Amplitude-modulated Tones Research Articles

On the Effectiveness of airborne infrasound in eliciting vestibular-evoked myogenic responses

The use of airborne infrasound and other stimuli to elicit (cervical) vestibular-evoked myogenic potentials (cVEMPs) was studied to address the common proposition that infrasound may efficiently stimulate the vestibular system, an effect which may underlie the so-called wind-turbine syndrome. cVEMPs were measured for both ears of 15 normal-hearing subjects using three types of airborne sound stimulation: (1) 500-Hz tone bursts (transient); (2) 500-Hz sinusoidally amplitude-modulated tones at a 40-Hz rate (SAM); and (3) low-frequency and infrasound pure tones (LF/IS). The two former stimulation types served as control and allowed a systematic comparison with (3). It was found that SAM stimulation is effective and appears to be comparable to transient stimulation, as was previously observed in a yet small number of studies. Although the vestibular system is reported to be highly sensitive to low-frequency mechanical vibration, airborne LF/IS stimulation at ∼80–90-phon loudness levels did not elicit significant saccular vestibular responses.

Classification of auditory selective attention using spatial coherence and modular attention index

Background and Objective: Brain-Computer Interfaces (BCIs) based on auditory selective attention have been receiving much attention because i) they are useful for completely paralyzed users since they do not require muscular effort or gaze and ii) focusing attention is a natural human ability. Several techniques - such as recently developed Spatial Coherence (SC) - have been proposed in order to optimize the BCI procedure. Thus, this work aims at investigating and comparing two strategies based on spatial coherence detection: contralateral and modular classifiers. The latter is a new method using modular attention index. The new classifier was developed to implement an auditory BCI where a volunteer makes binary choices using selective attention under the amplitude-modulated tones stimulation.Methods: Contralateral and modular classifiers were applied to the electroencephalogram (EEG) recorded from 144 subjects under the BCI protocol. The best set of parameters (carriers of the stimulus, channels and trials of signal) for this BCI was investigated taking into consideration the hit rate and the information transfer rate.Results: The best result obtained using the modular classifier was a hit rate of 91.67% and information transfer rate of 6.74 bits/min using 0.5 kHz/4.0 kHz as stimuli and three windows (5.10 sec of EEG signal). These results were obtained with five electrodes (C3, P3, F8, P4, O2) using exhaustive search to identify regions with greater coherence.Conclusion: The modular classifier - using electroencephalogram channels from the central, frontal, occipital and parietal areas - improves the performance of auditory BCIs based on selective attention.

Auditory cortex responses to interaural time differences in the envelope of low-frequency sound, recorded with MEG in young and older listeners

Interaural time and intensity differences (ITD and IID) are important cues in binaural hearing and allow for sound localization, improving speech understanding in noise and reverberation, and integrating sound sources in the auditory scene. Whereas previous research showed that the upper-frequency limit for ITD detection in the fine structure of sound declines in aging, the processing of envelope ITD in low-frequency amplitude modulated (AM) sound and the related brain responses are less understood. This study investigated the cortical processing of envelope ITD and compared the results with previous findings about the fine-structure ITD. In two experiments, participants listened to 40-Hz AM tones containing sudden changes in the envelope ITD. Multiple MEG responses were analyzed, including the auditory evoked N1 responses, elicited both by sound onsets and ITD changes, and 40-Hz responses, elicited by the AM. The first experiment with healthy young adults revealed a substantial decline in the magnitudes of the ITD change N1 response, and the 40-Hz phase resets at higher carrier frequencies, suggesting a similar frequency characteristic as observed for fine structure ITD. The amplitude of the 40-Hz ASSR declined only gradually with increasing carrier frequency, and it was excluded as a confounding factor in the decline in the ITD response. Larger responses to outward ITD changes than inward changes, here first reported for envelope ITD, were another characteristics that were similar to fine-structure ITD. A second experiment with groups of young and older listeners examined the effects of aging and concurrent noise on the cortical envelope ITD responses. One important research question was, whether binaural cues are accessible in noise. Behavioural tests showed an age-related hearing loss in the older group and decreased performance in envelope ITD detection and speech-in-noise (SIN) understanding. Binaural hearing and SIN performance were correlated with one other, but not with hearing loss. The frequency limit for envelope ITD was reduced in older listeners similarly as previously found for fine structure ITD, and older listeners were more susceptible to concurrent multi-talker noise. The similarities between responses to envelope ITD and to fine structure ITD suggest that a common cortical code exists for the envelope and fine structure ITD. The dependency on the carrier frequency suggests that envelope ITD processing at the subcortical level requires stimulus phase locking, which might be reduced in aging.

