Changes In Interaural Correlation Research Articles

Human auditory steady-state responses to changes in interaural correlation

Steady-state responses were evoked by noise stimuli that alternated between two levels of interaural correlation ρ at a frequency f m. With ρ alternating between +1 and 0, responses at f m dropped steeply above 4 Hz, but persisted up to 64 Hz. Two time constants of 47 and 4.4 ms with delays of 198 and 36 ms, respectively, were obtained by fitting responses to a transfer function based on symmetric exponential windows. The longer time constant, possibly reflecting cortical integration, is consistent with perceptual binaural “sluggishness”. The shorter time constant may reflect running cross-correlation in the high brainstem or primary auditory cortex. Responses at 2 f m peaked with an amplitude of 848 ± 479 nV ( f m = 4 Hz). Investigation of this robust response revealed that: (1) changes in ρ and lateralization evoked similar responses, suggesting a common neural origin, (2) response was most dependent on stimulus frequencies below 1000 Hz, but frequencies up to 4000 Hz also contributed, and (3) when ρ alternated between [0.2–1] and 0, response amplitude varied linearly with ρ, and the physiological response threshold was close to the average behavioral threshold ( ρ = 0.31). This steady-state response may prove useful in the objective investigation of binaural hearing.

Binaural detection of 500-Hz tones in broadband and in narrowband masking noise: Effects of signal/masker duration and forward masking fringes

NoSpi thresholds for a 500-Hz tonal signal were measured with broadband and with narrowband maskers using a single-interval adaptive matrix procedure [C. Kaernbach, J Acoust. Soc. Am. 88, 2645-2655 (1990)]. The purpose of the study was to investigate and to account for the effects on thresholds of varying the durations of the signals and maskers and the durations of forward masking fringes that preceded the occurrence of signal-plus-noise. For detection in both broadband and narrowband noise, the addition of brief forward fringes of masking noise resulted in elevations in threshold for the shortest signal durations. Longer forward fringes led to larger decreases in threshold when the masker was broadband as compared to when the masker was narrowband. The complex patterning of the data was explained by the operation of: (1) "predetection" temporal integration associated with peripheral auditory filtering; (2) duration-dependent, across-frequency influences that differentially affect broadband and narrowband NoSpi thresholds, (3) "post-detection" temporal integration associated with the central binaural mechanism, and (4) consideration of the detection thresholds in terms of changes in interaural correlation rather than in terms of signal level or signal-to-noise ratio, per se.

Auditory Midbrain and Nerve Responses to Sinusoidal Variations in Interaural Correlation

The human sensitivity to interaural temporal differences in the acoustic waveforms to the two ears shows remarkable acuity but is also very sluggish. Fast changes in binaural parameters are not detectable, and this inability contrasts sharply with the excellent temporal resolution of the monaural auditory system. We studied the response of binaural neurons in the inferior colliculus of the cat to sinusoidal changes in the interaural correlation of broadband noise. Responses to the same waveforms were also obtained from auditory nerve fibers and further analyzed with a coincidence analysis. Overall, the auditory nerve and inferior colliculus showed a similar ability to code changes in interaural correlation. This ability extended to modulation frequencies an order of magnitude higher than the highest frequencies detected binaurally in humans. We conclude that binaural sluggishness is not caused by a lack of temporal encoding of fast binaural changes at the level of the midbrain. We hypothesize that there is no neural substrate at the level of the midbrain or higher to read out this temporal code and that this constitutes a low-pass "sluggishness" filter.

