Sound Segregation Research Articles

Age-Related Changes in Processing Simultaneous Amplitude Modulated Sounds Assessed Using Envelope Following Responses.

Listening conditions in the real world involve segregating the stimuli of interest from competing auditory stimuli that differ in their sound level and spectral content. It is in these conditions of complex spectro-temporal processing that listeners with age-related hearing loss experience the most difficulties. Envelope following responses (EFRs) provide objective neurophysiological measures of auditory processing. EFRs were obtained to two simultaneous sinusoidally amplitude modulated (sAM) tones from young and aged Fischer-344 rats. One was held at a fixed suprathreshold sound level (sAM1FL) while the second varied in sound level (sAM2VL) and carrier frequency. EFR amplitudes to sAM1FL in the young decreased with signal-to-noise ratio (SNR), and this reduction was more pronounced when the sAM2VL carrier frequency was spectrally separated from sAM1FL. Aged animals showed similar trends, while having decreased overall response amplitudes compared to the young. These results were replicated using an established computational model of the auditory nerve. The trends observed in the EFRs were shown to be due to the contributions of the low-frequency tails of high-frequency neurons, rather than neurons tuned to the sAM1FL carrier frequency. Modeling changes in threshold and neural loss reproduced some of the changes seen with age, but accuracy improved when combined with an additional decrease representing synaptic loss of auditory nerve neurons. Sound segregation in this case derives primarily from peripheral processing, regardless of age. Contributions by more central neural mechanisms are likely to occur only at low SNRs.

Everyday Listening Performance of Children Before and After Receiving a Second Cochlear Implant: Results Using the Parent Version of the Speech, Spatial, and Qualities of Hearing Scale.

To evaluate change in individual children's performance in general areas of everyday listening following sequential bilateral implantation, and to identify the specific types of listening scenarios in which performance change occurred. The first hypothesis was that parent performance ratings for their child would be higher in the bilateral versus unilateral implant condition for each section of the speech, spatial and qualities of hearing scale for parents, viz.: speech perception, spatial hearing, and qualities of hearing. The second hypothesis was that the rating for the participant group would be higher in the bilateral condition for speech perception items involving group conversation or background noise, spatial hearing items, and qualities of hearing items focused on sound segregation or listening effort. Children receiving sequential bilateral implants at the Royal Victorian Eye and Ear Hospital and fulfilling selection criteria (primarily no significant cognitive or developmental delays, and oral English language skills of child and parent sufficient for completing assessments) were invited to participate in a wider project evaluating outcomes. The assessment protocol for older children included the speech, spatial, and qualities of hearing scale for parents. All children (n = 20; ages 4 to 15 years) whose parents completed the scale preoperatively and at 24-months postoperatively were included in this study. Ratings obtained preoperatively in the unilateral implant condition (or unilateral implant plus hearing aid for 4 participants) were compared with those obtained postoperatively in the bilateral implant condition. Bilateral ratings were significantly higher than unilateral ratings on the speech section for 12 children (W ≥ 7.0; p ≤ 0.03), on the spatial section for 13 children (W ≥ 15.0; p ≤ 0.03), and on the qualities of hearing section for 9 children (W ≥ 15.0; p ≤ 0.047). The difference between conditions was unrelated to time between implants or age at bilateral implantation (r ≤ 0.4; p ≥ 0.082). The median bilateral ratings for the participant group were higher for all eight speech perception items, including, as predicted, those involving group conversation and/or background noise (W ≥ 37.5; p ≤ 0.043). Also, as predicted, the median bilateral ratings for the participant group were higher for all six spatial hearing items (W ≥ 88.0; p ≤ 0.014), and for qualities of hearing items related to sound segregation (W ≥ 94.0; p ≤ 0.029), but not for those related to listening effort (W ≤ 92.0; p ≥ 0.112). Seventy-five percentage of parents perceived change in their child's daily listening performance postoperatively, and 25% perceived change across all three listening areas. For the overall participant group, the parents perceived a change in performance in the majority of specific listening scenarios, although change was limited in the qualities of hearing section, including no change in listening effort. Previous research suggests postoperative change was likely due to the headshadow effect and improved spatial hearing. Additional contributions may have been made by binaural summation, redundancy, and unmasking. For these participants, differences between device conditions may have been limited by their relatively old age at implantation, delay between implants, and limited bilateral experience. These results will provide valuable information to families during preoperative counseling and postoperative discussions about expected progress and evident benefit.