Impact of stimulus bandwidth on the estimate of the upper-frequency limit of hearing in toothed whales

No consensus on the stimuli used in toothed whale hearing tests using evoked potential methods currently exists. However, stimulus bandwidth should directly affect determination of the upper-frequency limit (UFL) of hearing because of the steep reduction in hearing sensitivity at frequencies immediately below the UFL—the broader the stimulus bandwidth, the greater the amount of spectral “splatter” into lower frequencies where hearing is more sensitive. To test this hypothesis, variability in auditory evoked potential thresholds to tone-pips (with variable bandwidth) and amplitude-modulated tones was determined in dolphins near their UFL of hearing and ½ to 1 octave below where hearing is more sensitive. Subjects included both normal and hearing-impaired dolphins. At frequencies where hearing was sensitive and the audiogram was relatively flat, negligible changes in the hearing threshold with changing stimulus bandwidth were observed. Conversely, thresholds near the UFL of hearing declined with increasing stimulus bandwidth, regardless of whether dolphins had normal hearing or hearing impairment, and resulted in broadened hearing bandwidth estimates. Thus, it is recommended that stimulus bandwidth be reported in hearing tests where the UFL is tested since stimulus bandwidth affects thresholds through spectral splatter into lower frequency bands where hearing is more sensitive.

Amplitude Modulation May Be Confused with Infrasound

Environmental infrasound is usually accompanied by low-frequency (LF) sounds. Considering that inner hair cell transduction equals half-wave rectification, activity of low-frequency auditory nerve fibres may be indistinguishable whether elicited by LF sound that is amplitude-modulated at an infrasonic rate, or LF sound that is superimposed onto infrasound that biases the basilar membrane position. We tested whether listeners are able to distinguish a 63-Hz carrier tone, amplitude modulated at 8 Hz, from a 63-Hz pure tone that was perceptually loudness-modulated by an 8-Hz biasing tone. Using a maximum-likelihood procedure, 12 participants first adjusted the intensity of the 8-Hz tone so that the perceived modulation of the pure tone matched a reference amplitude-modulated tone. Both stimuli types were then presented in random order, and participants had to identify presentations which contained the infrasound tone. About half the participants performed close to chance. The best had 81% correct. Experiments with a 125-Hz carrier tone gave similar results. Although performance may improve in a 2-interval discrimination task, this would not be representative of real listening conditions. Results suggest that slowly amplitude-modulated LF sounds may underlie complaints about environmental infrasound, where measured infrasound levels are well below sensation threshold.

Brain Computer Interface Using Modulation of Auditory Steady-State Response with Help of Stochastic Resonance.

This paper proposes an eye-movement independent brain computer interface based on the modulations of auditory steady-state response (ASSR-BCI) to amplitude-modulated (AM) tones elicited by paying selective attention to one of the two AM tones. Moreover, the proposed ASSR-BCI exploits a stochastic resonance effect to improve the signal separation and attained the mean classification accuracy of 77 % across nine normal subjects under a noise-added condition with sound pressures 60 dB for the two tones and 30 dB for the noise added to the two AM tones. Results from information transfer rate and its inter-individual difference suggest that it may be adequate to set an inter-trial interval at 2∽3 s for a trial time length. It is consequently feasible to develop a practical eye-movement-independent BCI available in eyes-closed state by optimizing the parameters such as the trial time length and electrode sites each user.

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.

Effects of age on sensitivity to interaural time differences in envelope and fine structure, individually and in combination.

