Sara Misurelli

Studies on bilateral cochlear implants at the University of Wisconsin's Binaural Hearing and Speech Laboratory.

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 article we describe the benefits from BiCIs compared with a single cochlear implant (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 to 14 yr. 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 in children with bilateral cochlear implants and with normal hearing: Effect of target-interferer similarity

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 (+90° and -90°) or asymmetrically (both at +90°, 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.

Does Bilateral Experience Lead to Improved Spatial Unmasking of Speech in Children Who Use Bilateral Cochlear Implants

Hypothesis: In children with bilateral cochlear implants (BiCIs), experience over a 1 to 3-year period can improve speech understanding and spatial unmasking of speech. Background: One reason for providing children with BiCIs is to improve spatial hearing abilities. Little is known about changes in performance with added bilateral experience, and the relation between sound localization and spatial unmasking of speech. Methods: Twenty children with BiCIs participated. Testing was conducted typically within a year of bilateral activation, and at 1, 2, or 3 follow-up annual intervals. All testing was done while children listened with both devices activated. Target speech was presented from front (co-located); interfering speech was from front, right (asymmetrical), or right and left (symmetrical). Speech reception thresholds (SRTs) were measured in each condition. Spatial release from masking (SRM) was quantified as the difference in SRTs between conditions with interferers at 0 degrees and 90 degrees. For 11 of the children, data are also compared with sound localization measures obtained on the same visit to the laboratory but published elsewhere. Results: Change in SRM with bilateral experience varied; some children showed improvement and others did not. Regression analyses identified relationships between SRTs and SRM. Comparison of the SRM with localization data suggests little evidence for correlations between the two spatial tasks. Conclusion: In children with BiCIs spatial hearing mechanisms involved in SRM and sound localization may be different. Reasons for reduced SRM include asymmetry between the ears, and individual differences in the ability to inhibit interfering information, switch and/or sustain attention.

Source segregation in noisy environments by children with normal hearing and bilateral cochlear implants.

Spatial release from making (SRM) refers to the improvement in speech intelligibility measured when target and maskers are spatially separated, as opposed to when they are co‐located. SRM is affected by numerous factors that are of interest in this study. Asymmetrical placement of maskers to one side of the head leads to SRM as large as 12 dB in normal‐hearing (NH) adults, due to a combination of monaural head‐shadow and binaural interaction effects. In addition, SRM is affected by the content of the masker and its similarity to the talker. To better study these effects in children, we recently measured SRM when maskers were either symmetrically or asymmetrically distributed in the horizontal plane to the right and left. In addition, the sex of the talker and thus its similarity to the masker were varied. In the NH groups, children between the ages of 4–6 and 7–9 years and adults were tested. In addition, children who are deaf and use bilateral cochlear implants, also 4–6 and 7–9 years of age, participate...

Spatial Release From Masking in 2-Year-Olds With Normal Hearing and With Bilateral Cochlear Implants:

This study evaluated spatial release from masking (SRM) in 2- to 3-year-old children who are deaf and were implanted with bilateral cochlear implants (BiCIs), and in age-matched normal-hearing (NH) toddlers. Here, we examined whether early activation of bilateral hearing has the potential to promote SRM that is similar to age-matched NH children. Listeners were 13 NH toddlers and 13 toddlers with BiCIs, ages 27 to 36 months. Speech reception thresholds (SRTs) were measured for target speech in front (0°) and for competitors that were either Colocated in front (0°) or Separated toward the right (+90°). SRM was computed as the difference between SRTs in the front versus in the asymmetrical condition. Results show that SRTs were higher in the BiCI than NH group in all conditions. Both groups had higher SRTs in the Colocated and Separated conditions compared with Quiet, indicating masking. SRM was significant only in the NH group. In the BiCI group, the group effect of SRM was not significant, likely limited by the small sample size; however, all but two children had SRM values within the NH range. This work shows that to some extent, the ability to use spatial cues for source segregation develops by age 2 to 3 in NH children and is attainable in most of the children in the BiCI group. There is potential for the paradigm used here to be used in clinical settings to evaluate outcomes of bilateral hearing in very young children.

