Binaural Display Research Articles

The Rapid Decline in Interaural-Time-Difference Sensitivity for Pure Tones Can Be Explained by Peripheral Filtering.

The interaural time difference (ITD) is a primary horizontal-plane sound localization cue computed in the auditory brainstem. ITDs are accessible in the temporal fine structure of pure tones with a frequency of no higher than about 1400Hz. How listeners' ITD sensitivity transitions from very best sensitivity near 700Hz to impossible to detect within 1 octave currently lacks a fully compelling physiological explanation. Here, it was hypothesized that the rapid decline in ITD sensitivity is dictated not by a central neural limitation but by initial peripheral sound encoding, specifically, the low-frequency (apical) portion of the cochlear excitation pattern produced by a pure tone. ITD sensitivity was measured in 16 normal-hearing listeners as a joint function of frequency (900-1500Hz) and level (10-50dB sensation level). Performance decreased with increasing frequency and decreasing sound level. The slope of performance decline was 90dB/octave, consistent with the low-frequency slope of the cochlear excitation pattern. Fine-structure ITD sensitivity near 1400Hz may be conveyed primarily by "off-frequency" activation of neurons tuned to lower frequencies near 700Hz. Physiologically, this could be realized by having neurons sensitive to fine-structure ITD up to only about 700Hz. A more extreme model would have only a single narrow channel near 700Hz that conveys fine-structure ITDs. Such a model is a major simplification and departure from the classic formulation of the binaural display, which consists of a matrix of neurons tuned to a wide range of relevant frequencies and ITDs.

Detection of early reflections from a binaural activity map using neural networks.

Human listeners localize sounds to their sources despite competing directional cues from early room reflections. Binaural activity maps computed from a running signal can provide useful information about the presence of room reflections, but must be inspected visually to estimate auditory cues. A model was constructed using machine learning to validate the presence of and perform the extraction of these cues. The model uses the running signal output of a binaurally integrated cross-correlation/autocorrelation mechanism (BICAM) to analyze a lead/lag stimulus and generate a binaural activity map. System reflections are visually presented on the binaural display as correlation peaks with increased amplitude. Three independent neural networks estimate the location of the direct sound, the time delay of the reflection, and the location of the reflection from binaural activity maps displayed by BICAM. Depending on the task, neural network accuracies on test data sets vary from 84.1% to 98.5%.

Human interaural time difference thresholds for sine tones: The high-frequency limit

The smallest detectable interaural time difference (ITD) for sine tones was measured for four human listeners to determine the dependence on tone frequency. At low frequencies, 250-700 Hz, threshold ITDs were approximately inversely proportional to tone frequency. At mid-frequencies, 700-1000 Hz, threshold ITDs were smallest. At high frequencies, above 1000 Hz, thresholds increased faster than exponentially with increasing frequency becoming unmeasurably high just above 1400 Hz. A model for ITD detection began with a biophysically based computational model for a medial superior olive (MSO) neuron that produced robust ITD responses up to 1000 Hz, and demonstrated a dramatic reduction in ITD-dependence from 1000 to 1500 Hz. Rate-ITD functions from the MSO model became inputs to binaural display models-both place based and rate-difference based. A place-based, centroid model with a rigid internal threshold reproduced almost all features of the human data. A signal-detection version of this model reproduced the high-frequency divergence but badly underestimated low-frequency thresholds. A rate-difference model incorporating fast contralateral inhibition reproduced the major features of the human threshold data except for the divergence. A combined, hybrid model could reproduce all the threshold data.

Lateralization produced by interaural temporal and intensitive disparities of high-frequency, raised-sine stimuli: Data and modeling

An acoustic pointing task was used to measure extents of laterality produced by combinations of ongoing envelope-based interaural temporal disparities (ITDs) and interaural intensitive disparities (IIDs) of 4-kHz-centered raised-sine stimuli [Bernstein and Trahiotis, J. Acoust. Soc. Am. 125, 3234-3242 (2009),] while varying, parametrically, their peakedness, depth of modulation, and frequency of modulation. The study was designed to assess whether IIDs act as "weights" within the putative "binaural display" at high spectral frequencies (where the envelopes convey ITD-information) as appears to be the case at low spectral frequencies (where the waveforms, i.e., fine-structure and envelopes, convey ITD-information). The data indicate that envelope-based IIDs do principally act as weights and that they appear to exert their influence on lateral position independently of the influence of ITDs. Quantitative analyses revealed that an augmented form of the cross-correlation-based "position-variable" model of Stern and Shear [J. Acoust. Soc. Am. 100, 2278-2288 (1996)] accounted for 94% of the variance in the data. This success notwithstanding, for a small subset of the data, predictions could be improved by assuming that the listeners utilized information within auditory filters having center frequencies above 4 kHz.

