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• many neurons in the central nucleus of the inferior colliculus are sensitive to sweeps of interaural time difference in tones or broadband noise. • response to such sweeps can match the response to static stimuli but can also drastically differ. • the difference in response to dynamic versus static stimuli predominantly reflects a neuron's temporal response pattern. • a roughly equal number of neurons showed build-up and adapting patterns. Many neurons in the central nucleus of the inferior colliculus (IC) show sensitivity to interaural time differences (ITDs), which is thought to be relayed from the brainstem. However, studies with interaural phase modulation of pure tones showed that IC neurons have a sensitivity to changes in ITD that is not present at the level of the brainstem. This sensitivity has been interpreted as a form of sensitivity to motion. A new type of stimulus is used here to study the sensitivity of IC neurons to dynamic changes in ITD, in which broad- or narrowband stimuli are swept through a range of ITDs with arbitrary start-ITD, end-ITD, speed, and direction. Extracellular recordings were obtained under barbiturate anesthesia in the cat. We applied the same analyses as previously introduced for the study of responses to tones. We find effects of motion which are similar to those described in response to interaural phase modulation of tones. The size of the effects strongly depended on the motion parameters but was overall smaller than reported for tones. We found that the effects of motion could largely be explained by the temporal response pattern of the neuron such as adaptation and build-up. Our data add to previous evidence questioning true coding of motion at the level of the IC. [ABSTRACT FROM AUTHOR]

• There was a statistically significant relationship between IPD-mediated ACC amplitude and behavioral ITD sensitivity for 250 Hz. • ACC N1-P2 responses were significantly higher for 250 Hz versus 1000 Hz. • ACC N1-P2 responses grow with IPD. • The IPD-mediated ACC response may be useful as a biomarker for ITD sensitivity in clinical populations. Combining cochlear implants with binaural acoustic hearing via preserved hearing in the implanted ear(s) is commonly referred to as combined electric and acoustic stimulation (EAS). EAS fittings can provide patients with significant benefit for speech recognition in complex noise, perceived listening difficulty, and horizontal-plane localization as compared to traditional bimodal hearing conditions with contralateral and monaural acoustic hearing. However, EAS benefit varies across patients and the degree of benefit is not reliably related to the underlying audiogram. Previous research has indicated that EAS benefit for speech recognition in complex listening scenarios and localization is significantly correlated with the patients' binaural cue sensitivity, namely interaural time differences (ITD). In the context of pure tones, interaural phase differences (IPD) and ITD can be understood as two perspectives on the same phenomenon. Through simple mathematical conversion, one can be transformed into the other, illustrating their inherent interrelation for spatial hearing abilities. However, assessing binaural cue sensitivity is not part of a clinical assessment battery as psychophysical tasks are time consuming, require training to achieve performance asymptote, and specialized programming and software all of which render this clinically unfeasible. In this study, we investigated the possibility of using an objective measure of binaural cue sensitivity by the acoustic change complex (ACC) via imposition of an IPD of varying degrees at stimulus midpoint. Ten adult listeners with normal hearing were assessed on tasks of behavioral and objective binaural cue sensitivity for carrier frequencies of 250 and 1000 Hz. Results suggest that 1) ACC amplitude increases with IPD; 2) ACC-based IPD sensitivity for 250 Hz is significantly correlated with behavioral ITD sensitivity; 3) Participants were more sensitive to IPDs at 250 Hz as compared to 1000 Hz. Thus, this objective measure of IPD sensitivity may hold clinical application for pre- and post-operative assessment for individuals meeting candidacy indications for cochlear implantation with low-frequency acoustic hearing preservation as this relatively quick and objective measure may provide clinicians with information identifying patients most likely to derive benefit from EAS technology. [ABSTRACT FROM AUTHOR]

Abstract The capacity of the auditory system to extract spatial information relies principally on the detection and interpretation of binaural cues, i.e., differences in the time of arrival or level of the sound between the two ears. In this review, we consider the effects of unilateral or asymmetric hearing loss on spatial hearing, with a focus on the adaptive changes in the brain that may help to compensate for an imbalance in input between the ears. Unilateral hearing loss during development weakens the brain's representation of the deprived ear, and this may outlast the restoration of function in that ear and therefore impair performance on tasks such as sound localization and spatial release from masking that rely on binaural processing. However, loss of hearing in one ear also triggers a reweighting of the cues used for sound localization, resulting in increased dependence on the spectral cues provided by the other ear for localization in azimuth, as well as adjustments in binaural sensitivity that help to offset the imbalance in inputs between the two ears. These adaptive strategies enable the developing auditory system to compensate to a large degree for asymmetric hearing loss, thereby maintaining accurate sound localization. They can also be leveraged by training following hearing loss in adulthood. Although further research is needed to determine whether this plasticity can generalize to more realistic listening conditions and to other tasks, such as spatial unmasking, the capacity of the auditory system to undergo these adaptive changes has important implications for rehabilitation strategies in the hearing impaired. Highlights • Unilateral hearing loss in infancy can disrupt spatial hearing, even after binaural inputs are restored. • Plasticity in the developing brain enables substantial recovery in sound localization accuracy. • Adaptation to unilateral hearing loss is based on reweighting of monaural spectral cues and binaural plasticity. • Training on auditory tasks can partially compensate for unilateral hearing loss, highlighting potential therapies. [ABSTRACT FROM AUTHOR]

