Changes In Interaural Level Differences Research Articles

Synchronizing Automatic Gain Control in Bilateral Cochlear Implants Mitigates Dynamic Localization Deficits Introduced by Independent Bilateral Compression.

The independence of left and right automatic gain controls (AGCs) used in cochlear implants can distort interaural level differences and thereby compromise dynamic sound source localization. We assessed the degree to which synchronizing left and right AGCs mitigates those difficulties as indicated by listeners' ability to use the changes in interaural level differences that come with head movements to avoid front-back reversals (FBRs). Broadband noise stimuli were presented from one of six equally spaced loudspeakers surrounding the listener. Sound source identification was tested for stimuli presented at 70 dBA (above AGC threshold) for 10 bilateral cochlear implant patients, under conditions where (1) patients remained stationary and (2) free head movements within ±30° were encouraged. These conditions were repeated for both synchronized and independent AGCs. The same conditions were run at 50 dBA, below the AGC threshold, to assess listeners' baseline performance when AGCs were not engaged. In this way, the expected high variability in listener performance could be separated from effects of independent AGCs to reveal the degree to which synchronizing AGCs could restore localization performance to what it was without AGC compression. The mean rate of FBRs was higher for sound stimuli presented at 70 dBA with independent AGCs, both with and without head movements, than at 50 dBA, suggesting that when AGCs were independently engaged they contributed to poorer front-back localization. When listeners remained stationary, synchronizing AGCs did not significantly reduce the rate of FBRs. When AGCs were independent at 70 dBA, head movements did not have a significant effect on the rate of FBRs. Head movements did have a significant group effect on the rate of FBRs at 50 dBA when AGCs were not engaged and at 70 dBA when AGCs were synchronized. Synchronization of AGCs, together with head movements, reduced the rate of FBRs to approximately what it was in the 50-dBA baseline condition. Synchronizing AGCs also had a significant group effect on listeners' overall percent correct localization. Synchronizing AGCs allowed for listeners to mitigate front-back confusions introduced by unsynchronized AGCs when head motion was permitted, returning individual listener performance to roughly what it was in the 50-dBA baseline condition when AGCs were not engaged. Synchronization of AGCs did not overcome localization deficiencies which were observed when AGCs were not engaged, and which are therefore unrelated to AGC compression.

Further simulations of the effect of cochlear-implant pre-processing and head movement on interaural level differences.

We simulated the effect of several automatic gain control (AGC) and AGC-like systems and head movement on the output levels, and resulting interaural level differences (ILDs) produced by bilateral cochlear-implant (CI) processors. The simulated AGC systems included unlinked AGCs with a range of parameter settings, linked AGCs, and two proprietary multi-channel systems used in contemporary CIs. The results show that over the range of values used clinically, the parameters that most strongly affect dynamic ILDs are the release time and compression ratio. Linking AGCs preserves ILDs at the expense of monaural level changes and, possibly, comfortable listening level. Multichannel AGCs can whiten output spectra, and/or distort the dynamic changes in ILD that occur during and after head movement. We propose that an unlinked compressor with a ratio of approximately 3:1 and a release time of 300–500 ms can preserve the shape of dynamic ILDs, without causing large spectral distortions or sacrificing listening comfort.

Evaluation of near field distance perception in virtual environments

Binaural synthesis of spatial audio for virtual and augmented environments is typically performed by convolving an anechoic signal with impulse responses measured at the listener's ears using a source at a fixed distance. This method is accurate for sounds presented at the measured impulse response's distance, and for more distant sources, decreasing intensity provides a reasonable approximation. However, sources nearer than 1 m provide additional distance cues. The head-shadow effect becomes markedly stronger and exponential sound intensity falloff comes into play, exaggerating inter-aural level differences. Also, the angle to each ear diverges, changing the directionally dependent spectral filtering of the pinnae. Changes in inter-aural level differences can be approximated across listeners, but changes in pinna cues with changes in distance are highly individual, and thus less easily approximated. Reproducing some or all of these cues for each individual may be necessary to create a convincing percept of very near objects in virtual and augmented reality environments. The present work aims to determine the importance of each cue for static and dynamic sources. Listening tests have been conducted in a virtual environment using generic and individual head related impulse responses, with and without near-field compensation.

