Interaural Delay Research Articles

The temporal mismatch across listening sides affects cortical auditory evoked responses in normal hearing listeners and cochlear implant users with contralateral acoustic hearing

Combining a cochlear implant with contralateral acoustic hearing typically enhances speech understanding, although this improvement varies among CI users and can lead to an interference effect. This variability may be associated with the effectiveness of the integration between electric and acoustic stimulation, which might be affected by the temporal mismatch between the two listening sides. Finding methods to compensate for the temporal mismatch might contribute to the optimal adjustment of bimodal devices and to improve hearing in CI users with contralateral acoustic hearing.The current study investigates cortical auditory evoked potentials (CAEPs) in normal hearing listeners (NH) and CI users with contralateral acoustic hearing. In NH, the amplitude of the N1 peak and the maximum phase locking value (PLV) were analyzed under monaural, binaural, and binaural temporally mismatched conditions. In CI users, CAEPs were measured when listening with CI only (CIS_only), acoustically only (AS_only) and with both sides together (CIS+AS). When listening with CIS+AS, various interaural delays were introduced between the electric and acoustic stimuli.In NH listeners, interaural temporal mismatch resulted in decreased N1 amplitude and PLV. Moreover, PLV is suggested as a more sensitive measure to investigate the integration of information between the two listening sides.CI users showed varied N1 latencies between the AS_only and CIS_only listening conditions, with increased N1 amplitude when the temporal mismatch was compensated. A tendency towards increased PLV was also observed, however, to a lesser extent than in NH listeners, suggesting a limited integration between electric and acoustic stimulation.This work highlights the potential of CAEPs measurement to investigate cortical processing of the information between two listening sides in NH and bimodal CI users.

The Influence of Asymmetric Hearing Loss on Peripheral and Central Auditory Processing Abilities in Patients With Vestibular Schwannoma.

Asymmetric or unilateral hearing loss (AHL) may cause irreversible changes in the processing of acoustic signals in the auditory system. We aim to provide a comprehensive view of the auditory processing abilities for subjects with acquired AHL, and to examine the influence of AHL on speech perception under difficult conditions, and on auditory temporal and intensity processing. We examined peripheral and central auditory functions for 25 subjects with AHL resulting from vestibular schwannoma, and compared them to those from 24 normal-hearing controls that were matched with the AHL subjects in mean age and hearing thresholds in the healthy ear. Besides the basic hearing threshold assessment, the tests comprised the detection of tones and gaps in a continuous noise, comprehension of speech in babble noise, binaural interactions, difference limen of intensity, and detection of frequency modulation. For the AHL subjects, the selected tests were performed separately for the healthy and diseased ear. We observed that binaural speech comprehension, gap detection, and frequency modulation detection abilities were dominated by the healthy ear and were comparable for both groups. The AHL subjects were less sensitive to interaural delays, however, they exhibited a higher sensitivity to sound level, as indicated by lower difference limen of intensity and a higher sensitivity to interaural intensity difference. Correlations between the individual test scores indicated that speech comprehension by the AHL subjects was associated with different auditory processing mechanisms than for the control subjects. The data suggest that AHL influences both peripheral and central auditory processing abilities and that speech comprehension under difficult conditions relies on different mechanisms for the AHL subjects than for normal-hearing controls.

Impact of processing-latency induced interaural delay and level discrepancy on sensitivity to interaural level differences in cochlear implant users.

This study investigated whether an interaural delay, e.g. caused by the processing latency of a hearing device, can affect sensitivity to interaural level differences (ILDs) in normal hearing subjects or cochlear implant (CI) users with contralateral normal hearing (SSD-CI). Sensitivity to ILD was measured in 10SSD-CI subjects and in 24normal hearing subjects. The stimulus was a noise burst presented via headphones and via a direct cable connection (CI). ILD sensitivity was measured for different interaural delays in the range induced by hearing devices. ILD sensitivity was correlated with results obtained in a sound localization task using seven loudspeakers in the frontal horizontal plane. In the normal hearing subjects the sensitivity to interaural level differences deteriorated significantly with increasing interaural delays. In the CI group, no significant effect of interaural delays on ILD sensitivity was found. The NH subjects were significantly more sensitive to ILDs. The mean localization error in the CI group was 10.8° higher than in the normal hearing group. No correlation between sound localization ability and ILD sensitivity was found. Interaural delays influence the perception of ILDs. For normal hearing subjects a significant decrement in sensitivity to ILD was measured. The effect could not be confirmed in the tested SSD-CI group, probably due to a small subject group with large variations. The temporal matching of the two sides may be beneficial for ILD processing and thus sound localization for CI patients. However, further studies are needed for verification.

