Processing Of Interaural Time Differences Research Articles

Auditory Spatial Illusion – A Psychoacoustic Study in Young Adults

The Franssen illusion, or Franssen effect (FE), is one of the auditory spatial illusions. Few studies have explored the FE, and the mechanisms underlying it remain unknown. The present study was conducted to clarify the FE occurrence with different tasks and presentation modes in young adults. It also sought to investigate possible neurophysiological similarities between interaural time difference (ITD) cue processing and FE perception. FE perception was evaluated using two different tasks and two presentation modes (i.e., insert phones and loudspeakers). Sound reflections (reverberation) were presented in the diffuse field (loudspeaker mode). ITD performance was investigated using different stimuli delivered via insert phones. No significant difference between the two FE perception tasks was found ( F 1,25 = 0.138, p = 0.713). However, the FE perception showed a significant difference between the two presentation modes (F 1,25 = 434.03, p < 0.001). Spearman’s correlation did not reveal a significant relationship between FE perception and ITD scores ( p > 0.05). The current findings show the importance of reverberation in the FE occurrence. Also, the non-significant correlation between the results of the behavioral binaural temporal resolution test and FE perception in young people with normal temporal resolution may indicate that room reflections (reverberation) complicate the ability to process ITDs (rather than poor ITD processing for the “steady state” portion of signal).

Is the rapid decline in interaural time difference sensitivity above 700 Hz explained by downward spread of excitation into the frequency dominant region?

Low-frequency temporal-fine-structure interaural time differences (ITDs) are so important that they almost completely dominate localization of broadband sounds. This “Dominant Region” occurs at ∼700 Hz and also coincides with best ITD discrimination sensitivity. Above this frequency range, ITD sensitivity worsens, until ∼1400 Hz where it rapidly becomes impossible. Explaining how listeners’ ITD sensitivity transitions from best sensitivity to impossible to detect within ∼1 octave challenges models of ITD processing. One possible explanation is that listeners continue to use binaural information from the Dominant Region even for higher-frequency pure tones. Within this explanation, the steep decline in ITD sensitivity reflects the high-frequency limb of peripheral frequency tuning for neurons within the Dominant Region (i.e., downward spread of excitation). To test this hypothesis, we measured the upper frequency limit of ITD processing as a function of stimulus level. Normal-hearing listeners were tested on a left-right ITD discrimination task. Stimuli were pure tones with 500–1500Hz frequencies and 10–50 dB sensation levels. It was hypothesized that the upper frequency limit of ITD processing would decrease in frequency with decreasing level, which would indicate that the downward spread of excitation into the Dominant Region at least partially explains the rapid decline in ITD sensitivity.

Neonatal Deafening Selectively Degrades the Sensitivity to Interaural Time Differences of Electrical Stimuli in Low-Frequency Pathways in Rats.

We examined the effect of neonatal deafening on frequency-specific pathways for processing of interaural time differences (ITDs) in cochlear-implant stimuli. Animal studies have demonstrated differences in neural ITD sensitivity in the inferior colliculus (IC) depending on the intracochlear location of intracochlear stimulating electrodes. We used neonatally deafened (ND) rats of both sexes and recorded the responses of single neurons in the IC to electrical stimuli with ITDs delivered to the apical or basal cochlea and compared them with acutely deafened (AD) rats of both sexes with normal hearing (NH) during development. We found that neonatal deafness significantly impacted the ITD sensitivity and the ITD tuning patterns restricted to apically driven IC neurons. In ND rats, the ITD sensitivity of apically driven neurons is reduced to values similar to basally driven neurons. The prevalence of ITD-sensitive apical neurons with a peak-shaped ITD tuning curve, which may reflect predominant input from the medial superior olivary (MSO) complex, in ND rats was diminished compared with that in AD rats (67%, AD vs 40%, ND). Conversely, monotonic-type responses rarely occurred in AD rats (14%) but were approximately equally as prevalent as peak-type tuning curves in ND rats (42%). Nevertheless, in ND rats, the ITD at the maximum slope of the ITD tuning curve was still more concentrated within the physiological ITD range in apically driven than in basally driven neurons. These results indicate that the development of high ITD sensitivity processed by low-frequency pathways depends on normal auditory experience and associated biases in ITD tuning strategies.

