This report highlights research projects relevant to binaural and spatial hearing in adults and children. In the past decade we have made progress in understanding the impact of bilateral cochlear implants (BiCIs) on performance in adults and children. However, BiCI users typically do not perform as well as normal hearing (NH) listeners. In this paper we describe the benefits from BiCIs compared with a single CI, focusing on measures of spatial hearing and speech understanding in noise. We highlight the fact that in BiCI listening the devices in the two ears are not coordinated, thus binaural spatial cues that are available to NH listeners are not available to BiCI users. Through the use of research processors that carefully control the stimulus delivered to each electrode in each ear, we are able to preserve binaural cues and deliver them with fidelity to BiCI users. Results from those studies are discussed as well, with a focus on the effect of age at onset of deafness and plasticity of binaural sensitivity. Our work with children has expanded both in number of subjects tested and age range included. We have now tested dozens of children ranging in age from 2-14 years. Our findings suggest that spatial hearing abilities emerge with bilateral experience. While we originally focused on studying performance in free-field, where real world listening experiments are conducted, more recently we have begun to conduct studies under carefully controlled binaural stimulation conditions with children as well. We have also studied language acquisition and speech perception and production in young CI users. Finally, a running theme of this research program is the systematic investigation of the numerous factors that contribute to spatial and binaural hearing in BiCI users. By using CI simulations (with vocoders) and studying NH listeners under degraded listening conditions, we are able to tease apart limitations due to the hardware/software of the CI systems from limitations due to neural pathology.

Spatial release from masking (SRM) was measured in groups of children with bilateral cochlear implants (BiCIs, average ages 6.0 and 7.9 yr) and with normal hearing (NH, average ages 5.0 and 7.8 yr). Speech reception thresholds (SRTs) were measured for target speech in front (0°), and interferers in front, distributed asymmetrically toward the right (+90°/+90°) or distributed symmetrically toward the right and left (+90°/-90°). In the asymmetrical condition both monaural "better ear" and binaural cues are available. In the symmetrical condition, listeners rely heavily on binaural cues to segregate sources. SRM was computed as the difference between SRTs in the front condition and SRTs in either the asymmetrical or symmetrical conditions. Results showed that asymmetrical SRM was smaller in BiCI users than NH children. Furthermore, NH children showed symmetrical SRM, suggesting they are able to use binaural cues for source segregation, whereas children with BiCIs had minimal or absent symmetrical SRM. These findings suggest that children who receive BiCIs can segregate speech from noise under conditions that maximize monaural better ear cues. Limitations in the CI devices likely play an important role in limiting SRM. Thus, improvement in spatial hearing abilities in children with BiCIs may require binaural processing strategies.

In recent years, cochlear implants (CIs) have been provided in growing numbers to people with not only bilateral deafness but also to people with unilateral hearing loss, at times in order to alleviate tinnitus. This study presents audiological data from 15 adult participants (ages 48 ± 12 years) with single sided deafness. Results are presented from 9/15 adults, who received a CI (SSD-CI) in the deaf ear and were tested in Acoustic or Acoustic + CI hearing modes, and 6/15 adults who are planning to receive a CI, and were tested in the unilateral condition only. Testing included (1) audiometric measures of threshold, (2) speech understanding for CNC words and AzBIO sentences, (3) tinnitus handicap inventory, (4) sound localization with stationary sound sources, and (5) perceived auditory motion. Results showed that when listening to sentences in quiet, performance was excellent in the Acoustic and Acoustic + CI conditions. In noise, performance was similar between Acoustic and Acoustic + CI conditions in 4/6 participants tested, and slightly worse in the Acoustic + CI in 2/6 participants. In some cases, the CI provided reduced tinnitus handicap scores. When testing sound localization ability, the Acoustic + CI condition resulted in improved sound localization RMS error of 29.2° (SD: ±6.7°) compared to 56.6° (SD: ±16.5°) in the Acoustic-only condition. Preliminary results suggest that the perception of motion direction, whereby subjects are required to process and compare directional cues across multiple locations, is impaired when compared with that of normal hearing subjects.

In complex auditory environments, it is often difficult to separate a target talker from interfering speech. For normal hearing (NH) adult listeners, similarity between the target and interfering speech leads to increased difficulty in separating them; that is, informational masking occurs due to confusability of the target and interferers. This study investigated performance of children with bilateral cochlear implants (BiCIs) when target and interferers were either same-sex (male) talkers, or different-sex talkers (male target, female interferer). Comparisons between children with BiCIs and NH, when matched for age, were also conducted. Speech intelligibility was measured for target and interferers spatially co-located, or spatially separated with the interferers positioned symmetrically (þ90 and À90 ) or asymmetrically (both at þ90 , right). Spatial release from masking (SRM) was computed as the difference between co-located and separated conditions. Within group BiCI comparisons revealed that in the co-located condition speech intelligibility was worse with the same-sex vs different-sex stimuli. There was also a trend for more SRM with the same-sex vs different-sex stimuli. When comparing BiCI to NH listeners, SRM was larger for the NH groups, suggesting that NH children are better able to make use of spatial cues to improve speech understanding in noise.

Adults with bilateral cochlear implants (BiCIs) receive benefits in localizing stationary sounds when listening with two implants compared with one; however, sound localization ability is significantly poorer when compared to normal hearing (NH) listeners. Little is known about localizing sound sources in motion, which occurs in typical everyday listening situations. The authors considered the possibility that sound motion may improve sound localization in BiCI users by providing multiple places of information. Alternatively, the ability to compare multiple spatial locations may be compromised in BiCI users due to degradation of binaural cues, and thus result in poorer performance relative to NH adults. In this study, the authors assessed listeners' abilities to distinguish between sounds that appear to be moving vs stationary, and track the angular range and direction of moving sounds. Stimuli were bandpass-filtered (150-6000 Hz) noise bursts of different durations, panned over an array of loudspeakers. Overall, the results showed that BiCI users were poorer than NH adults in (i) distinguishing between a moving vs stationary sound, (ii) correctly identifying the direction of movement, and (iii) tracking the range of movement. These findings suggest that conventional cochlear implant processors are not able to fully provide the cues necessary for perceiving auditory motion correctly. V