Changes In Interaural Correlation Research Articles

Does sensitivity to interaural correlation changes correlate with binaural fusion performance?

The purpose of this study was to assess whether and to what degree sensitivity to changes in interaural correlation (IC) in bilateral cochlear implant (CI) listeners is related to the overall degree of binaural fusion those listeners experience. Compared to typical-hearing listeners, CI users often report perceiving a reduced degree of binaural fusion. Because IC is a strong determiner of perceived fusion, degraded perception of fusion might be coupled with a relative insensitivity to changes in IC. To investigate this, two experiments were conducted. First, discrimination of IC changes was measured using the method of constant stimuli in which CI listeners identified the “target” interval containing an IC differing from the “reference” values of either 0.7 or 1.0. Stimuli were generated by modulating 1-kHz pulse trains by speech-derived envelopes the IC of which were manipulated. The same stimuli were used in a second procedure in which the perceived degree of fusion was quantified for ICs varying between 0.4–1.0. Preliminary results from both studies suggest that relative insensitivity to changes in IC are associated with a lesser degree of perceived fusion, overall.

Responses to dichotic tone-in-noise stimuli in the inferior colliculus.

Human listeners are more sensitive to tones embedded in diotic noise when the tones are out-of-phase at the two ears (N0Sπ) than when they are in-phase (N0S0). The difference between the tone-detection thresholds for these two conditions is referred to as the binaural masking level difference (BMLD) and reflects a benefit of binaural processing. Detection in the N0Sπ condition has been explained in modeling studies by changes in interaural correlation (IAC), but this model has only been directly tested physiologically for low frequencies. Here, the IAC-based hypothesis for binaural detection was examined across a wide range of frequencies and masker levels using recordings in the awake rabbit inferior colliculus (IC). IAC-based cues were strongly correlated with neural responses to N0Sπ stimuli. Additionally, average rate-based thresholds were calculated for both N0S0 and N0Sπ conditions. The rate-based neural BMLD at 500 Hz matched rabbit behavioral data, but the trend of neural BMLDs across frequency differed from that of humans.

Large group differences in binaural sensitivity are represented in preattentive responses from auditory cortex.

Correlated sounds presented to two ears are perceived as compact and centrally lateralized, whereas decorrelation between ears leads to intracranial image widening. Though most listeners have fine resolution for perceptual changes in interaural correlation (IAC), some investigators have reported large variability in IAC thresholds, and some normal-hearing listeners even exhibit seemingly debilitating IAC thresholds. It is unknown whether or not this variability across individuals and outlier manifestations are a product of task difficulty, poor training, or a neural deficit in the binaural auditory system. The purpose of this study was first to identify listeners with normal and abnormal IAC resolution, second to evaluate the neural responses elicited by IAC changes, and third to use a well-established model of binaural processing to determine a potential explanation for observed individual variability. Nineteen subjects were enrolled in the study, eight of whom were identified as poor performers in the IAC-threshold task. Global scalp responses (N1 and P2 amplitudes of an auditory change complex) in the individuals with poor IAC behavioral thresholds were significantly smaller than for listeners with better IAC resolution. Source-localized evoked responses confirmed this group effect in multiple subdivisions of the auditory cortex, including Heschl's gyrus, planum temporale, and the temporal sulcus. In combination with binaural modeling results, this study provides objective electrophysiological evidence of a binaural processing deficit linked to internal noise, that corresponds to very poor IAC thresholds in listeners that otherwise have normal audiometric profiles and lack spatial hearing complaints.NEW & NOTEWORTHY Group differences in the perception of interaural correlation (IAC) were observed in human adults with normal audiometric sensitivity. These differences were reflected in cortical-evoked activity measured via electroencephalography (EEG). For some participants, weak representation of the binaural cue at the cortical level in preattentive N1-P2 cortical responses may be indicative of a potential processing deficit. Such a deficit may be related to a poorly understood condition known as hidden hearing loss.

Impaired interaural correlation processing in people with schizophrenia.