Sequential streaming, binaural cues and lateralization.

Interaural time differences (ITDs) and interaural level differences (ILDs) associated with monaural spectral differences (coloration) enable the localization of sound sources. The influence of these spatial cues as well as their relative importance on obligatory stream segregation were assessed in experiment 1. A temporal discrimination task favored by integration was used to measure obligatory stream segregation for sequences of speech-shaped noises. Binaural and monaural differences associated with different spatial positions increased discrimination thresholds, indicating that spatial cues can induce stream segregation. The results also demonstrated that ITDs and coloration were relatively more important cues compared to ILDs. Experiment 2 questioned whether sound segregation takes place at the level of acoustic cue extraction (ITD per se) or at the level of object formation (perceived azimuth). A difference in ITDs between stimuli was introduced either consistently or inconsistently across frequencies, leading to clearly lateralized sounds or blurred lateralization, respectively. Conditions with ITDs and clearly perceived azimuths induced significantly more segregation than the condition with ITDs but reduced lateralization. The results suggested that segregation was mainly based on a difference in lateralization, although the extraction of ITDs might have also helped segregation up to a ceiling magnitude.

Concurrent sound segregation impairments in schizophrenia: The contribution of auditory-specific and general cognitive factors

The present study sought to test whether perceptual segregation of concurrently played sounds is impaired in schizophrenia (SZ), whether impairment in sound segregation predicts difficulties with a real-world speech-in-noise task, and whether auditory-specific or general cognitive processing accounts for sound segregation problems. Participants with SZ and healthy controls (HCs) performed a mistuned harmonic segregation task during recording of event-related potentials (ERPs). Participants also performed a brief speech-in-noise task. Participants with SZ showed deficits in the mistuned harmonic task and the speech-in-noise task, compared to HCs. No deficit in SZ was found in the ERP component related to mistuned harmonic segregation at around 150ms (the object-related negativity or ORN), but instead showed a deficit in processing at around 400ms (the P4 response). However, regression analyses showed that indexes of education level and general cognitive function were the best predictors of sound segregation difficulties, suggesting non-auditory specific causes of concurrent sound segregation problems in SZ.

Attentional Capacity Limits Gap Detection during Concurrent Sound Segregation.

Detecting a brief silent interval (i.e., a gap) is more difficult when listeners perceive two concurrent sounds rather than one in a sound containing a mistuned harmonic in otherwise in-tune harmonics. This impairment in gap detection may reflect the interaction of low-level encoding or the division of attention between two sound objects, both of which could interfere with signal detection. To distinguish between these two alternatives, we compared ERPs during active and passive listening with complex harmonic tones that could include a gap, a mistuned harmonic, both features, or neither. During active listening, participants indicated whether they heard a gap irrespective of mistuning. During passive listening, participants watched a subtitled muted movie of their choice while the same sounds were presented. Gap detection was impaired when the complex sounds included a mistuned harmonic that popped out as a separate object. The ERP analysis revealed an early gap-related activity that was little affected by mistuning during the active or passive listening condition. However, during active listening, there was a marked decrease in the late positive wave that was thought to index attention and response-related processes. These results suggest that the limitation in detecting the gap is related to attentional processing, possibly divided attention induced by the concurrent sound objects, rather than deficits in preattentional sensory encoding.

Rapid Task-Related Plasticity of Spectrotemporal Receptive Fields in the Auditory Midbrain.

