Differential Adaptation in Azimuth and Elevation to Acute Monaural Spatial Hearing after Training with Visual Feedback

Abstract

Sound localization in the horizontal plane (azimuth) relies mainly on binaural difference cues in sound level and arrival time. Blocking one ear will perturb these cues, and may strongly affect azimuth performance of the listener. However, single-sided deaf listeners, as well as acutely single-sided plugged normal-hearing subjects, often use a combination of (ambiguous) monaural head-shadow cues, impoverished binaural level-difference cues, and (veridical, but limited) pinna- and head-related spectral cues to estimate source azimuth. To what extent listeners can adjust the relative contributions of these different cues is unknown, as the mechanisms underlying adaptive processes to acute monauralization are still unclear. By providing visual feedback during a brief training session with a high-pass (HP) filtered sound at a fixed sound level, we investigated the ability of listeners to adapt to their erroneous sound-localization percepts. We show that acutely plugged listeners rapidly adjusted the relative contributions of perceived sound level, and the spectral and distorted binaural cues, to improve their localization performance in azimuth also for different sound levels and locations than those experienced during training. Interestingly, our results also show that this acute cue-reweighting led to poorer localization performance in elevation, which was in line with the acoustic–spatial information provided during training. We conclude that the human auditory system rapidly readjusts the weighting of all relevant localization cues, to adequately respond to the demands of the current acoustic environment, even if the adjustments may hamper veridical localization performance in the real world.

Changes in Sound Localization Performance of Single-Sided Deaf Listeners after Visual Feedback Training in Azimuth

“…A slight localization improvement was observed for the higher-intensity sounds, as the MAE was significantly reduced, because of the reduced bias. These response patterns, albeit preliminary, seem to differ markedly from the results obtained with the acute unilateral plugged hearing condition described in [1]. Pre-Adaptation Post-Adaptation Figure 8: Summarized statistics of regression analysis for the three participants for pre-adaptation and postadaptation.…”

“…We show that the training had rapidly readjusted the contributions of monaural cues and internal priors, which resulted to be imposed by the multisensory information provided during the training. We compare the results with the strategies found for the acute monaural hearing condition of normal-hearing listeners, described in an earlier study [1].…”

“…The 58 fixation points and raw head-position signals thus obtained were used to train two three-layer neural networks (one for azimuth, one for elevation) that served to calibrate the head-position data, using the Bayesian regularization implementation of the back-propagation algorithm (MatLab; Neural Networks Toolbox) to avoid overfitting [29]. In each sound-localization experiment, the listener started a trial by fixating the central LED (azimuth and elevation both zero; Figure 3, [1]). After a pseudo-random period between 1.5˘2.0 sec, the fixation LED was extinguished, and an auditory stimulus was presented 400 msec later.…”

“…We recently showed that normal-hearing human listeners rapidly learn to remap the acoustic spectral cues for elevation during a short training session to a limited number of locations, both with and without visual feedback [13]. In a follow-up study [1], we subsequently demonstrated that the auditory system can also swiftly reweight its internal priors regarding the binaural difference cues and monaural head-shadow cues, to improve localization performance in azimuth, in response to acutely imposed monaural hearing (ear-canal plug and muff over one ear).…”

“…In this paper, we set out to study short-term adaptive behavior of chronic single sided deaf (CSSD) listeners, and compare the results with acute conductive unilateral plugged (ACUP) listening for normal-hearing subjects, as described in the previous study [1]. In the latter group, the manipulation perturbed the highly robust binaural differences cues for sound-source azimuth, which led to an immediate and dramatic degradation of sound-localization performance in the horizontal plane, with a large localization bias towards the hearing ear.…”