Realising the potential of cochlear implants

A capacity to adapt to altered inputs is critical for maintaining perceptual abilities following sensory impairments. We have shown that the auditory system can compensate for changes in the balance of inputs between the two ears, both during development and in later life, and maintain accurate sound localization in spite of reduced hearing in one ear. Our research in animals and humans has demonstrated that this can be achieved either through adaptive shifts in neuronal sensitivity to the altered binaural cues or by reweighting different spatial cues according to their relative reliability. The degree to which each strategy for adaptation is used depends on the way that subjects are trained, suggesting that targeted training strategies may be helpful for restoring sound localization in clinical populations according to their residual hearing ability and therefore the localization cues that remain available. Importantly, our recent work suggests that adaptation to asymmetric hearing loss generalizes to untrained sounds and to more challenging listening conditions. Furthermore, we have shown that visual information can influence auditory spatial judgments and the weighting of different spatial cues, and that a multisensory training paradigm can help to improve auditory spatial processing and perception following bilateral cochlear implantation. 

Professor Andrew King FMedSci FRS, University of Oxford, UK

Professor Andrew King FMedSci FRS, University of Oxford, UK

Andrew King is a Wellcome Principal Research Fellow and Professor of Neurophysiology at the University of Oxford and the Director of the Centre for Integrative Neuroscience in the Department of Physiology, Anatomy and Genetics. He studied physiology at King’s College London and obtained his PhD from the National Institute for Medical Research. He has worked at the University of Oxford since then, with a spell at the Eye Research Institute in Boston, and his research has been supported by fellowships from the SERC, the Lister Institute of Preventive Medicine and the Wellcome Trust. Andrew was awarded the Wellcome Prize in Physiology and is a Fellow of the Royal Society, the Academy of Medical Sciences, and the Physiological Society. His group studies how the auditory brain adapts to the rapidly changing statistics that characterize real-life soundscapes, integrates other sensory and motor-related signals, and learns to compensate for the altered auditory inputs resulting from hearing impairment.

Congenital deafness affects the functional and anatomical properties of the auditory system (Kral et al, 2019, Ann Rev Neurosci). It has been suggested that congenital deafness affects predominantly corticocortical functional connections within but also beyond the auditory system (the connectome model of deafness, Kral et al, 2016, Lancet Neurol). This may lead to increased risk of cognitive deficits in deaf children. Indeed, so-called induced responses, indicative of corticocortical interactions, were reduced in the auditory cortex of congenitally deaf cats (Yusuf et al, 2017, Brain). Here Dr Kral’s group directly investigated effective connectivity between primary and secondary areas of congenitally deaf cats (CDC).

In adult hearing cats (HC) and CDCs, responses to acoustic and electric stimulation (through a cochlear implant) were compared. Recordings were in the primary auditory field (A1) and the higher order posterior auditory field (PAF) using multielectrode arrays. Penetrations were histologically reconstructed. For effective connectivity pairwise phase consistency, weighted phase-lag index and nonparametric Granger causality were determined and compared. Additionally, spike-field coherence was computed between different recording sites.

CDCs demonstrated a substantially reduced stimulus-related corticocortical coupling in the connectivity measures used. Largest deficits were observed in sensory-related top-down interactions, in the alpha and beta band (Yusuf et al, 2021, Front Neurosci). Furthermore, spike-field coherence revealed a decoupling of supragranular and infragranular layers in A1 (Yusuf et al, 2022, Front Syst Neurosci), likely responsible for dystrophic changes in the deep cortical layers (Berger et al, 2017, J Comp Neurol). The data document that corticocortical interactions are dependent on developmental hearing experience. These observations suggests that the congenitally deaf brain cannot incorporate top-down prediction information into auditory processing and thus have a deficient predictive processing.