The effect of listener motion on localization of tones in a room

Listeners reported the perceived azimuths of 36 sine-tone sources in a room. For some trials, listener motion was forbidden and for others unstructured head and torso motion was encouraged with the listener remaining seated. Tone frequencies were 500, 1000, 2000, and 4000 Hz. Onsets and offsets were masked, leaving a steady-state duration of 7.6 s for exploration. It was found that the benefits of motion on source localization accuracy were clearly frequency dependent. Dynamic interaural cues were obtained from probe-microphone ear-canal recordings. Interaural cues were combined with head-tracker orientation data to predict perceived azimuths according to diverse hypotheses. Responses and interaural cues for trials without motion were used to predict a dynamic inferred location within the room for runs with motion and to identify stable inferences. Analysis of front/back confusions was conducted based on the regression between changes in interaural level differences and changes in head angle sampled at 76 instants. Except for 4000 Hz, the slope of the regression was significantly different depending on whether responses were in the front or back of the room. For interaural time difference, significance was limited to 500 and 1000 Hz. [Work supported by the AFOSR.]

Uncertainty in binaural hearing linked to inherent envelope fluctuations

It was recently shown that supra-threshold changes in interaural level differences (ILDs) in 1/3-octave bands of noise were more quickly and reliably perceived when carried by high-frequency (4000 Hz) rather than low-frequency (500 Hz) noise. For fixed-octave ratio noises, however, center frequency differences are accompanied by differences in inherent envelope modulations stemming from differences in absolute bandwidth. We propose that envelope fluctuations (rather than frequency differences) are a more parsimonious explanation of response latency and certainty. In the current study we investigate the hypothesis that for noisier envelopes, there is less certainty that an ILD is a true diversion from noise perceived at center. Differences in inherent envelope modulation strength were controlled using three types of stimuli: low-noise noise (with negligible envelope fluctuations), high-noise noise, and an equal mixture of the two. Perception of ILD changes was measured using anticipatory eye movements elicited by ILD changes. Results confirm that sensitivity and response latency to ILD changes were affected by degree of envelope fluctuation. In addition, high-noise noise provoked more phantom percepts, which were nearly absent for low-noise noise. Finally, we propose a mathematical model of accumulating certainty in this task as a function of time and degree of envelope fluctuation.

The Effects of Asymmetric Hearing on Bilateral Brainstem Function: Findings in Children with Bimodal (Electric and Acoustic) Hearing

As implantation criteria are broadening to include children with asymmetric hearing loss, it is important to determine the degree of residual hearing needed to protect the bilateral auditory pathways for binaural hearing and whether there is a sensitive period in development for implantation in these children. We have been studying these questions in a growing cohort of children. In the present study, auditory brainstem responses were recorded in 21 children who had 2.2 ± 2.2 years of bimodal hearing. Responses were evoked by 11-Hz acoustic clicks presented to the non-implanted ear and with biphasic electric pulses presented to the implanted ear. Twelve of these children also completed a behavioural task in which they were asked to which side of their heads bilaterally presented clicks/pulses that varied in interaural level or timing lateralized. All children experienced a delay in the non-implanted ear that resulted in 2.0 ± 0.35 ms longer peak latencies. These were further prolonged in 7 children as measured by longer interwave latencies from this ear than from the implanted ear. Despite large asymmetries in timing of brainstem activity between the two ears, all children perceived changes in interaural level differences. They were unable to detect differences in interaural timing cues. Symmetric brainstem function suggests bilateral development was preserved in some children. Future work will explore whether these children have better potential for developing binaural hearing using bimodal input.