Use of non-individualized head-related transfer functions to measure spatial release from masking in children with normal hearing

Spatial hearing studies with children have typically been conducted using loudspeakers in laboratories. However, loudspeaker arrays are rare in clinics due to high cost and technical set-up requirements. The use of virtual auditory space (VAS) with non-individualized head-related transfer functions (HRTFs) can increase the feasibility of assessing spatial hearing abilities in clinical settings. A novel paradigm for measuring spatial release from masking (SRM) was developed using non-individualized HRTFs. This paradigm measures the minimum angular separation needed between target and masker to achieve a 20% increase in target speech intelligibility. First, the 50% speech reception threshold (SRT) was measured with target and masker co-located to one side. Then, the masker position was adaptively changed to achieve 70.7% intelligibility while maintaining the signal-to-noise ratio at the level of the co-located SRT. To verify the use of non-individualized HRTFs, normal-hearing children were tested (1) using a loudspeaker array and (2) in headphone-based VAS created using KEMAR HRTFs measured in the same setup as (1). Preliminary results showed that co-located SRTs and target-masker angle separation to achieve a 20% SRM were similar in loudspeaker array and in headphone-based VAS. This suggests that non-individualized HRTFs might be used in an SRM task for clinical testing.Spatial hearing studies with children have typically been conducted using loudspeakers in laboratories. However, loudspeaker arrays are rare in clinics due to high cost and technical set-up requirements. The use of virtual auditory space (VAS) with non-individualized head-related transfer functions (HRTFs) can increase the feasibility of assessing spatial hearing abilities in clinical settings. A novel paradigm for measuring spatial release from masking (SRM) was developed using non-individualized HRTFs. This paradigm measures the minimum angular separation needed between target and masker to achieve a 20% increase in target speech intelligibility. First, the 50% speech reception threshold (SRT) was measured with target and masker co-located to one side. Then, the masker position was adaptively changed to achieve 70.7% intelligibility while maintaining the signal-to-noise ratio at the level of the co-located SRT. To verify the use of non-individualized HRTFs, normal-hearing children were tested (1) using ...

Hearing with Cochlear Implants and Hearing Aids in Complex Auditory Scenes

One of the most important tasks that humans face is communication in complex, noisy acoustic environments. In this chapter, the focus is on populations of children and adult listeners who suffer from hearing loss and are fitted with cochlear implants (CIs) and/or hearing aids (HAs) in order to hear. The clinical trend is to provide patients with the ability to hear in both ears. This trend to stimulate patients in both ears has stemmed from decades of research with normal-hearing (NH) listeners, demonstrating the importance of binaural and spatial cues for segregating multiple sound sources. There are important effects due to the type of stimuli used, testing parameters, and auditory task utilized. The review of research in hearing impaired populations notes auditory cues that are potentially available to users of CIs and HAs. In addition, there is discussion of limitations resulting from the ways that devices handle auditory cues, auditory deprivation, and other factors that are inherently problematic for these patients.

The impact of cochlear implantation on spatial hearing and listening effort

We studied the potential benefits of bilateral hearing in cochlear implants (CI) users, and in patients with single-sided deafness (SSD) who receive a CI in the deaf ear and have normal hearing in the other ear. We hypothesized that listening effort, measured with pupil dilation, can reveal benefits of bilateral hearing that may not be consistently observed when localization or spatial release from masking (SRM) are measured. Result from 12 bilateral CI users showed reduction in listening effort with bilateral hearing compared to the poor ear or better ear alone. In patients with SSD, benefits of adding a CI to a normal hearing ear can emerge over a protracted period of a year or longer. In addition, for at least some of the subjects bilateral hearing (adding a CI to the normal hearing ear) produced release from listening effort even in conditions where SRM was not observed. That is, speech intelligibility did not always improve with spatial separation of target and competing speech, but pupil dilation wa...