Professor Andrej Kral, Hannover Medical School, Germany

Professor Andrej Kral, Hannover Medical School, Germany

Andrej Kral studied general medicine in Bratislava, Slovakia (Comenius University, MD 1993, PhD 1998). After his PhD he focused on neuroscience of cochlear implants and brain development in deafness (Institute of Sensory Physiology, J.W.Goethe University, Frankfurt am Main). He was appointed associate professor of physiology (‘Priv-Doz’, J.W.Goethe University) in 2002. From 2004 to 2009 he was Professor of Neurophysiology at the University of Hamburg. Since 2009 he has been Chaired Professor of Auditory Neuroscience at the Medical University Hannover and the director of the Department of Experimental Otology. Since 2004 he has been Adjunct Professor of Neuroscience and Cognition at the University of Texas at Dallas, USA, and since 2018 Professor of Systems Neuroscience at Macquarie University, Sydney, Australia. He serves as chair of the PhD Program ‘Auditory Sciences’ in Hannover. A Kral is an elected member of the German National Academy of Science and of the international Collegium Oto-Rhino-Laryngologicum Amicitiae Sacrum (CORLAS).

To successfully design devices for the human body, engineers often view the body itself as the ideal design template. Similarly, for individuals missing a limb, the development of artificial prosthetic limbs often centers on embodiment as the goal: focusing device design and control on becoming more like our biological bodies. But ultimately, the success of artificial limb will critically depend on its neural representation in our brains. Importantly, neurocognitive resources might differ radically, depending on the user’s life experiences and needs. Here I will present a series of studies where we investigated the neural basis of artificial limb use for both substitution and augmentation technologies. We find that contrary to folk wisdom, the brain does not assimilate neural representations for the artificial limb with those for the biological body, creating opportunities for novel technological interfaces. Collectively, these studies suggest that although, in principle, opportunities exist for harnessing hand neural and cognitive resources to control artificial limbs, alternative non-biomimetic approaches could be also well suited for successful human-device interface.  

Professor Tamar Makin, University of Cambridge, UK

Professor Tamar Makin, University of Cambridge, UK

Dr Tamar Makin is a Programme Leader at the MRC Cognition and Brain Unit at Cambridge University and the leader of the Plasticity Lab. She was previously a Professor of Cognitive Neuroscience at UCL’s Institute of Cognitive Neuroscience. Her main interest is in understanding how our body representation changes in the brain (brain plasticity). Her primary model for this work is studying individuals with a hand loss. Tamar graduated from the Brain and Behavioural Sciences programme at the Hebrew University of Jerusalem in 2009. She was then awarded several career development fellowships to establish her research programme on brain plasticity in amputees at the University of Oxford, first as Research Fellow and later as a Principal Investigator. She joined the faculty of UCL in 2016 and moved to Cambridge in 2022. She is currently supported by the European Research Council (Starting and Consolidator Grants), the Wellcome Trust (Senior Research Fellow) and the UK Engineering and Physical Sciences Research Council. 

Cochlear implant (CI) users with a prelingual onset of hearing loss show poor sensitivity to interaural time differences (ITDs), an important cue for sound localization and speech reception in noise. Similarly, neural ITD sensitivity in the inferior colliculus (IC) of neonatally-deafened animals is degraded compared with animals deafened as adults. Here, neonatally-deafened rabbits were provided with chronic bilateral CI stimulation to investigate whether auditory experience through bilateral CIs during development can reverse the effect of early-onset deafness on ITD sensitivity. In the early-deaf rabbits that received chronic stimulation during development, the prevalence of ITD sensitive neurons was restored to the level of adult-deaf rabbits. Also neural ITD sensitivity in the early-deaf and stimulated rabbits improved partially. In contrast, chronic CI stimulation did not improve temporal coding in early-deaf rabbits. These findings highlight the importance of auditory experience during development on the maturation of binaural circuitry.

Dr Yoojin Chung, Harvard Medical School, USA

Dr Yoojin Chung, Harvard Medical School, USA

Dr. Yoojin Chung obtained her bachelor’s degree in electrical engineering at Seoul National University (Seoul, South Korea) and her doctorate in biomedical engineering at Boston University (Boston, USA). As an Instructor in Otolaryngology at Harvard Medical School and Massachusetts Eye and Ear Infirmary (Boston, USA), Dr. Chung focused on understanding how the auditory system processes sounds presented through cochlear implants (CI) with the goal of improving the perceptual performance in CI.  
Currently she is a senior scientist at Decibel Therapeutics developing gene therapies to repair and restore hearing and balance function. 

