Electrophysiological and psychophysical asymmetries in sensitivity to interaural correlation steps

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.

Acoustically Controlled Binaural Auditory Training with Vocal Duets: Assessment and Effectiveness

OBJECTIVES

We aimed to evaluate the effectiveness of a binaural auditory training program with vocal duets by comparing skills through outcomes from behavioral and electrophysiological assessment instruments at three moments: before the intervention, moment one (M1); immediately after training, moment two (M2); and 3 months after, moment three (M3).

METHODS

This interventional, longitudinal, prospective, and uncontrolled study was approved by our Research Ethics Committee. Binaural auditory training with vocal duets (ATVD) was applied in 10 adults with normal audiometric thresholds and auditory processing disorders. ATVD used four different vocals of a public domain song sung in a cappella as stimuli. Participants were asked to register any perceived difference in frequency for each syllable of the song during 30-minute sessions twice a week. The number of sessions required ranged from 12 (6 hours) to 20 (10 hours).

RESULTS

Regarding behavioral tests, the dichotic consonant-vowel test showed significant evidence of an improved advantage in the left ear (LE) in the non-forced condition and a significant reduction in the number of errors at M2 and M3 in the forced left condition. The speech-in-noise test and frequency pattern test showed a significant reduction in impaired results at M2 and M3. Electrophysiological results showed a significant increase in the LE amplitude in the P3 long-latency auditory evoked potentials test, as well as a decrease in the auditory brainstem response test (III-V and I-V inter-peak latencies in the right ear and wave I and I-III inter-peak latencies in LE).

CONCLUSION

The effectiveness of ATVD was evidenced, and the results were maintained after 3 months.

Electrophysiological and psychophysical asymmetries in sensitivity to interaural correlation gaps and implications for binaural integration time

Brief deviations of interaural correlation (IAC) can provide valuable cues for detection, segregation and localization of acoustic signals. This study investigated the processing of such "binaural gaps" in continuously running noise (100-2000 Hz), in comparison to silent "monaural gaps", by measuring late auditory evoked potentials (LAEPs) and perceptual thresholds with novel, iteratively optimized stimuli. Mean perceptual binaural gap duration thresholds exhibited a major asymmetry: they were substantially shorter for uncorrelated gaps in correlated and anticorrelated reference noise (1.75 ms and 4.1 ms) than for correlated and anticorrelated gaps in uncorrelated reference noise (26.5 ms and 39.0 ms). The thresholds also showed a minor asymmetry: they were shorter in the positive than in the negative IAC range. The mean behavioral threshold for monaural gaps was 5.5 ms. For all five gap types, the amplitude of LAEP components N1 and P2 increased linearly with the logarithm of gap duration. While perceptual and electrophysiological thresholds matched for monaural gaps, LAEP thresholds were about twice as long as perceptual thresholds for uncorrelated gaps, but half as long for correlated and anticorrelated gaps. Nevertheless, LAEP thresholds showed the same asymmetries as perceptual thresholds. For gap durations below 30 ms, LAEPs were dominated by the processing of the leading edge of a gap. For longer gap durations, in contrast, both the leading and the lagging edge of a gap contributed to the evoked response. Formulae for the equivalent rectangular duration (ERD) of the binaural system's temporal window were derived for three common window shapes. The psychophysical ERD was 68 ms for diotic and about 40 ms for anti- and uncorrelated noise. After a nonlinear Z-transform of the stimulus IAC prior to temporal integration, ERDs were about 10 ms for reference correlations of ±1 and 80 ms for uncorrelated reference. Hence, a physiologically motivated peripheral nonlinearity changed the rank order of ERDs across experimental conditions in a plausible manner.

