Diagnosing Contributions of Sensory and Cognitive Deficits to Hearing Dysfunction in Blast-Exposed/TBI Service Members
Abstract
Many Service members returning from deployment complain of problems communicating in noisy settings, especially when they are trying to understand speech when there are competing sounds. However, when they go to the audiological clinic, they are often found to have "normal hearing," meaning that they can detect quiet sounds as easily as healthy young adults without hearing dysfunction. To date, the reasons for these kinds of hearing difficulties are not well understood, and, therefore, there is no evidence-based approach to treating such problems. Anecdotally, blast exposure, which many returning Service members have suffered, seems to cause or exacerbate these kinds of complaints. An ongoing study of the prevalence of hearing dysfunction in Service members at Walter Reed National Military Medical Center (WRNMMC) on the impact of hearing dysfunction on fitness for duty suggests that as many as 40% of Service members deployed to Iraq or Afghanistan experienced high-intensity blasts and that roughly 40% of these blast-exposed personnel show functional hearing deficits, despite having normal hearing thresholds, leading to the staggering suggestion that 15%-20% of returning Service members may fall into this category of listeners with "normal hearing" who nonetheless have difficulty communicating in everyday environments. The current project builds on emerging evidence that suggest that in listeners with "normal" hearing, there are at least two quite distinct deficits that can contribute to problems understanding speech amidst competing sources, but through very different mechanisms. The first, a sensory deficit, is thought to arise from a loss in the number of auditory nerve fibers conveying sound from the cochlea to the brain. This "auditory neuropathy," which is now well documented in animals, does not always affect hearing thresholds, but degrades the fidelity with which auditory information is encoded. Auditory neuropathy is associated with noise exposure, which may explain why Service members are particularly prone to experiencing difficulty. The second, a cortical deficit, is thought to arise from damage to cortical structures that are engaged when a listener is trying to focus attention on one sound and ignore competing sounds. This process of directing attention changes the information that the brain extracts from a scene, but depends on many different neural regions coordinating their activity in precise ways. Blast exposure experienced by Service members is likely damaging parts of the brain (and connections between brain regions) that are critical for controlling this "auditory selective attention," thus leading to communication disorders. The first and second goals of this project are to test Service members who have clinically "normal hearing" (based on current audiometric tests) to determine how both sensory and cortical deficits (respectively) contribute to their problems. The third goal is to develop rapid, reliable, and repeatable tests that can separately quantify sensory coding fidelity and the ability of an individual listener to cortically control selective auditory attention. Five to eight years from now, such tests could be in the clinic, revolutionizing how audiologists think about hearing function, and how they counsel their clients. Results from the proposed study will build new understanding of the mechanisms that lead to hearing dysfunction that is currently "invisible." By uncovering the core deficiencies leading to difficulties communicating in noisy settings and developing robust diagnostic tests, new interventions and therapies may be developed that target specific deficits in individual patients. Results are likely to aid in the development of appropriate evaluations of auditory fitness for duty, ensuring more Service members are operating effectively in their military roles. In Veteran Service members, where age-related hearing loss is likely to be exacerbated by noise and
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 04, 2016
- Source ID
- W81XWH1510490
Entities
People
- Barbara Shinn-Cunningham
Organizations
- Boston University
- United States Army