The effects of aging and hearing loss on the across-frequency processing of interaural time differences

Aging and hearing loss cause speech understanding deficits, particularly in the presence of competing sounds, as well as temporal processing deficits, including binaural encoding of interaural time differences (ITDs). However, there is little understanding of the independent effects of aging and hearing loss on across-frequency ITD processing, which demonstrates a clear dependence on frequency weighting or “dominant” region around 600–700Hz. We hypothesize that aging and hearing loss will reduce extent of laterality of ITDs and will shift the frequency weighting lower. Young normal-hearing, older normal-hearing, and older hearing-impaired listeners will be tested on an intracranial lateralization task. The stimuli will consist of narrowband noises. They will have upper frequency boundaries = 500, 600, 700, or 800 Hz, bandwidths = 50, 100, 200, or 400 Hz, and ITDs = 0, ±0.5, ±1, ±1.5, and ±2 ms. These findings will help us better understand the role of the dominant region in across-frequency ITD processing, and how age- and hearing-loss-related changes in ITD processing may contribute to spatial-hearing deficits.

Ambiguous lateralization of complex binaural stimuli emerge from neurons constrained to physiologically relevant ranges of ITD and frequency—The binaural aperture

Although we perceive a spatially contiguous, integrated world, sensory systems sample information discretely via small spatial and narrow frequency receptive fields (RFs) in vision and audition, respectively. Undersampling of stimuli by narrow RFs produces ambiguous representations (e.g., the classic “aperture problem” that visual motion cannot be uniquely determined by one RF). Here we posit a similar problem for encoding interaural time differences (ITDs). Current models of ITD processing propose large ensembles of ITD-sensitive neurons spanning wide ranges of preferred ITD and characteristic frequency (CF). To account for anomalous lateralization of narrowband but robust lateralization of broadband stimuli, such models require two paradoxical assumptions that have recently come into question: (1) the existence of neurons preferring larger than physiologically possible ITDs; and (2) population biases overemphasizing small ITDs. Here, we consider an alternative account based on a highly constrained ITDxCF aperture through which the stimuli are perceived. ITDs bounded by ±π interaural phase difference and CFs by dominance weighting, where ∼600-750 Hz contributes most to lateralization. A more detailed consideration of this “binaural aperture” sheds light on how ambiguous lateralization data emerge from limited ensembles of neurons constrained to physiologically relevant ranges of ITD and frequency. [R01-DC011555 (DJT), R01-DC014948 (MJG), and R01-DC016643 (GCS).]

The Relationship Between Interaural Insertion-Depth Differences, Scalar Location, and Interaural Time-Difference Processing in Adult Bilateral Cochlear-Implant Listeners.

Sensitivity to interaural time differences (ITDs) in acoustic hearing involves comparison of interaurally frequency-matched inputs. Bilateral cochlear-implant arrays are, however, only approximately aligned in angular insertion depth and scalar location across the cochleae. Interaural place-of-stimulation mismatch therefore has the potential to impact binaural perception. ITD left-right discrimination thresholds were examined in 23 postlingually-deafened adult bilateral cochlear-implant listeners, using low-rate constant-amplitude pulse trains presented via direct stimulation to single electrodes in each ear. Angular insertion depth and scalar location measured from computed-tomography (CT) scans were used to quantify interaural mismatch, and their association with binaural performance was assessed. Number-matched electrodes displayed a median interaural insertion-depth mismatch of 18° and generally yielded best or near-best ITD discrimination thresholds. Two listeners whose discrimination thresholds did not show this pattern were confirmed via CT to have atypical array placement. Listeners with more number-matched electrode pairs located in the scala tympani displayed better thresholds than listeners with fewer such pairs. ITD tuning curves as a function of interaural electrode separation were broad; bandwidths at twice the threshold minimum averaged 10.5 electrodes (equivalent to 5.9 mm for a Cochlear-brand pre-curved array). Larger angular insertion-depth differences were associated with wider bandwidths. Wide ITD tuning curve bandwidths appear to be a product of both monopolar stimulation and angular insertion-depth mismatch. Cases of good ITD sensitivity with very wide bandwidths suggest that precise matching of insertion depth is not critical for discrimination thresholds. Further prioritizing scala tympani location at implantation should, however, benefit ITD sensitivity.