Abstract Over the years, the effect of aging on auditory function has been investigated in animal models and humans in an effort to characterize age-related changes in both perception and physiology. Here, we review how aging may impact neural encoding and processing of binaural and spatial cues in human listeners with a focus on recent work by the authors as well as others. Age-related declines in monaural temporal processing, as estimated from measures of gap detection and temporal fine structure discrimination, have been associated with poorer performance on binaural tasks that require precise temporal processing. In lateralization and localization tasks, as well as in the detection of signals in noise, marked age-related changes have been demonstrated in both behavioral and electrophysiological measures and have been attributed to declines in neural synchrony and reduced central inhibition with advancing age. Evidence for such mechanisms, however, are influenced by the task (passive vs. attending) and the stimulus paradigm (e.g., static vs. continuous with dynamic change). That is, cortical auditory evoked potentials (CAEP) measured in response to static interaural time differences (ITDs) are larger in older versus younger listeners, consistent with reduced inhibition, while continuous stimuli with dynamic ITD changes lead to smaller responses in older compared to younger adults, suggestive of poorer neural synchrony. Additionally, the distribution of cortical activity is broader and less asymmetric in older than younger adults, consistent with the hemispheric asymmetry reduction in older adults model of cognitive aging. When older listeners attend to selected target locations in the free field, their CAEP components (N1, P2, P3) are again consistently smaller relative to younger listeners, and the reduced asymmetry in the distribution of cortical activity is maintained. As this research matures, proper neural biomarkers for changes in spatial hearing can provide objective evidence of impairment and targets for remediation. Future research should focus on the development and evaluation of effective approaches for remediating these spatial processing deficits associated with aging and hearing loss. Highlights • Aging leads to declines in processing monaural and binaural cues. • Poor spatial hearing in older adults is linked to a decline in temporal processing. • Reduced central inhibition in older adults is evident from response to static ITDs. • Poor neural synchrony in older adults is evident from response to dynamic ITDs. • Consistently less hemispheric asymmetry in older adults relative to younger adults. [ABSTRACT FROM AUTHOR]

Some blind people use echoes to detect discrete, silent objects to support their spatial orientation/navigation, independence, safety and wellbeing. The acoustical features that people use for this are not well understood. Listening to changes in spectral shape due to the presence of an object could be important for object detection and avoidance, especially at short range, although it is currently not known whether it is possible with echolocation-related sounds. Bands of noise were convolved with recordings of binaural impulse responses of objects in an anechoic chamber to create ‘virtual objects’, which were analysed and played to sighted and blind listeners inexperienced in echolocation. The sounds were also manipulated to remove cues unrelated to spectral shape. Most listeners could accurately detect hard flat objects using changes in spectral shape. The useful spectral changes for object detection occurred above approximately 3 kHz, as with object localisation. However, energy in the sounds below 3 kHz was required to exploit changes in spectral shape for object detection, whereas energy below 3 kHz impaired object localisation. Further recordings showed that the spectral changes were diminished by room reverberation. While good high-frequency hearing is generally important for echolocation, the optimal echo-generating stimulus will probably depend on the task. [ABSTRACT FROM AUTHOR]

Binaural notched-noise experiments indicate a reduced frequency selectivity of the binaural system compared to monaural processing. The present study investigates how auditory efferent activation (via the medial olivocochlear system) affects binaural frequency selectivity in normal-hearing listeners. Thresholds were measured for a 1-kHz signal embedded in a diotic notched-noise masker for various notch widths. The signal was either presented in phase (diotic) or in antiphase (dichotic), gated with the noise. Stimulus duration was 25 ms, in order to avoid efferent activation due to the masker or the signal. A bandpass-filtered noise precursor was presented prior to the masker and signal stimuli to activate the efferent system. The silent interval between the precursor and the masker-signal complex was 50 ms. For comparison, thresholds for detectability of the masked signal were also measured in a baseline condition without the precursor and, in addition, without the masker. On average, the results of the baseline condition indicate an effectively wider binaural filter, as expected. For both signal phases, the addition of the precursor results in effectively wider filters, which is in agreement with the hypothesis that cochlear gain is reduced due to the presence of the precursor. [ABSTRACT FROM AUTHOR]

