By Emily Leclerc, Waisman Science Writer
The Waisman Center has been at the forefront of research on cochlear implants and hearing science for more than two decades. Cochlear implants are a surgically implanted device that can allow individuals with hearing loss, particularly those who are deaf, to hear again. This is the first and only sense that scientists and physicians have been able to replace. None of the other senses can be replaced with biomedical intervention. “Cochlear implants are the most remarkable feat of modern biomedical engineering,” Ruth Litovsky, PhD, Waisman investigator, Oros Bascom Chair, professor of communication sciences and disorders, and an associate dean in the College of Letters and Sciences. says.
Despite cochlear implants’ impressive ability to restore hearing, the technology is not perfect and CIs have some significant limitations that researchers like Litovsky, are trying to better understand and work to improve. For example, the signal CIs send to the brain can be hard to interpret because it is missing some of the information that ‘typical’ sounds contain. Noisy environments make it particularly hard for cochlear implant users to understand speech and to know which sound to focus on. These issues are often very challenging for those who have cochlear implants.
Waisman has two investigators, Litovsky and Carlos Benítez-Barrera, who are studying how to improve outcomes in patients with cochlear implants and how to have these medical devices provide better information to augment an individual’s ability to communicate.
Inner workings of the cochlear implant
Cochlear implants work by stimulating the sensory organ inside the ear, the cochlea, with electrodes. The cochlea is a small delicate organ in the shape of a spiral that is situated in the inner ear, past the ear drum and middle ear. “Inside the cochlea there are tens of thousands of hair cells arranged in that spiral shape. Along the membrane that holds those hair cells, different places are sensitive to frequencies that range from high to low. A common cause of deafness is loss of function of these hair cells,” says Litovsky, a preeminent expert on cochlear implants. A cochlear implant is designed to essentially take the place of those hair cells as much as possible.
The speech processor portion of the implant, which sits on the outside of the ear similar to a hearing aid, takes in sound and breaks it down into what are called bands or channels. The bands are then sent to a stimulator, which is attached to the skull, that converts acoustic sounds into electrical pulses. The pulses are then sent through the 12 to 22 electrodes implanted in the cochlea. “Somehow the brain figures out that there is meaningful speech in those pulses of electricity. It is stunning,” Litovsky says.
Working together to localize sound
Litovsky, who has been with the Waisman Center for more than twenty years, has been working on finding ways to make cochlear implants better at utilizing binaural cues. Binaural cues are information that the brain takes from both ears to determine where sounds are coming from, otherwise known as localizing sounds. Sound localization is important for safety and orientation. It can make it easier to focus on particular sounds in a noisy environment, like someone speaking to you in a loud restaurant. “The brain acts like a machine that computes where the sounds are coming from,” Litovsky says.
For individuals with cochlear implants, deciphering speech in a noisy environment can be particularly hard because the implants do not convey most of the binaural cues that hearing brains pick up on. Litovsky has dedicated much of her research to rectifying this issue. “We temporarily remove their clinical processors and fit them with research ones, to introduce good binaural cues. It turns out that the signals arriving at the two ears are poorly synchronized in clinical processors, i.e., binaural information is poorly represented. With our research tools we can carefully control binaural cues arriving at the electrodes,” Litovsky says. “In the real world, the implants are not capturing all of those beautiful binaural cues that a hearing brain does. The question has been can they capture them if I force the devices to communicate and synchronize? Can the brain of somebody with cochlear implants use that information?”
Litovsky continues to work on these questions and ways to better understand how to redesign cochlear implants to utilize binaural cues. To add on to her long-standing work, she is now beginning studies using neuroimaging to better understand how the structure of the brain in individuals with hearing loss is related to the behavior they are displaying. Using electroencephalogram (EEG) and functional near-infrared spectroscopy (fNIRS), Litovsky and her lab wish to better illuminate the neural mechanisms in cochlear implant wearers to understand how neural representation connects to behavior.
New investigator helps cut through the noise
Carlos Benítez-Barrera, PhD, assistant professor of communication sciences and disorders and a Waisman investigator since 2022, is similarly working to help individuals with cochlear implants be better able to hear speech and noise in more cluttered environments. “Cochlear implants only have so many electrodes so you are only stimulating certain regions of the cochlea. The signal reaching the ears is somewhat distorted and it can be tough to make sense of it. When their environment gets too noisy it’s really hard to know what’s going on,” Benítez-Barrera says.
In class or in more controlled environments it is easier to control background noise which gives individuals with cochlear implants better access to someone speaking. But the larger world is a noisy place and that can limit these individuals’ access to language, Benítez-Barrera says. He is using EEG to compare how the brain in children with typical hearing processes speech in noisy environments with how the brain processes speech in children with hearing loss. Understanding the differences will point Benítez-Barrera toward how to help children and adults with hearing loss encode speech more efficiently.
In addition to this line of research, Benítez-Barrera is also investigating how this trouble hearing speech amidst background noise impacts a child’s language development. “We know that [individuals with cochlear implants] struggle listening in noise. So, if they live in a very noisy house, does this have an impact on their language development compared to another child with hearing loss that lives in a quieter house?” he says.
Connecting hearing and intellectual and developmental disabilities (IDDs)
Hearing loss and cochlear implant research may not seem like they have a direct link to intellectual and developmental disabilities, but both Litovsky and Benítez-Barrera see important connections between their work and IDDs. Several IDDs are often accompanied by some level of hearing loss. In individuals with Down syndrome, for example, it is estimated that the incidence of hearing loss is around 70%. “Hearing loss on top of a developmental or neurodevelopmental cognitive disability creates an even greater issue with access to language, communication skills, and brain development,” Litovsky says. Both Litovsky and Benítez-Barrera have research projects focused on hearing loss in individuals with Down syndrome. Their work in hearing loss and the improvement of cochlear implants occupies an important intersection between hearing ability, communication, and intellectual and developmental disabilities.
“I think that hearing disorders are often under-appreciated, under-assessed, and not well understood not just in typical people but also in populations that have disabilities. Having access to sound builds better brain networks involved in communication,” Litovsky says. “Mainstreaming people with hearing loss in society and diversifying access to resources in audiology has been widely recognized by the NIH (National Institutes of Health) and other funding agencies as an urgent area of priority. But there remains a huge need to advocate for people with hearing loss in many aspects of our society. Our work plays an important role in helping to advance the Waisman Center’s mission.”
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