Effects of place of stimulation on the interaural time difference sensitivity in bilateral electrical intracochlear stimulations: Neurophysiological study in a rat model.

We examined the sensitivity of the neurons in the inferior colliculus (IC) in male and female rats to the interaural time differences (ITDs) conveyed in electrical pulse trains. Using bipolar pairs of electrodes that selectively activate the auditory nerve fibers at different intracochlear locations, we assessed whether the responses to electrical stimulation with ITDs in different frequency regions were processed differently. Most well-isolated single units responded to the electrical stimulation in only one of the apical or basal cochlear regions, and they were classified as either apical or basal units. Regardless of the cochlear stimulating location, more than 70% of both apical and basal units were sensitive to ITDs of electrical stimulation. However, the pulse rate dependence of neural ITD sensitivity differed significantly depending on the location of the stimulation. Moreover, ITD discrimination thresholds and the relative incidence of ITD tuning type markedly differed between units activated by apical and basal stimulations. With apical stimulation, IC neurons had a higher incidence of peak-type ITD function, which mostly exhibited the steepest position of the tuning curve within the rat's physiological ITD range of ±160μs and, accordingly, had better ITD discrimination thresholds than those with basal stimulation. These results support the idea that ITD processing in the IC might be determined by functionally segregated frequency-specific pathways from the cochlea to the auditory midbrain.

The aperture problem for binaural hearing can explain ambiguous lateralization of complex binaural stimuli

Although we perceive a spatially contiguous, integrated world, sensory systems sample information discretely via small spatial and narrow frequency receptive fields (RFs) in vision and audition, respectively. Undersampling of stimuli by narrow RFs produces ambiguous representations (e.g., the classic “aperture problem” that visual motion cannot be uniquely determined by one RF). Here, we posit a similar problem for encoding low-frequency (<1500 Hz) interaural time differences (ITDs). Current models of ITD processing propose large ensembles of ITD-sensitive neurons spanning wide ranges of preferred ITD and characteristic frequency (CF). To account for anomalous lateralization of narrowband but robust lateralization of broadband stimuli, such models require two paradoxical assumptions that have recently come into question; (1) the existence of neurons preferring larger than physiologically possible ITDs, and (2) population biases overemphasizing small ITDs. Here, we consider an alternative account based on a narrow and weighted ITDxCF aperture through which the stimuli are perceived. ITDs are bounded by ±π interaural phase difference and CFs by dominance weighting, where ∼600–750 Hz contributes most to lateralization. A more detailed consideration of this “binaural aperture” sheds light on how lateralization data emerge from limited ensembles of neurons constrained to physiologically relevant ranges of ITD and frequency.