Different fields of the auditory cortex can be distinguished by the extent and level tolerance of spatial selectivity. The mechanisms underlying the range of spatial tuning properties observed across cortical fields are unclear. Here, this issue was addressed in the pallid bat because its auditory cortex contains two segregated regions of response selectivity that serve two different behaviors: echolocation for obstacle avoidance and localization of prey-generated noise. This provides the unique opportunity to examine mechanisms of spatial properties in two functionally distinct regions. Previous studies have shown that spatial selectivity of neurons in the region selective for noise (noise-selective region, NSR) is level tolerant and shaped by interaural level difference (ILD) selectivity. In contrast, spatial selectivity of neurons in the echolocation region (‘FM sweep-selective region’ or FMSR) is strongly level dependent with many neurons responding to multiple distinct spatial locations for louder sounds. To determine the mechanisms underlying such level dependence, frequency, azimuth, rate-level responses and ILD selectivity were measured from the same FMSR neurons. The majority (∼75%) of FMSR neurons were monaural (ILD insensitive). Azimuth tuning curves expanded or split into multiple peaks with increasing sound level in a manner that was predicted by the rate-level response of neurons. These data suggest that azimuth selectivity of FMSR neurons depends more on monaural ear directionality and rate-level responses. The pallid bat cortex utilizes segregated monaural and binaural regions to process echoes and prey-generated noise. Together the pallid bat FMSR/NSR data provide mechanistic explanations for a broad range of spatial tuning properties seen across species. [ABSTRACT FROM AUTHOR]

Abstract: It is well established that the responses of binaural auditory neurons can adapt and change dramatically depending on the nature of a preceding sound. Examples of how the effects of ensuing stimuli play a functional role in auditory processing include motion sensitivity and precedence-like effects. To date, these types of effects have been documented at the level of the midbrain and above. Little is known about sensitivity to ensuing stimuli below in the superior olivary nuclei where binaural response properties are first established. Here we report on single cell responses in the gerbil lateral superior olive, the initial site where sensitivity to interaural level differences is established. In contrast to our expectations we found a robust sensitivity to ensuing stimuli. The majority of the cells we tested (86%), showed substantial suppression and/or enhancement to a designated target stimulus, depending on the nature of a preceding stimulus. Hence, sensitivity to ensuing stimuli is already established at the first synaptic station of binaural processing. [Copyright &y& Elsevier]

Abstract: Sound pressure level changes can affect the timing of spiketrains. Timing of spiketrains is critical for sensitivity to interaural timing differences (ITDs). Interaural level differences (ILDs) can therefore affect the ITD cue. It has been hypothesized that ILDs may be coded indirectly through a peripheral conversion of level to time (but it should be cautioned that the changes in phase with SPL in low-CF AN fibers of the cat are more complicated) (Jeffress, L.A., 1948. A place theory of sound localization. J. Comp. Physiol. Psychol. 41, 35–39). We tested this conversion by recording from auditory nerve fibers to broadband noise at different SPLs. For each fiber, correlograms were constructed to compare timing to fine-structure across SPLs. We find generally a decrease in the time delay between spikes and the stimulus with increasing SPL. However, the magnitudes of the shift in time are surprisingly small, and dependent on characteristic frequency (CF): the largest shifts are approximately 10μs/dB and occur at the lowest CFs. Nevertheless, the effects of level on spike timing are systematic and of a magnitude to which the binaural system is sensitive. Thus, even though the results indicate that ILD is not traded for ITD in a simple way, the possibility that low-frequency ILDs affect the binaural percept via a peripheral level-to-time conversion cannot be excluded. [Copyright &y& Elsevier]

Abstract: Despite extensive subcortical processing, the auditory cortex is believed to be essential for normal sound localization. However, we still have a poor understanding of how auditory spatial information is encoded in the cortex and of the relative contribution of different cortical areas to spatial hearing. We investigated the behavioral consequences of inactivating ferret primary auditory cortex (A1) on auditory localization by implanting a sustained release polymer containing the GABAA agonist muscimol bilaterally over A1. Silencing A1 led to a reversible deficit in the localization of brief noise bursts in both the horizontal and vertical planes. In other ferrets, large bilateral lesions of the auditory cortex, which extended beyond A1, produced more severe and persistent localization deficits. To investigate the processing of spatial information by high-frequency A1 neurons, we measured their binaural-level functions and used individualized virtual acoustic space stimuli to record their spatial receptive fields (SRFs) in anesthetized ferrets. We observed the existence of a continuum of response properties, with most neurons preferring contralateral sound locations. In many cases, the SRFs could be explained by a simple linear interaction between the acoustical properties of the head and external ears and the binaural frequency tuning of the neurons. Azimuth response profiles recorded in awake ferrets were very similar and further analysis suggested that the slopes of these functions and location-dependent variations in spike timing are the main information-bearing parameters. Studies of sensory plasticity can also provide valuable insights into the role of different brain areas and the way in which information is represented within them. For example, stimulus-specific training allows accurate auditory localization by adult ferrets to be relearned after manipulating binaural cues by occluding one ear. Reversible inactivation of A1 resulted in slower and less complete adaptation than in normal controls, whereas selective lesions of the descending corticocollicular pathway prevented any improvement in performance. These results reveal a role for auditory cortex in training-induced plasticity of auditory localization, which could be mediated by descending cortical pathways. [Copyright &y& Elsevier]