Detection of transient changes in interaural correlation is based on the temporal precision of the central representations of acoustic signals. Whether schizophrenia impairs the temporal precision in the interaural correlation process is not clear. In both participants with schizophrenia and matched healthy-control participants, this study examined the detection of a break in interaural correlation (BIC, a change in interaural correlation from 1 to 0 and back to 1), including the longest interaural delay at which a BIC was just audible, representing the temporal extent of the primitive auditory memory (PAM). Moreover, BIC-induced electroencephalograms (EEGs) and the relationships between the early binaural psychoacoustic processing and higher cognitive functions, which were assessed by the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), were examined. The results showed that compared to healthy controls, participants with schizophrenia exhibited poorer BIC detection, PAM and RBANS score. Both the BIC-detection accuracy and the PAM extent were correlated with the RBANS score. Moreover, participants with schizophrenia showed weaker BIC-induced N1-P2 amplitude which was correlated with both theta-band power and inter-trial phase coherence. These results suggested that schizophrenia impairs the temporal precision of the central representations of acoustic signals, affecting both interaural correlation processing and higher-order cognitions.

Effects of interaural delay, center frequency, and no more than "slight" hearing loss on precision of binaural processing: Empirical data and quantitative modeling.

This study assessed how precision of binaural processing is affected by center frequency (CF), interaural temporal disparity (ITD), and listeners' hearing status. Tonal signals and 100-Hz-wide Gaussian noise maskers were employed at CFs ranging between 250 and 8000 Hz, in octave steps. In addition, for CFs of 2000, 4000, and 8000 Hz, transposed maskers and signals were employed. All listeners had no greater than "slight" hearing losses (i.e., no thresholds greater than 25 dB HL). Across all CFs and ITDs tested, binaural detection thresholds were elevated for listeners whose absolute thresholds at 4 kHz exceeded 7.5 dB HL. That outcome is consistent with results from Bernstein and Trahiotis [(2016). J. Acoust. Soc. Am. 140, 3540-3548]. Quantitative predictions of binaural detection thresholds derived via a comprehensive interaural correlation-based model of binaural processing were highly accurate across the entire set of data. The modeling results suggest that elevated thresholds from listeners having small hearing losses stem from elevated levels of stimulus-dependent, additive internal noise. They do not appear to stem from increased levels of noise within the central binaural comparator or from reduced sensitivity to changes in interaural correlation produced by the addition of the signal to the masker.

Binaural detection-based estimates of precision of coding of interaural temporal disparities across center frequency

This presentation reports binaural detection data obtained using three masking configurations previously shown, in toto, to reveal: (1) whether listeners can compensate internally for external interaural temporal disparities (ITDs) and (2) the precision with which such compensation of ITD changes as a function of reference ITD [L. R. Bernstein and C. Trahiotis, J. Acoust. Soc. Am. 138, EL474-EL479 (2015)]. The current experiment extended that paradigm in order to obtain binaural detection data at center frequencies that ranged in octave steps from 250 Hz to 8 kHz. Results indicate that, at all center frequencies, listeners are able to compensate internally for externally imposed ITDs. Such compensation allows them to outperform what would be expected were they only utilizing changes in interaural correlation produced by the addition of the tonal signal to the noise masker. The data will be discussed in terms of quantitative analyses that show how detection performance is constrained by two distinct sources of interaurally and mutually uncorrelated internal noise: one having a power that increases with the power of the external masker and the other having a power that grows with the magnitude of the ITD.

Effects of level roving and overall level on correlation change discrimination in naïve and trained listeners

Sensitivity to interaural differences is remarkably variable across individuals, even highly trained listeners. In experiment 1, we measured sensitivity to changes in interaural correlation in 28 naïve listeners to determine the expected inter-individual variability for this task. Stimuli were 65-dB-A, 10-Hz narrowband noises with a 500- or 4000-Hz center frequency. Stimuli were tested either without level roving, or with ±5 dB of level roving in an effort to force listeners to attend to binaural width/movement cues rather than binaural loudness cues. At 500 Hz without level roving, the performance for the naïve listeners was much worse than the performance for the highly trained listeners previously reported in the literature. The addition of level roving significantly degraded performance, suggesting a close relationship between detection of decorrelation and loudness. At 4 kHz with and without level roving, none of the naïve listeners could perform the task. In experiment 2, we measured correlation change sensitivity as a function of overall level in trained listeners. Performance improved as overall level increased to about 65 dB-A, then worsened for more intense levels. The data will be discussed in terms of confusions between width and loudness cues, and in terms of Weber’s law.