Provision of interaural time difference information in chronic intracochlear electrical stimulation enhances neural sensitivity to these differences in neonatally deafened cats

Although performance with bilateral cochlear implants is superior to that with a unilateral implant, bilateral implantees have poor performance in sound localisation and in speech discrimination in noise compared to normal hearing subjects. Studies of the neural processing of interaural time differences (ITDs) in the inferior colliculus (IC) of long-term deaf animals, show substantial degradation compared to that in normal hearing animals. It is not known whether this degradation can be ameliorated by chronic cochlear electrical stimulation, but such amelioration is unlikely to be achieved using current clinical speech processors and cochlear implants, which do not provide good ITD cues. We therefore developed a custom sound processor to deliver salient ITDs for chronic bilateral intra-cochlear electrical stimulation in a cat model of neonatal deafness, to determine if long-term exposure to salient ITDs would prevent degradation of ITD processing. We compared the sensitivity to ITDs in cochlear electrical stimuli of neurons in the IC of cats chronically stimulated with our custom ITD-aware sound processor with sensitivity in acutely deafened cats with normal hearing development and in cats chronically stimulated with a clinical stimulator and sound processor. Animals that experienced stimulation with our custom ITD-aware sound processor had significantly higher neural sensitivity to ITDs than those that received stimulation from clinical sound processors. There was no significant difference between animals received no stimulation and those that received stimulation from clinical sound processors, consistent with findings from clinical cochlear implant users. This result suggests that development and use of clinical ITD-aware sound processing strategies from a young age may promote ITD sensitivity in the clinical population.

Development of Binaural Sensitivity: Eye Gaze as a Measure of Real-time Processing.

Children localize sounds using binaural cues when navigating everyday auditory environments. While sensitivity to binaural cues reaches maturity by 8–10 years of age, large individual variability has been observed in the just-noticeable-difference (JND) thresholds for interaural time difference (ITD) among children in this age range. To understand the development of binaural sensitivity beyond JND thresholds, the “looking-while-listening” paradigm was adapted in this study to reveal the real-time decision-making behavior during ITD processing. Children ages 8–14 years with normal hearing (NH) and a group of young NH adults were tested. This novel paradigm combined eye gaze tracking with behavioral psychoacoustics to estimate ITD JNDs in a two-alternative forced-choice discrimination task. Results from simultaneous eye gaze recordings during ITD processing suggested that children had adult-like ITD JNDs, but they demonstrated immature decision-making strategies. While the time course of arriving at the initial fixation and final decision in providing a judgment of the ITD direction was similar, children exhibited more uncertainty than adults during decision-making. Specifically, children made more fixation changes, particularly when tested using small ITD magnitudes, between the target and non-target response options prior to finalizing a judgment. These findings suggest that, while children may exhibit adult-like sensitivity to ITDs, their eye gaze behavior reveals that the processing of this binaural cue is still developing through late childhood.

Effects of rate and age in processing interaural time and level differences in normal-hearing and bilateral cochlear-implant listeners.

Bilateral cochlear implants (BICIs) provide improved sound localization and speech understanding in noise compared to unilateral CIs. However, normal-hearing (NH) listeners demonstrate superior binaural processing abilities compared to BICI listeners. This investigation sought to understand differences between NH and BICI listeners' processing of interaural time differences (ITDs) and interaural level differences (ILDs) as a function of fine-structure and envelope rate using an intracranial lateralization task. The NH listeners were presented band-limited acoustical pulse trains and sinusoidally amplitude-modulated tones using headphones, and the BICI listeners were presented single-electrode electrical pulse trains using direct stimulation. Lateralization range increased as fine-structure rate increased for ILDs in BICI listeners. Lateralization range decreased for rates above 100 Hz for fine-structure ITDs, but decreased for rates lower or higher than 100 Hz for envelope ITDs in both groups. Lateralization ranges for ITDs were smaller for BICI listeners on average. After controlling for age, older listeners showed smaller lateralization ranges and BICI listeners had a more rapid decline for ITD sensitivity at 300 pulses per second. This work suggests that age confounds comparisons between NH and BICI listeners in temporal processing tasks and that some NH-BICI binaural processing differences persist even when age differences are adequately addressed.