The auditory motion aftereffect (aMAE) can be induced in listeners after repeated presentation of a horizontally moving sound source. Aftereffects have also been found for the individual acoustic consequences of source motion such as amplitude or frequency modulations (AM, FM). No study, however, has investigated whether combining these changes would enhance the magnitude of the aMAE, which has appeared otherwise weak relative to its visual counterpart. AM, FM and binaural changes can occur simultaneously when sources move along common translational trajectories rather than the restricted rotational paths used in previous adaptation studies. This raises the question whether the observed weakness of the aMAE is due to the improper stimulation of units responsive to the entire macrostructure induced by translational motion. The hypothesis is tested here that if integrated motion detectors exist, then including lawful amplitude and frequency changes in adapting stimuli may enhance aftereffects. Though results indicate that interaurally moving stimuli in general induce an aMAE, the acoustic macrostructure of translational motion does not appear to increase the aftereffect. A simple cross-correlation model is used to illustrate that such acoustic modulations may allow brainstem auditory centers time to recover from adaptation to translational motion. [Copyright &y& Elsevier]

• Hearing loss is associated with less binaural frequency selectivity. • Sound level does not change binaural frequency selectivity. • Binaural frequency selectivity is likely affected by other deafness-related factors. Pitch is an important cue that allows the auditory system to distinguish between sound sources. Pitch cues are less useful when listeners are not able to discriminate different pitches between the two ears, a problem encountered by listeners with hearing impairment (HI). Many listeners with HI will fuse the pitch of two dichotically presented tones over a larger range of interaural frequency disparities, i.e., have a broader fusion range, than listeners with normal hearing (NH). One potential explanation for broader fusion in listeners with HI is that hearing aids stimulate at high sound levels. The present study investigated effects of overall sound levels on pitch fusion in listeners with NH. It was hypothesized that if sound level increased, then fusion range would increase. Fusion ranges were measured by presenting a fixed frequency tone to a reference ear simultaneously with a variable frequency tone to the opposite ear and finding the range of frequencies that were fused with the reference frequency. No significant effects of sound level (comfortable level ± 15 dB) on fusion range were found, even when tested within the range of levels where some listeners with HI show large fusion ranges. Results suggest that increased sound level does not explain increased fusion range in listeners with HI and imply that other factors associated with hearing loss might play a larger role. [ABSTRACT FROM AUTHOR]

Sequential stream segregation on the basis of binaural 'ear-of-entry', modulation rate and electrode place-of-stimulation cues was investigated in bilateral cochlear implant (CI) listeners using a rhythm anisochrony detection task. Sequences of alternating 'A' and 'B' bursts were presented via direct electrical stimulation and comprised either an isochronous timing structure or an anisochronous structure that was generated by delaying just the 'B' bursts. 'B' delay thresholds that enabled rhythm anisochrony detection were determined. Higher thresholds were assumed to indicate a greater likelihood of stream segregation, resulting specifically from stream integration breakdown. Results averaged across subjects showed that thresholds were significantly higher when monaural 'A' and 'B' bursts were presented contralaterally rather than ipsilaterally, and that diotic presentation of 'A', with a monaural 'B', yielded intermediate thresholds. When presented monaurally and ipsilaterally, higher thresholds were also found when successive bursts had mismatched rather than matched modulation rates. In agreement with previous studies, average delay thresholds also increased as electrode separation between bursts increased when presented ipsilaterally. No interactions were found between ear-of-entry, modulation rate and place-of-stimulation. However, combining moderate electrode difference cues with either diotic-'A' ear-of-entry cues or modulation-rate mismatch cues did yield greater threshold increases than observed with any of those cues alone. The results from the present study indicate that sequential stream segregation can be elicited in bilateral CI users by differences in the signal across ears (binaural cues), in modulation rate (monaural cues) and in place-of-stimulation (monaural cues), and that those differences can be combined to further increase segregation. • Bilateral cochlear implant users may be able to exploit differences in the signal received at their two ears (i.e. binaural cues) to perform sequential stream segregation. • Cochlear implant users may be able to exploit differences in modulation rate within the same ear to perform sequential stream segregation. • In line with previous findings, cochlear implant users may be able to exploit differences in place-of-stimulation (i.e. electrode place) to perform sequential stream segregation. [ABSTRACT FROM AUTHOR]