Interaural Intensity Research Articles

Interaural intensity difference and ear advantage in listening to dichotic consonant–vowel syllable pairs

The right-ear advantage (REA) is typically observed in verbal dichotic listening, indicating a left hemisphere superiority for speech processing. The REA could be thought of as a bottom–up, stimulus-driven laterality effect, caused by the preponderance of the contralateral neural fibers from the right ear to the auditory/speech processing areas in the left temporal lobe. The REA can, however, be modified by explicitly requiring the listeners to focus their attention alternatively on the left or right-ear stimuli, thus either countering or enhancing the bottom–up processes through top–down attentional control. In the present study, we manipulated the strength of the bottom–up REA by inducing an intensity difference between the right-ear and left-ear speech inputs in order to make the REA either weaker (left-ear input > right-ear input) or stronger (left-ear input < right-ear input) and also examined how this manipulation affected the top–down attention modulation effects. Twenty healthy participants listened to dichotic presentations of consonant–vowel syllable pairs with different attention instructions. The results showed that the interaural intensity difference significantly affected the ear advantage in the predicted way. It also interacted with the top–down control effects, attentional control having a stronger effect when attending to the ear that had a weaker sound intensity, as compared to when the intensities were equal.

Forward-masked monaural and interaural intensity discrimination

Intensity-discrimination thresholds were measured for a 25-ms, 6-kHz pure tone for pedestal levels from 40 to 90 dB sound pressure level (SPL) with and without a forward masker (100-ms narrowband Gaussian noise, N(0)=70 dB). When the masker was present, the masker and probe were separated by 100 ms of silence. Unmasked and masked thresholds were measured in a two-interval monaural procedure and, separately, in a single-interval interaural procedure in which the pedestal and incremented pedestals were presented simultaneously to opposite ears. While the monaural thresholds were elevated markedly by the forward masker for mid-level pedestals, interaural thresholds were nearly unaffected by the masker across pedestal levels. The results argue against the notion that the monaural elevation in forward-masked thresholds is due to degraded encoding of intensity information at early stages of auditory processing.

Spatial orientation in the bushcricket Leptophyes punctatissima (Phaneropterinae; Orthoptera): III. Peripheral directionality and central nervous processing of spatial cues

We examined peripheral and central nervous cues underlying the ability of the bushcricket Leptophyes punctatissima to orient to elevated and depressed sound sources broadcasting the female acoustic reply. The peripheral spatial directionality of the ear was measured physiologically using monaural preparations of an auditory interneuron (T-fibre). In the azimuth, maximal interaural intensity differences of 18 dB occur between ipsi- and contralateral stimulation. With increasing elevation or depression of the sound sources, IIDs decrease systematically and reach zero with the source exactly above or below the preparation. Bilateral, simultaneous recordings of the activity of the pair of interneurons allowed determining the binaural discharge differences which occur in response to the extremely short (1 ms) female reply. These discharge differences are large (four action potentials/stimulus) and reliable in the azimuth with lateral stimulation, and decrease gradually with more frontal stimulation. With elevation and depression of sound sources these differences again decrease to one action potential/stimulus at 60 degrees or 75 degrees elevation, and lateral stimulus angles of about 60 degrees . We also calculated the reliability with which a receiver could correctly determine the location of the sound source. We discuss these quantitative measures in relation to the spatial phonotactic behaviour of male L. punctatissima.

Understanding Intervention for (C)APD: As Easy as A-B-C

Understanding Intervention for (C)APD: As Easy as A-B-C

Using bat-modelled sonar as a navigational tool in virtual environments

Bats are able to use active sonar as a mechanism for locating object in three dimensions and for generating spatial maps of their environments. Humans use passive sound cues to detect features of the space they occupy, as well as react to the spatial location of objects which generate sound. The system described in this paper allows free-ranging humans to locate a virtual sound location using active sonar. An emitted pulse, centred on the users head, serves as an intensity and time marker. The return pulse is rendered at the virtual target location and emitted after a time delay corresponding to the two-way path from sender to target and back again. The sonar system is modelled on those of bats, using ultrasonic frequency-modulated signals reflected from simple targets. The model uses the reflectivity characteristics of ultrasound, but the frequency and temporal structure used are scaled, with the speed of sound being set to 8.5 ms −1 to bring the frequency range and temporal resolution within the capabilities of the human auditory system. Orientation with respect to the ensonified target is achieved by time-of-flight time delays to give target range, and binaural location information derived from interaural timing differences, interaural intensity differences, and head-related transfer functions. Subjects performed significantly better at a localization task when given temporal data based on echo delays with an outgoing reference pulse than without a reference pulse. Frequency-modulated signals sweeping from 1.5 kHz–100 Hz over 500 ms provide the best localization cues, and users found them significantly easier to locate than continuous sounds.