Processing of interaural time differences in normal-hearing subjects and cochlear implant users with FSP and HDCIS coding strategy

Sound localization is necessary for social interaction and remains challenging for bilateral cochlear implant (CI) users. This study investigated the influence of fine structure processing (FSP) coding strategies on temporal accuracy and source localization. The ability to discriminate between different interaural time differences (ITD) was measured objectively and compared. Fifteen normal-hearing subjects and five CI users participated in this study. Electrophysiological recording of mismatch negativity (MMN) and psychoacoustic tests with headphones and loudspeakers were conducted to measure and compare the discrimination of ITDs. In normal-hearing subjects the discrimination threshold for ITD was 83-117 µs. Localization ability in a free sound field was below the limit of resolution. Asignificant MMN was measured. CI users showed amean angle detection error of more than 30° and adiscrimination threshold between 1100 and 2100 µs. Due to artifacts, no clear MMN component could be recorded. This study shows that MMN can be used as an objective measure of ITD discrimination in normal-hearing participants. An indication that improvements in directional hearing can be achieved with the transfer of fine structure could not be provided by this study.

Revisiting perceived intracranial lateralization for stimuli with interaural time differences that are larger than the head

The interaural time difference (ITD) is the primary sound-localization cue for humans. While realistic sound sources have energy across a wide frequency range, the ear performs a narrowband frequency decomposition, and within each band are ambiguities in the interaural cross-correlation (an index of the signal ITD). These ambiguities are thought to be resolved by the approximate consistency of ITDs across frequency, or “straightness.” However, straightness has not been evaluated over a wide range of stimuli. Therefore, normal-hearing listeners reported the intracranial lateralization of narrowband noises and tone complexes to better evaluate across-frequency ITD processing. In a new modification of the typical paradigm, listeners were encouraged to give multiple responses if split images were perceived. ITDs larger than those naturally produced by the head (~750 ms) best demonstrate straightness because across-frequency comparison is necessary to resolve the interaural phase ambiguities, which is why ITDs as large as 1500 ms were applied. Straightness effects reported previously for narrowband noises were replicated, and were broadly similar for complex tones. The extent of lateralization of 1500-ms ITDs and the occurrence of split images were difficult to account for using simple lateralization-based models for smaller ITDs.

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

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

Localization of broadband sounds carrying interaural time differences: Effects of frequency, reference location, and interaural coherence.

The auditory processes involved in the localization of sounds in rooms are still poorly understood. The present study investigated the auditory system's across-frequency processing of interaural time differences (ITDs) and the impact of the interaural coherence (IC) of the stimuli in ITD discrimination and localization. First, ITD discrimination thresholds were measured as a function of signal frequency, reference ITD, and IC using critical-band wide noises. The resulting data were fitted with a set of analytical functions and ITD weights were derived using concepts from signal detection theory. Inspired by the weighted-image model [Stern, Zeiberg, and Trahiotis. (1988). J. Acoust. Soc. Am. 84, 156-165], the derived ITD weights were then integrated in a simplified localization model using an optimal combination of ITD information across frequency. To verify this model, a series of localization experiments were conducted using broadband noise in which ITD and IC were varied across frequency. The model predictions were in good agreement with the experimental data, supporting the assumption that the auditory system performs a weighted integration of ITD information across frequency to localize a sound source. The results could be valuable for the design of new paradigms to measure localization in more complex acoustic conditions and may provide constraints for future localization models.

Across Species "Natural Ablation" Reveals the Brainstem Source of a Noninvasive Biomarker of Binaural Hearing.

