Objectives-We investigated the ability of single-sided deaf listeners implanted with a cochlear implant (SSD-CI) to (i) determine the front-back and left-right location of sound sources presented from loudspeakers surrounding the listener and (ii) use small head rotations to further improve their localization performance. The resulting behavioral data were used for further analyses investigating the value of so-called "monaural" spectral shape cues for front-back sound source localization.Design-Eight SSD-CI patients were tested with their CI on and off. Eight NH listeners, with one ear plugged during the experiment, and another group of eight NH listeners, with neither ear plugged, were also tested. Gaussian noises of three-second duration were bandpass filtered to 2-8 kHz and presented from one of six loudspeakers surrounding the listener, spaced 60° apart. Perceived sound source localization was tested under conditions where the patients faced forward with the head stationary, and under conditions where they rotated their heads between ±30°. Results-(i)Under stationary listener conditions, unilaterally-plugged NH listeners and SSD-CI listeners (with their CIs both on and off) were nearly at chance in determining the front-back location of high-frequency sound sources. (ii) Allowing rotational head movements improved performance in both the front-back and left-right dimensions for all listeners. (iii) For SSD-CI patients with their CI turned off, head rotations substantially reduced front-back reversals, and the combination of turning on the CI with head rotations led to near-perfect resolution of front-back sound source location. (iv) Turning on the CI also improved left-right localization performance. (v) As expected, NH listeners with both ears unplugged localized to the correct front/back and leftright hemifields both with and without head movements.Conclusions-Although SSD-CI listeners demonstrate a relatively poor ability to distinguish the front-back location of sound sources when their head is stationary, their performance is substantially improved with head movements. Most of this improvement occurs when the CI is off, suggesting that the NH ear does most of the "work" in this regard, though some additional gain is introduced with turning the CI on. During head turns, these listeners appear to primarily rely on comparing changes in head position to changes in monaural level (ML) cues produced by the direction-dependent attenuation of high-frequency sounds that result from acoustic head

This study investigated the effects of unilateral hearing loss (UHL), of either conductive or sensorineural origin, on stereo sound localization and related visual bias in listeners with normal hearing, short-term (acute) UHL, and chronic UHL. Time-delay-based stereophony was used to isolate interaural-time-difference cues for sound source localization in free field. Listeners with acute moderate (<40 dB for tens of minutes) and chronic severe (>50 dB for more than 10 years) UHL showed poor localization and compressed auditory space that favored the intact ear. Listeners with chronic moderate (<50 dB for more than 12 years) UHL performed near normal. These results show that the auditory spatial mechanisms that allow stereo localization become less sensitive to moderate UHL in the long term. Presenting LED flashes at either the same or a different location as the sound source elicited visual bias in all groups but to different degrees. Hearing loss led to increased visual bias, especially on the impaired side, for the severe and acute UHL listeners, suggesting that vision plays a compensatory role in restoring perceptual spatial symmetry.

Horizontal sound localization in free field requires integration of interaural time (ITD) and level (ILD) differences, in making accurate spatial judgments. Recently, we showed that listeners demonstrated great variability in localizing a stereo sound source (Montagne and Zhou, JASA, 2016). We hypothesized that this variability might arise from conflicting sidedness between ITDs and ILDs within and/or across frequency bands. To test this hypothesis, here we generated a new set of stimuli with variable spatial congruence between ITDs and ILDs by adding a constant inter-channel level cue ( + /- 5 dB) either aligned with or opposed to the inter-channel timing cue (from -1 to 1 msec). In Experiment 1 listeners responded to 15-ms broadband noise bursts. Response variability decreased when the inter-channel timing and level cues were spatially congruent and increased when they were not. In Experiment 2 listeners responded to low- and high-pass filtered noise (1.5 kHz cutoff) for the spatially incongruent stimuli only. Response variability was much reduced but the perceived source location consistently pointed to the “wrong side,” favoring the level cue. Together, the new results suggest a significant weighting role for ILDs (generated from level-based stereophony) in determining the lateral position of a stereo image.