Auditory Evoked Potentials in the Detection of Interaural Intensity Differences in Children and Adults

The goal of the current study was to identify neurophysiological indices of binaural processing in children with no history of hearing loss or listening problems. The results will guide our efforts to diagnose children for whom impaired binaural processing may contribute to difficulty understanding speech in adverse listening environments. Our main aim was to document the cortical auditory evoked potentials (AEPs) in response to interaural intensity differences (IIDs) in children. It is well known, however, that the morphology of AEPs is substantially different in children and adults. Comparison of AEPs in response to IIDs between children and adults allowed us to evaluate age-related differences in the AEP waveform of binaural processing. Nine children (ages 7 yr 0 mo to 9 yr 4 mo) and 11 adults (23 to 34 yr) with normal hearing and no known or suspected neurological or academic problems listened to click stimuli under earphones. Click trains consisted of broadband noise of 1-msec duration with a click rate of 100 Hz. In the experimental condition (IID-present) 50-msec intervals containing an interaural intensity difference of 20 dB were introduced periodically in the continuous stream of otherwise diotic click trains. The diotic trains alternated in intensity between 50 and 70 dB peSPL. In the baseline condition (IID-absent), the same continuous diotic click stream utilized in the IID-present condition was presented with no IID. Finally, for comparison with existing literature on AEPs in children and adults, we presented monaural click trains of 50-msec duration, like those used in the IID stimulus (but with no continuous stream) to the left ear at 70 dB peSPL, with an interstimulus interval of 750 msec. Stimuli were presented in separate blocks for each stimulus type and AEPs were recorded in a passive listening condition. A prominent AEP activation was present in both age groups for the IID-present condition; the IID-absent condition did not evoke substantial AEPs. Adult waveform characteristics of the AEPs to monaural clicks and IID-present around 100 and 200 msec were comparable to previous reports. The children demonstrated the expected AEP activation patterns in response to monaural clicks (i.e., positivity around 100 msec, followed by prominent negativity around 250 msec); however their AEP waveforms to IIDs were mainly comprised of a prolonged positivity around 200 to 250 msec after stimulus onset. A two-step temporal-spatial Principal Component Analysis (PCA) was used to evaluate the temporal (time) and spatial (electrode location) composition of the AEP waveforms in children and adults in response to IID-present and IID-absent conditions. Separate repeated-measures ANOVAs with factor scores as the dependent variable were conducted for each temporal factor (TF) representing the waveform deflections around 100, 200 and 250 msec (i.e., TF110, TF200, and TF255) at the frontocentral spatial factor (SF1). Significantly greater negative activation was observed in adults than in children in response to IID-present for TF110. The IID-present condition evoked a significantly greater waveform inflection for TF200 in both age groups than IID-absent. A positive going activation for TF255 was observed in the IID-present condition in children but not in adults. This study compared obligatory AEPs in response to binaural processing of IIDs in children and adults with normal hearing. The morphology of the AEP waveform in children was different for monaural clicks and IID-present stimuli. The difference between AEPs for monaural clicks and IID-present did not occur in adults. It is likely that polarity reversal of the AEPs in response to the IID accounts for the observed AEP morphology in children.