Effects of Dynamic-Range Compression on the Spatial Attributes of Sounds in Normal-Hearing Listeners

Dynamic-range compression is routinely used in bilaterally fitted hearing devices. The objective of this study was to investigate how compression applied independently at each ear affects spatial perception in normal-hearing listeners and to relate the effects to changes in binaural cues caused by the compression for different types of sound. A semantic-differential method was used to measure the spatial attributes of sounds. Eleven normal-hearing participants responded to questions addressing certainty of location, diffuseness, movement, image splits, and externalization of sounds. Responses were given on seven-point scales between pairs of opposing terms. Stimuli included speech and a range of synthetic sounds with varying characteristics. Head-related transfer functions were used to simulate a source at an azimuth of -60° or +60°. Three processing conditions were compared: (1) an unprocessed reference condition; (2) fast-acting, wide-dynamic-range compression operating independently at each ear; and (3) imposition of a static bias in interaural level difference (ILD) equivalent to that generated by the compression under steady state conditions. All processing was applied in a high-frequency channel above 2 kHz. The three processing conditions were compared separately in two bandwidth conditions: a high-pass condition in which the high-frequency channel was presented to listeners in isolation and a full-bandwidth condition in which the high-frequency channel was recombined with the unprocessed low-frequency channel. Hierarchical cluster analysis was used to group related questions based on similarity of participants' responses. This led to the calculation of composite scores for four spatial attributes: "diffuseness," "movement," "image split," and "externalization." Compared with the unprocessed condition, fast-acting compression significantly increased diffuseness, movement, and image-split scores and significantly reduced externalization scores. The effects of compression were greater when listeners heard the high-frequency channel in isolation than when it was recombined with the unprocessed low-frequency channel. The effects were apparent only for sounds containing gradual onsets and offsets, including speech. Dynamic compression had a much more pronounced effect on the spatial attributes of sounds than imposition of a static bias in ILD. Fast-acting compression at high frequencies operating independently at each ear can adversely affect the spatial attributes of sounds in normal-hearing listeners by increasing diffuseness, increasing or giving rise to a sense of movement, causing images to split, and affecting the externalization of sounds. The effects are reduced, but not eliminated, when listeners have access to undisturbed low-frequency cues. Sounds containing gradual onsets and offsets, including speech, are most affected. The effects arise primarily as a result of relatively slow changes in ILD that are generated as the sound level at one or both ears crosses the compression threshold. The results may have implications for the use of compression in bilaterally fitted hearing devices, specifically in relation to spatial perception in dynamic situations.

Lateralization of Interimplant Timing and Level Differences in Children Who Use Bilateral Cochlear Implants

Interaural level differences (ILD) and interaural timing differences (ITD) are important cues for locating sounds in space. Adult bilateral cochlear implant (CI) users use ILDs more effectively than ITDs. Few studies investigated the ability of children who use bilateral CIs to make use of these binaural cues. Our working hypothesis was that children using bilateral CIs are able to perceive changes in ITDs and ILDs similar to their normal-hearing (NH) peers. Participants were two groups of children; 19 bilateral implant users (CI) and nine NH children. The children in the CI group had received a second CI after 4.9 +/- 2.8 yrs of unilateral use. Children performed a four alternative forced-choice lateralization task in which they were asked to describe stimuli as coming from the left side, right side, middle of the head, or from both sides simultaneously. Stimuli were 500 msec trains of electrical pulses delivered to apical electrode no. 18 (CI group) or clicks (NH group) presented 11 times per second with either ITDs (0, 400, 1000, or 2000 microsec delay between sides) or level differences (0, 10, or 20 Current Units (CI group) or 0, 10, or 20 dB (NH group) difference between sides). ITDs were presented using current levels that were balanced using left and right electrically evoked brain stem responses. Stimulus levels evoking response amplitudes that were most similar were used. Responses from children in the CI group changed significantly with changes in ILD of bilateral stimuli, but not with changes in ITD. Responses from children in the CI group were significantly different from those in the NH group in three ways. Children in the CI group perceived bilaterally presented electrical pulses: (1) to come from the second implanted side more often than the first, (2) to rarely come from the middle, and (3) to come from both sides of the head simultaneously. Perceived changes in lateralization with ILD changes were correlated with differences in amplitudes of electrically evoked brain stem responses by the left versus right CI. The results of this study illustrate that children who use bilateral CIs can lateralize stimuli on the basis of level cues, but have difficulty interpreting interimplant timing differences. Perceived lateralization of bilaterally presented stimuli to the second implanted side in many of the stimulus conditions may relate to the use of different device generations between sides. Further differences from normal lateralization responses could be due to abnormal binaural processing, possibly resulting from a period of unilateral hearing before the provision of a second implant or due to insufficiently matched interimplant stimuli. It may be possible to use objective measures such as electrically evoked auditory brain stem responses wave eV amplitudes to provide balanced levels of bilateral stimulation in children who have had no binaural hearing experience.