Sensitivity to envelope-based interaural delays at high frequencies: center frequency affects the envelope rate-limitation.

Sensitivity to ongoing interaural temporal disparities (ITDs) was measured using bandpass-filtered pulse trains centered at 4600, 6500, or 9200 Hz. Save for minor differences in the exact center frequencies, those target stimuli were those employed by Majdak and Laback [J. Acoust. Soc. Am. 125, 3903-3913 (2009)]. At each center frequency, threshold ITD was measured for pulse repetition rates ranging from 64 to 609 Hz. The results and quantitative predictions by a cross-correlation-based model indicated that (1) at most pulse repetition rates, threshold ITD increased with center frequency, (2) the cutoff frequency of the putative envelope low-pass filter that determines sensitivity to ITD at high envelope rates appears to be inversely related to center frequency, and (3) both outcomes were accounted for by assuming that, independent of the center frequency, the listeners' decision variable was a constant criterion change in interaural correlation of the stimuli as processed internally. The finding of an inverse relation between center frequency and the envelope rate limitation, while consistent with much prior literature, runs counter to the conclusion reached by Majdak and Laback.

The Interaural Time Difference Pathway: a Comparison of Spectral Bandwidth and Correlation Sensitivity at Three Anatomical Levels

Temporal differences between the two ears are critical for spatial hearing. They can be described along axes of interaural time difference (ITD) and interaural correlation, and their processing starts in the brainstem with the convergence of monaural pathways which are tuned in frequency and which carry temporal information. In previous studies, we examined the bandwidth (BW) of frequency tuning at two stages: the auditory nerve (AN) and inferior colliculus (IC), and showed that BW depends on characteristic frequency (CF) but that there is no difference in the mean BW of these two structures when measured in a binaural, temporal framework. This suggested that there is little frequency convergence in the ITD pathway between AN and IC and that frequency selectivity determined by the cochlear filter is preserved up to the IC. Unexpectedly, we found that AN and IC neurons can be similar in CF and BW, yet responses to changes in interaural correlation in the IC were different than expected from coincidence patterns ("pseudo-binaural" responses) in the AN. To better understand this, we here examine the responses of bushy cells, which provide monaural inputs to binaural neurons. Using broadband noise, we measured BW and correlation sensitivity in the cat trapezoid body (TB), which contains the axons of bushy cells. This allowed us to compare these two metrics at three stages in the ITD pathway. We found that BWs in the TB are similar to those in the AN and IC. However, TB neurons were found to be more sensitive to changes in stimulus correlation than AN or IC neurons. This is consistent with findings that show that TB fibers are more temporally precise than AN fibers, but is surprising because it suggests that the temporal information available monaurally is not fully exploited binaurally.

The effect of interaural fluctuation rate on correlation change discrimination.

While bilateral cochlear implants (CIs) provide some binaural benefits, these benefits are limited compared to those observed in normal-hearing (NH) listeners. The large frequency-to-electrode allocation bandwidths (BWs) in CIs compared to auditory filter BWs in NH listeners increases the interaural fluctuation rate available for binaural unmasking, which may limit binaural benefits. The purpose of this work was to investigate the effect of interaural fluctuation rate on correlation change discrimination and binaural masking-level differences in NH listeners presented a CI simulation using a pulsed-sine vocoder. In experiment 1, correlation-change just-noticeable differences (JNDs) and tone-in-noise thresholds were measured for narrowband noises with different BWs and center frequencies (CFs). The results suggest that the BW, CF, and/or interaural fluctuation rate are important factors for correlation change discrimination. In experiment 2, the interaural fluctuation rate was systematically varied and dissociated from changes in BW and CF by using a pulsed-sine vocoder. Results indicated that the interaural fluctuation rate did not affect correlation change JNDs for correlated reference noises; however, slow interaural fluctuations increased correlation change JNDs for uncorrelated reference noises. In experiment 3, the BW, CF, and vocoder pulse rate were varied while interaural fluctuation rate was held constant. JNDs increased for increasing BW and decreased for increasing CF. In summary, relatively fast interaural fluctuation rates are not detrimental for detecting changes in interaural correlation. Thus, limiting factors to binaural benefits in CI listeners could be a result of other temporal and/or spectral deficiencies from electrical stimulation.

Auditory Training: Evidence for Neural Plasticity in Older Adults.

