Interactive Effects of Aging and Blast Exposure on Central Auditory System

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Many blast-exposed Veterans report difficulty in complex listening situations (e.g., understanding speech in noise) despite normal or near-normal performance on standard audiological tests. This suggests a deficit in central auditory processing. Indeed, blast-exposed Veterans perform abnormally on central auditory tests, particularly those assessing temporal processing. A similar pattern is observed in middle age adults who often have good peripheral hearing but show deficits on tests of temporal processing and speech understanding in noise. Age-related auditory deficits have been linked to functional and anatomical changes in the central auditory system. The physiologic origin of auditory deficits in blast-exposure is currently unknown, although the similarity in behavioral profiles between aging and blast exposure suggests similar mechanisms may be affected. It is therefore possible that the auditory deficits experienced by blast-exposed Veterans will intensify as they enter middle age and beyond. This has not been investigated systematically. The present study will determine: (a) whether there is an interaction between aging and blast exposure on temporal processing and speech recognition (Specific Aim 1); (b) whether age- and/or blast-related differences in behavioral performance can be linked to neuroanatomical changes within or outside the auditory system (Specific Aim 2); and (c) whether aging, blast exposure, or their interaction lead to changes in functional representations of speech in the auditory cortex (Specific Aim 3). Groups of 50 blast-exposed and 50 unexposed Veterans will be tested. Young (18-39 years) and middle age (40-60 years) Veterans will be represented in equal numbers within each blast exposure group. Each participant will be assessed on a number of temporal processing and speech recognition tests known to reveal deficits in middle age listeners. Gray and white matter morphology will be assessed using T1- and diffusion-weighted magnetic resonance imaging (MRI). Finally, cortical activity will be measured during a competing speech task using functional MRI. We hypothesize that aging and blast exposure will produce interactive effects on performance in temporal processing and speech tests, and that performance will be linked to changes in the structural and functional integrity of the central auditory nervous system. We will test for alternatives, e.g., that auditory performance is linked to cognitive ability or brain health outside the auditory system. We expect our findings will: (a) lead to improvements in diagnosis and classification of central auditory deficits; and (b) motivate focused rehabilitation strategies. This study will build on the candidate’s previous research using psychophysical and neuroimaging techniques to examine speech processing in listeners with normal hearing and sensorineural hearing loss. A strong mentorship team will contribute to research and career development. Dr. Marjorie Leek is the primary mentor and will provide guidance on behavioral testing in clinical populations (particularly blast exposure) and general oversight of the career development plan. Co-mentor Dr. Barbara Holshouser will coordinate imaging activities and provide expert training in neuroimaging including advanced imaging modalities, post-processing, and MR physics. Co- mentor Dr. Bruce Lyeth will provide research education in experimental traumatic brain injury (TBI) research. Co-mentor Dr. Grace Lee will provide training in cognitive assessment. Drs. Joseph Helpern and Christopher Stecker will provide consultation in diffusion-weighted imaging and functional auditory imaging, respectively. Training activities will include: workshops on grant writing, imaging methods, and TBI; consultation with research groups and clinicians involved in neuropsychology, audiology and TBI; and visits to Dr. Lyeth’s laboratory. The candidate will apply for NIH R01 and VA RR&D Merit Award funding toward the end of the award period. The candidate’s short-term career goals are to gain experience with new clinical populations, learn new neuroimaging modalities, and establish a background in TBI research. The candidate’s long-term career goal is to become an independent VA investigator in auditory rehabilitation science.