Interaural Level Difference Discrimination Thresholds for Single Neurons in the Lateral Superior Olive

The lateral superior olive (LSO) is one of the earliest sites in the auditory pathway that is involved in processing acoustical cues to sound location. Here, we tested the hypothesis that LSO neurons can signal small changes in interaural level differences (ILDs), a cue to horizontal sound location, of pure tones based on discharge rate consistent with psychophysical performance in the discrimination of ILDs. Neural thresholds for ILD discrimination were determined from the discharge rates and associated response variability of single units in response to 300 ms tones in the LSO of barbiturate-anesthetized cats using detection theory. Neural response variability was well described by a power function of the mean rate, both in individual neurons and collectively; LSO neurons were less variable than expected from a Poisson process. Compared with psychophysical data, the best-threshold ILDs of single LSO neurons were comparable with or better than behavior over the full range of frequencies (0.3-35 kHz) and pedestal ILDs (+/-25 dB) explored in this study. With a pedestal ILD of 0 dB, ILD increments of 1 dB could be discriminated by some neurons, with a median of 4.35 dB across neurons. For pedestal ILDs away from 0 dB, the best-threshold ILDs were as low as 0.5 dB, with a median of 2.3 dB. These findings support the hypothesis that the LSO plays a role in the extraction of ILD, and that the representation of ILD by LSO neurons may set a lower bound on the behavioral sensitivity to ILDs.

Response Patterns Along an Isofrequency Contour in Cat Primary Auditory Cortex (AI) to Stimuli Varying in Average and Interaural Levels

The topographical response of a portion of an isofrequency contour in primary cat auditory cortex (AI) to a series of monaural and binaural stimuli was studied. Responses of single neurons to monaural and a matrix of binaural characteristic frequency tones, varying in average binaural level (ABL) and interaural level differences (ILD), were recorded. The topography of responses to monaural and binaural stimuli was appreciably different. Patches of cells that responded monotonically to increments in ABL alternated with patches that responded nonmonotonically to ABL. The patches were between 0.4 and 1 mm in length along an isofrequency contour. Differences were found among monotonic patches and among nonmonotonic patches. Topographically, activated and silent populations of neurons varied with both changes in ILD and changes in ABL, suggesting that the area of responsive units may underlie the coding of sound level and sound location.

The effect of frequency modulation of the ability to judge dynamic changes in interaural level differences

The cues for apparent auditory motion include dynamic interaural temporal differences, level changes, and frequency modulation (FM) [Rosenblum, Perception 16, 175–186 (1987)]. The manner in which these cues interact was examined in a task in which listeners judged the point at which a stimulus having dynamic changes in interaural level difference (ILD) passed through the intracranial midline. In base-line conditions, stimuli were 2.0–2.5-s, 0.5- or 2-kHz sinusoids presented through headphones. In experimental conditions, a frequency-modulated tone was added. Frequency was modulated by a linear ramp or according to a Doppler-based algorithm. The midpoint of the frequency-sweep occurred either at the instant that the ILD passed through 0 dB, acting as a task cue, or at some other point. FM rate and magnitude, temporal position of frequency-sweep midpoint, and direction of apparent movement were randomized across trials. Results indicated that the presence of FM had a significant effect on the perception of intracranial midline. This effect was obtained across FM parameter values. Results were considered in terms of how dynamic cues for perceived motion perceptually interact. [Work supported by NIH and AFOSR.]