Improvements in digital amplification, cochlear implants, and other innovations have extended the potential for improving hearing function; yet, there remains a need for further hearing improvement in challenging listening situations, such as when trying to understand speech in noise or when listening to music. Here, we review evidence from animal and human models of plasticity in the brain's ability to process speech and other meaningful stimuli. We considered studies targeting populations of younger through older adults, emphasizing studies that have employed randomized controlled designs and have made connections between neural and behavioral changes. Overall results indicate that the brain remains malleable through older adulthood, provided that treatment algorithms have been modified to allow for changes in learning with age. Improvements in speech-in-noise perception and cognition function accompany neural changes in auditory processing. The training-related improvements noted across studies support the need to consider auditory training strategies in the management of individuals who express concerns about hearing in difficult listening situations. Given evidence from studies engaging the brain's reward centers, future research should consider how these centers can be naturally activated during training.

The role of envelope statistics in detecting changes in interaural correlation.

The role of envelope statistics in binaural masking-level differences (BMLDs) and correlation change detection was investigated in normal-hearing listeners. Thresholds and just-noticeable differences (JNDs) were measured for different bandwidths and center frequencies (500, 2000, 4000, and 8000 Hz) using Gaussian noises (GNs) and low-fluctuation noises (LFNs). At a 500-Hz center frequency, GN NoSo thresholds were higher than, NoSπ thresholds were lower than, and correlation change detection JNDs were the same as LFN thresholds and JNDs. At higher center frequencies, GN NoSπ thresholds were the same or higher than LFN thresholds and GN correlation change detection JNDs were much smaller than LFN JNDs. Using a pulsed sine vocoder, a second experiment was performed to investigate if binaural adaptation might contribute to the difference in GN and LFN detection. There was no effect of pulse rate, thus providing no clear evidence that binaural adaptation plays a role in these tasks. Both a cross-correlation model and a model that utilized the fluctuations in the interaural differences could explain a majority of the variance in the LFN correlation change JNDs.

Dynamic binaural-correlation processing in rats' inferior colliculus, medial geniculate body, and primary auditory cortex

Interaural correlation processing is critical for grouping and segregating auditory streams under reverberant environments with multiple sources. Although detection of dynamic changes in interaural correlation has been extensively studied in the field of psychoacoustics, the underlying neural mechanism remains largely unknown. In this study, frequency-following responses (FFRs) to narrow-band noises were measured at various levels of the auditory system in rats, including the inferior colliculus (IC), ventral division of the medial geniculate body (MGB), and primary auditory cortex (A1). The results of Experiment 1 show that FFRs recorded in the IC were affected by both interaural correlation and the interaural time difference (ITD). Moreover, results of Experiment 2 show that a break in interaural correlation (BIC) could elicit marked FFRs in each of the three central auditory structures, and the BIC-induced FFRs were significantly affected by the ITD. The results suggest that the rat's central auditory system is able to resolve and compare fast changes in fine-structure details of arbitrary noises presented at the two ears.

Age‐related changes in the binaural temporal window.

Substantial data indicate that monaural temporal processing, sound localization ability, and spatial release from masking decline with advancing age. Physiological and psychophysical evidence indicate that the auditory system is quite sensitive to patterns of interaural cross correlation (IACC) as determined by the combined input from the two ears. Here we test the hypothesis that age‐related changes in temporal processing lead to an elongation of the binaural temporal window as measured with an interaural correlation change interval (ICCI) task. The ICCI threshold was measured as a function of the duration change in interaural correlation. Band pass (400–3200 Hz) noise bursts were separated into three segments. Initial and final segments were interaurally uncorrelated while central segments were interaurally uncorrelated in the standard intervals and interaurally correlated in the signal interval. The duration of the central segment was adaptively varied to determine ICCI threshold. The just noticeable difference in static interaural correlation also was measured. The perception of static and dynamic changes in interaural correlation as well as estimates of the equivalent rectangular duration of the binaural temporal window will be compared across groups of younger and older adult subjects.