Previous research has demonstrated that auditory cortical neurons can modify their receptive fields when animals engage in auditory detection tasks. We tested for this form of task-related plasticity in the inferior colliculus (IC) of ferrets trained to detect a pure tone target in a sequence of noise distractors that did not overlap in time. During behavior, responses were suppressed at the target tone frequency in approximately half of IC neurons relative to the passive state. This suppression often resulted from a combination of a local tuning change and a global change in overall excitability. Local and global suppression were stronger when the target frequency was aligned to neuronal best frequency. Local suppression in the IC was indistinguishable from that described previously in auditory cortex, while global suppression was unique to the IC. The results demonstrate that engaging in an auditory task can change selectivity for task-relevant features in the midbrain, an area where these effects have not been reported previously. Significance statement: Previous studies have demonstrated that the receptive fields of cortical neurons are modified when animals engage in auditory behaviors, a process that is hypothesized to provide the basis for segregating sound sources in an auditory scene. This study demonstrates for the first time that receptive fields of neurons in the midbrain inferior colliculus are also modified during behavior. The magnitude of the tuning changes is similar to previous reports in cortex. These results support a hierarchical model of behaviorally driven sound segregation that begins in the subcortical auditory network.

Sound stream segregation: a neuromorphic approach to solve the "cocktail party problem" in real-time.

The human auditory system has the ability to segregate complex auditory scenes into a foreground component and a background, allowing us to listen to specific speech sounds from a mixture of sounds. Selective attention plays a crucial role in this process, colloquially known as the “cocktail party effect.” It has not been possible to build a machine that can emulate this human ability in real-time. Here, we have developed a framework for the implementation of a neuromorphic sound segregation algorithm in a Field Programmable Gate Array (FPGA). This algorithm is based on the principles of temporal coherence and uses an attention signal to separate a target sound stream from background noise. Temporal coherence implies that auditory features belonging to the same sound source are coherently modulated and evoke highly correlated neural response patterns. The basis for this form of sound segregation is that responses from pairs of channels that are strongly positively correlated belong to the same stream, while channels that are uncorrelated or anti-correlated belong to different streams. In our framework, we have used a neuromorphic cochlea as a frontend sound analyser to extract spatial information of the sound input, which then passes through band pass filters that extract the sound envelope at various modulation rates. Further stages include feature extraction and mask generation, which is finally used to reconstruct the targeted sound. Using sample tonal and speech mixtures, we show that our FPGA architecture is able to segregate sound sources in real-time. The accuracy of segregation is indicated by the high signal-to-noise ratio (SNR) of the segregated stream (90, 77, and 55 dB for simple tone, complex tone, and speech, respectively) as compared to the SNR of the mixture waveform (0 dB). This system may be easily extended for the segregation of complex speech signals, and may thus find various applications in electronic devices such as for sound segregation and speech recognition.

Neural realignment of spatially separated sound components.

Natural auditory scenes often consist of several sound sources overlapping in time, but separated in space. Yet, location is not fully exploited in auditory grouping: spatially separated sounds can get perceptually fused into a single auditory object and this leads to difficulties in the identification and localization of concurrent sounds. Here, the brain mechanisms responsible for grouping across spatial locations were explored in magnetoencephalography (MEG) recordings. The results show that the cortical representation of a vowel spatially separated into two locations reflects the perceived location of the speech sound rather than the physical locations of the individual components. In other words, the auditory scene is neurally rearranged to bring components into spatial alignment when they were deemed to belong to the same object. This renders the original spatial information unavailable at the level of the auditory cortex and may contribute to difficulties in concurrent sound segregation.

Enhanced segregation of concurrent sounds with similar spectral uncertainties in individuals with autism spectrum disorder.

