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Recent Advances in Hearing and Balance Research

Hair Cells

  • Scientists have identified TMC1, TMC2, TMHS, and TMIE as proteins important in the conversion of sound-evoked mechanical motion in the inner ear into electric signals to the brain. This knowledge has fundamentally advanced our understanding of how hair cells work.9-15
  • High-throughput RNA-sequencing has provided scientists with new insights into the distinct molecular characteristics that occur during the formation of different cell types in the organ of Corti, including hair cells. This information may aid in development of cell-based therapies for treating hearing loss and balance disorders.16-20
  • Scientists found that a group of gene regulators called Regulatory Factor Xs (RFXs) helps to drive genes that are preferentially active in hair cells in mice. The researchers concluded that the RFX gene regulators, while not crucial early in the development of hair cells, are necessary for the cells' maturation and long-term survival.21
  • Scientists have used proteomics to identify new proteins expressed in hair cell stereociliary bundles. This approach has revealed new insights into hair cell function 22, 23 and identified new components of the hair bundle necessary for hearing and balance.24

Development and Regeneration

  • Wnt signaling and Lgr5-expression have been shown to be key for the generation of hair cells in the developing cochlea.25, 26
  • Scientists have developed an in vitro technique to turn embryonic stem cells into inner ear hair cells and supporting cells. This technique is well suited for high-throughput screening of drugs for hair cell regeneration.27
  • Antisense oligonucleotides have been used to rescue hearing and balance function in a mouse model of human deafness.28
  • In the research laboratory, it is now possible to prevent hearing loss and stimulate repair or regenerate sensory cells of the inner ear by transdifferentiating or directly reprogramming cells, or by using gene therapy in animal models.29-31

Hearing Loss

  • Damage to spiral ganglion neurons or their synapses in the inner ear may contribute to hearing loss. Scientists have discovered that the synapses between cochlear nerve fibers and inner hair cells are the most vulnerable elements in noise-induced and age-related hearing loss and nerve fibers with high response thresholds are the first to degenerate, which likely contributes to problems with hearing in noisy environments.32-37
  • Scientists have determined that unmyelinated type II sensory fibers innervating outer hair cells respond to cellular damage resulting from loud sound and thus may serve as the nociceptors of the inner ear.38, 39
  • Dozens of new gene defects responsible for hereditary hearing loss have been identified in recent years, including mutations in the first microRNA (miR-96) involved in hearing loss.40, 41
  • The combination of using whole exome sequencing (a technique for sequencing all the expressed genes in a genome) and hearing testing is ushering in a new area of personalized diagnoses, opportunity for earlier intervention, and ultimately, treatment for individuals with hearing loss.42-50
  • Gene therapy is being used to correct gene defects that cause hereditary hearing loss and restore auditory function in animal models.51-53
  • The use of high-throughput screening in zebrafish is leading to the discovery of new protective compounds that will help diminish or prevent noise- or drug-induced hearing impairment.54-57
  • Proof-of-principle studies have shown that small molecules delivered to the cochlea after noise damage can lead to some hair cell regeneration and some functional recovery.58
  • Preliminary studies suggest that, in older adults, hearing impairment is associated with cognitive decline, dementia, and depression. Estimated declines are greatest in participants who do not wear a hearing aid. Although data do not currently support a causative relationship, they support future research on causation and potential for reversal with interventions for treatment of hearing loss.59, 60
  • Scientists have identified the genetic bases for accelerated age-related hearing loss in humans.61
  • Research has shown that genetically producing overexpression of proteins called neurotrophins in the inner ear can elicit regeneration of cochlear synapses after noise damage.62

Otitis Media

  • Research has advanced understanding of cell signaling and gene expression patterns of the innate immune system in response to an ear infection (otitis media). 63, 64
  • The study of microbial genomes has provided a cost-effective and high-throughput tool to determine genome content of a bacterium that causes ear infections.65
  • Scientists have identified and characterized new vaccine candidates with the potential for preventing ear infections.66-68
  • To better treat ear infections, scientists have developed a new, noninvasive drug delivery system for the administration of antibiotics and anti-inflammatory agents across the eardrum.69, 70
  • Researchers have described how the inflammation induced by bacterial infections treated with aminoglycoside antibiotics potentiates the undesirable side effect of hearing loss.71