The effects of age and interaural delay on detecting a change in interaural correlation: The role of temporal jitter

Duration thresholds for detecting a change in interaural correlation (from 0 to 1, or from 1 to 0) in the initial portion of a 1-second, broadband noise (0–10 kHz) were determined for younger and older adults in a two-interval, two-alternative forced choice paradigm as a function of the interaural delay between the noise bursts presented to each ear. When the interaural delay was 0 ms, older adults found it harder to detect a change in correlation from 0 to 1 than from 1 to 0. For younger adults, however, this pattern was reversed. For interaural delays greater than 0 ms, both younger adults and older adults found it easier to detect a change in interaural correlation from 0 to 1 for short interaural delays (1 ms) with the reverse being true for longer interaural delays (5 ms). It is shown that this pattern of results is expected if temporal jitter (loss of neural synchrony in the auditory system) increases with age and with interaural delay. The implications of these results for age-related changes in stream segregation are discussed.

The Effects of Aging and Interaural Delay on the Detection of a Break in the Interaural Correlation between Two Sounds

In noisy, reverberant environments, older adults often find it difficult to process acoustic signals, possibly because their ability to differentiate reflected waves that belong to a source from those generated by other sources diminishes with age. Therefore, older adults may be less efficient than younger adults at parsing the auditory scene into its component sound sources. To parse the auditory scene into its component sources the listener has to be able to group correlated waves coming from different directions (the direct wave and its reflections off of environmental surfaces). Because detecting a change in correlation is an important component of scene parsing, this study examined whether there is an age-related deficit in detecting break in correlation (BIC) between the noises presented over left and right headphones or over left and right loudspeakers, where a BIC refers to a change in interaural correlation from 1 to zero and then a return to 1. In experiment 1, we determined the shortest BIC duration at which 10 younger and 10 older adults could detect the BIC in the middle of identical noises (bandwidth = 10 kHz; duration = 1 sec) presented simultaneously to the left and right ears over headphones or played simultaneously over loudspeakers positioned 45 degrees to the left and right of the listeners. In experiment 2, we determined the longest delay between the left-side noise and the right-side noise, at which a 100 ms BIC presented in the middle of the noise could be detected in 10 younger and 8 older adults. The results of experiment 1 show that younger participants could detect significantly shorter BICs than older participants independent of whether the noises were presented over headphones or loudspeakers. The results of experiment 2 show that younger participants could detect the 100 ms BIC at significantly longer interaural delays than older participants. Also, for both age groups, detecting the BIC was easier under the loudspeaker-stimulation condition than the headphone-stimulation condition. Moreover, in the loudspeaker condition, the spectral cues arising from interactions between correlated sound sources seemed to be of greater benefit to younger than to older participants. The age-related decrease in sensitivity to a BIC indicates that older adults are less able than younger adults to detect a change in correlation in an ongoing sound. The inability of older adults to detect the 100 ms BIC as readily as younger adults, when the noise arriving at one ear is delayed relative to the noise arriving at the other ear, suggests that the representation of aspects of the sound's waveform decays more rapidly in older adult than in younger adult listeners. Moreover, these age-related deficits are not related to listeners' audiograms. In addition, younger adults seem to be much better than older adults at using the spectral cue provided by comb filtering to detect the BIC when there is a delay between the noises presented over loudspeakers. The more rapid decay of waveform details, combined with the lesser sensitivity to change in correlation and to spectral cues, suggest that older adults may not be as capable as younger adults in parsing auditory scenes.

Comparison of Bandwidths in the Inferior Colliculus and the Auditory Nerve. II: Measurement Using a Temporally Manipulated Stimulus

To localize low-frequency sounds, humans rely on an interaural comparison of the temporally encoded sound waveform after peripheral filtering. This process can be compared with cross-correlation. For a broadband stimulus, after filtering, the correlation function has a damped oscillatory shape where the periodicity reflects the filter's center frequency and the damping reflects the bandwidth (BW). The physiological equivalent of the correlation function is the noise delay (ND) function, which is obtained from binaural cells by measuring response rate to broadband noise with varying interaural time delays (ITDs). For monaural neurons, delay functions are obtained by counting coincidences for varying delays across spike trains obtained to the same stimulus. Previously, we showed that BWs in monaural and binaural neurons were similar. However, earlier work showed that the damping of delay functions differs significantly between these two populations. Here, we address this paradox by looking at the role of sensitivity to changes in interaural correlation. We measured delay and correlation functions in the cat inferior colliculus (IC) and auditory nerve (AN). We find that, at a population level, AN and IC neurons with similar characteristic frequencies (CF) and BWs can have different responses to changes in correlation. Notably, binaural neurons often show compression, which is not found in the AN and which makes the shape of delay functions more invariant with CF at the level of the IC than at the AN. We conclude that binaural sensitivity is more dependent on correlation sensitivity than has hitherto been appreciated and that the mechanisms underlying correlation sensitivity should be addressed in future studies.