Sensitivity to interaural time differences (ITDs) in envelope and temporal fine structure (TFS) of amplitude-modulated (AM) tones was assessed for young and older subjects, all with clinically normal hearing at the carrier frequencies of 250 and 500 Hz. Some subjects had hearing loss at higher frequencies. In experiment 1, thresholds for detecting changes in ITD were measured when the ITD was present in the TFS alone (ITDTFS), the envelope alone (ITDENV), or both (ITDTFS/ENV). Thresholds tended to be higher for the older than for the young subjects. ITDENV thresholds were much higher than ITDTFS thresholds, while ITDTFS/ENV thresholds were similar to ITDTFS thresholds. ITDTFS thresholds were lower than ITD thresholds obtained with an unmodulated pure tone, indicating that uninformative AM can improve ITDTFS discrimination. In experiment 2, equally detectable values of ITDTFS and ITDENV were combined so as to give consistent or inconsistent lateralization. There were large individual differences, but several subjects gave scores that were much higher than would be expected from the optimal combination of independent sources of information, even for the inconsistent condition. It is suggested that ITDTFS and ITDENV cues are processed partly independently, but that both cues influence lateralization judgments, even when one cue is uninformative.

Context effects in modulation masking

Neal Viemeister is especially known for his contributions to our understanding of perception of amplitude modulation. Modulation masking and signal-to-noise ratio in the modulation domain have been shown to be strong predictors of speech intelligibility in noise. This study uses 40-ms amplitude-modulated tones embedded in a simultaneous 100-ms noise masker to show that for a given type of simultaneous masker, modulation masking can vary significantly depending on the type of stimulus that precedes the masked target. Three types of precursor were used to investigate these context effects, a 2-octave noise centered on the signal frequency (the same as the masker), a 7-component inharmonic complex tone, and a pure tone with a frequency equal to that of the target. All precursors had a duration of 400 ms and were presented in close temporal proximity at levels equal to those of the noise masker. The noise precursor produced on average a 7-8 dB [20log(m)] release from modulation masking. In contrast no change in the amount of masking and a small release of masking were observed for the pure-tone and complex-tone precursors, respectively. Mechanisms that may underlie the observed context effects will be discussed. [Work supported by NIH grant R01 DC015462.]

Reduced low-frequency power and phase locking reflect restoration in the auditory continuity illusion.

Interruptions in auditory input can be perceptually restored if they coincide with a masking sound, resulting in a continuity illusion. Previous studies have shown that this continuity illusion is associated with reduced low-frequency neural oscillations in the auditory cortex. However, the precise contribution of oscillatory amplitude changes and phase alignment to auditory restoration remains unclear. Using electroencephalography, we investigated induced power changes and phase locking in response to 3Hz amplitude-modulated tones during the interval of an interrupting noise. We experimentally manipulated both the physical continuity of the tone (continuous vs. interrupted) and the masking potential of the noise (notched vs. full). We observed an attenuation of 3Hz power during continuity illusions in comparison with both continuous tones and veridically perceived interrupted tones. This illusion-related suppression of low-frequency oscillations likely reflects a blurring of auditory object boundaries that supports continuity perception. We further observed increased 3Hz phase locking during fully masked continuous tones compared with the other conditions. This low-frequency phase alignment may reflect the neural registration of the interrupting noise as a newly appearing object, whereas during continuity illusions, a spectral portion of this noise is delegated to filling the interruption. Taken together, our findings suggest that the suppression of slow cortical oscillations in both the power and phase domains supports perceptual restoration of interruptions in auditory input.

Subcortical amplitude modulation encoding deficits suggest evidence of cochlear synaptopathy in normal-hearing 18-19 year olds with higher lifetime noise exposure.

Noise exposure and aging can damage cochlear synapses required for suprathreshold listening, even when cochlear structures needed for hearing at threshold remain unaffected. To control for effects of aging, behavioral amplitude modulation (AM) detection and subcortical envelope following responses (EFRs) to AM tones in 25 age-restricted (18-19 years) participants with normal thresholds, but different self-reported noise exposure histories were studied. Participants with more noise exposure had smaller EFRs and tended to have poorer AM detection than less-exposed individuals. Simulations of the EFR using a well-established cochlear model were consistent with more synaptopathy in participants reporting greater noise exposure.

40Hz auditory steady-state response in schizophrenia: Sensitivity to stimulation type (clicks versus flutter amplitude-modulated tones).