Posttransient shifts in auditory lateralization

Presenting an intense (e.g., 80-dB [SPL]) "transient" (e.g., 50-msec) inducer to the ear reduces the loudness of subsequent signals at or near the frequency of the inducer. In this study, we ask whether similar inducers also affect lateralization. In two experiments, we asked how inducing tones presented to one ear (the exposed ear) affect judgments of the lateral position of subsequent target tones having various interaural intensity differences. In Experiment 1, inducers had the same frequency as the targets, and, as predicted, reduced the tendency to lateralize the targets to the exposed ear. In Experiment 2, the frequency of the inducers and the target differed (different critical bands), thereby eliminating the effect on lateralization. These results are consistent with the hypothesis that inducers temporarily reduce the magnitude of the representation of intensity signals in the frequency region around them and that this reduction occurs, at least partly, peripherally to the site at which binaural intensity differences are encoded. The results imply further that the reduction in loudness previously reported under similar stimulus conditions reflects a more general reduction of intensity-based information in hearing.

Development of functional organization of the pallid bat auditory cortex

The primary auditory cortex is characterized by a tonotopic map and a clustered organization of binaural properties. The factors involved in the development of overlain representation of these two properties are unclear. We addressed this issue in the auditory cortex of the pallid bat. The adult pallid bat cortex contains a systematic relationship between best frequency (BF) and binaural properties. Most neurons with BF <30 kHz are binaurally inhibited (EO/I), while most neurons with BF >30 kHz are monaural (EO). As in other species, binaural properties are clustered. The EO/I cluster contains a systematic map of interaural intensity difference (IID) sensitivity. We asked if these properties are present at the time the bat acquires its full audible range (postnatal day [P] 15). Tonotopy, relationship between BF and binaural properties, and the map of IID sensitivity are adult-like at P15. However, binaural facilitation is only observed in pups older than P25. Frequency selectivity shows a BF-dependent sharpening during development. Thus, overlain representation of binaural properties and tonotopy in the pallid bat cortex is remarkably adult-like at an age when the full audible range is first present, suggesting an experience-independent development of overlapping feature maps.

Interaction between interaural time and intensity differences in the central auditory system: A psychophysical approach

Interaction between interaural time and intensity differences in the central auditory system: A psychophysical approach

Neural mechanisms for calculation of the interaural intensity difference in the chicken auditory brainstem

Interaural time differences in the envelope of a sound (envelope ITDs) can potentially provide spatial information at high frequencies where interaural phase differences (IPDs) are not available. Interaural intensity differences (IIDs) also provide important spatial information at high frequencies. Both IIDs and envelope ITDs can influence spatial perception at high frequencies, but behavioral and physiological studies suggest that IIDs dominate perception. This study examines envelope ITD sensitivity in the auditory cortex of the pallid bat, a species that uses passive sound localization at the low end of its audible range to find prey. Its auditory system is entirely ‘high-frequency’ in that phase-locking does not occur at the low end of its audible range. If the bat uses ITDs, they must be derived from the envelope of the signal. A previous study of envelope ITD sensitivity in its inferior colliculus (IC) reported that neurons are sensitive to the small ±70 μs range of available ITDs. This study extends these findings to the cortical level to assess the transformation of ITD sensitivity and the binaural response properties that underlie this sensitivity. Two measures of sensitivity were used. The dynamic ITD range measures the range of ITDs over which the maximum response of a neuron decreases by 80%. When presented with square-wave amplitude-modulated tones statically delayed in arrival time, the average dynamic ITD range in the IC is 304 μs, but dropped to 175 μs in auditory cortex. IC neurons average a 38% change in maximum response over the relevant ITD range, while cortical neurons average a 67% change. Also measured were time–intensity trading ratios, which index the extent to which a change in IID can cause a shift the dynamic ITD range. Average trading ratios are approximately the same in the IC and auditory cortex (17.9 μs/dB vs. 16.7 μs/dB, respectively). Binaural interactions changed from the IC to auditory cortex. In IC, ITD sensitivity is an inhibitory, subtractive process in which ITDs reduce the response evoked by contralateral monaural stimulation. In the auditory cortex, both binaural inhibition and facilitation occur. In the majority of cortical neurons, IID and ITD functions were remarkably similar in shape, having stepped, step-peaked or peaked functions. The binaural interactions (inhibition and/or facilitation) evoked by ITDs and IIDs were also typically similar. These results suggest that IIDs and envelope ITDs are having similar effects on output of the same binaural comparator system.

