Unilateral Conductive Hearing Loss Disrupts the Developmental Refinement of Binaural Processing in the Rat Primary Auditory Cortex

Binaural hearing is critically important for the perception of sound spatial locations. The primary auditory cortex (AI) has been demonstrated to be necessary for sound localization. However, after hearing onset, how the processing of binaural cues by AI neurons develops, and how the binaural processing of AI neurons is affected by reversible unilateral conductive hearing loss (RUCHL), are not fully elucidated. Here, we determined the binaural processing of AI neurons in four groups of rats: postnatal day (P) 14–18 rats, P19–30 rats, P57–70 adult rats, and RUCHL rats (P57–70) with RUCHL during P14–30. We recorded the responses of AI neurons to both monaural and binaural stimuli with variations in interaural level differences (ILDs) and average binaural levels. We found that the monaural response types, the binaural interaction types, and the distributions of the best ILDs of AI neurons in P14–18 rats are already adult-like. However, after hearing onset, there exist developmental refinements in the binaural processing of AI neurons, which are exhibited by the increase in the degree of binaural interaction, and the increase in the sensitivity and selectivity to ILDs. RUCHL during early hearing development affects monaural response types, decreases the degree of binaural interactions, and decreases both the selectivity and sensitivity to ILDs of AI neurons in adulthood. These new evidences help us to understand the refinements and plasticity in the binaural processing of AI neurons during hearing development, and might enhance our understanding in the neuronal mechanism of developmental changes in auditory spatial perception.

Binaural Reproduction Based on Bilateral Ambisonics

Binaural reproduction of high-quality spatial sound has gained considerable interest with the recent technology developments in virtual and augmented reality. The reproduction of binaural signals in the Spherical-Harmonics (SH) domain using Ambisonics is now a well-established methodology, with flexible binaural processing realized using SH representations of the sound-field and the Head-Related Transfer Function (HRTF). However, in most practical cases, the binaural reproduction is order-limited, which introduces truncation errors that have a detrimental effect on the perception of the reproduced signals, mainly due to the truncation of the HRTF. Recently, it has been shown that manipulating the HRTF phase component, by ear-alignment, significantly reduces its effective SH order while preserving its phase information, which may be beneficial for alleviating the above detrimental effect. Incorporating the ear-aligned HRTF into the binaural reproduction process has been suggested by using Bilateral Ambisonics, which is an Ambisonics representation of the sound-field formulated at the two ears. While this method imposes challenges on acquiring the sound-field, and specifically, on applying head-rotations, it leads to a significant reduction in errors caused by the limited-order reproduction, which yields a substantial improvement in the perceived binaural reproduction quality even with first order SH.

Clinical Importance of Binaural Information: Extending Auditory Assessment in Clinical Populations Using a Portable Testing Platform

Purpose The purpose of this study is to use variability on tests of basic auditory processing to allow identification of those tests that could be used clinically to describe functional hearing ability beyond the pure-tone audiogram and clinical speech-in-noise tests. Method Psychoacoustic tests implemented using the Portable Automated Rapid Testing system on a calibrated iPad were evaluated for nine young normal-hearing participants ( M age = 21.3, SD = 2.5) and seven hearing-impaired participants ( M age = 64.9, SD = 13.5). Participants completed 10 psychoacoustic subtests in a quiet room. Correlational analyses were used to compare performance on the psychoacoustic test battery with performance on a clinical speech-in-noise test and with the 4-frequency pure-tone average (4FreqPTA). Results Spectral processing ability was highly correlated with 4FreqPTA, and temporal processing ability showed minimal variability across the hearing-impaired group. Tests involving binaural processing captured variability across hearing-impaired listeners not associated with 4FreqPTA or speech-in-noise performance. Conclusions Tests that capture the ability to use binaural cues may add information to what current clinical protocols reveal about patients with auditory complaints. Further testing with a larger sample size is needed to confirm the need for binaural measurements and to develop normative data for clinical settings.

Glycinergic axonal inhibition subserves acute spatial sensitivity to sudden increases in sound intensity

Locomotion generates adventitious sounds which enable detection and localization of predators and prey. Such sounds contain brisk changes or transients in amplitude. We investigated the hypothesis that ill-understood temporal specializations in binaural circuits subserve lateralization of such sound transients, based on different time of arrival at the ears (interaural time differences, ITDs). We find that Lateral Superior Olive (LSO) neurons show exquisite ITD-sensitivity, reflecting extreme precision and reliability of excitatory and inhibitory postsynaptic potentials, in contrast to Medial Superior Olive neurons, traditionally viewed as the ultimate ITD-detectors. In vivo, inhibition blocks LSO excitation over an extremely short window, which, in vitro, required synaptically-evoked inhibition. Light and electron microscopy revealed inhibitory synapses on the axon initial segment as the structural basis of this observation. These results reveal a neural vetoing mechanism with extreme temporal and spatial precision and establish the LSO as the primary nucleus for binaural processing of sound transients.