When acoustic signals from different sound sources are mixed upon arrival at the ears, the auditory system organizes these acoustic elements by their features. This study shows that individuals with autism spectrum disorder (ASD) performed better in terms of hearing a target sequence among distractors that had similar spectral uncertainties. Their superior performance in this task indicates an enhanced discrimination between auditory streams with the same spectral uncertainties but different spectro-temporal details. The enhanced discrimination of acoustic components may be related to the absence of the automatic grouping of acoustic components with the same features, which results in difficulties in speech perception in a noisy environment. On the other hand, the ASD group and the control group had similar performance in hearing a target sequence among distractors that had different spatial cues defined by interaural intensity differences.

Triboacoustic Localization for Mobile Device: Improving Accuracy & Noise Clustering

<div class=""abs_img""> <img src=""[disp_template_path]/JRM/abst-image/00270002/10.jpg"" width=""300"" /> Mobile localization: grouping</div> Mobile devices are caught in an inverse relationship between mobility and ease of use. This paper presents incremental technology related to a previously proposed method of mobile yet easy-to-use input using triboacoustic signals generated from the action of a user tracing shapes on surfaces in environment. Mobile devices must function in various environments, thus requiring that they be immune to noise interference. We propose an improvement in accuracy and automated multiple sound source segregation supported by experiments evidencing the proposal’ s effectiveness, results of which show the proposal’ s accuracy and capability have merit and should be pursued further. </span>

The cocktail-party problem revisited: early processing and selection of multi-talker speech.

How do we recognize what one person is saying when others are speaking at the same time? This review summarizes widespread research in psychoacoustics, auditory scene analysis, and attention, all dealing with early processing and selection of speech, which has been stimulated by this question. Important effects occurring at the peripheral and brainstem levels are mutual masking of sounds and “unmasking” resulting from binaural listening. Psychoacoustic models have been developed that can predict these effects accurately, albeit using computational approaches rather than approximations of neural processing. Grouping—the segregation and streaming of sounds—represents a subsequent processing stage that interacts closely with attention. Sounds can be easily grouped—and subsequently selected—using primitive features such as spatial location and fundamental frequency. More complex processing is required when lexical, syntactic, or semantic information is used. Whereas it is now clear that such processing can take place preattentively, there also is evidence that the processing depth depends on the task-relevancy of the sound. This is consistent with the presence of a feedback loop in attentional control, triggering enhancement of to-be-selected input. Despite recent progress, there are still many unresolved issues: there is a need for integrative models that are neurophysiologically plausible, for research into grouping based on other than spatial or voice-related cues, for studies explicitly addressing endogenous and exogenous attention, for an explanation of the remarkable sluggishness of attention focused on dynamically changing sounds, and for research elucidating the distinction between binaural speech perception and sound localization.

Performance ratings for children using bilateral cochlear implants obtained with the Speech, Spatial, and Other Qualities of Hearing Scale for parents

Introduction Researchers at The University of Melbourne are evaluating the broader outcomes of bilateral implantation in early implanted school-age children, including effects on parent and child stress (Sarant and Garrard, 2013), language (Sarant et al., 2014), psychosocial functioning, academic achievement, and everyday listening. The Speech, Spatial, and Other Qualities of Hearing Scale (SSQ) is a three-part adult questionnaire (http://www.ihr.mrc.ac.uk/products/ display/ssq; Gatehouse and Noble, 2004). The SSQ focusses mainly on hearing functions requiring the binaural system and is therefore suitable for evaluating everyday listening performance with bilateral implants. The SSQ-P is an adapted version for use with parents of children with hearing impairment (Galvin and Noble, 2013). The first section of the SSQ focusses on speech perception in different listening scenarios, such as in group conversation in a noisy environment, or a single speaker in a reverberant environment. The second section focusses on spatial hearing, in this case the direction of movement and location of sound sources, including the speaker in a group conversation. The final section focusses on other qualities of hearing, including sound segregation, listening effort, and sound and object identification.