Hearing Aids

  • Advanced digital technology hearing aids provide noise reduction, directional hearing, and feedback suppression. Binaural hearing aids further improve sound source localization and spatial separation.72

Cochlear Implants and Other Implantable Hearing Devices

  • Hybrid devices that combine both electric and acoustic stimulation allow individuals with preserved low-frequency hearing and un-aidable high frequency loss to utilize a combination device that includes a cochlear implant for stimulation of high frequencies and a hearing aid to enhance residual low frequency hearing.73-75
  • Scientists are studying further expansion of cochlear implant candidacy in individuals with unilateral deafness who received a cochlear implant. They showed significant improvement in speech perception performance in quiet and in noise after implantation.76 Another study has shown the benefit of cochlear implants in reducing tinnitus in individuals with unilateral hearing loss.77
  • More focused electrical stimulation can improve performance for existing cochlear implant users by limiting the overlap between the number of neurons stimulated by different sound frequencies.78, 79
  • For individuals in whom cochlear implantation is not an option, auditory brainstem implants now offer an alternative.80

Balance Disorders

  • Similar to the benefit of cochlear implants, vestibular implants provide a means of stimulating the afferent nerves within semicircular canals of the inner ear vestibular system. The vestibular prosthesis can mimic the natural vestibular signals 81 to the brain without causing surrounding tissue damage.82 A variety of vestibular disorders can potentially be treated with such a prosthesis.83

Tinnitus

  • When cochlear hearing loss occurs, the brain becomes more sensitive to sound to compensate for the reduced peripheral input. Too much sensitivity can make everyday sounds seem too loud (hyperacusis) or can cause ringing in the ear (tinnitus). 84
  • Tinnitus and hyperacusis likely involve distributed neural networks that connect multiple brain regions rather than one discrete region. Increased connection and activity between auditory areas of the brain and those associated with emotion, memory, attention, arousal, and spatial location may contribute to some of the maladaptive features of these disorders (e.g., anxiety or fear).85-89
  • Improved understanding of the disordered processes that cause tinnitus is leading to better treatments. Animal model studies have identified tinnitus-associated neural changes that commence at the cochlea and extend to more central portions of the brain that process sound. Maladaptive changes in nerve cell behavior likely underlie these changes, resulting in increased spontaneous nerve cell firing rates and synchrony (firing together) among nerve cells in parts of the brain that process sound, possibly resulting in a person “hearing” a sound when no sound stimulus is present. Scientists are currently conducting clinical trials to test the effectiveness of drugs that change the way nerve cells fire to treat acute tinnitus in people. Other new approaches including brain stimulation, such as rTMS (repetitive transcranial magnetic stimulation) 90, hold some promise. Scientists have also had some success with vagal nerve stimulation to eliminate or minimize abnormal nerve cell circuits in individuals with tinnitus. Research has shown that, after cochlear damage, upregulation of somatosensory input to the cochlear nucleus may follow reduction in auditory nerve input, resulting in heightened cochlear nucleus cell responses to somatosensory stimulation. Animals known to have tinnitus have been shown to demonstrate changes in auditory-somatosensory integration, providing a possible mechanism for the treatment of individuals with tinnitus.91, 92

Auditory and Vestibular Processing

  • Scientists have been able to determine which speech stimuli cause brain activity by making electrophysiological recordings from electrodes placed on the human brain’s surface. This advance has high significance for the future development of objective ways to measure ability in the parts of the brain that produce and process speech in individuals with normal hearing and hearing impairment.93-95
  • Several studies have established that the auditory cortex represents only the sounds of interest and is less affected by the presence of background noise than peripheral auditory neurons in the ears. These findings are crucial for understanding the mechanisms for signal detection in unfavorable listening conditions and the detrimental consequences of even mild hearing loss on those capacities.96-98
  • Scientists have made important discoveries to describe the ion channels responsible for transmitting signals to the brain that help us detect our balance and orientation in space.99, 100
  • Scientists have integrated their study of auditory and vestibular activity with other sensory systems to advance our understanding of how the nervous system combines and jointly encodes input of sound, sight, and position to improve the ability to orient ourselves with objects around us, while maintaining gaze and posture.101-106

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Last Updated Date: 
January 27, 2017