). A low-frequency chirp (100-850 Hz) at stimulation levels between 40 and 80 dB normalized hearing level was presented to 12 normal hearing subjects. During presentation of each stimulus, the level of a high-pass noise with a low cutoff frequency of 1100 Hz was varied between 0 and 25 dB signal-to-noise ratio (SNR) by using 5 dB steps (at 0 dB SNR the same level of both the chirp and the masker in dB sound pressure level was presented). Measurements without masking were used as a reference. In all masking conditions, the latency of wave V was significantly increased compared to unmasked ABRs. The amplitude of wave V decreased when reaching the effective and therefore optimal masking level. Accordingly, in order to ensure place specificity of the ABR, ipsilateral masking is essential. At lower stimulus levels the SNR can be substantially increased (i.e., the masker level decreased) without loss of place specificity.

Role of static and dynamic interaural differences in the localization of spatially segregated sounds

We clarify the role of interaural time difference (ITD) and interaural level difference (ILD) in the localization of spatially segregated sounds. Listeners were asked to distinguish between two sources of white noise having various ITDs/ILDs under head-still and head-rotating conditions. For the former, the segregated rate reached 80% for an ITD of 0.3 ms, when the ITD of other sound source corresponded to the other side. The segregated rate also reached 80% when the ILD between the two noise sources corresponded to an angular difference of 30°. Under head-rotating conditions, the segregated rate reached almost 100%, regardless of the ITDs of the two noise sources. This was because when only one white noise source was used, it was perceived as separate high and low sound images. In confirmations with low- and high-pass noises, the lower-frequency sound image was contained for <1.7 kHz and the higher-frequency one was contained for >1.2 kHz. In addition, the segregated rate was essentially the same for the ILD under head-still conditions when the ILDs of two noise sources corresponded to the separate side. The sound image was integrated into a single one when the ILDs of two noise sources corresponded to the same side.

Using ILD or ITD Cues for Sound Source Localization and Speech Understanding in a Complex Listening Environment by Listeners With Bilateral and With Hearing-Preservation Cochlear Implants.

To assess the role of interaural time differences and interaural level differences in (a) sound-source localization, and (b) speech understanding in a cocktail party listening environment for listeners with bilateral cochlear implants (CIs) and for listeners with hearing-preservation CIs. Eleven bilateral listeners with MED-EL (Durham, NC) CIs and 8 listeners with hearing-preservation CIs with symmetrical low frequency, acoustic hearing using the MED-EL or Cochlear device were evaluated using 2 tests designed to task binaural hearing, localization, and a simulated cocktail party. Access to interaural cues for localization was constrained by the use of low-pass, high-pass, and wideband noise stimuli. Sound-source localization accuracy for listeners with bilateral CIs in response to the high-pass noise stimulus and sound-source localization accuracy for the listeners with hearing-preservation CIs in response to the low-pass noise stimulus did not differ significantly. Speech understanding in a cocktail party listening environment improved for all listeners when interaural cues, either interaural time difference or interaural level difference, were available. The findings of the current study indicate that similar degrees of benefit to sound-source localization and speech understanding in complex listening environments are possible with 2 very different rehabilitation strategies: the provision of bilateral CIs and the preservation of hearing.

Temporary unilateral hearing loss during development impairs behavioral and neural sensitivity to interaural level difference cues for sound localization

Children who experience persistent conductive hearing loss (CHL) early in life often display binaural hearing impairments that persist long after CHL is resolved, suggesting abnormal central auditory development. Abnormal sensitivity to interaural level differences (ILDs) is particularly likely as a CHL (such as an ear infection) can attenuate sound in the affected ear by >30 dB, dramatically distorting ILD cues. Here, we quantified the effects of unilateral CHL on (1) behavioral spatial acuity and (2) neural information processing of ILD cues in the guinea pig auditory midbrain (inferior colliculus, IC) using the mathematical framework of Fisher information (FI). Animals raised with unilateral CHL displayed larger minimum audible angles for high-pass noise compared to age-matched controls, suggesting impaired ILD sensitivity. Based on acoustic directional transfer function measurements, ILD discrimination thresholds were elevated by ~3–6 dB. Following behavior, extracellular recordings were made in the I...