Cochlear efferent feedback balances interaural sensitivity

Neurons in the lateral superior olive (LSO) compute sound location based on differences in interaural intensity, coded in ascending signals from the two cochleas. Unilateral destruction of the neuronal feedback from the LSO to the cochlea, the lateral olivocochlear efferents, disrupted the normal interaural correlation in response amplitudes to sounds of equal intensity. Thus, lateral olivocochlear feedback maintains the binaural balance in neural excitability required for accurate localization of sounds in space.

Parallel thalamocortical pathways for echolocation and passive sound localization in a gleaning bat, Antrozous pallidus

We present evidence for parallel auditory thalamocortical pathways that serve two different behaviors. The pallid bat listens for prey-generated noise (5-35 kHz) to localize prey, while reserving echolocation [downward frequency-modulated (FM) sweeps, 60-30 kHz] for obstacle avoidance. Its auditory cortex contains a tonotopic map representing frequencies from 6 to 70 kHz. The high-frequency (BF > 30 kHz) representation is dominated by FM sweep-selective neurons, whereas most neurons tuned to lower frequencies prefer noise. Retrograde tracer injections into these physiologically distinct cortical regions revealed that the high-frequency region receives input from the suprageniculate (SG) nucleus, but not the ventral division of the medial geniculate body (MGBv), in all experiments (n = 9). In contrast, the low-frequency region receives tonotopically organized input from the MGBv in all experiments (n = 16). Labeling in the SG was observed in only two of these experiments. Both cortical regions also receive sparse inputs from medial (MGBm) and parts of the dorsal division (MGBd) outside the SG. These results show that the low- and high-frequency regions of a single tonotopic map receive dominant inputs from different thalamic divisions. Within the low-frequency region, most neurons are binaurally inhibited, and an orderly map of interaural intensity difference (IID) sensitivity is present. We show that the input to the IID map arises from topographically organized projections from the MGBv. As observed in other species, a frequency-dependent organization is observed in the lateromedial direction in the MGBv. These data demonstrate that MGBv-to-auditory cortex connections are organized with respect to both frequency and binaural selectivity.

The effect of optokinetic stimulation on orientation of sound lateralization

Conclusions. The current study demonstrates that sound lateralization sensitivity during interaural time difference (ITD) discrimination may be altered by optokinetic (OK) stimulation, and that sound lateralization sensitivity of ITD discrimination may be more susceptible to OK stimulation than that of interaural intensity difference (IID) discrimination. These data suggest that nystagmus or the sensation of self-rotation induced by OK stimulation influences auditory afferent information such as sound lateralization. Objective. Using dichotic sound, the effect of optokinetic stimulation on the orientation of sound lateralization was investigated in humans. Materials and methods. Subjects were studied by testing ITD and IID discrimination during OK stimulation. Results. At 90°/s of the light stripes angular velocity the amplitudes for the ITD discrimination tests during OK stimulation were significantly greater than those either before the beginning of OK stimulation or at 30°/s (p <0.05). No significant difference in the amplitude for the IID discrimination test was observed between the results obtained before and during OK stimulation. During OK stimulation, all subjects felt that their perceptual body axes shifted toward the quick phase of OK nystagmus. In 8 of 12 subjects, the median line of amplitude for the ITD discrimination test shifted to the quick phase side of the OK nystagmus.

Deafferentation induces novel axonal projections in the auditory brainstem after hearing onset

Deafferentation of neural tissue can result in cell death, morphological changes, and/or alterations in sources of innervation. These changes often occur during a limited period of development. In the auditory brainstem, the ventral cochlear nucleus (VCN) projects to the contralateral but not ipsilateral medial nucleus of the trapezoid body (MNTB). This pathway is part of a circuit that computes interaural intensity differences used in sound localization. Previous studies have shown that, after the cochlea is removed early in postnatal development, cells in the VCN on the deafferented side die, and the intact VCN innervates MNTB on both sides of the brain. These changes after cochlea removal are limited to an early postnatal period that preceeds hearing onset. In this study, we lesioned the VCN directly to evaluate plasticity in axonal pathways after hearing onset. We found that novel projections from the intact VCN to ipsilateral MNTB emerge after lesions performed as late as postnatal day 25. The morphological sequence of events is similar to that seen during the initial development of this pathway. These data suggest that plasticity in the auditory brainstem is possible when pathways are challenged with denervation of target nuclei. The results show that the opportunity for plasticity in auditory brainstem circuitry is more prolonged than previously thought and that novel pathways can form after the normal pathways are fully mature and functional. Moreover, sensitive periods for changes in individual pathways are independently regulated.