The binaural interaction component (BIC) of the auditory brainstem response is a noninvasive electroencephalographic signature of neural processing of binaural sounds. Despite its potential as a clinical biomarker, the neural structures and mechanism that generate the BIC are not known. We explore here the hypothesis that the BIC emerges from excitatory-inhibitory interactions in auditory brainstem neurons. We measured the BIC in response to click stimuli while varying interaural time differences (ITDs) in subjects of either sex from five animal species. Species had head sizes spanning a 3.5-fold range and correspondingly large variations in the sizes of the auditory brainstem nuclei known to process binaural sounds [the medial superior olive (MSO) and the lateral superior olive (LSO)]. The BIC was reliably elicited in all species, including those that have small or inexistent MSOs. In addition, the range of ITDs where BIC was elicited was independent of animal species, suggesting that the BIC is not a reflection of the processing of ITDs per se. Finally, we provide a model of the amplitude and latency of the BIC peak, which is based on excitatory-inhibitory synaptic interactions, without assuming any specific arrangement of delay lines. Our results show that the BIC is preserved across species ranging from mice to humans. We argue that this is the result of generic excitatory-inhibitory synaptic interactions at the level of the LSO, and thus best seen as reflecting the integration of binaural inputs as opposed to their spatial properties.SIGNIFICANCE STATEMENT Noninvasive electrophysiological measures of sensory system activity are critical for the objective clinical diagnosis of human sensory processing deficits. The binaural component of sound-evoked auditory brainstem responses is one such measure of binaural auditory coding fidelity in the early stages of the auditory system. Yet, the precise neurons that lead to this evoked potential are not fully understood. This paper provides a comparative study of this potential in different mammals and shows that it is preserved across species, from mice to men, despite large variations in morphology and neuroanatomy. Our results confirm its relevance to the assessment of binaural hearing integrity in humans and demonstrates how it can be used to bridge the gap between rodent models and humans.

Comparisons of electric and acoustic ITD coding in normal hearing gerbils

Small differences in the arrival time of sound between the two ears [interaural time differences (ITDs)] provide important cues for directional hearing and speech understanding in noise. Deaf subjects with bilateral cochlear implants (CIs) show relatively poor sensitivity to ITDs. It is unclear whether these limitations are due to differences in binaural brain circuits activated by electric compared to acoustic stimulation, mismatches in electric activation site across ears, or deafness-induced degradations in neural ITD processing. To identify potential limitations of electric ITD coding, we compared electric and acoustic ITD coding (i.e., ITD tuning and discrimination thresholds) in the same population of auditory brainstem and midbrain neurons in normal hearing gerbils. When compared in the same neurons, ITD coding to acoustic stimulation did not predict coding to electric stimulation. However, on a population level, neurons demonstrated surprising similarities in acoustic and electric ITD processing. ...

A model of the neuronal processing of interaural time disparities

The auditory system of humans and other mammals is able to use interaural time and intensity differences as well as spectral cues to localize sound sources. One important cue for localizing low-frequency sound sources in the horizontal plane are interaural time differences (ITDs), which are first analyzed in the medial superior olive (MSO) in the brainstem. Results from electrophysiological and psychoacoustic studies suggest ITD encoding in the relative activities of neuronal populations in the two brain hemispheres. This contribution first explores the neuronal encoding of fine-structure ITDs using a physiologically motivated spiking neuronal-network model of the mammalian MSO. Results from this model confirm robust ITD encoding in the relative activity of the MSOs in both hemispheres. Based on the neuronal-network simulations, a simple probabilistic model of subsequent ITD processing is proposed. This simplified model connects the neuronal responses of the two hemispheres with different hearing sensation properties, such as lateral position, spatial extent, or number of spatially separable sensations. First predictions from the model are discussed with regard to the results of a series of psychoacoustic experiments on the lateralization of pure-tone impulses presented via headphones.

Influence of envelope waveform on ITD sensitivity of neurons in the auditory midbrain.