Human Auditory Cortical Processing of Changes in Interaural Correlation

Sensitivity to the similarity of the acoustic waveforms at the two ears, and specifically to changes in similarity, is crucial to auditory scene analysis and extraction of objects from background. Here, we use the high temporal resolution of magnetoencephalography to investigate the dynamics of cortical processing of changes in interaural correlation, a measure of interaural similarity, and compare them with behavior. Stimuli are interaurally correlated or uncorrelated wideband noise, immediately followed by the same noise with intermediate degrees of interaural correlation. Behaviorally, listeners' sensitivity to changes in interaural correlation is asymmetrical. Listeners are faster and better at detecting transitions from correlated noise than transitions from uncorrelated noise. The cortical response to the change in correlation is characterized by an activation sequence starting from approximately 50 ms after change. The strength of this response parallels behavioral performance: auditory cortical mechanisms are much less sensitive to transitions from uncorrelated noise than from correlated noise. In each case, sensitivity increases with interaural correlation difference. Brain responses to transitions from uncorrelated noise lag those from correlated noise by approximately 80 ms, which may be the neural correlate of the observed behavioral response time differences. Importantly, we demonstrate differences in location and time course of neural processing: transitions from correlated noise are processed by a distinct neural population, and with greater speed, than transitions from uncorrelated noise.

Detection of static and dynamic changes in interaural correlation

This study examines the relation between a static and a dynamic measure of interaural correlation discrimination: (1) the just noticeable difference (JND) in interaural correlation and (2) the minimum detectable duration of a fixed interaural correlation change embedded within a single noise-burst of a given reference correlation. For the first task, JNDs were obtained from reference interaural correlations of + 1, -1, and from 0 interaural correlation in either the positive or negative direction. For the dynamic task, duration thresholds were obtained for a brief target noise of +1, -1, and 0 interaural correlation embedded in reference marker noise of +1, -1, and 0 interaural correlation. Performance with a reference interaural correlation of +1 was significantly better than with a reference correlation of -1. Similarly, when the reference noise was interaurally uncorrelated, discrimination was significantly better for a target correlation change towards +1 than towards -1. Thus, for both static and dynamic tasks, interaural correlation discrimination in the positive range was significantly better than in the negative range. Using the two measures, the length of a binaural temporal window was estimated. Its equivalent rectangular duration (ERD) was approximately 86 ms and independent of the interaural correlation configuration.

Interaural correlation discrimination: II. Relation to binaural unmasking.

Many theoretical models of binaural interaction assume that sensitivity to interaural correlation underlies binaural unmasking. This paper explores the extent to which sensitivity to changes in interaural correlation implied by results from binaural detection experiments are consistent with sensitivity to changes in interaural correlation implied by results from binaural detection experiments are consistent with sensitivity to changes in interaural correlation measured directly in correlation discrimination experiments. The vehicle for this exploration is a simplified model of the underlying processes assumed by many models of binaural unmasking for the detection of narrow-band signals in the presence of broadband noise. Consideration is given to psychometric function slopes, detection thresholds, bandwidth effects, duration effects, level effects, and interaural-parameter effects. Although many of the results obtained from our analysis are consistent with the notion that the cue in binaural detection tasks is a change in interaural correlation, some significant inconsistencies are noted.

Performance in several binaural-interaction experiments.

The relationship between interaural correlation discrimination and binaural detection was investigated using common experimental procedures and common subjects. Psychometric functions were obtained for four normal-hearing subjects at 500 and 4000 Hz using third-octave noise signals for the correlation discrimination experiment, and pure-tone signals and third-octave noise maskers for the detection experiment. Results from these two measurements, which were compared by expressing the signal-to-noise ratio as an equivalent change in interaural correlation, support the idea that interaural correlation discrimination and binaural detection are closely related. Since large intersubject differences in binaural performance were observed in these experiments, interaural-time, interaural-intensity, and monaural-intensity discrimination were measured in a second experiment. The results of the second experiment show large intersubject differences for the interaural tasks, but not for the monaural task.

Binaural Listening and Interaural Noise Cross Correlation

The identification of different interaural correlations was examined over a range of reference correlations. Interaural correlations were produced by the method of Licklider and Dzendolet in which three independent noise sources were combined into two outputs. The change in interaural correlation (squared) required for 75% correct identification varied from about 0.4 for a reference correlation of 0.0 to about 0.04 for a reference correlation of 1.0.