The spatio-temporal brain dynamics of processing and integrating sound localization cues in humans

Interaural intensity and time differences (IID and ITD) are two binaural auditory cues for localizing sounds in space. This study investigated the spatio-temporal brain mechanisms for processing and integrating IID and ITD cues in humans. Auditory-evoked potentials were recorded, while subjects passively listened to noise bursts lateralized with IID, ITD or both cues simultaneously, as well as a more frequent centrally presented noise. In a separate psychophysical experiment, subjects actively discriminated lateralized from centrally presented stimuli. IID and ITD cues elicited different electric field topographies starting at ∼ 75 ms post-stimulus onset, indicative of the engagement of distinct cortical networks. By contrast, no performance differences were observed between IID and ITD cues during the psychophysical experiment. Subjects did, however, respond significantly faster and more accurately when both cues were presented simultaneously. This performance facilitation exceeded predictions from probability summation, suggestive of interactions in neural processing of IID and ITD cues. Supra-additive neural response interactions as well as topographic modulations were indeed observed ∼ 200 ms post-stimulus for the comparison of responses to the simultaneous presentation of both cues with the mean of those to separate IID and ITD cues. Source estimations revealed differential processing of IID and ITD cues initially within superior temporal cortices and also at later stages within temporo-parietal and inferior frontal cortices. Differences were principally in terms of hemispheric lateralization. The collective psychophysical and electrophysiological results support the hypothesis that IID and ITD cues are processed by distinct, but interacting, cortical networks that can in turn facilitate auditory localization.

Role of hyperpolarization-activated conductances in the lateral superior olive: A modeling study

This modeling study examines the possible functional roles of two hyperpolarization-activated conductances in lateral superior olive (LSO) principal neurons. Inputs of these LSO neurons are transformed into an output, which provides a firing-rate code for a certain interaural sound intensity difference (IID) range. Recent experimental studies have found pharmacological evidence for the presence of both the Gh conductance as well as the inwardly rectifying outward GKIR conductance in the LSO. We addressed the question of how these conductances influence the dynamic range (IID versus firing rate). We used computer simulations of both a point-neuron model and a two-compartmental model to investigate this issue, and to determine the role of these conductances in setting the dynamic range of these neurons. The width of the dynamic regime, the frequency-current (f-I) function, first-spike latency, subthreshold oscillations and the interplay between the two hyperpolarization activated conductances are discussed in detail. The in vivo non-monotonic IID-firing rate function in a subpopulation of LSO neurons is in good correspondence with our simulation predictions. Two compartmental model simulation results suggest segregation of Gh and GKIR conductances on different compartments, as this spatial configuration could explain certain experimental results.

Binaural Response Properties of Low-Frequency Neurons in the Gerbil Dorsal Nucleus of the Lateral Lemniscus

Differences in intensity and arrival time of sounds at the two ears, interaural intensity and time differences (IID, ITD), are the chief cues for sound localization. Both cues are initially processed in the superior olivary complex (SOC), which projects to the dorsal nucleus of the lateral lemniscus (DNLL) and the auditory midbrain. Here we present basic response properties of low-frequency (< 2 kHz) DNLL neurons and their binaural sensitivity to ITDs and IIDs in the anesthetized gerbil. We found many neurons showing binaural properties similar to those reported for SOC neurons. IID-properties were similar to that of the contralateral lateral superior olive (LSO). A majority of cells had an ITD sensitivity resembling that of either the ipsilateral medial superior olive (MSO) or the contralateral LSO. A smaller number of cells displayed intermediate types of ITD sensitivity. In neurons with MSO-like response ITDs that evoked maximal discharges were mostly outside of the range of ITDs the gerbil naturally experiences. The maxima of the first derivative of their ITD-functions (steepest slope), however, were well within the physiological range of ITDs. This finding is consistent with the concept of a population rather than a place code for ITDs. Moreover, we describe several other binaural properties as well as physiological and anatomical evidence for a small but significant input from the contralateral MSO. The large number of ITD-sensitive low-frequency neurons implicates a substantial role for the DNLL in ITD processing and promotes this nucleus as a suitable model for further studies on ITD-coding.