Robust auditory localization using probabilistic inference and coherence-based weighting of interaural cues

Robust sound source localization is performed by the human auditory system even in challenging acoustic conditions and in previously unencountered, complex scenarios. Here a computational binaural localization model is proposed that possesses mechanisms for handling of corrupted or unreliable localization cues and generalization across different acoustic situations. Central to the model is the use of interaural coherence, measured as interaural vector strength (IVS), to dynamically weight the importance of observed interaural phase (IPD) and level (ILD) differences in frequency bands up to 1.4 kHz. This is accomplished through formulation of a probabilistic model in which the ILD and IPD distributions pertaining to a specific source location are dependent on observed interaural coherence. Bayesian computation of the direction-of-arrival probability map naturally leads to coherence-weighted integration of location cues across frequency and time. Results confirm the model's validity through statistical analyses of interaural parameter values. Simulated localization experiments show that even data points with low reliability (i.e., low IVS) can be exploited to enhance localization performance. A temporal integration length of at least 200 ms is required to gain a benefit; this is in accordance with previous psychoacoustic findings on temporal integration of spatial cues in the human auditory system.

Objective measures of binaural masking level differences and comodulation masking release based on late auditory evoked potentials

The audibility of important sounds is often hampered due to the presence of other masking sounds. The present study investigates if a correlate of the audibility of a tone masked by noise is found in late auditory evoked potentials measured from human listeners. The audibility of the target sound at a fixed physical intensity is varied by introducing auditory cues of (i) interaural target signal phase disparity and (ii) coherent masker level fluctuations in different frequency regions. In agreement with previous studies, psychoacoustical experiments showed that both stimulus manipulations result in a masking release (i: binaural masking level difference; ii: comodulation masking release) compared to a condition where those cues are not present. Late auditory evoked potentials (N1, P2) were recorded for the stimuli at a constant masker level, but different signal levels within the same set of listeners who participated in the psychoacoustical experiment. The data indicate differences in N1 and P2 between stimuli with and without interaural phase disparities. However, differences for stimuli with and without coherent masker modulation were only found for P2, i.e., only P2 is sensitive to the increase in audibility, irrespective of the cue that caused the masking release. The amplitude of P2 is consistent with the psychoacoustical finding of an addition of the masking releases when both cues are present. Even though it cannot be concluded where along the auditory pathway the audibility is represented, the P2 component of auditory evoked potentials is a candidate for an objective measure of audibility in the human auditory system.

Interaural correlation discrimination from diffuse field reference correlations

Just noticeable differences in interaural correlation (ρ-jnds) from diffuse sound field reference correlations are obtained. In a three-interval, three-alternative forced-choice procedure, ρ-jnds are measured for positive and negative deviations from nine narrowband reference conditions. Stimuli are 1 equivalent rectangular bandwidth wide noise bursts with center frequencies between 165 and 1500 Hz. The frequency dependent reference correlation (ρref) is determined by the simulated interaural correlation under ideal diffuse sound field conditions. Results show that the interaural correlation at threshold for deviation toward the positive correlation range follows the reference curve in a nonlinear fashion. For deviation toward the negative correlation range the interaural correlation at threshold is further afar the reference curve and does not markedly resemble its trend. The results indicate that the previously found asymmetry for correlation discrimination from uncorrelated broadband stimuli to the positive and negative correlation range becomes less pronounced for narrowband stimuli. For positive deviation, the highest jnds are found for the region where the reference curve occupies the global minimum in ρref; despite that, the interaural correlation at threshold for positive deviation exhibits its lowest value at that point.

Apparent auditory source width insensitivity in older hearing-impaired individuals

Previous studies have shown a loss in the precision of horizontal localization responses of older hearing-impaired (HI) individuals, along with potentially poorer neural representations of sound-source location. These deficits could be the result or corollary of greater difficulties in discriminating spatial images, and the insensitivity to punctate sound sources. This hypothesis was tested in three headphone-presentation experiments varying interaural coherence (IC), the cue most associated with apparent auditory source width. First, thresholds for differences in IC were measured for a broad sampling of participants. Older HI participants were significantly worse at discriminating IC across reference values than younger normal-hearing participants. These results are consistent with senescent increases in temporal jitter. Performance decreased with age, a finding corroborated in a second discrimination experiment using a separate group of participants matched for hearing loss. This group also completed a third, visual experiment, with both a cross-mapping task where they drew the size of the sound they heard and the identification task where they chose the image that best corresponded to what they heard. The results from the visual tasks indicate that older HI individuals do not hear punctate images and are relatively insensitive to changes in width based on IC.