Auditory steady-state response (ASSR) at 40Hz has been proposed as a potential biomarker for schizophrenia. The ASSR studies in patients have used click stimulation or amplitude-modulated tones. However, the sensitivity of 40Hz ASSRs to different stimulation types in the same group of patients has not been previously evaluated. Two stimulation types for ASSRs were tested in this study: (1) 40Hz clicks and (2) flutter-amplitude modulated tones. The mean phase-locking index, evoked amplitude and event-related spectral perturbation values were compared between schizophrenia patients (n=26) and healthy controls (n=20). Both stimulation types resulted in the observation of impaired phase-locking and power measures of late (200-500ms) 40Hz ASSR in patients compared to healthy controls. The early-latency (0-100ms) 40Hz ASSR part was diminished in the schizophrenia group in response to clicks only. The late-latency 40Hz ASSR parameters obtained through different stimulation types correlated in healthy subjects but not in patients. We conclude that flutter amplitude-modulated tone stimulation, due to its potential to reveal late-latency entrainment deficits, is suitable for use in clinical populations. Careful consideration of experimental stimulation settings can contribute to the interpretation of ASSR deficits and utilization as a potential biomarker.

Comparison of univariate and multivariate magnitude-squared coherences in the detection of human 40-Hz auditory steady-state evoked responses

Abstract Objective response detection (ORD) techniques for evaluating bioelectrical evoked responses in the electroencephalogram (EEG) are based on statistical criteria rather than on visual inspection. Hence, they do not depend on human evaluation, which is often a subjective approach. Furthermore, since such techniques do not involve heuristic approaches, they may be more easily implemented and used in automatic systems. The Magnitude-Squared Coherence (MSC), together with its recently developed multivariate extension (the multiple magnitude-squared coherence – MMSC), have been pointed out as one of the most efficient ORD techniques for detecting steady-state responses in the EEG. In this work, both MSC and MMSC were applied to EEG signals collected during auditory stimulation in order to allow comparison in the detection of auditory steady-state responses (ASSRs). The stimuli consisted of 40 Hz amplitude-modulated tones delivered binaurally in the intensity of 50 dB SPL (sound pressure level). The best result was obtained by using MMSC in the two-electrode set C4 and Fz. This configuration led to a 0.92-detection ratio, within 111.55 s in average to detect each response and kept the false alarm ratio under 0.05. The average improvement in performance was about 11% when compared to the MSC. These results allow concluding that the detection protocol of 40 Hz ASSRs can be improved by using MMSC in multichannel EEG analysis when compared to the traditional univariate MSC approach.

Erratum to: A Modeling Study of the Responses of the Lateral Superior Olive to Ipsilateral Sinusoidally Amplitude-Modulated Tones.

The lateral superior olive (LSO) is a brainstem nucleus that is classically understood to encode binaural information in high-frequency sounds. Previous studies have shown that LSO cells are sensitive to envelope interaural time difference in sinusoidally amplitude-modulated (SAM) tones (Joris and Yin, J Neurophysiol 73:1043–1062, 1995; Joris, J Neurophysiol 76:2137–2156, 1996) and that a subpopulation of LSO neurons exhibit low-threshold potassium currents mediated by Kv1 channels (Barnes-Davies et al., Eur J Neurosci 19:325–333, 2004). It has also been shown that in many LSO cells the average response rate to ipsilateral SAM tones decreases with modulation frequency above a few hundred Hertz (Joris and Yin, J Neurophysiol 79:253–269, 1998). This low-pass feature is not directly inherited from the inputs to the LSO since the response rate of these input neurons changes little with increasing modulation frequency. In the current study, an LSO cell model is developed to investigate mechanisms consistent with the responses described above, notably the emergent rate decrease with increasing frequency. The mechanisms explored included the effects of after-hyperpolarization (AHP) channels, the dynamics of low-threshold potassium channels (KLT), and the effects of background inhibition. In the model, AHP channels alone were not sufficient to induce the observed rate decrease at high modulation frequencies. The model also suggests that the background inhibition alone, possibly from the medial nucleus of the trapezoid body, can account for the small rate decrease seen in some LSO neurons, but could not explain the large rate decrease seen in other LSO neurons at high modulation frequencies. In contrast, both the small and large rate decreases were replicated when KLT channels were included in the LSO neuron model. These results support the conclusion that KLT channels may play a major role in the large rate decreases seen in some units and that background inhibition may be a contributing factor, a factor that could be adequate for small decreases.