Detection of dynamic changes in interaural delay by older adults (L).

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.

The relationship between interaural delay in binaural gap detection and sensitivity to temporal fine structure in young adults with or without musical training experience.

Humans can detect the presence of a break in interaural correlation (BIC, also called binaural gap) even if a large interaural time delay (ITD) is introduced, which is important for detecting, recognizing, and localizing sounds in everyday environments. To investigate the relationship between interaural delay in binaural gap detection and the sensitivity of temporal fine structure (TFS), 40 young college students with normal hearing took the BIC delay threshold test, the TFS1 test (the test of monaural TFS sensitivity), and the TFS-AF test (the test of binaural TFS sensitivity). All participants were asked whether they had any musical training experience in their childhood. Results showed that the BIC delay threshold was significantly correlated with the TFS1 test (r =−0.426, p = 0.006), but not with the TFS-AF performance (r =−0.005, p = 0.997). The correlation between BIC delay threshold and monaural TFS sensitivity was observed in the non-music training group (r =−0.508, p = 0.010), but not in the music training group (r =−0.290, p = 0.295). These findings suggest that the interaural delay in binaural gap detection is related to the monaural sensitivity of TFS, this significant correlation was mainly found in young adults without musical training experience.

A Computational Model for Evaluating Transient Auditory Storage of Acoustic Features in Normal Listeners.

Humans are able to detect an instantaneous change in correlation, demonstrating an ability to temporally process extremely rapid changes in interaural configurations. This temporal dynamic is correlated with human listeners’ ability to store acoustic features in a transient auditory manner. The present study investigated whether the ability of transient auditory storage of acoustic features was affected by the interaural delay, which was assessed by measuring the sensitivity for detecting the instantaneous change in correlation for both wideband and narrowband correlated noise with various interaural delays. Furthermore, whether an instantaneous change in correlation between correlated interaural narrowband or wideband noise was detectable when introducing the longest interaural delay was investigated. Then, an auditory computational description model was applied to explore the relationship between wideband and narrowband simulation noise with various center frequencies in the auditory processes of lower-level transient memory of acoustic features. The computing results indicate that low-frequency information dominated perception and was more distinguishable in length than the high-frequency components, and the longest interaural delay for narrowband noise signals was highly correlated with that for wideband noise signals in the dynamic process of auditory perception.

Binaural Synthetic Aperture Imaging of the Field of Audition as the Head Rotates and Localisation Perception of Monophonic Sound Listened to through Headphones

A human listening to monophonic sound through headphones perceives the sound to emanate from a point inside the head at the auditory centre at effectively zero range. The extent to which this is predicted by synthetic-aperture calculation performed in response to head rotation is explored. The instantaneous angle between the auditory axis and the acoustic source, lambda, for the zero inter-aural time delay imposed by headphones is 90°. The lambda hyperbolic cone simplifies to the auditory median plane, which intersects a spherical surface centred on the auditory centre, along a prime meridian lambda circle. In a two-dimensional (2-D) synthetic-aperture computation, points of intersection of all lambda circles as the head rotates constitute solutions to the directions to acoustic sources. Geometrically, lambda circles cannot intersect at a point representing the auditory centre; nevertheless, 2-D synthetic aperture images for a pure turn of the head and for a pure lateral tilt yield solutions as pairs of points on opposite sides of the head. These can reasonably be interpreted to be perceived at the sums of the position vectors of the pairs of points on the acoustic image, i.e., at the auditory centre. But, a turn of the head on which a fixed lateral tilt of the auditory axis is concomitant (as in species of owl) yields a 2-D synthetic-aperture image without solution. However, extending a 2-D synthetic aperture calculation to a three-dimensional (3-D) calculation will generate a 3-D acoustic image of the field of audition that robustly yields the expected solution.

Impaired interaural correlation processing in people with schizophrenia.