Better time–intensity trade revealed by bilateral giant magnetostrictive bone conduction

Sound lateralization tests were performed to compare the magnet coil bone-conduction headphone with the giant magnetostrictive bone-conduction headphone using 18 healthy participants. Although, no significant difference between these bone-conduction headphones was obtained for the interaural time difference and interaural intensity difference, a significant difference was obtained for the time-intensity trade. This revealed that the difference between the headphones is apparent in the integration of the heterogeneous sensations of the time and intensity difference at the cognitive level, but no difference is apparent between the homogeneous sensations of the discrimination of interaural time difference or interaural intensity difference at the sensory level. It was concluded that the difference at the cognitive level indicates the better performance of the giant magnetostrictive headphone.

Comparative study of fixed time versus intensity trade and fixed intensity versus time trade tests in sound lateralization

Objective In the perception of sound lateralization a sound source exists in the side that the sound reaches an ear earlier in time or louder in intensity than the other ear. It is an imaginary phenomenon where the direction of sound is lateralized by an interaural time difference (ITD) and an interaural intensity difference (IID) of sound by both ears. It is speculated from pathophysiological findings that ITD and IID are processed by different pathways, but it has been not yet proved which of them is predominant in sound lateralization. The time difference can be traded by the intensity difference (fixed time versus intensity trade), and vice versa (fixed intensity versus time trade). In order to investigate predominance in this trade, we measured possible differences in effects by ITD and IID using two opposite trade phenomena of time versus intensity trade and intensity versus time trade. Methods The fixed time versus intensity trade test for selected narrow-band noise was investigated in 30 subjects with normal hearing. Using headphones, the subjects were instructed to push the button when the sound bias generated by fixed ITD (4 dB, 6 dB, 8 dB, 10 dB, 12 dB, 14 dB, 16 dB, 18 dB, 20 dB) was traded by gradually increased IID. The plot figure with fixed ITD and required IID to trade was made. It was compared with similar fixed intensity versus time trade test in 12 subjects with normal hearing. The fixed ITD (200–600 μs) was traded by gradually increased IID and the plot figure was made. Results In fixed time versus intensity trade test, each fixed ITD could be traded with an average of 6 dB of IID. On the other hand, in fixed intensity versus time trade test, ITD required to trade fixed IID increased, in proportion to fixed IID increased. Conclusion It is concluded that predominance exists. In fixed time versus intensity trade test, the uniform amount of IID is required to trade the different ITD. However, in the novel fixed intensity versus time trade test, the phenomenon was in completely different manner that ITD in proportion to the given IID is required to trade.

音像定位能力に対する垂直視運動刺激の影響

Using dichotic sound, the effects of optokinetic vertical stimulation on the orientation of horizontal sound lateralization was investigated in 10 healthy subjects. Subjects were given vertical optokinetic (OK) stimulation, and interaural time differences (ITD) and interaural intensity differences (IID) were discriminated, respectively. At a light stripes velocity of 90°/sec, the amplitudes for the ITD discrimination tests during OK stimulation were significantly greater than those before the beginning of OK stimulation, those at a light stripes velocity of 30°/sec, and those at a light stripes accelerated velocity of 4°/sec2 (P<0.05). On the other side, at a light stripes velocity of 90°/ sec, the amplitudes for the IID discrimination tests during OK stimulation were only significantly greater than those at a light stripes accelerated velocity of 4°/sec2 (P<0.05). The median line of amplitude for both ITD and IID discrimination tests did not shift at a light stripes velocity of 30°/sec, 90°/sec, and 4°/sec2. The present study demonstrated that sound horizontal lateralization sensitivity may be influenced by vertical OK stimulation. These findings also suggest that horizontal sound lateralization is more dependent on high-speed vertical OK stimulation than low-speed vertical OK stimulation. Therefore, we hypothesized that there is some interaction between the subcortical pathway of high-speed vertical OK stimulation and the sound lateralization pathway. The results of the present study will help in the understanding of space perception, comprising multiple sensory inputs.