Hierarchical neurocomputations underlying concurrent sound segregation: Connecting periphery to percept

Natural soundscapes often contain multiple sound sources at any given time. Numerous studies have reported that in human observers, the perception and identification of concurrent sounds is paralleled by specific changes in cortical event-related potentials (ERPs). Although these studies provide a window into the cerebral mechanisms governing sound segregation, little is known about the subcortical neural architecture and hierarchy of neurocomputations that lead to this robust perceptual process. Using computational modeling, scalp-recorded brainstem/cortical ERPs, and human psychophysics, we demonstrate that a primary cue for sound segregation, i.e., harmonicity, is encoded at the auditory nerve level within tens of milliseconds after the onset of sound and is maintained, largely untransformed, in phase-locked activity of the rostral brainstem. As then indexed by auditory cortical responses, (in)harmonicity is coded in the signature and magnitude of the cortical object-related negativity (ORN) response (150–200ms). The salience of the resulting percept is then captured in a discrete, categorical-like coding scheme by a late negativity response (N5; ~500ms latency), just prior to the elicitation of a behavioral judgment. Subcortical activity correlated with cortical evoked responses such that weaker phase-locked brainstem responses (lower neural harmonicity) generated larger ORN amplitude, reflecting the cortical registration of multiple sound objects. Studying multiple brain indices simultaneously helps illuminate the mechanisms and time-course of neural processing underlying concurrent sound segregation and may lead to further development and refinement of physiologically driven models of auditory scene analysis.

A function for binaural integration in auditory grouping and segregation in the inferior colliculus.

Responses of neurons to binaural, harmonic complex stimuli in urethane-anesthetized guinea pig inferior colliculus (IC) are reported. To assess the binaural integration of harmonicity cues for sound segregation and grouping, responses were measured to harmonic complexes with different fundamental frequencies presented to each ear. Simultaneously gated harmonic stimuli with fundamental frequencies of 125 Hz and 145 Hz were presented to the left and right ears, respectively, and recordings made from 96 neurons with characteristic frequencies >2 kHz in the central nucleus of the IC. Of these units, 70 responded continuously throughout the stimulus and were excited by the stimulus at the contralateral ear. The stimulus at the ipsilateral ear excited (EE: 14%; 10/70), inhibited (EI: 33%; 23/70), or had no significant effect (EO: 53%; 37/70), defined by the effect on firing rate. The neurons phase locked to the temporal envelope at each ear to varying degrees depending on signal level. Many of the cells (predominantly EO) were dominated by the response to the contralateral stimulus. Another group (predominantly EI) synchronized to the contralateral stimulus and were suppressed by the ipsilateral stimulus in a phasic manner. A third group synchronized to the stimuli at both ears (predominantly EE). Finally, a group only responded when the waveform peaks from each ear coincided. We conclude that these groups of neurons represent different "streams" of information but exhibit modifications of the response rather than encoding a feature of the stimulus, like pitch.

Neural representation of concurrent harmonic sounds in monkey primary auditory cortex: implications for models of auditory scene analysis.

The ability to attend to a particular sound in a noisy environment is an essential aspect of hearing. To accomplish this feat, the auditory system must segregate sounds that overlap in frequency and time. Many natural sounds, such as human voices, consist of harmonics of a common fundamental frequency (F0). Such harmonic complex tones (HCTs) evoke a pitch corresponding to their F0. A difference in pitch between simultaneous HCTs provides a powerful cue for their segregation. The neural mechanisms underlying concurrent sound segregation based on pitch differences are poorly understood. Here, we examined neural responses in monkey primary auditory cortex (A1) to two concurrent HCTs that differed in F0 such that they are heard as two separate "auditory objects" with distinct pitches. We found that A1 can resolve, via a rate-place code, the lower harmonics of both HCTs, a prerequisite for deriving their pitches and for their perceptual segregation. Onset asynchrony between the HCTs enhanced the neural representation of their harmonics, paralleling their improved perceptual segregation in humans. Pitches of the concurrent HCTs could also be temporally represented by neuronal phase-locking at their respective F0s. Furthermore, a model of A1 responses using harmonic templates could qualitatively reproduce psychophysical data on concurrent sound segregation in humans. Finally, we identified a possible intracortical homolog of the "object-related negativity" recorded noninvasively in humans, which correlates with the perceptual segregation of concurrent sounds. Findings indicate that A1 contains sufficient spectral and temporal information for segregating concurrent sounds based on differences in pitch.