Detection of transient changes in interaural correlation is based on the temporal precision of the central representations of acoustic signals. Whether schizophrenia impairs the temporal precision in the interaural correlation process is not clear. In both participants with schizophrenia and matched healthy-control participants, this study examined the detection of a break in interaural correlation (BIC, a change in interaural correlation from 1 to 0 and back to 1), including the longest interaural delay at which a BIC was just audible, representing the temporal extent of the primitive auditory memory (PAM). Moreover, BIC-induced electroencephalograms (EEGs) and the relationships between the early binaural psychoacoustic processing and higher cognitive functions, which were assessed by the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), were examined. The results showed that compared to healthy controls, participants with schizophrenia exhibited poorer BIC detection, PAM and RBANS score. Both the BIC-detection accuracy and the PAM extent were correlated with the RBANS score. Moreover, participants with schizophrenia showed weaker BIC-induced N1-P2 amplitude which was correlated with both theta-band power and inter-trial phase coherence. These results suggested that schizophrenia impairs the temporal precision of the central representations of acoustic signals, affecting both interaural correlation processing and higher-order cognitions.

Sensitivity to a Break in Interaural Correlation in Frequency-Gliding Noises.

This study was to investigate whether human listeners are able to detect a binaurally uncorrelated arbitrary-noise fragment embedded in binaurally identical arbitrary-noise markers [a break in correlation, break in interaural correlation (BIAC)] in either frequency-constant (frequency-steady) or frequency-varied (unidirectionally frequency gliding) noise. Ten participants with normal hearing were tested in Experiment 1 for up-gliding, down-gliding, and frequency-steady noises. Twenty-one participants with normal hearing were tested in Experiment 2a for both up-gliding and frequency-steady noises. Another nineteen participants with normal hearing were tested in Experiment 2b for both down-gliding and frequency-steady noises. Listeners were able to detect a BIAC in the frequency-steady noise (center frequency = 400 Hz) and two types of frequency-gliding noises (center frequency: between 100 and 1,600 Hz). The duration threshold for detecting the BIAC in frequency-gliding noises was significantly longer than that in the frequency-steady noise (Experiment 1), and the longest interaural delay at which a duration-fixed BIAC (200 ms) in frequency-gliding noises could be detected was significantly shorter than that in the frequency-steady noise (Experiment 2). Although human listeners can detect a BIAC in frequency-gliding noises, their sensitivity to a BIAC in frequency-gliding noises is much lower than that in frequency-steady noise.

Interaural delay modulates the prepulse inhibition of the startle reflex induced by binaural gap in humans

This study used prepulse inhibition (PPI) of the acoustic startle reflex as a physiological measure to determine the sensitivity to a break in interaural correlation (BIC) in binaural gap detection and compared this physiological measure with the traditional behavioral measure. Results show that BIC could be used as a prepulse to induce PPI at different interaural delays. Furthermore, the PPIs induced by BIC at an interaural delay of 4 ms, but not 0, 2, or 8 ms, were correlated with the BIC delay threshold. These findings suggest that the BIC-induced PPI paradigm may serve as an objective measure of binaural gap detection.

Synaptic excitation underlies processing of paired stimulation in the mouse inferior colliculus.

The inferior colliculus (IC) receives inputs from the ascending auditory pathway and helps localize the sound source by shaping neurons' responses. However, the contributions of excitatory or inhibitory synaptic inputs evoked by paired binaural stimuli with different inter-stimulus intervals to auditory responses of IC neurons remain unclear. Here, we firstly investigated the IC neuronal response to the paired binaural stimuli with different inter-stimulus intervals using in vivo loose-patch recordings in anesthetized C57BL/6 mice. It was found that the total acoustic evoked spikes remained unchanged under microsecond interval conditions, but persistent suppression would be observed when the time intervals were extended. We further studied the paired binaural stimuli evoked excitatory/inhibitory inputs using in vivo whole-cell voltage-clamp techniques and blockage of the auditory nerve. The amplitudes of the contralateral excitatory inputs could be suppressed, unaffected or facilitated as the interaural delay varied. In contrast, contralateral inhibitory inputs and ipsilateral synaptic inputs remained almost unchanged. Most IC neurons exhibited the suppression of contralateral excitatory inputs over the interval range of dozens of milliseconds. The facilitative effect was generated by the summation of contralateral and ipsilateral excitation. Suppression and facilitation were completely abolished when ipsilateral auditory nerve was blocked pharmacologically, indicating that these effects were exerted by ipsilateral stimulation. These results suggested that the IC would inherit the binaural inputs integrated at the brainstem as well as within the IC and synaptic excitations, modulated by ipsilateral stimulation, underlie the binaural acoustic response.

