Objective Assessment of Auditory Pathway Integrity and Functional Hearing Abilities

Background: The U.S. involvement in Iraq and Afghanistan has resulted in unprecedented amounts of trauma to the auditory system. Even in cases where noise-exposed individuals are not suffering from severe neurological problems, the DoD and VA health care communities have reported concomitant damage to peripheral, central, and cognitive auditory structures from excessive amounts of noise. This damage may manifest itself in patients complaining of difficulty understanding speech in complex backgrounds while at the same time presenting with normal to near-normal audiometric thresholds. Standard clinical audiometric tests fail to properly diagnose the true extent of hearing damage, and current rehabilitation strategies do not restore hearing to a functioning status appropriate for military readiness. Therefore, auditory trauma poses a great challenge to VA and DoD audiologists. To address this problem, biotechnology companies are working on techniques to restore hearing through regeneration of cochlear hair cells. This proposal seeks to develop and validate an objective battery of tests to track the various stages of reintegration and reorganization necessary to restore hearing to a functional state appropriate for active-duty service.Objective/Hypotheses: This study will address the critical need for a functional assessment battery of tests that is able to track the progress of hearing restoration strategies involving hair cell regeneration. The study will validate an assessment strategy that measures the physical integrity of auditory pathways as well as the return to normal hearing function.Specific Aims: (1) Identify objective pre-attentive and unbiased measures that differentiate between normal hearing systems and those with auditory dysfunction, and establish expected values of those measures for varying degrees of hearing loss. (2) Assess differences between individual’s objective test measures between sessions to determine reliability of each of the proposed test measures. (3) Evaluate the ability to predict functional measures of speech in noise performance from objective measures of the physical integrity and functional capabilities of the auditory system.Study Design: This study uses standard audiological mid-level and cortical EEG measures to assess the restoration of hearing following newly developed therapeutic interventions. Three groups of listeners with normal, mild, and moderate-to-severe auditory thresholds will be evaluated. By measuring auditory system integrity and function as a function of hearing loss, the study provides benchmarks throughout the restoration process. Initial progress of hearing restoration may be limited to cochlea mechanics and auditory nerve stimulation sufficient to demonstrate an awareness to sound but not sufficient to identify words or segregate multiple speakers and other acoustic objects within a complex listening environment. Importantly, the proposed assessment battery will allow researchers to demonstrate success in future FDA clinical trials even in cases where restoration of hearing is incomplete. In addition to developing and evaluating the functional assessment battery across a wide range of hearing loss, the stability and reliability of the assessment battery will be assessed by having a subset of subjects repeat the battery. Finally, analyses will be carried out to determine the extent to which results from the various electrophysiological measures can predict a standard speech-in-noise test.Military Benefit/Impact: It is projected that in the next 1.5 to 3 years, FDA clinical trials will begin on new biotechnologies that can restore damaged hair cells within the cochlea. However, evidence that these new hair cells correctly reintegrate with the auditory system to restore not only the awareness of sound but also the basic functionality of hearing necessary to return to active duty is scant. This proposal seeks to develop and validate a functional assessment battery that will track the progress of reintegration and cortical reorganization by measuring the physical integrity of auditory pathways from cochlea to cortex as well as measuring functional capability for both basic auditory building blocks, such as frequency and intensity discrimination, and more complex discriminations requiring sensitivity to modulations in the background noise and awareness of the spatial locations of multiple auditory objects.Translation: Development of an objective test battery that more thoroughly assesses auditory integrity and function will provide investigators superior validation techniques for the success of hair cell restoration through the function of the auditory system from the periphery through the cortex. However, its use within audiology clinics for patients receiving other hearing restoration treatments (hearing aids, cochlear implants, auditory training) will also prove beneficial by more fully assessing auditory dysfunction and patients’ needs.