Resolution of interaural time differences in the avian sound localization circuit-a modeling study.

Interaural time differences (ITDs) are a main cue for sound localization and sound segregation. A dominant model to study ITD detection is the sound localization circuitry in the avian auditory brainstem. Neurons in nucleus laminaris (NL) receive auditory information from both ears via the avian cochlear nucleus magnocellularis (NM) and compare the relative timing of these inputs. Timing of these inputs is crucial, as ITDs in the microsecond range must be discriminated and encoded. We modeled ITD sensitivity of single NL neurons based on previously published data and determined the minimum resolvable ITD for neurons in NL. The minimum resolvable ITD is too large to allow for discrimination by single NL neurons of naturally occurring ITDs for very low frequencies. For high frequency NL neurons (>1 kHz) our calculated ITD resolutions fall well within the natural range of ITDs and approach values of below 10 μs. We show that different parts of the ITD tuning function offer different resolution in ITD coding, suggesting that information derived from both parts may be used for downstream processing. A place code may be used for sound location at frequencies above 500 Hz, but our data suggest the slope of the ITD tuning curve ought to be used for ITD discrimination by single NL neurons at the lowest frequencies. Our results provide an important measure of the necessary temporal window of binaural inputs for future studies on the mechanisms and development of neuronal computation of temporally precise information in this important system. In particular, our data establish the temporal precision needed for conduction time regulation along NM axons.

Sound-by-sound thalamic stimulation modulates midbrain auditory excitability and relative binaural sensitivity in frogs.

Descending circuitry can modulate auditory processing, biasing sensitivity to particular stimulus parameters and locations. Using awake in vivo single unit recordings, this study tested whether electrical stimulation of the thalamus modulates auditory excitability and relative binaural sensitivity in neurons of the amphibian midbrain. In addition, by using electrical stimuli that were either longer than the acoustic stimuli (i.e., seconds) or presented on a sound-by-sound basis (ms), experiments addressed whether the form of modulation depended on the temporal structure of the electrical stimulus. Following long duration electrical stimulation (3–10 s of 20 Hz square pulses), excitability (spikes/acoustic stimulus) to free-field noise stimuli decreased by 32%, but returned over 600 s. In contrast, sound-by-sound electrical stimulation using a single 2 ms duration electrical pulse 25 ms before each noise stimulus caused faster and varied forms of modulation: modulation lasted <2 s and, in different cells, excitability either decreased, increased or shifted in latency. Within cells, the modulatory effect of sound-by-sound electrical stimulation varied between different acoustic stimuli, including for different male calls, suggesting modulation is specific to certain stimulus attributes. For binaural units, modulation depended on the ear of input, as sound-by-sound electrical stimulation preceding dichotic acoustic stimulation caused asymmetric modulatory effects: sensitivity shifted for sounds at only one ear, or by different relative amounts for both ears. This caused a change in the relative difference in binaural sensitivity. Thus, sound-by-sound electrical stimulation revealed fast and ear-specific (i.e., lateralized) auditory modulation that is potentially suited to shifts in auditory attention during sound segregation in the auditory scene.