Speech Comprehension and Its Relation to Other Auditory Parameters in Elderly Patients With Tinnitus.

Deteriorated speech comprehension is a common manifestation of the age-related decline of auditory functions (presbycusis). It could be assumed that when presbycusis is accompanied by tinnitus, general hearing functions, and particularly comprehension of speech in quiet and speech in noise (SIN), will be significantly affected. In this study, speech comprehension ability and other parameters of auditory function were assessed in elderly subjects with (T, n = 25) and without (NT, n = 26) tinnitus, aiming for examination of both peripheral and central auditory processing. Apart from high-frequency audiograms in quiet and in background noise, speech recognition thresholds in silence or in competitive babble noise, and the ability to understand temporally gated speech (GS), we measured also sensitivity to frequency modulation (FM) and interaural delay, gap detection thresholds (GDT), or the difference limens of intensity. The results show that in elderly participants matched by age (mean ages around 68 years), cognitive status (median MoCA scores around 27), and hearing thresholds [median pure-tone averages (PTA) around 16 dB hearing loss (HL)], tinnitus per se has little influence on speech comprehension. The tinnitus patients also show similar GDT, sensitivity to interaural intensity difference, and sensitivity to FM as the NT subjects. Despite these similarities, nevertheless, significant differences in auditory processing have been found in the tinnitus participants: a worse ability to detect tones in noise, a higher sensitivity to intensity changes, and a higher sensitivity to interaural time differences. Additional correlation analyses further revealed that speech comprehension in the T subjects is dependent on the sensitivity to temporal modulation and interaural time delay (ITD), while these correlations are weak and non-significant in the NT subjects. Therefore, despite similarities in average speech comprehension and several other parameters of auditory function, elderly people with tinnitus exhibit different auditory processing, particularly at a suprathreshold level. The results also suggest that speech comprehension ability of elderly tinnitus patients relies more on temporal features of the sound stimuli, especially under difficult conditions, compared to elderly people without tinnitus.

Correlation between Auditory Brainstem Response Characteristics and Hearing Recovery in Sudden Deafness

Background and Objectives Auditory brainstem response (ABR) can be utilized to verify the hearing threshold and determine the existence of retrocochlear pathology in sudden deafness. However, little is known about the significance of ABR characteristics as a prognostic factor in sudden deafness. The purpose of this study is to evaluate which characteristics of ABR is associated with the prognosis of sudden deafness. Subjects and Method We studied patients who were diagnosed with unilateral sudden deafness from January 2017 to May 2018. ABR results of click stimuli at 90 dB nHL were analyzed by the latency of wave I, III, and V. Results A total of 113 patients (55 men and 58 women) participated in the study. Hearing recovery was observed in 94 cases (83.2%). The absolute latency of wave I, III, and V was prolonged in the affected ears when compared with the unaffected ears. Hearing improvement was only correlated to the interaural latency delay of wave I (Pearson’s r=-0.278, p＜0.05). Prolonged interaural latency delay was noted from complete toward slight hearing recovery group. When the hearing outcome was evaluated by the interaural latency delay of wave I at 0.2 ms, the result of under 0.2 ms was significantly better than that over 0.2 ms and no visible wave I. Conclusion Prolonged interaural delay of wave I over 0.2 ms and no visible wave I in ABR showed worse hearing outcome in sudden deafness. This finding may provide ABR as a potential prognostic indicator in sudden deafness. Key words: Auditory ㆍ Evoked potentials ㆍ Latency

A modification of the scale illusion into a detection task for assessment of binaural streaming.