Computed-tomography estimates of interaural mismatch in insertion depth and scalar location in bilateral cochlear-implant users

Hypothesis: We hypothesized that the bilateral cochlear-implant (BI-CI) users would have a range of interaural insertion-depth mismatch because of different physical placements or characteristics of the arrays, but less than half of electrodes would have less than 75° or 3 mm of interaural insertion-depth mismatch. We also hypothesized that interaural insertion-depth mismatch would be more prevalent nearer the apex, when electrodes were located outside of scala tympani (i.e., possible interaural scalar mismatch), and when the arrays were a mix of pre-curved and straight types. Background: Brainstem neurons in the superior olivary complex are exquisitely sensitive to interaural differences, the cues to sound localization. These binaurally sensitive neurons rely on interaurally place-of-stimulation-matched inputs at the periphery. BI-CI users may have interaural differences in insertion depth and scalar location, causing interaural place-of-stimulation mismatch that impairs binaural abilities. Methods: Insertion depths and scalar locations were calculated from temporal-bone computed-tomography (CT) scans of 107 BI-CI users (27 Advanced Bionics, 62 Cochlear, and 18 Med-El). Each subject had either both pre-curved, both straight, or one of each type of array (mixed). Results: The median interaural insertion-depth mismatch was 23.4° or 1.3 mm. Relatively large interaural insertion-depth mismatch sufficient to disrupt binaural processing occurred for about 15% of electrode pairs [defined as >75° (13.0% of electrode pairs) or >3 mm (19.0% of electrode pairs)]. There was a significant three-way interaction of insertion depth, scalar location, and array type. Interaural insertion-depth mismatch was most prevalent when electrode pairs were more apically located, electrode pairs had interaural scalar mismatch (i.e., one in Scala Tympani, one in Scala Vestibuli), and when the arrays were both pre-curved. Conclusion: Large interaural insertion-depth mismatch can occur in BI-CI users. For new BI-CI users, improved surgical techniques to avoid interaural insertion-depth and scalar mismatch is recommended. For existing BI-CI users with interaural insertion-depth mismatch, interaural alignment of clinical frequency allocation tables by an audiologist might remediate any negative consequences to spatial-hearing abilities.

Interaural place-of-stimulation mismatch estimates using CT scans and binaural perception, but not pitch, are consistent in cochlear-implant users

ABSTRACTBilateral cochlear implants (BI-CIs) or a CI for single-sided deafness (SSD; one normally functioning acoustic ear) can partially restore spatial-hearing abilities including sound localization and speech understanding when there are competing sounds. However for these populations, frequency information is not explicitly aligned across the ears, resulting in interaural place-of-stimulation mismatch. This diminishes spatial-hearing abilities because binaural encoding occurs in interaurally frequency-matched neurons. This study examined whether plasticity – the reorganization of central neural pathways over time – can compensate for peripheral interaural place mismatch. We hypothesized differential plasticity across two systems: none for binaural processing but adaptation toward the frequencies delivered by the specific electrodes for sequential pitch perception. Interaural place mismatch was evaluated in 43 human subjects (20 BI-CI and 23 SSD-CI, both sexes) using interaural-time-difference (ITD) discrimination (simultaneous bilateral stimulation), place-pitch ranking (sequential bilateral stimulation), and physical electrode- location estimates from computed-tomography (CT) scans. On average, CT scans revealed relatively little BI-CI interaural place mismatch (26° insertion-angle mismatch), but relatively large SSD-CI mismatch, particularly at the apical end of the array (166° for an electrode tuned to 300 Hz, decreasing to 14° at 7000 Hz). ITD and CT measurements were in agreement, suggesting little binaural-system plasticity to mismatch. The pitch measurements did not agree with the binaural and CT measurements, suggesting plasticity for pitch encoding or procedural biases. The combined results show that binaural processing may be optimized by using CT-scan information, but not pitch measurements, to program the CI frequency allocation to reduce interaural place mismatch.SIGNIFICANCE STATEMENTPlacement of electrode arrays in users of cochlear implants (CIs; bionic auditory prostheses that partially restore hearing) does not align the frequency information to acoustic neural encoding across the ears. This interaural place-of-stimulation mismatch diminishes spatial hearing abilities. This study shows that for experienced adult CI users with two CIs or with one CI and one normal-hearing ear, the best possible binaural sensitivity occurs when the same cochlear location is stimulated in both ears. This means that binaural brainstem pathways do not experience “plasticity” to compensate for interaural place mismatch – i.e., they do not reorganize to respond to input from different cochlear places. Therefore, explicit correction of interaural place mismatch by a clinician is necessary to derive maximum spatial-hearing benefits.

Different pattern of auditory processing lateralization in musicians and non-musicians

The aim of the study was an evaluation of different pattern of auditory processing lateralization in musicians and non-musicians. 41 people aged 20-46 participated in the experiment, from which two research groups were selected: musicians ‒ instrumentalists professionally active (N: 21) and non-musicians (N: 20). All of them were right-handed. The dichotic listening test (Kurkowski 2007) was used to assess the laterality of external stimuli. The examination showed the superiority of right-ear perception or binaural speech processing. In the study of non-focused attention, musicians achieved a similar rate of correct responses for the left and right ear, which indicates binaural processing, where they gave more correct responses for the left ear and fewer correct responses for the right ear than non-musicians. The difference between the groups is statistically significant. In the study focused on the right ear, both groups obtained similar high scores. In the left-ear study the musicians gave more correct responses from the perception of stimuli to the left ear than non-musicians. This research confirmed different pattern of auditory processing lateralization in musicians and non-musicians.