Effects of Non-Blast mTBI on Binaural Processing and Speech Understanding in Noise

Mild traumatic brain injury (mTBI) resulting from blast exposure or non-blast incidents such as falls, sports inju- ries, or motor vehicle accidents is prevalent in Operation Enduring Freedom and Operation Iraqi Freedom Vet- erans. In addition, mTBIs represent the leading causes of brain injury in the general US population. Individuals with a history of these brain injuries often report substantial difficulties understanding speech in noise despite having normal peripheral hearing sensitivity. Research using behavioral central auditory processing measures have established that Veterans and non-Veterans with a history of mTBI have binaural processing deficits that likely impact speech understanding in complex listening environments. However, the behavioral measures used in previous studies are unable to effectively tease apart how mTBI effects sensory or cognitive mecha- nisms underlying impaired binaural processing abilities and how these deficits relate to speech understanding in complex listening environments. This lack of knowledge severely limits our understanding of the neurophysi- ological consequences of mTBI and impedes the development of effective, individualized clinical diagnosis and treatment strategies for this patient population. The long-term goal of our research program is to develop tar- geted auditory assessment and rehabilitation tools for individuals with brain injury who have substantial audi- tory difficulties. Our specific objectives for this CDA-2 proposal are to identify the effects of mTBI on sensory and cognitive processing mechanisms and to identify bottom-up and top-down neural coding deficits that are predictive of speech understanding in complex listening environments in this clinical population. Specifically, we aim to identify patterns of bottom-up sensory and top-down cognitive neural coding deficits from electro- physiological measures designed to assess the neural encoding of binaural temporal cues and electrophysio- logical measures designed to assess the cognitive modulation of cortical responses to binaural speech stimuli. This proposal also aims to determine relationships between electrophysiological responses and behavioral measures of speech understanding in complex listening environments. We hypothesize that some Veteran and non-Veteran participants with mTBI will demonstrate degraded neural responses in electrophysiological measures designed to assess the bottom-up neural encoding of binaural auditory cues and those that are de- signed to assess the top-down cognitive modulation of cortical responses to speech. We further hypothesize that a combination of these electrophysiological measures will best predict performance on behavioral measures that assess speech understanding in complex listening environments and self-reported auditory diffi- culties. The expected outcomes of this research proposal include a better understanding of effects of mTBI on top-down sensory and bottom-up cognitive auditory processes that impact the perception of speech in noise. These results will have a significant positive impact by identifying neural mechanisms that should be prioritized in future studies that aim to further delineate the mechanistic and functional effects of mTBI. Together, this work will provide strong evidence for processing mechanisms that should be targeted in future research on the development of clinical auditory assessment and rehabilitation methods for civilians and Veterans with mTBI.