Interaural time differences (ITDs) conveyed by the modulated envelopes of high-frequency sounds can serve as a cue for localizing a sound source. Klein-Hennig et al. (J Acoust Soc Am 129: 3856, 2011) demonstrated the envelope attack (the rate at which stimulus energy in the envelope increases) and the duration of the pause (the interval between successive envelope pulses) as important factors affecting sensitivity to envelope ITDs in human listeners. Modulated sounds with rapid attacks and long pauses produce the lowest ITD discrimination thresholds. The duration of the envelope's sustained component (sustain) and the rate at which stimulus energy falls at the offset of the envelope (decay) are only minor factors. We assessed the responses of 71 single neurons, recorded from the midbrains of 15 urethane-anesthetized tri-colored guinea pigs, to envelope shapes in which the four envelope components, i.e., attack, sustain, decay, and pause, were systematically varied. We confirmed the importance of the attack and pause components in generating ITD-sensitive responses. Analysis of neural firing rates demonstrated more neurons (49/71) show ITD sensitivity in response to "damped" stimuli (fast attack and slow decay) compared with "ramped" stimuli (slow attack and fast decay) (14/71). Furthermore, the lowest threshold for the damped stimulus (91 μs) was lower by a factor of 4 than that for the temporally reversed ramped envelope shape (407 μs). The data confirm the importance of fast attacks and optimal pause durations in generating sensitivity to ITDs conveyed in the modulated envelopes of high-frequency sounds and are incompatible with models of ITD processing based on the integration of sound energy over time.NEW & NOTEWORTHY Using single-neuron electrophysiology, we show that the precise shape of a sound's "energy envelope" is a critical factor in determining how well midbrain neurons are able to convey information about auditory spatial cues. Consistent with human behavioral performance, sounds with rapidly rising energy and relatively long intervals between energy bursts are best at conveying spatial information. The data suggest specific sound energy patterns that might best be applied to hearing devices to aid spatial listening.

Cortical Representation of Interaural Time Difference Is Impaired by Deafness in Development: Evidence from Children with Early Long-term Access to Sound through Bilateral Cochlear Implants Provided Simultaneously.

Accurate use of interaural time differences (ITDs) for spatial hearing may require access to bilateral auditory input during sensitive periods in human development. Providing bilateral cochlear implants (CIs) simultaneously promotes symmetrical development of bilateral auditory pathways but does not support normal ITD sensitivity. Thus, although binaural interactions are established by bilateral CIs in the auditory brainstem, potential deficits in cortical processing of ITDs remain. Cortical ITD processing in children with simultaneous bilateral CIs and normal hearing with similar time-in-sound was explored in the present study. Cortical activity evoked by bilateral stimuli with varying ITDs (0, ±0.4, ±1 ms) was recorded using multichannel electroencephalography. Source analyses indicated dominant activity in the right auditory cortex in both groups but limited ITD processing in children with bilateral CIs. In normal-hearing children, adult-like processing patterns were found underlying the immature P1 (∼100 ms) response peak with reduced activity in the auditory cortex ipsilateral to the leading ITD. Further, the left cortex showed a stronger preference than the right cortex for stimuli leading from the contralateral hemifield. By contrast, children with CIs demonstrated reduced ITD-related changes in both auditory cortices. Decreased parieto-occipital activity, possibly involved in spatial processing, was also revealed in children with CIs. Thus, simultaneous bilateral implantation in young children maintains right cortical dominance during binaural processing but does not fully overcome effects of deafness using present CI devices. Protection of bilateral pathways through simultaneous implantation might be capitalized for ITD processing with signal processing advances, which more consistently represent binaural timing cues.SIGNIFICANCE STATEMENT Multichannel electroencephalography demonstrated impairment of binaural processing in children who are deaf despite early access to bilateral auditory input by first finding that foundations for binaural hearing are normally established during early stages of cortical development. Although 4- to 7-year-old children with normal hearing had immature cortical responses, adult patterns in cortical coding of binaural timing cues were measured. Second, children receiving two cochlear implants in the same surgery maintained normal-like input from both ears, but this did not support significant effects of binaural timing cues in either auditory cortex. Deficits in parieto-occiptal areas further suggested impairment in spatial processing. Results indicate that cochlear implants working independently in each ear do not fully overcome deafness-related binaural processing deficits, even after long-term experience.