Spiking neural network model of cortical auditory source segregation

Humans have the remarkable ability to tune into a particular voice even in loud noisy environments. The neural underpinnings of this amazing perceptual phenomenon are not yet fully understood. Recent EcoG [1] and MEG studies [2] have established that the neural representation of the attended speaker’s speech is much stronger than the unattended (distractor) speech when human subjects are asked to pay attention to a target speaker in a mixture of speech. How the brain sieves through the mixture waveform to enhance the target speaker’s speech and attenuate the background acoustic scene is still being investigated. In this work, we propose a spiking neural network architecture based on the theory of temporal coherence [3] to achieve auditory source segregation. Our model does not require training on the background noise or prior exposure to the target speech. Along with using bottom-up spectro-temporal features and pitch features, the model can also accommodate top-down attentional mechanisms to generate segregated phase locked neural representations to target speaker’s speech envelope. The model comprises of a feature extraction stage followed by clustering stage. The feature extraction stage mimics the auditory pathway starting from a cochlear representation followed by a multi-resolution analysis of the cochlear output using a bank of band-pass filters (cortical stage), to provide a rich timbre representation. Dominant pitch tracks are extracted from the sound mixture and processed through the same set of band-pass filters as the timbre channels. The output of the feature extraction stage comprising of the pitch and timbre channels are transduced into a spike-based representation using leaky integrate and fire neurons with time constants tuned to the bandwidth of the multi-resolution band-pass filters. The clustering stage comprises of a bank of coincidence detector neurons. Using the pitch signals as anchors the coincidence detector neurons can segregate the two sources from the mixture timbre representation. Thus, the output of the coincidence detector neurons comprises only of responses phase locked to the envelope of a single source. This model does not require any weight learning, is unsupervised and can segregate sources online. Previous studies on correlation based sound segregation employed network of neurons with intrinsic oscillator dynamics [4]. In this work, clustering of features belonging to a single source is driven only by the temporal coherence of spectro-temporal features of the given source. This spike-based representation provides an easy mechanism to group coherent features, which otherwise would require computationally expensive numerical routines for online, adaptive principal components analysis. Future work is aimed at reconstructing the speech waveform from the segregated spike trains.

Effects of multiple congruent cues on concurrent sound segregation during passive and active listening: An event-related potential (ERP) study

In two experiments, we assessed the effects of combining different cues of concurrent sound segregation on the object-related negativity (ORN) and the P400 event-related potential components. Participants were presented with sequences of complex tones, half of which contained some manipulation: one or two harmonic partials were mistuned, delayed, or presented from a different location than the rest. In separate conditions, one, two, or three of these manipulations were combined. Participants watched a silent movie (passive listening) or reported after each tone whether they perceived one or two concurrent sounds (active listening). ORN was found in almost all conditions except for location difference alone during passive listening. Combining several cues or manipulating more than one partial consistently led to sub-additive effects on the ORN amplitude. These results support the view that ORN reflects a combined, feature-unspecific assessment of the auditory system regarding the contribution of two sources to the incoming sound.

Effects of spectral degradation on attentional modulation of auditory responses to continuous speech

Auditory attention enhances phase-locked responses to the temporal envelope of the attended speech stream in a competing background. This study investigates how cortical responses are affected by spectral degradation and the extent to which low-frequency fine-structure cues facilitate sound segregation. Two competing speech streams were presented diotically to normal-hearing subjects. Each subject was tested with unprocessed speech, vocoder speech (8 to 64 channels), and six-channel high-frequency vocoder + lowpass filtered speech simulating electric-acoustic stimulation (EAS). Ongoing EEG responses were measured in each condition. Cross-correlation between speech envelope and EEG responses was calculated at different time lags. For unprocessed speech, the cross-correlation function showed opposite signs of correlations between the attended and unattended speech, supporting the neural mechanism of suppression of the competing speech stream. This suppression mechanism, however, was only evident for the 32- and 64-channel vocoder conditions, but not for the 8- and 16-channel conditions. This indicates that greater frequency resolution is required for sound segregation. For EAS, the pattern of cortical responses was similar to that of the eight-channel vocoder condition, although speech intelligibility was higher in EAS than vocoder speech. Consistent with previous psychoacoustic evidence, these neural results further argue against the segregation mechanism for EAS benefit.