Age-related changes in envelope-following responses at equalized peripheral or central activation

Previous work has debated about the comparisons of hearing abilities faced with alterations in hearing thresholds and evoked potentials between groups following acoustic trauma- or age-related changes. This study compares envelope-following responses (EFRs) of young and aged rats when sound levels were matched according to (1) wave I amplitudes of auditory brainstem responses (ABRs) elicited by 8-kHz tones or (2) EFR amplitudes evoked by sinusoidally amplitude-modulated (SAM) tones at 100% depth. Matched wave I amplitudes across age corresponded to approximately 20-dB sound level differences. For matched wave I, no age-related differences were observed in wave V amplitudes. However, EFRs recorded in silence were enhanced with aging at 100% but not at 25% depth, consistent with enhanced central gain in aging. For matched EFRs, there were no age-related differences in EFRs of amplitude modulation (AM) depth and AM frequency processing. These results suggest novel, objective measures beyond threshold to compensate for differences in auditory nerve activation and to differentiate peripheral and central contributions of EFRs.

Amplitude-modulation detection and speech recognition in normal and hearing-impaired listeners

Even though temporal speech envelopes may form a salient cue when listening to speech in noisy backgrounds, the relationship between speech intelligibility and the sensitivity to detecting temporal envelopes (i.e., amplitude modulation detection) is not well understood. This study measured speech reception thresholds in quiet, stationary and speech-modulated noise in three listener groups: young (yNH) and older normal-hearing (oNH) listeners and hearing-impaired (HI) listeners. In addition to broadband speech and noise signals, we adopted low and high-pass filtered versions of the stimuli to study the contribution of different coding mechanisms in basal and apical cochlear regions. For the same listeners, amplitude-modulation (AM) detection thresholds were measured in quiet and in the presence of broadband masking noise for 70 dB AM tones of 0.5, 2 and 4 kHz with either 100 or 5 Hz modulation frequency. Even though group trends were clearer than individual differences, AM detection thresholds showed a relationship to speech recognition in the low-pass filtered speech-modulated noise condition, and for yNH and oNH listeners in the high-pass filtered condition. Overall, this study sheds light on the importance of temporal envelope coding sensitivity for speech recognition and its relationship to near-and supra-threshold hearing deficits.

Psychometric amplitude-modulation detection thresholds in chinchillas before and after moderate noise exposure

Recent findings from animal studies suggest that moderate-level acoustic overexposure can produce permanent cochlear synaptopathy while not significantly affecting hearing thresholds in quiet. It has been hypothesized that this hidden hearing loss may underlie difficulties some listeners have in noisy situations even with normal audiograms. However, this hypothesis has not been tested directly due to difficulties measuring behavior in animal models for which cochlear synaptopathy has been demonstrated, and the inability to measure cochlear synaptopathy directly in humans. We recently established a mammalian model (chinchilla) that has corresponding neural and behavioral amplitude-modulation (AM) detection thresholds in line with human behavioral thresholds, and which shows cochlear synaptopathy following moderate noise exposure. Here, behavioral AM-detection thresholds were measured in six chinchillas, before and after noise exposure, using the method of constant stimuli. Animals were trained to discriminate a sinusoidal AM (SAM) tone (4-kHz carrier) from a pure tone, in the presence of a notched-noise masker. Behavioral thresholds before noise exposure were consistent across individual animals and were in the range from -25 to -15 dB. Preliminary data collected following noise exposure do not show a substantial effect on AM-detection thresholds in this simple task. [Work supported by NIH grant R01-DC009838.]

Envelope following responses to Schroeder-phase harmonic stimuli

Recent studies suggest that peripheral nonlinearities only partially account for the envelope-following response (EFR) recorded at the scalp, with additional contributions from broadly tuned neurons in the central nervous system. The present study investigated this by measuring EFR to low and high-frequency stimuli designed to (1) optimize within-channel phase synchrony to produce maximal envelope modulation depth on the basilar membrane, or (2) optimize phase synchrony across multiple frequency channels to maximize envelope-related activity arising in the central nervous system. EFR was recorded in normal-hearing listeners using Schroeder-phase stimuli with harmonic components constrained to low and high-frequency ranges, as well to multi-frequency sinusoidally amplitude modulated tones as a function of component phase. Estimates of basilar membrane phase curvature varied across individuals. Response amplitudes and phase-locking values for the EFR varied as a function of carrier frequency range and component phase. The present results are consistent with an understanding of scalp-recorded EFR as a measure of central envelope processing extending beyond the initial encoding of the stimulus envelope arising in the periphery. The scalp-recorded EFR is thus not a direct index of envelope encoding in the auditory nerve.

