The ability to hear and respond to auditory stimuli in our environment is crucial to our well-being and survival, such as understanding what others are saying in a conversation at work and reacting to the honks of a car coming down the street. Studies have shown that we need input to both ears to accurately identify where auditory cues are coming from.1-3 While people with normal hearing can accomplish this fairly easily, pinpointing the location of a sound source can be challenging for those with hearing loss, even if they are using bilateral cochlear implants.4-6
One factor contributing to this difficulty is that current cochlear implant processors are not coordinated and discard temporal fine structure, the rapid oscillations close to the center frequency, which limits the access to interaural time differences for bilateral cochlear implant (BiCI) users. Interaural time differences result from differences in the travel time from a sound source to the ears closer to and farther from the source, and provide a cue to the direction or angle of the sound source from the head.7 Manufacturers are trying to improve signal processing of bilateral devices to correct this problem, but so far those efforts have not led to tangible changes.
Research will be crucial to driving the development of better technology, and that's what Ruth Litovsky, PhD, a professor and the department chair of the communication sciences and disorders at the University of Wisconsin-Madison, hopes to achieve with her work on the topic. She and her colleagues at the University of Wisconsin-Madison decided to address a neglected but important area in sound localization research—BiCI users’ ability to track a moving sound source. Much of the literature to date has only looked at sound localization of stationary sources. “Studying auditory motion is a more realistic approach to understanding how people with hearing loss function in realistic listening situations,” Litovsky said. “Past research only focused on stationary sounds, but in the real world we are often faced with tracking information from sounds that are in motion.”
Litovsky's study used a tiered system to measure the ability of adults who are deaf and hear through bilateral cochlear implants and that of people with normal hearing to track auditory motion.8 They had stationary sounds for comparison as well. Participants were first asked to indicate whether the sound was stationary or moving, and, if the sound was moving, to identify its direction and range of motion. “We hypothesized that having to track moving sound may lead to poorer spatial hearing performance in BiCI users because they do not receive spatial cues with fine grained resolution,” Litovsky said.
Prior studies comparing the stationary sound localization ability of people with normal hearing with that of BiCI users found that those who had no hearing impairment far outperformed the other group, and that's what Litovsky and her colleagues anticipated and discovered for auditory motion tracking in their study. Both the normal hearing and BiCI groups performed best in the condition with the greatest sound movement. There was great variability in BiCI users’ ability to distinguish between a moving and stationary sound source, and they generally overshot the actual range of sound movement, especially for longer stimulus duration.
Not all findings were as they expected, however. “Given how poorly cochlear implants represent localization cues, we were surprised to see that participants with cochlear implants did have some ability to recognize if sound sources were moving v. stationary with location separations of 40 degrees,” Litovsky said. “This is much worse than normal hearing listeners, but they were not guessing all the time, which shows that in the real world when tracking moving sounds like a moving bus, they could have some ability to recognize if a sound is moving or stationary.”
Litovsky also wants to highlight in her research the importance of recognizing that people who are deaf and use bilateral cochlear implants perform much better when using both implants than when only using a single implant. “Without both implants, they would be guessing as to where the sounds are coming from,” she said. “However, these findings suggest that conventional cochlear implant processors are not able to fully provide the cues necessary for perceiving auditory motion correctly.”
Despite the shortcomings of current CI technology regarding sound localization and spatial hearing, audiologists can still apply the insights from this study to help patients who are BiCI users improve their hearing experience. “In counseling patients, audiologists should discuss how well patients think they are able to localize sounds, especially moving sound sources,” Litovsky said. “Similar to the importance of stationary sounds, mapping cochlear implants in both ears in a manner that balances the loudness across the ears can most likely improve detection of auditory motion.”
Research on this topic is ongoing, and Litovsky said another study on a different population of CI users is already underway. “We are working on these areas with people who are deaf in one ear and normal hearing in the other ear (single-sided deaf with a cochlear implant),” she said. “We are currently working on analyzing our results in this population and hope to publish them very soon.”