Binaural streaming by frequency-proximity was investigated without subjective listener-feedback by modifying the scale illusion of Deutsch [J. Acoust. Soc. Am. 57, 1156-1160 (1975)] into a detection-task. Nineteen listeners had to detect one deviation within a repeating melody stream, while simultaneously presented with a randomized distractor stream. Every second note in each stream was presented to the opposite ear, requiring binaural streaming to detect the deviant. Listeners performed well in this test but adding interaural delay or timbre-difference let the listeners group by lateralization instead. This confirms the grouping by frequency-proximity. The method could be used to investigate binaural streaming in hearing-impaired patients, where interaural percepts might differ.

“Straightness” versus “briefness” in binaural cue extraction

The neural mechanisms that detect and encode interaural delays have elicited intense interest for many decades, from Jeffress’s [J. Comput. Physiol. Psychol. 41, 35–39 (1948)] original “place theory of sound localization,” its modeling via frequency-specific cross-correlation starting with Colburn [JASA 54, 1458–1470 (1973)], and continuing to this day. An important advance was the emphasis of cross-correlation peaks that align across frequency [“straightness” ; Stern et al., JASA 84, 156–165 (1988)]. While narrowband cross-correlation produces multiple peaks and is therefore inherently ambiguous, straightness detection accounts for resolution of that ambiguity in broadband stimuli by combining binaural outputs across frequency. An alternative account is suggested by the effects of temporal fluctuations, such as onsets, on the inputs to binaural processing. Zurek [JASA 67, 953–964 (1980)] described how emphasizing such events (e.g., by “windowing” the input) also resolves interaural ambiguities. Recent psychophysical and physiological evidence supports that view, strongly suggesting that binaural processing does, in fact, occur in brief windows triggered by envelope fluctuations such as onsets and intrinsic fluctuations in bands of noise. This talk investigates the possibility that the resulting temporal sparsity of binaural inputs (“briefness” ) might account for bandwidth effects even within single frequency channels, i.e. without a second level of “straightness” detection.

Internally coupled middle ears enhance the range of interaural time differences heard by the chicken.

Interaural time differences (ITDs) are one of several principal cues for localizing sounds. However, ITDs are in the sub-millisecond range for most animals. Because the neural processing of such small ITDs pushes the limit of temporal resolution, the precise ITD range for a given species and its usefulness - relative to other localization cues - has been a powerful selective force in the evolution of the neural circuits involved. Birds and other non-mammals have internally coupled middle ears working as pressure-difference receivers that may significantly enhance ITDs, depending on the precise properties of the interaural connection. Here, the extent of this internal coupling was investigated in chickens, specifically under the same experimental conditions as typically used in investigations of the neurophysiology of ITD-coding circuits, i.e. with headphone stimulation and skull openings. Cochlear microphonics (CM) were recorded simultaneously from both ears of anesthetized chickens under monaural and binaural stimulation, using pure tones from 0.1 to 3 kHz. Interaural transmission peaked at 1.5 kHz at a loss of only -5.5 dB; the mean interaural delay was 264 µs. CM amplitude was strongly modulated as a function of ITD, confirming significant interaural coupling. The 'ITD heard' derived from the CM phases in both ears showed enhancement, compared with the acoustic stimuli, by a factor of up to 1.8. However, the experimental conditions impaired interaural transmission at low frequencies (<1 kHz). I identify factors that need to be considered when interpreting neurophysiological data obtained under these conditions and relating them to the natural free-field condition.

Representation of Multiple Acoustic Sources in a Virtual Image of the Field of Audition from Binaural Synthetic Aperture Processing as the Head is Turned

The representation of multiple acoustic sources in a virtual image of the field of audition based on binaural synthetic-aperture computation (SAC) is described through use of simulated inter-aural time delay (ITD) data. Directions to the acoustic sources may be extracted from the image. ITDs for multiple acoustic sources at an effective instant in time are implied for example by multiple peaks in the coefficients of a short-time base (≈2.25 ms for an antennae separation of 0.15 m) cross correlation function (CCF) of acoustic signals received at the antennae. The CCF coefficients for such peaks at the time delays measured for a given orientation of the head are then distended over lambda circles in a short-time base instantaneous acoustic image of the field of audition. Numerous successive short-time base images of the field of audition generated as the head is turned are integrated into a mid-time base (up to say 0.5 s) acoustic image of the field of audition. This integration as the head turns constitutes a SAC. The intersections of many lambda circles at points in the SAC acoustic image generate maxima in the integrated CCF coefficient values recorded in the image. The positions of the maxima represent the directions to acoustic sources. The locations of acoustic sources so derived provide input for a process managing the long-time base (&gt;10s of seconds) acoustic image of the field of audition representing the robot’s persistent acoustic environmental world view. The virtual images could optionally be displayed on monitors external to the robot to assist system debugging and inspire ongoing development.