Stimulus Context Affects Auditory Cortical Responses to Changes in Interaural Correlation

We use magnetoencephalography to study human auditory cortical processing of changes in interaural correlation (IAC). We studied transitions from correlated (identical signals at the 2 ears) to uncorrelated (different signals at the 2 ears) or vice versa for two types of wide-band noise stimuli: CHANGE signals contained a single IAC change (or none) and ALT signals alternated between correlated and uncorrelated at a constant rate. The relevant transitions, from correlated to uncorrelated or vice versa, are physically identical in both stimulus conditions, but auditory cortical response patterns differed substantially. CHANGE stimuli exhibited a response asymmetry in their temporal dynamics and magnetic field morphology according to the direction of change. Distinct field patterns indicate the involvement of separate neural substrates for processing, and distinct latencies are suggestive of different temporal integration windows. In contrast, the temporal dynamics of responses to change in the ALT stimuli did not differ substantially according to the direction of change. Notably, the uncorrelated-to-correlated transition in the ALT stimuli showed a first deflection approximately 90 ms earlier than for the same transition in the CHANGE stimuli and with an opposite magnetic field distribution. This finding suggests that as early as 50 ms after the onset of an IAC transition, a given physical change is processed differentially depending on stimulus context. Consequently, even early cortical activation cannot be interpreted independently of the specific long-term stimulus context used in the experiment.

Age‐related declines in the precedence effect and the detection of the dynamic change in interaural correlation

Elderly adults experience greater difficulties with sound localization tasks involving the precedence effect (PE) than young adults (Cranford et al., 1990). In this study, the echo threshold of the PE and the threshold for detecting the dynamic change in interaural correlation were investigated in 12 young adults and 8 elderly adults with clinically normal hearing. For studying the threshold of detecting the dynamic change in interaural correlation, participants were presented with either binaurally‐uncorrelated or binaurally‐anti‐phase noise fragments, which were inserted in the middle of two interaurally identical (correlated) broadband noise markers (duration=900 ms) delivered by headphones. Either the duration of the noise fragment or the ITD of the noise markers was manipulated. Compared to young participants, elderly participants had both higher echo thresholds and higher fragment‐duration thresholds at various ITDs. The echo threshold was significantly correlated with the fragment‐duration threshol...

Detection of interaural correlation by neurons in the superior olivary complex, inferior colliculus and auditory cortex of the unanesthetized rabbit

A critical binaural cue important for sound localization and detection of signals in noise is the interaural time difference (ITD), or difference in the time of arrival of sounds at each ear. The ITD can be determined by cross-correlating the sounds at the two ears and finding the ITD where the correlation is maximal. The amount of interaural correlation is affected by properties of spaces and can therefore be used to assess spatial attributes. To examine the neural basis for sensitivity to the overall level of the interaural correlation, we identified subcollicular neurons and neurons in the inferior colliculus (IC) and auditory cortex of unanesthetized rabbits that were sensitive to ITDs and examined their responses as the interaural correlation was varied. Neurons at each brain level could show linear or non-linear responses to changes in interaural correlation. The direction of the non-linearities in most neurons was to increase the slope of the response change for correlations near 1.0. The proportion of neurons with non-linear responses was similar in subcollicular and IC neurons but increased in the auditory cortex. Non-linear response functions to interaural correlation were not related to the type of response as determined by the tuning to ITDs across frequencies. The responses to interaural correlation were also not related to the frequency tuning of the neuron, unlike the responses to ITD, which broadens for neurons tuned to lower frequencies. The neural discriminibility of the ITD using frozen noise in the best neurons was similar to the behavioral acuity in humans at a reference correlation of 1.0. However, for other reference ITDs the neural discriminibility was more linear and generally better than the human discriminibility of the interaural correlation, suggesting that stimulus rather than neural variability is the basis for the decline in human performance at lower levels of interaural correlation.