Our lab studies children and adults who receive bilateral cochlear implants (BiCI), or adults with single-sided deafness receiving a CI in the deaf ear (SSD-CI). Bilateral hearing typically improves localization of sounds and segregation of speech from background noise compared with unilateral hearing. However, patients typically perform worse than normal hearing listeners. We use several approaches to understand mechanisms driving gaps in performance, including asymmetry in sensitivity to monaural information such as modulation detection. Further interaural mismatch in place of stimulation along the cochlea, age at onset of deafness and age at implantation contribute to recovery of binaural hearing. Because CI processors do not preserve binaural cues with fidelity, we use research processors to generate multi-channel binaural stimulation strategies that introduce different rates of stimulation across the electrode arrays, thereby preserving rates that are important for both binaural sensitivity and speech understanding. In addition, eye gaze studies reveal developmental factors that in decision-making that are not observed with measures of threshold. Finally, pupillometry studies provide insight into the impact of integrating inputs from two ears, whereby in some instances improved performance with two ears can be “costly” in the listening effort domain.

Professor Ruth Litovsky, University of Wisconsin-Madison, USA

Professor Ruth Litovsky, University of Wisconsin-Madison, USA

Ruth Litovsky is Oros Family chair, department chair and director of the Binaural hearing and speech lab at the Waisman Center. She received her PhD in psychology and psychoacoustics and completed a postdoctoral fellowship in auditory neuroscience. She has published over 120 papers and book chapters. Her research focuses hearing abilities covering lifespan of humans to include infants and elderly adults. Her research program focuses on hearing restoration with cochlear implants, in particular the impact of bilateral implantation, and more recently single-sided deafness. Her lab also uses pupillometry and eye gaze measures to examine ‘real time’ processing of information and listening effort during perceptual tasks. Her lab is also beginning to study auditory function in conjunction with cognition and language in young adults with Down Syndrome.

Professor Litovsky teaches courses in undergraduate and graduate programs, serves on numerous national and international grant review panels and editorial boards, and in various positions of leadership in the research community. She is President Elect of the Association for Research in Otolaryngology. She has received a number of awards, including a Fulbright Senior Fellowship. Her lab has over 20 students, postdoctoral fellows, engineers and audiologists. Her ongoing research program has been funded for over 20 years by grants from the NIH-NIDCD. 

 

Bilateral cochlear implants (CIs) greatly improve the spatial hearing of CI users compare to those with only one implant. However substantial gaps still exist between bilateral CI users and normal hearing listeners in different aspects, such as their lateralization and localization performance. Computer models are expected to partially reveal the possible reasons behind such gaps, by dissecting the specific contribution of each stage along the whole processing chain. In order to provide a model framework for predicating bilateral CI users’ lateralization or localization performance in a variety of experimental scenarios, here a recently published electrically stimulated single excitation-inhibition (EI) model neuron (Hu et al. 2022, JARO) was extend to population-EI-model neurons. The proposed model framework includes (a) parameter initialization, (b) binaural signal generating, (c) switchable CI processing, (d) auditory nerve (AN)- and EI- neuron processing, and (e) decision model stages. For demonstration purposes, several bilateral CIs experiments were simulated, such as pulse rate limitation of ITD sensitivity; left/right discrimination or lateralization; free-field localization with and without two independent automatic gain controls (AGCs). In general, the model framework could capture the average performance of all selected experiments even with the same EI-model units and a very simplified decision model.

Dr Hongmei Hu, Carl von Ossietzky-Universität Oldenburg, Germany

Dr Hongmei Hu, Carl von Ossietzky-Universität Oldenburg, Germany

After achieving her PhD in Information and Signal Processing in Southeast University (China), Dr Hongmei Hu moved to the United Kingdom and worked as a Marie Curie senior researcher in Southampton University in 2010. Then she was promoted from a lecturer to a tenured associate professor in Jiangsu University, China in 2012 where she received both her Bachelor and Master degrees. For personal interests, Dr Hongmei Hu moved to Carl von Ossietzky-Universität Oldenburg in Germany in 2013 to continue her researches in the field of the binaural hearing, especially focusing on further developing new techniques for bilateral cochlear implants (CIs). Her current main research threads are: 1) subjective perception (psychoacoustic) and objective (Electroencephalography, EEG) measurements based fitting strategies for CI users; 2) advanced signal processing and electric modelling for CIs; 3) speech in noise for tonal and non-tonal languages using our newly developed Mandarin and the exist Matrix sentence tests.