Spectrotemporal window of binaural integration in auditory object formation

Binaural integration of interaural temporal information is essential for sound source localization and segregation. Current models of binaural interaction have shown that accurate sound localization in the horizontal plane depends on the resolution of phase ambiguous information by across-frequency integration. However, as such models are mostly static, it is not clear how proximate in time binaural events in different frequency channels should occur to form an auditory object with a unique lateral position. The present study examined the spectrotemporal window required for effective integration of binaural cues across frequency to form the perception of a stationary position. In Experiment 1, listeners judged whether dichotic frequency-modulated (FM) sweeps with a constant large nominal interaural delay (1500 μs), whose perceived laterality was ambiguous depending on the sweep rate (1500, 3000, 6000, and 12,000 Hz/s), produced a percept of continuous motion or a stationary image. Motion detection performance, indexed by d-prime (d') values, showed a clear effect of sweep rate, with auditory motion effects most pronounced for low sweep rates, and a punctate stationary image at high rates. Experiment 2 examined the effect of modulation rate (0.5, 3, 20, and 50 Hz) on lateralizing sinusoidally frequency-modulated (SFM) tones to confirm the effect of sweep rate on motion detection, independent of signal duration. Lateralization accuracy increased with increasing modulation rate up to 20 Hz and saturated at 50 Hz, with poorest performance occurring below 3 Hz depending on modulator phase. Using the transition point where percepts changed from motion to stationary images, we estimated a spectrotemporal integration window of approximately 150 ms per octave required for effective integration of interaural temporal cues across frequency channels. A Monte Carlo simulation based on a cross-correlation model of binaural interaction predicted 90% of the variance on perceptual motion detection performance as a function of FM sweep rate. Findings suggest that the rate of frequency channel convergence of binaural cues is essential to binaural lateralization.

The Precedence Effect: Spectral, Temporal, and Intensitive Interactions

Within the phenomenon termed the precedence effect, location dominance of the leading signal has been associated with the time-course of spatial cues, with stimulus onset playing a dominant role. For a continuous stimulus consisting of lead and lag components, each with different interaural delays, a pattern of interaural level differences and interaural time delays will be created that varies in a specific manner as a function of frequency. In principle, provided that a sufficiently large spectral interval is available, this type of frequency dependence allows one to infer the direction of the leading sound source. We investigated precedence using such lead-lag stimuli and asked listeners to judge the lateral position(s) of the stimuli, depending on spectral content. Results suggest that with such stimuli, lateralization is dominated by interaural cues available within and across auditory filters. In addition, the data suggest there exists a ‘lateralization bias’ that depends on the patterning of interaural cues that exist across frequency.

Effects of interaural delay, center frequency, and no more than ”slight” hearing loss on binaural processing: Behavioral data and quantitative analyses

Recent neurophysiological studies concerning “hidden hearing loss” have motivated investigators to attempt to uncover behaviorally-measured auditory deficits that might exist in human listeners who would be categorized via standard audiometry as having normal hearing or, at most, “slight” hearing loss. We hypothesized that behavioral measures of binaural auditory performance could reveal the presence of deficits that may not be discoverable via standard audiometric testing or via measures of monaural processing. That hypothesis was based on the notion that monaural (perhaps, peripheral) deficits would be effectively multiplied via binaural interaction. Groups of listeners, all having no greater than “slight” hearing loss were tested. Binaural detection thresholds were elevated across a wide range of center frequencies for listeners whose absolute thresholds at 4 kHz exceed 7.5 dB HL. Quantitative predictions made via an interaural correlation-based model of binaural processing suggest that the elevated binaural detection thresholds observed for listeners having slightly elevated absolute thresholds stem solely from their having elevated levels of internal noise. They appear to stem neither from reduced sensitivity to signal-dependent changes in information derived from the stimuli as processed internally nor from increased noise along the listener’s putative internal delay line.