Empirical and theoretical findings concerning how interaural intensitive disparities affect lateralization for high- versus low -frequency stimuli.

This presentation concerns empirical and theoretical findings concerning the potency of interaural intensitive disparities (IIDs) at low versus high spectral frequencies. In one context, an acoustic pointing task was used to measure how IIDs affect extents of laterality. Laterality was measured for 4-kHz-centered stimuli with reinforcing or opposing combinations of IIDs and interaural temporal disparities (ITDs). The goal was to assess whether IIDs act as scalars or “weights” within the putative “binaural display” at high spectral frequencies (where the envelopes convey ITD-information) as they have been repeatedly shown to operate at low spectral frequencies (where the waveforms convey ITD-information). They do. The data were accounted for via an augmentation of the cross-correlation-based “position-variable” modeling approach developed by Stern and Shear [J. Acoust. Soc. Am. 100, 2278–2288 (1996)] to account for ITD-based lateralization at low spectral frequencies. In a second context, laterality produced by IIDs (ITD=0) was measured for stimuli centered at either 4 kHz or 500 Hz. The novel finding was that IIDs consistently produced larger extents of laterality at 4 kHz than they did at 500 Hz. [Work supported by research Grant No. NIH DC-04147 from the National Institute on Deafness and Other Communication Disorders, National Institutes of Health.]

Spatial resolution and the binaural display.

An important feature of a Jeffress‐type binaural display is that it not only allows for localization of a single sound source but it also allows for resolution of multiple sound sources. Here an experiment is reported which directly addresses spatial resolution with a paradigm similar to the notched‐noise paradigm which quantifies spectral resolution. Listeners are required to detect a band‐pass signal at a frontal position in the presence of two independent band‐stop maskers symmetrically arranged around the signal in space and frequency. Data show that signal detection does not improve when the spatial separation between the signal and the maskers is increased from ± 10 to ± 90 deg. When listeners are turned by 90 deg, i.e., the signal is presented from the left, release from masking is only observed when the maskers are moved into the right hemisphere, an effect closely related to BMLDs observed under headphones. The current data show that auditory spatial resolution of simultaneous sound sources is very bad and in line with a “two channel model,” which allows for spatial acuity but not for spatial resolution. Follow‐up experiments quantify the extent to which this can be compensated for by the recently shown high binaural temporal resolution.

An introduction to the physiological, psychological, and modeling aspects of the binaural display.

Binaural hearing is a model system to understand behavioral outcomes through models based on physiological measurements. Internal or compensating delays are one component that is generally present in binaural models. These delays are typically modeled as a continuum of pure time delays. We will review the physiological evidence for the distribution of these delays in different mammals, their (in)dependence on stimulus parameters, and we will briefly address their possible origin. Besides in physiology, the binaural display also plays a central role in describing psychoacoustic data. In a typical model of binaural processing, the binaural display is an intermediate stage in which activity is represented as a function of the internal interaural delay, computed by a cross‐correlation process, and frequency. Such models have been applied successfully to a variety of lateralization experiments and to a great variety of binaural signal detection conditions. Besides introducing these elements of binaural models, we will also shortly address the question of whether there is a strict need for behavioral models to contain a continuous internal delay axis.

A sonar aid to enhance spatial perception of the blind: engineering design and evaluation

The design of an air sonar device with a new form of binaural display is described which aids the blind in perceiving their environment. Some of the limitations of knowledge of human perception and the influence this has on a specification for the device are discussed. Inherent limitations in the binaural aid both in terms of technology development and performance are also explained. The paper described what is expected of the man-machine control system in a mobility setting and discusses the technique of evaluating a manmachinesystem so as to assess the machine performance.