Serotonergic modulation of synapses in the developing gerbil lateral superior olive.

The lateral superior olive (LSO) is a primary site of binaural convergence that responds selectively to changes in interaural level difference (ILD) by integrating ipsilateral excitatory and contralateral inhibitory inputs. The circuit matures during the first three postnatal weeks, undergoing several structural and functional changes that are influenced by afferent activity. Therefore modulation of synaptic activity by neuromodulators may participate in the maturation of this circuit. The present study describes robust effects of serotonin (5-HT) on LSO synaptic function. Using whole cell voltage-clamp recording from gerbil LSO neurons (postnatal days 6-13) in an in vitro slice preparation, we have identified several distinct forms of serotonergic modulation of spontaneous and evoked synaptic transmission. First, 1-2 min application of 5-HT (100 microM) activated prolonged bursts of spontaneous inhibitory postsynaptic currents (IPSCs). However, there was an age-dependent decline, such that this effect rarely was observed beyond postnatal day 8. 5-HT apparently increased the excitability of inhibitory afferents, because 5-HT-induced IPSCs were blocked by tetrodotoxin. A second effect of 5-HT was to depress rapidly and profoundly the amplitude of electrically evoked excitatory postsynaptic currents (EPSCs). In contrast, 5-HT also depressed evoked IPSCs but to a significantly lesser degree. The receptor subtypes mediating these effects were examined using specific 5-HT agonists and antagonists. A 5-HT1 agonist, 5-carboxamidotryptamine, produced EPSC depression but did not induce spontaneous IPSCs. A 5-HT2 agonist, alpha-Me-5-HT, reproduced all the observed effects of 5-HT (PSC depression as well as induction of spontaneous IPSCs), whereas a 5-HT2 antagonist, ketanserin, blocked the induction of spontaneous IPSCs. Therefore induction of spontaneous IPSCs is mediated by 5-HT2 receptors, whereas both 5-HT1 and 5-HT2 receptor types contribute to PSC depression. Serotonergic modulation of LSO synapses may have consequences for both developmental plasticity and auditory function. Serotonergic induction of IPSCs was observed primarily in young animals and thus may represent a mechanism for amplifying the activity of inhibitory synapses in LSO during a period of use-dependent plasticity in postnatal development. PSC depression, which preferentially affects excitation, is a potential mechanism for modulation of ILD tuning.

Across‐frequency interaural pattern discrimination

The ability of observers to detect changes in interaural intensity difference across frequencies was tested using multitone complexes. The tonal components ranged in frequency from 200 to 5000 Hz, with amplitudes randomly drawn from a Gaussian distribution having a mean of 60 dB SPL and a given variance. For the standard stimulus, variance was zero, providing equal‐amplitude tones. The signal stimulus had a nonzero variance. Because independently drawn signal stimuli were presented to the left and right ears, interaural level differences varied randomly across frequency. Percent correct signal detections were measured as a function of the variance of the signal stimulus. To encourage discrimination based on the pattern of interaural level differences across frequency rather than the change from diotic (standard) to dichotic (standard+signal), the overall levels of the stimuli presented to each ear were independently chosen from a 7‐dB range. Because standard and standard+signal power spectra differed, monaural cues to the presence of the signal were available. Thus psychometric functions were separately obtained using the same signal stimulus at both ears. Results indicate that the detectability of changes in interaural level differences across frequency is nearly the same as the detectability of changes in power spectra for diotic presentations. [Work funded by NIH.]