Bilateral cochlear implants (CIs) have provided hearing advantages over use of a single CI in children with deafness in both ears. Yet, the goal of promoting normal binaural/spatial hearing development in these children remains elusive.  Evidence of remaining abnormalities in children using bilateral CIs comes from behavioral measures of impaired sound localization and sensitivity to binaural cues. These challenges may reflect abnormalities in integration of bilateral CI input in the auditory brainstem and cortex, including disruptions in functional connectivity, as measured with electrophysiology in children. We suggest that mismatches in bilateral CI input contribute to binaural/spatial hearing challenges by creating an aural preference for one ear when implantation of the opposite ear is delayed or allowing ongoing asymmetries or mismatches in stimulation of the two ears. While the former problem can be addressed through early access to CI in each candidate ear, there is no present protocol to harmonize the separate program parameters and processing of independent CIs.  Present studies seek to identify these sources of mismatched bilateral CI input and to provide methods that might be used to resolve these challenges to binaural hearing development.

Professor Karen Gordon, The Hospital for Sick Children, University of Toronto, Canada

Professor Karen Gordon, The Hospital for Sick Children, University of Toronto, Canada

Karen Gordon, PhD, is a Professor in the Department of Otolaryngology and a Graduate Faculty Member in the Institute of Medical Science at the University of Toronto.  She works at the Hospital for Sick Children in Toronto, Ontario, Canada, as a Senior Scientist in the Research Institute and an Audiologist in the Department of Communication Disorders.  She is Director of Research in Archie’s Cochlear Implant Laboratory and holds the Bastable-Potts Health Clinician Scientist Award in Hearing Impairment and Cochlear Americas Chair of Auditory Development. Karen’s research focuses on auditory development in children who are deaf and use auditory prostheses including cochlear implants.   Her work is supported by research funding from the Canadian Institutes of Health Research along with the Cochlear Americas Chair in Auditory Development and generous donations.  

The provision of a cochlear implant to patients who are single-sided deaf has allowed researchers to estimate the sound quality, or voice, of a cochlear implant. Clean signals can be directed to the implanted ear and then signals that might sound like the CI (candidate signals) can be directed to the normal hearing ear. Patients can then tell the researcher how close the candidate signals come to the sound of the CI. To create close approximations to CI sound quality, the group currently choose from the following operations: 1) flatten the intonation contour, 2) upshift F0, 3) upshift formant frequencies, 4) smear the signal in the frequency domain, 5) add a flange effect, 6) combine natural speech signals and noise or sine vocoders, 7) smear the signal in the time domain and 8) high pass, low pass or band-pass the signal. Altering natural speech signals using subsets of these operations has allowed us to create close matches to CI sound quality for many patients. A muffled sound quality dominates the matches for many patients while a higher ‘pitch’ created by an upshifted formant pattern or F0 dominates the matches for others. A small number of patients match to signals that are almost clean – an outcome that seems very unlikely.

Professor Michael Dorman

Professor Michael Dorman

Professor Dorman received his Ph.D. in Experimental Child and Developmental Psychology with a Linguistics minor from the University of Connecticut. For a period following his degree, and while on the faculty of the City University of New York, he was a staff scientist at Haskins Laboratories in New Haven, Connecticut working on problems of speech acoustics and speech motor control.  He then moved to Arizona State University where he is currently Professor Emeritus in the College of Health Solutions. Professor Dorman has over 200 publications in areas including speech perception, cortical lateralization of function, neural plasticity and cochlear implants.  His laboratory developed the AzBio sentence lists which are the industry standard for testing cochlear patients in the United States.