Aronoff's R01 grant aimed at maximizing binaural benefits
Bilateral cochlear implants are used to provide hearing to both ears for deaf children and adults, as well as provide binaural hearing. But the benefits of bilateral implants can be hampered by poor integration of the devices’ left and right inputs. Thanks to an R01 grant, Department of Speech and Hearing Science associate professor Justin Aronoff has a plan to combat that.
Aronoff was awarded a $1.57 million grant from the National Institute on Deafness and Other Communication Disorders for his project “The contributions of interaurally correlated signals and interaurally symmetric place of stimulation for the binaural auditory system.” The proposed study will provide insight into how the binaural auditory system combines signals from the two ears and lay the groundwork for a shift in how and when clinicians program bilateral cochlear implant users’ devices to maximize binaural benefits.
Aronoff has just begun data collection, and recently gave a demonstration of some of the study’s testing, with research assistant Simin “Tina” Soleimanifar as the subject.
In Aronoff’s lab, Tina, who does not have a cochlear implant, sat next to a scope where she can see the signal that is coming out of a cochlear implant.
“The first thing that we need to do when we're testing a cochlear implant patient is the same thing that you would do if you go into the (audiology) clinic,” Aronoff said. “And that's basically setting what are the comfort and safety levels.”
As Aronoff explains, the simulations of cochlear implants are not really simulations of what it would sound like to cochlear implant users. “Most of them are just simulations of what it would sound like to only have 22 notes on your piano,” he said, “but everything has to be done on those 22 notes. That's all you can hear. … (Renowned researcher) David Landsberger (said) listening with the cochlear implant is like playing the piano with a ping pong paddle.' That you're hitting a bunch of notes at once. And so if I turn off an electrode, that paddle gets a little wider for all the other notes. That's the way to think about it.”
To understand what the signal from the cochlear implant actually is, you need to use a scope. Aronoff said the scope is connected to breakout boards, which allow him to tap the output from each electrode and put it on a scope and record it, to make sure the signal is what he thinks it is. Different devices have a different number of electrodes, Aronoff said. He was working with a cochlear device during this test run, which has 22 electrodes. During the test, he gradually increases the amount of stimulation until Tina can see something on the scope.
Aronoff compared the electrodes to shining a flashlight beam.
“As you walk away from a wall that you're shining a flashlight beam on, the beam gets wider and wider. And these are fairly far away from the wall. What that means is if you have two flashlight beams right next to each other, they illuminate mostly the same spot on the wall. There's a little difference on the edges, but they're mostly overlapping. And that's what's happening as well with these electrodes. And so that's why when you go from one electrode to the next, you're stimulating most of the same neurons.”
One of the most important issues Aronoff hopes to tackle with this grant is about perception of interaural time differences (ITDs) and interaural level differences (ILDs), which limit the ability of bilateral cochlear implant users to localize sounds and understand speech in noisy environments.
“This is actually a big question of the grant,” he said. “We know for a pitch that it is very malleable. That over time whatever I tell you in your map, whichever electrodes get the same frequencies in the outside world will start sounding the same in terms of pitch. We don't know if that's true for ITDs and ILDs. That if the best electrodes paired together change over time or not. It definitely seems to be less malleable. We don't know if it's malleable at all. And that's a big purpose of this grant, to see if that correlated input only affects the pitch that you hear, or if it's affecting the entire auditory system.”
Another issue is that people who have two cochlear implants don’t always hear one coherent sound from the two ears. They will sometimes hear a left ear sound and a right ear sound, Aronoff said.
“If you're listening over headphones and one of them is bad, the way to tell is you lift one up. You can't be like, ‘Oh, I can hear it's the left one that's bad. You have to lift one up.' That's how well things fuse together into one perception. Now, for cochlear implant users, that's often not the case. They often do not have things fusing together completely. And so that's one thing that we look at. There's big benefits to it.”
The benefit of having bilateral cochlear implants is more than just having a backup if one implant goes out, Aronoff said. They will allow you to hear better in noisy environments.
“If you're listening to someone who's across the table from you and there's background noise, being able to spatially separate out where are the speakers from everyone else helps you. And having two ears gives you that ability. If you only have one ear, you cannot tell something's coming from the left or the right. So two ears is really what you need. And most cochlear implant users can localize reasonably well. Not as good as normal hearing listeners, but reasonably well. So that's a big benefit of having two ears as well. There's other things in terms of when someone comes up on one side of you. If it's on the side that doesn't have a cochlear implant, you might not even know they're talking to you. There's a lot of benefits of having two instead of one.
Getting a good measure of fusion has been one of the more challenging aspects of the project, Aronoff said, since fusion is a central idea to the grant, and because everyone has a different idea of what fusion means.
“A lot of the other things are largely predicated on this idea that you hear it as a coherent sound,” he said. “You can't localize a sound if it sounds like it's coming from both ears. And so, yeah, fusion is very central to this grant. And so we have a lot of experiments where we are looking at that fusion and how different things affect it. “