Auditory fear conditioning modifies steady-state evoked potentials in the rat inferior colliculus

The rat inferior colliculus (IC) is a major midbrain relay for ascending inputs from the auditory brain stem and has been suggested to play a key role in the processing of aversive sounds. Previous studies have demonstrated that auditory fear conditioning (AFC) potentiates transient responses to brief tones in the IC, but it remains unexplored whether AFC modifies responses to sustained periodic acoustic stimulation-a type of response called the steady-state evoked potential (SSEP). Here we used an amplitude-modulated tone-a 10-kHz tone with a sinusoidal amplitude modulation of 53.7 Hz-as the conditioning stimulus (CS) in an AFC protocol (5 CSs per day in 3 consecutive days) while recording local field potentials (LFPs) from the IC. In the preconditioning session (day 1), the CS elicited prominent 53.7-Hz SSEPs. In the training session (day 2), foot shocks occurred at the end of each CS (paired group) or randomized in the inter-CS interval (unpaired group). In the test session (day 3), SSEPs markedly differed from preconditioning in the paired group: in the first two trials the phase to which the SSEP coupled to the CS amplitude envelope shifted ~90°; in the last two trials the SSEP power and the coherence of SSEP with the CS amplitude envelope increased. LFP power decreased in frequency bands other than 53.7 Hz. In the unpaired group, SSEPs did not change in the test compared with preconditioning. Our results show that AFC causes dissociated changes in the phase and power of SSEP in the IC.NEW & NOTEWORTHY Local field potential oscillations in the inferior colliculus follow the amplitude envelope of an amplitude-modulated tone, originating a neural response called the steady-state evoked potential. We show that auditory fear conditioning of an amplitude-modulated tone modifies two parameters of the steady-state evoked potentials in the inferior colliculus: first the phase to which the evoked oscillation couples to the amplitude-modulated tone shifts; subsequently, the evoked oscillation power increases along with its coherence with the amplitude-modulated tone.

Cortical Measures of Binaural Processing Predict Spatial Release from Masking Performance.

Binaural sensitivity is an important contributor to the ability to understand speech in adverse acoustical environments such as restaurants and other social gatherings. The ability to accurately report on binaural percepts is not commonly measured, however, as extensive training is required before reliable measures can be obtained. Here, we investigated the use of auditory evoked potentials (AEPs) as a rapid physiological indicator of detection of interaural phase differences (IPDs) by assessing cortical responses to 180° IPDs embedded in amplitude-modulated carrier tones. We predicted that decrements in encoding of IPDs would be evident in middle age, with further declines found with advancing age and hearing loss. Thus, participants in experiment #1 were young to middle-aged adults with relatively good hearing thresholds while participants in experiment #2 were older individuals with typical age-related hearing loss. Results revealed that while many of the participants in experiment #1 could encode IPDs in stimuli up to 1,000 Hz, few of the participants in experiment #2 had discernable responses to stimuli above 750 Hz. These results are consistent with previous studies that have found that aging and hearing loss impose frequency limits on the ability to encode interaural phase information present in the fine structure of auditory stimuli. We further hypothesized that AEP measures of binaural sensitivity would be predictive of participants' ability to benefit from spatial separation between sound sources, a phenomenon known as spatial release from masking (SRM) which depends upon binaural cues. Results indicate that not only were objective IPD measures well correlated with and predictive of behavioral SRM measures in both experiments, but that they provided much stronger predictive value than age or hearing loss. Overall, the present work shows that objective measures of the encoding of interaural phase information can be readily obtained using commonly available AEP equipment, allowing accurate determination of the degree to which binaural sensitivity has been reduced in individual listeners due to aging and/or hearing loss. In fact, objective AEP measures of interaural phase encoding are actually better predictors of SRM in speech-in-speech conditions than are age, hearing loss, or the combination of age and hearing loss.